The Origin of the Pterosaur Sternal Complex

It is a common and mistaken paradigm that pterosaurs appeared out of nowhere, seemingly unrelated to other prehistoric reptiles. Those who say this (and the list is long) also judiciously avoid any discussions of pterosaurs as lizards or fenestrasaurs. Their investments in the outmoded and unsupportable archosaur hypothesis have not provided answers — and never will. Here we will take a look at the development of the sternal complex of pterosaurs evolving from the most parsimonious sister taxa yet discovered (Peters 2000a, 2007).

The Pectoral Girdle in Huehuecuetzpalli
The story begins with Huehuecuetzpalli (Reynoso 1998), a basal tritosaurid lizard with a fairly typical pectoral girdle (Figure 1). A T-shaped interclavicle and sinuous tapered clavicles anteriorly framed the short scapula and fenestrated but otherwise discoidal coracoid. A broad sternum was located at the posterior tip of the interclavicle. The coracoid was free to rotate between the clavicles, interclavicle and sternum, increasing the range of motion of the humerus.

The Pectoral Girdle in Cosesaurus
Several changes to this pattern can be seen in the basal fenestrasaur and tritosaur, Cosesaurus (Figure 1). The interclavicle developed an anterior process. The sternum moved anteriorly, now dorsal to the transverse processes of the interclavicle. The clavicles were shorter, no wider than the sternum and aligned with the anterior rim of the sternum. The coracoids were relatively larger and considerably narrower as the anterior fenestrations expanded until just the quadrant-shaped posterior rim remained. The scapula was strap-shaped with a long posterior process extending over several more dorsal ribs. With the sternum leading edge now anterior to the interclavicle trailing edge, the coracoids had no room to move and their ventral stems became socketed and essentially immobile, resembling the configuration in birds and serving as a precursor to the configuration in pterosaurs.


Tritosaur pectoral girdles demonstrating the evolution and migration of the sternal elements to produce a sternal complex.

Figure 3. Tritosaur pectoral girdles demonstrating the evolution and migration of the sternal elements to produce a sternal complex. Figure 1. The evolution of the pterosaur pectoral girdle and sternal complex featuring Huehuecuetzpalli, Cosesaurus, Longisquama, and the basal pterosaur, MPUM 6009.

The Pectoral Girdle in Longisquama
In Longsiquama (Figure 1) the interclavicle, clavicles and sternum are closely integrated, as in pterosaurs. Distinct from all other tetrapods, the clavicles curved posteriorly, extending to the posterior rim of the crescent-shaped sternum, which they frame. The cruciform interclavicle extended ventrally to form a small keel. Taphonomically displaced to beneath the throat, the overlapping clavicles were mistaken by Jones et al. (2000) for a bird-like furcula (fused clavicles in birds).

The Pectoral Girdle in Pterosaurs
In basal pterosaurs (Figure 1) there were few changes from the Longisquama pattern. So the sternal complex (Wild 1994), like many other aspects of pterosaur morphology, had evolved before the advent of large pterosaurian wings (Peters 2002, contra Bennett 2008).

All these changes could never have taken place if Cosesaurus was restricted to a typical quadrupedal configuration. The forelimbs had to become elevated from the substrate in a bipedal configuration, as imagined (based on morphology) in its phylogenetic predecessors, Lacertulus (Carroll and Thompson 1982) and Huehuecuetzpalli — and as evidence by matching Cosesaurus pedes to Rotodactylus tracks (Peters 2000b) which were ocassionally bipedal. Cosesaurus had a pectoral complex essentially and mechanically identical to that of pterosaurs (and broadly similar to that of birds). So it seems likely that it was also flapping, probably in some sort of territorial or mating ritual, long before gliding and flying were possible in its descendant taxa, Sharovipteryx, Longisquama and pterosaurs.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Bennett SC 2008. Morphological evolution of the forelimb of pterosaurs: myology and function. Pp. 127–141 in E. Buffetaut & D.W.E. Hone (eds.), Flugsaurier: pterosaur papers in honour of Peter Wellnhofer. Zitteliana, B28.
Carroll and Thompson 1982. A bipedal lizardlike reptile fro the Karroo. Journal of Palaeontology 56:1-10.
Elgin RA, Hone DWE and Frey E 2011. The extent of the pterosaur flight membrane. Acta Palaeontologica Polonica doi: 10.4202/app.2009.0145 online pdf
Jones TD et al 2000. Nonavian Feathers in a Late Triassic Archosaur. Science 288 (5474): 2202–2205. doi:10.1126/science.288.5474.2202. PMID 10864867.
Peters D 2000a. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Peters D 2000b. Description and Interpretation of Interphalangeal Lines in Tetrapods.  Ichnos 7:11-41.
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. Historical Biology 15: 277-301.
Peters D 2007. The origin and radiation of the Pterosauria. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.
Reynoso V-H 1998. Huehuecuetzpalli mixtecus gen. et sp. nov: a basal squamate (Reptilia) from the Early Cretaceous of Tepexi de Rodríguez, Central México. Philosophical Transactions of the Royal Society, London B 353:477-500.
Sharov AG 1970. A peculiar reptile from the lower Triassic of Fergana. Paleontologiceskij Zurnal (1): 127–130.
Wild R 1993. A juvenile specimen of Eudimorphodon ranzii Zambelli (Reptilia, Pterosauria) from the upper Triassic (Norian) of Bergamo. Rivisita Museo Civico di Scienze Naturali “E. Caffi” Bergamo 16: 95-120.

Sharovipteryx and the Origin of Pterosaurs

The bizarre hind-wing glider, Sharovipteryx mirabilis, has confounded and intrigued paleontologists since 1971 when Alexander G. Sharov first described it. The most prominent aspects of the fossil are the extremely long hind limbs, trailed by extensive extradermal membranes called uropatagia. While several readily visible aspects of Sharovipteryx immediately recall pterosaurs (attenuated tail, longer tibia than femur, hollow bones, elongated ilium. sacrum of way more than two vertebrae, elongated pedal 5.1, dermal membranes, among others), the reduced forelimbs were cause for concern in a pterosaur sister taxon.


Sharovipteryx mirabilis

Figure 1. Sharovipteryx mirabilis in various views. Click to learn more.

Poorly preserved?
has been described as “poorly preserved” even though the finest details are preserved in its extradermal membranes. The skull is crushed, but all the details are visible. Insects are preserved with and within Sharovipteryx. A beetle lies nearby. A winged ant or wasp is found in the skull. Shcherbakov (2008) described the Lagerstätte formation from which Sharovipteryx was found. The paleoenvironment (Madygen Formation, Osh Region in Kyrgyzstan, Early Triassic, 228 mya) may be reconstructed as an intermontane river valley in seasonally arid climate, with mineralized oxbow lakes and ephemeral ponds on the floodplain. After amber, it may be the best formation for preserving insects.

Early Errors
Peters (2000) attempted to trace the skull elements of Sharovipteryx, but I assumed the split at the back of the skull must have been a pterygoid. Instead it represents a domed cranium. In my rookie year as a paleontologist, I made several mistakes, this one among them. Later I was able to discern and correct my error. I also found more elements of the forelimb. All these can be seen here. No one else has attempted a detailed tracing and identification of the elements before or since.

Subsequent Corrections
There is word that some further preparation has occurred, according to Hone and Benton (2007), who reported, “In any case, the true arms of Sharovipteryx have now been found buried in the matrix (R. R. Reisz, pers. comm., 2003) and this confirms that Peters (2000) supposed arm was incorrectly identified.” It is not clear that Hone and Benton (2007) actually had access to the data itself, but in their zeal to discredit Peters (2000) they latched onto this hearsay. Unfortunately, the Reisz data have not been made available and have not been published in the eight years that have followed. Concurrently, as mentioned earlier, I was able to correct earlier mistakes. The forelimb elements I found matched left to right and fell in line in all morphological aspects between the two sister taxa of Sharovipteryx, Cosesaurus and Longisquama (Peters 2006). These included a tiny pteroid and preaxial carpal, bones otherwise found only in fenestrasaurs, including pterosaurs. I also identified prepubic bones and a hyper-elongated ilium in Sharovipteryx. These traits are also restricted to fenestrasaurs including pterosaurs. Prepubes acted like elongated pubes, adducting the sprawling hind limbs.

The pelvis and prepubes of Sharovipteryx.

Figure 2. The pelvis and prepubes of Sharovipteryx.

Truncated Studies
Following his  studies of the uropatagia in Sordes (Unwin DM and Bakhurina NN 1994), Dr. David Unwin (2000a, b, c) flirted briefly with Sharovipteryx as a pterosaur sister taxon, but has ignored it ever since. Unfortunately in his update he used Sharov’s own figure from 1971, rather than providing an updated figure.

In his book, The Pterosaurs From Deep Time, Unwin 2006 asked if Sharovipteryx could be ancestral to pterosaurs, then answered, “Probably not. Because it is almost the same geological age as early pterosaurs and, with its remarkably long neck, already highly specialized.” While referencing nearly every other paper and worker on pterosaurs, all papers written by yours truly (Peters 2000 and 2002 among them) were overlooked and ignored. Rather he opted to continue the old paradigm that, “pterosaurs sit in splendid isolation, definitely related to, but somehow remote from, other diapsids.” Unwin said there was no antorbital fenestra and the arms were extraordinarily short and small. While the latter is true, the former is not. Unwin never performed a cladistic analysis with Sharovipteryx and other pterosaur ancestor candidates to test the results of Peters (2000).

Following Gans et al. (1987), Dyke et al. (2006) described Sharovipteryx as a “delta-wing” flyer. Unfortunately they provided no evidence of membranes anterior to the hindlimb, but imagined them instead.

With such small forelimbs and such long hindlimbs, Sharovipteryx would have been a full-time biped, a fact that has been largely overlooked. As a biped, Sharovipteryx could have done other things with its forelimbs, such as gliding and flapping.

Sharovipteryx would have been a consummate glider. The enlarged hyoids extended the neck skin into strakes (leading edge root extensions), an aerodynamic structure found on several modern jet fighters. The ribs extended laterally, forming a small round pancake. Manual digit 4 extended further than the other digits. Since both sister taxa (Cosesaurus and Longisquama) had trailing edge membranes, it is likely that Sharovipteryx also had them. Rather than a delta wing, the membranes had a deeper chord distally, creating a canard wing configuration.

Due to the stem-like coracoid and strap-like scapula, Sharoviptyerx likely flapped its forelimbs, not only to show excitement and attract attention when grounded, but to create thrust and lift when aloft. The large, fiber embedded uropatagia that trailed the sprawling hind limbs of Sharovipteryx provided the majority of lift and extended through the center of balance. Other tiny membranes extended anterior to the lower tibia and mid femur. Longisquama was similar in configuration, but with longer forelimbs. Pterosaurs were also similar, but with even longer wing fingers.

The hind legs of Sharovipteryx provide a good model for the configuration of most pterosaurs, sprawling in flight. On land, whenever the knees were lower than the hip socket, which was probably typical, the right angled knees returned the ankles to beneath the torso, as in pterosaurs.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Dyke G, Nudds RL and Rayner JMV 2006. Flight of Sharovipteryx mirabilis: the world’s first delta-winged glider. Journal of Evolutionary Biology.
Gans C, Darevski I, and Tatarinov LP 1987. Sharovipteryx. A reptilian glider? Paleobiology 13(4):415–426.
Hone DWE and Benton MJ 2007. An evaluation of the phylogenetic relationships of the pterosaurs to the archosauromorph reptiles. Journal of Systematic Palaeontology 5:465–469.
Peters, D. 2000b. Description and Interpretation of Interphalangeal Lines in Tetrapods.  Ichnos 7(1):11-41.
Peters D 2000.
A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing—with a twist. Historical Biology 15:277-301.
Peters D 2006. The Front Half of Sharovipteryx. Prehistoric Times 76: 10-11.
Shcherbakov DE 2008. Madygen, Triassic Lagerstätte number one, before and after Sharov. Alavesia 2:113-124. online pdf
Senter P 2003. Taxon Sampling Artifacts and the Phylogenetic Position of Aves. PhD dissertation. Northern Illinois University, 1-279.
Sharov AG 1971. New flying reptiles from the Mesozoic of Kazakhstan and Kirghizia. – Transactions of the Paleontological Institute, Akademia Nauk, USSR, Moscow, 130: 104–113 [in Russian].
Unwin DM 2000a. Sharovipteryx: what can it tell us about the origin of pterosaurs?
48th Symposium of Vertebrate Palaeontology and Comparative Anatomy, Portsmouth,
England, Monday 28 Aug. – Sunday 3 September.
Unwin DM 2000b. Sharovipteryx and its significance for the origin of the pterosaur
flight apparatus. 5th European Workshop on Vertebrate Palaeontology, 27.6.2000
– 1.7.2000, Staatliches Museum für Naturkunde Karlsruhe (SMNK) Erbprinzenstr.
13 D-76133 Karlsruhe Germany (
Unwin DM 2006. The Pterosaurs from Deep Time. Pi Press, New York, NY.
Unwin DM and Bakhurina NN 1994. Sordes pilosus and the nature of the pterosaur flight apparatus. Nature 371: 62-64.
Unwin DM, Alifanov VR and Benton MJ 2000. Enigmatic small reptiles from the Middle Triassic of Kirgizia, pp. 177–186. In: Benton M. J., Unwin D. M. & Kurochin E. “The age of Dinosaurs in Russia and Magnolia”, Cambridge University Press, Cambridge.

Longisquama and the Origin of Pterosaurs

Prequel: Longisquama Gets No Respect
(or the Lengths Scientists Will Go to Protect Pet Theories)

In their two-part paper on pterosaur origins Hone and Benton (2007, 2008) announced they would test whether pterosaurs nested more parsimoniously within the Archosauria (Bennett 1996) or the Prolacertiformes (Peters 2000). They used the technique of the supertree, gathering several trees together to come up with a larger, ostensibly more complete, tree. That permitted them to use the data of others without having to visit fossils. We’ll get back to their results (below), but first a short background study.

Bennett (1996) used suprageneric taxa, for the most part, and nested pterosaurs with Scleromochlus at the base of the Dinosauria + Lagosuchus (now Marasuchus). The Ornithosuchidae were basal to this clade. The Prolacertiformes were nested far toward the base of the tree. Earlier we discussed problems with these putative sisters here. Bennett (1996) did not consider CosesaurusSharovipteryx and Longisquama.


Figure 1. Click to enlarge. Fenestrasaurs including Cosesaurus, Sharovipteryx, Longisquama and pterosaurs

Peters (2000) tested the matrices of Bennett (1996) and two others (Jalil 1991 and Evans 1986) simply by adding Langobardisaurus and the fenestrasaurs, including CosesaurusSharovipteryx and Longisquama. Pterosaurs nested with these taxa, rather than any archosaur or archosauromorph, when given the opportunity. Peters (2000) erected the clade, the Fenestrasauria, because they shared the trait of an antorbital fenestra without a fossa, convergent with that of archosaurs.

The largest study to date on reptile interrelationships nested Longisquama and pterosaurs with lizards like Lacertulus, Meyasaurus and Huehuecuetzpalli, far from Prolacerta, archosauromorphs, Scleromochlus and archosaurs.

Getting Back to Where We Began
Hone and Benton (2007) discredited the data of Peters (2000) and elected not to include any of it in their supertree. That left only one study that included pterosaurs, Bennett (1996), in their supertree analysis. Having eliminated the opposing candidate data and the opposing candidate taxa, the results were predetermined. The results of Hone and Benton (2008) reflected the results of Bennett (1996). Sadly, the results also nested members of the Choristodera far from the Choristodera and members of the Lepidosauromorpha far from the Lepidosauromorpha, so the study had its problems. Moreover, Hone and Benton (2008) falsely gave credit for the prolacertiform hypothesis to Bennett (1996), after properly giving it to Peters (2000) in their earlier (2007) paper. And now you know  the lengths scientists will go to protect their pet theories.

The Back Half of Longisquama
Ever since Sharov (1971) reported that only the front half of Longisquama was visible, scientists stopped looking for it. Ironically, one of the plumes illustrated by Sharov(1971), the one not radiating like the others, was a tibia and femur. The subdivided “feather shafts” reported by Jones et al. (2000) were actually displaced toes subdivided by phalanges. Here, using the technique of DGS (digital graphic segregation) the back half of Longisquama is, at last, revealed.

The complete fossil of Longisquama.

Figure 2. Click to enlarge. The complete fossil of Longisquama.

The back half of Longisquama was overlooked for so long because the elements lined up with and were camouflaged by the plumes. Apparently Longisquama’s stomach exploded, or was torn up. The front third of Longisquama is undisturbed, the tail is undisturbed, but the hips are turned backwards and the legs and feet are rotated up to the dorsal vertebrae.

Longisquama in lateral view

Figure 3. Longisquama in lateral view, dorsal view and closeup of the skull. Like Microraptor, Longisquama glided/flew with similarly-sized wings both fore and aft.

Distinct from Cosesaurus
The skull of Longsiquama had a more constricted snout, which enhanced binocular vision. The orbits were larger. The teeth had larger cusps. The naris was probably larger. With increased bipedalism and active flapping, Longiquama probably experimented with aerobic metabolism. The cervicals were shorter and the dorsal series was longer, especially so near the hips and between the ilia. The sacrum curved dorsally 90 degrees, which elevated the attenuated tail. These vertebral modifications made Longsiquama similar to a lemur, which also leaps from tree to tree. Such a long torso provided more room for plumes, gave the back great flexibility, and provided more room for egg production. The pectoral girdle was little changed from Cosesaurus. The clavicles curved around the sternal complex and the sternal keel was deeper. Fused together the interclavicle, clavicles and sternum form a sternal complex, as in pterosaurs. During taphonomy the sternal complex ofLongisquama drifted to beneath the cervicals, exactly where the clavicles are found in non-fenestrasaur tetrapods, including birds. This has led to confusion because the clavicles overlapped giving the appearance of a bird-like furcula. As in Cosesaurus, the pterosaur-like pectoral girdle and socketed coracoids enabled Longisquama to flap and generate thrust during leaps. The pelvis was greatly elongated anteriorly and posteriorly with a posterior ilium rising along with the dorsally curved sacrum of seven vertebrae. The pubis and ischium were much deeper, which provided a much larger pelvic aperture to pass a much larger egg. The distal femur was concave and the proximal tibia convex, as in Sharovipteryx. Both the femur and tibia/fibula were more robust. The foot was relatively large with digits of increasing length laterally. Pedal digit V had a curved proximal phalanx.

Longisquama is famous for, and was named for, its dorsal plumes. Another set of plumes arose from its skull and neck. Former caudal hairs (in Cosesaurus) formed a tail vane in Longisquama. As in Sharovipteryx and pterosaurs, Longsiquama had a uropatagium trailing each of its hind limbs. Like Cosesaurus, Sharovipteryx and pterosaurs membranes trailed the forelimbs, too. This documents the origin of the pterosaur wing and proves that it developed distally on a flapping wing (Peters 2002) rather than proximally as a gliding membrane (contra Elgin, Hone and Frey in press) and certainly without wing pronation, loss of digit V, loss of ungual 4 and migration of metacarpals I-III to the anterior face of metacarpal IV (contra Bennett 2008).

Longisquama was overloaded with secondary sexual characteristics. From plumes to flapping arms, Longisquama was all about creating an exciting presentation unrivaled until the present-day bird-of-paradise. Longisquama had everything Cosesaurus had, only wildly exaggerated. With increased bipedalism and active flapping, Longiquama probably experienced the genesis of aerobic metabolism.

Figure 4. Click to enlarge. The origin of the pterosaur wing and the migration of the pteroid and preaxial carpal. A. Sphenodon. B. Huehuecuetzpalli. C. Cosesaurus. D. Sharovipteryx. E. Longisquama. F-H. The Milan specimen MPUM 6009, a basal pterosaur.

The Origin of the Pterosaur Wing
The elongated and robust finger four of Longisquama was also overlooked by all prior workers. Reconstructed here the hand of Longsiquama remains the best transitional example between Cosesaurus and pterosaurs. It is likely that digit 4 did not flex with the other three fingers in Longisquama because the PILs (parallel interphalangeal lines) were not continuous through digit 4, which also supported a pterosaur-like wing membrane, preserved along with the other soft tissue, the plumes.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Bennett SC 2008. Morphological evolution of the forelimb of pterosaurs: myology and function. Pp. 127–141 in E. Buffetaut & D.W.E. Hone (eds.), Flugsaurier: pterosaur papers in honour of Peter Wellnhofer. Zitteliana, B28.
Elgin RA, Hone DWE and Frey E 2011. The extent of the pterosaur flight membrane. Acta Palaeontologica Polonica doi: 10.4202/app.2009.0145 online pdf
Jones TD et al 2000. Nonavian Feathers in a Late Triassic Archosaur. Science 288 (5474): 2202–2205. doi:10.1126/science.288.5474.2202. PMID 10864867.
Martin LD 2004. A basal archosaurian origin for birds. Acta Zoologica Sinica 50(6): 978-990.
Peters D 2000. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. Historical Biology 15: 277-301.
Senter P 2003. Taxon Sampling Artifacts and the Phylogenetic Position of Aves. PhD dissertation. Northern Illinois University, DeKalb, IL, 1-279.
Senter P 2004. Phylogeny of Drepanosauridae (Reptilia: Diapsida) Journal of Systematic Palaeontology 2(3): 257-268.
Sharov AG 1970. A peculiar reptile from the lower Triassic of Fergana. Paleontologiceskij Zurnal (1): 127–130.


Which Way Did Pterosaur Fingers Flex?

Pterosaur fossils have been known for over 200 years and yet we still argue about which way the fingers flexed. Undisturbed or minimally disturbed fossils give some clues. Fossil handprints do too.

Most pterosaur fossils are found crushed and the finger claws (unguals) are crushed along with the rest of the body (Figure 1). Tall and thin, like the raptorial, tree-clinging claws they were, pterosaur claws were most often preserved broad side down, tips pointing anteriorly, medial faces exposed. And that’s the way they are traditionally reconstructed, paying little attention to the cylinder-shaped interphalangeal joints or evolutionary precedent.

Right hand of Shenzhoupterus, dorsal view.

Figure 1. Click to enlarge. Right hand of Shenzhoupterus, dorsal view. Note the crushing of the ungual broadside down and disarticulation of the various finger joints. If you ignored these facts you might want to orient the claws anteriorly as shown.

There are Two Hypotheses at Work Here
In the traditional model (Bennett 2008) metacarpals 1-3 were stacked with #1 on top and all three were bound to metacarpal 4. See Figures 2 and 3. The fingers flexed anteriorly (in flight). Any specimens in which metacarpals 1-3 were found lined up anterior to metacarpal 4, Bennett (2008) ascribed to the effects of gravity on the bones after death.

Pterosaur hand dorsal view

Figure 2. Pterosaur hands, dorsal view, the two opposing hypotheses. See Figure 3 for anterior views to see how Bennett (2008) intended the fingers to stack with #1 on top. The Bennett configuration orients the fingers facing up when the hands are adducted (brought together), which would be unsuitable for tree climbing/clinging. The Peters configuration, like clapping hands, points the fingertips together, ideal for tree climbing.

In the heretical yet more conservative model (Peters 2002) metacarpals 1-3 lined up side-by-side anterior to metacarpal 4 and the fingers flexed ventrally (as in all other tetrapods). See Figures 2 and 3. In this configuration only metacarpal 4 twisted 90 degrees axially so the palmar side faced posteriorly (in flight) to facilitate wing folding. The three small fingers did not change their configuration or orientation. The palmar side of fingers 1-3 continued to point ventrally in flight.

Peters (2002) reported: “…in certain Cretaceous forms [the medial three digits] rotated into the vertical plane and became closely appressed to the much larger metacarpal IV (Bennett, 1991; 2000b). From this configuration, they flexed anteriorly in a subhorizontal plane when the wing was extended.” I apologize for this. Unfortunately I was influenced by Dr. Bennett at the time. Subsequent studies helped me realize the error of this statement.

Bennett and Peters pterosaur finger orientation configurations

Figure 3. Bennett (2008) and Peters (2002) pterosaur finger orientation configurations. See Figure 2 for dorsal views. Note: Bennett wanted digit 1 dorsal as shown here, not as in Figure 2.

The Wellnhofer (1991) Twist
Wellnhofer (1991) lined up the metacarpals anteriorly, but also twisted the unguals anteriorly. Of course, this could be a problem in pterosaurs with fingers of similar lengths and does not take into account the various disarticulations at several finger joints.

The Evolution of the Bennett (2008) Configuration
Bennett (2008) imagined the evolution of the pterosaur hand (Figure 4) based on an imaginary taxon. He started with the supination of the entire arm, which rotated all the palmar surfaces anteriorly. Metacarpal 4 became thicker than the others as it supported a lengthening wing finger. Overlooked by Bennett (2008), but implicit in his arguments, the next step involved migration of the metacarpals 1-3 as a unit down the anterior face of a much larger metacarpal 4. The supination of the hand envisioned by Bennett (2008) ultimately included reversing the flexion and extension of digit 4 such that hyperextension folded the wing in his view. No other tetrapod has ever done this. Also note there is no space for the large wing extensor between the attached metacarpals (1-3 back-to-front with 4) in the Bennett (2008) configuration. Bennett (2008) also envisioned the early disappearance of ungual 5 on the wing, which, due to supination, also faced anteriorly in this configuration. However, the wing ungual was retained. Bennett (2008) also imagined the loss of manual digit 5, but manual digit 5 was retained. He did not envision an origin for the preaxial carpal and pteroid, which occurred as far back as Cosesaurus (Peters 2009).

Pterosaur finger orientation in lateral view

Figure 4. Pterosaur finger orientation in lateral view, the two hypotheses. There are several problems with the Bennett (2008) hypothesis, least of all it leaves no room for the big wing extensor tendon.

The Evolution of the Peters (2002) Configuration
Peters (2002) discussed the evolution of his pterosaur hand configuration (Figure 4) based on actual taxa, including Longisquama insignis. Between Longisquama and the first pterosaur manual digit 4 was rotated axially so that the palmar (flexor) surface became the new posterior surface to facilitate wing folding in the plane of the wing with hyperflexion. Digit 5, already reduced, became a vestige. It revolved, along with metacarpal 4, to the new dorsal side of the metacarpus. During the rotation, metacarpals 1-3 shifted to the ventral rim of metacarpal 4 while retaining their configuration. This left plenty of room for a large extensor tendon (Figure 4). Contra Bennett (2008), flexion remained flexion in all the fingers. There was no reversal of function. The preaxial carpal and pteroid first appeared in the fenestrasaur and pterosaur precursor, Cosesaurus (Peters 2009) having migrated to the medial wrist from the central carpus where they were identified as the two centralia seen in Sphenodon.

Here’s Where the Trouble Started
In many crushed fossils, like  Shenzhoupterus (Figure 1) and the left hand of Eudimorphodon (Figure 5), the claws point anteriorly because they are crushed broadside down. In order to do this, some finger joints must disarticulate and this is always observable.

But look what happens in the right hand of Eudimorphodon
In the same Eudimorphodon (Figure 5) the right hand has the palmar surface exposed. The metacarpals were lifted and flipped over the palmar surface of metacarpal 4, but metacarpal 3 remained attached to metacarpal 4. Moreover the unguals pointed posteromedially. According to Bennett (2008) this should not have been possible if metacarpals 1-3 were bound to the anterior face of metacarpal 4 and pointed anteriorly. Digit 1 should have been buried first and deepest, but it was not.

Eudimorphodon hands.

Figure 5. Eudimorphodon hands. The right hand preserves the metacarpals lined up anteriorly with only metacarpal 3 attached to metacarpal 4. The claws are disarticulated due to crushing. The right hand, preserved with its palmar side exposed shows what happens when the lighter digits 1-3 drift as a unit with metacarpal 3 moving the least because it was attached to metacarpal 4.

Santanadactylus hand and fingers

Figure 6. Click to enlarge. Santanadactylus hand with metacarpals preserved at a 45 degree angle to the anterior face of metacarpal 4.

Even in 3D Fossils There Can be Some Confusion
Here, in this 3D Santanadactylus hand, the metacarpals have been raised like a drawbridge, far from their original orientation (palmar side down) and close to being pressed against the large metacarpal 4, palmar side anterior. Such a configuration permits no space for the big tendon between the appressed surfaces of metacarpal 4 and the three small metacarpals. This is an excellent example of taphonomic lifting on the hinge at the metacarpal 3-4 interface from an origin with the palmar side down for metacarpals 1-3. It is the only configuration that permits the big extensor tendon of metacarpal 4 to run unimpeded dorsal to the three small metacarpals and their extensor tendons (Figure 4). When the extensor tendon rots or pops, there is nothing to prevent metacarpals 1-3 from rising and falling like an airplane elevator in the drifting sea currents.

The left manus of Pteranodon KUVP 49400

Figure 7. The left manus of Pteranodon KUVP 49400 in a rare anterior burial. Here the unguals were crushed in their natural orientation, palmar side down. Thanks to M. Everhart at for this image.

YPM 49400 – a Rare Anterior Burial and Posterior Exposure in Pteranodon
Here in a Pteranodon specimen YPM 49400 metacarpus was preserved anterior face down. This is a very rare burial. The manus was preserved intact and in its natural orientation, fingers 1-3 palmar side down and metacarpal 4 palmar (flexor) side now posterior for wing folding. The claws here pointed ventrally as in other tetrapods. The metacarpals lined up as in other tetrapods. Buried like this there was no chance for them to wave around or become disarticulated in the bottom currents. The proximal wing phalanx would have stood vertically erect (essentially the Z-axis) in this configuration, but it rotted, disarticulated and fell on its dorsum, exposing its ventral face.

Pteraichnus nipponensis

Figure 8. Pteraichnus nipponensis (Lee et al. 2009) along with a matching trackmaker, n23 of Wellnhofer 1970.

The Evidence of Ichnites
Pterosaur handprints (Figure 8) commonly preserve digits 1 and 2 laterally and digit 3 posteriorly. Sometimes digit 1 extends anteriorly (Lee et al. 2009). In the Bennett (2008) configuration, all three fingers would have extended posteriorly when quadrupedal because the arms were supinated. In the Peters (2002) configuration, all three fingers would have extended laterally when quadrupedal because the arms were neither supinated nor pronated, but in the neutral position.

Digit 3 Goes the Opposite Way
The key to orienting digit 3 posteriorly (and sometimes digit 1 anteriorly) goes back to the lizard ancestry of pterosaurs. The metacarpophalangeal joint of digit 3 is different than digit 2. Shaped more like a hemisphere than a cylinder, it permits digit orientation in several directions. With digit 3 directed posteriorly, digit 4 never touched the substrate. This evidence is in direct contrast with the configuration envisioned for the difficult to support wing launch hypothesis currently in favor also illustrated here.

On a Side Note
Bennett (2008) connects the extensor and flexors tendons to the proximal tips of the first wing phalanx. This is wrong. In lizards these tendons split in half, bypassing the nearest points and inserting further down the bone. As shown here, this permits complete wing folding, something the Bennett (2008) attachment is unable to do. We just learned about the evolution of the manus hand and pteroid from a Sphenodon-like ancestor here.

Bennett (2008) reported, “The reconstruction of the long extensor and flexor of thewingfinger suggests that there was no rotation of Mc IV about its long axis. If there had been a rotation, the tendon of m. flexor digiti quarti would have had to spiral posteriorly under the metacarpus to insert on the posterior process of wing phalanx 1 and the tendon of m. extensor digiti quarti longus to digit.” As mentioned above, it is a mistake to attach the flexor to the posterior process of wing phalanx 1, as Bennett (2008) proposes. Rather the insertion is further down the phalanx shaft, as in lizards, and in order to complete wing folding. Thus there would have been no problematic spiraling if metacarpal 4 rotated axially.

In Summary
All the evidence points to a configuration in which the hand of pterosaurs was configured the same as in all other tetrapods with the exception that the big wing finger was axially rotated so that the old palmar surface became the new posterior surface. This configuration is based on actual predecessor taxa, not figments of Bennett’s imagination. The new configuration creates a large channel for the massive extensor tendon that is missing from the Bennett configuration. The new configuration does not require the flexor side of the wing finger to become the extensor side and vice versa.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Bennett SC 2008. Morphological evolution of the wing of pterosaurs: myology and function. Zitteliana B28:127-141.
Lee YN, Azuma Y, Lee H-J, Shibata M, Lu J 2009. The first pterosaur trackways from Japan. Cretaceous Research 31, 263–267.
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. – Historical Biology 15: 277–301.
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29:1327-1330.
Wellnhofer P 1991. The Illustrated Encyclopedia of Pterosaurs. Salamander Books, Limited, London, 192 pp.

The Pterosaur Pteroid …and Preaxial Carpal

Nyctosaurus pteroid

Figure 1. Nyctosaurus pteroid and preaxial carpal

The Bone(s) At the Center of the Argument
The pteroid and the preaxial carpal are two unique wrist bones not seen in other tetrapods, only pterosaurs (and their kin). The pteroid is longer, pointier and tends to bend around the anterior distal radius (Figure 1). The preaxial carpal (always getting second billing) is short, stout and provided with a sesamoid in a dorsal pit (Bennett 2007). Dr. David Hone’s (2008) post “That troublesome pteroid” reported, “Thanks to a lack of direct pterosaurian ancestors in the fossil record [ed. note: Hone refuses to accept fenestrasaurs as pterosaur precursors] we are not sure where the pteroid comes from…” Workers have argued over the pteroid’s homology, articulation and orientation. Here are some answers based on the fossil record (Figure 1).

What is a Pteroid?
Early workers, like Goldfuss (1831) considered the pteroid digit #1 and the wing finger digit #5. Williston (1911) dismissed that hypothesis and wondered if the pteroid were 1) a misplaced first metacarpal or centrale; 2) a bone derived from sinew (tendon) or 3) an elongated sesamoid. Unwin et al. (1996) determined that the pteroid was a true bone, but could not determine its homology. Peters (2002) reported that the pteroid and preaxial carpal were homologs to centralia 1 and 2 of Romer (1956) because pterosaurs do not have centralia in their primitive positions. Peters (2009) found a tiny pteroid and preaxial carpal in Cosesaurus a phylogenetic precursor to pterosaurs. Here the pteroid and preaxial carpal are homologous with the centralia of Sphenodon, a taxon that shares a common ancestor with pterosaurs at the base of the Lepidosauria. Their migration of the centralia to the medial rim of the wrist is discussed below (Figure 2).

Figure 2. Click to enlarge. The origin of the pterosaur wing and the migration of the pteroid and preaxial carpal. A. Sphenodon. B. Huehuecuetzpalli. C. Cosesaurus. D. Sharovipteryx. E. Longisquama. F-H. The Milan specimen MPUM 6009, a basal pterosaur.

Pteroid Articulation
Early hypotheses placed the base of the pteroid in the dorsal pit of the preaxial carpal (Marsh 1882, Hankin and Waton 1914, Frey and Riess 1981, Padian 1984, Wilkinson et al. 2006). Bennett (2007) pointed out that the cup was preoccupied by a sesamoid and indicated that the pteroid articulated to the side of the preaxial carpal in a minor depression. Peters (2009) pointed out that Bennett’s (2007) own data demonstrated that the base of the pteroid actually articulated with the saddle-shaped joint on the proximal syncarpal (radiale) and only weakly contacted the proximal medial surface of the preaxial carpal.

Which Way Did the Pteroid Point?
Concerning its relative position some authors have postulated an anteriorly facing pteroid able to rotate to a variety of postions based on the suggested higher aerodynamic efficiency of a larger propatagium (Frey and Riess 1981; Wilkinson et al. 2006) and able to swivel while inserted within the preaxial cup. Both are wrong. All articulated pteroids face medially (Bennett 2007) on the anterior face of the radiale (proximal syncarpal, Peters 2009). The pteroid was a passive element, as are all carpals, moving only slightly as the propatagium was tightened or relaxed (Figure 6).

Carpus of Sphenodon

Figure 3. Click to Enlarge. Carpus of Sphenodon comparing the medial and lateral centralia to the pteroid (yellow) and preaxial carpal (blue) of pterosaurs.

How Did the Pteroid Come to Be?
A pterosaur precursor, Cosesaurus, also has a tiny pteroid, a preaxial carpal and its attendant sesamoid (Peters 2009), indicating a nonvolant origin for this unusual carpal configuration. Subsequent study has shown that Sharovipteryx and Longisquama also have a pteroid and preaxial carpal. Fenestrasaurs and all tritosaurs following Lacertulus and Huehuecuetzpalli lack centralia.

More primitive tetrapods, like Sphenodon, have centralia the same shapes as the pteroid and preaxial carpal in fenestrasaurs (Figure 3). The medial centrale in Sphenodon is long and pointed medially. The lateral centrale is short and stout. Neither are insertion or origin points for any muscles or tendons. They are only spacers, keeping the distal carpals apart from the proximal carpals. After migration in tritosaurs the distal carpals articulated with the proximal carpals. The intermedium and pisiform disappeared during the process, probably by fusion and reduction respectively.

Wing evolution in pterosaurs

Figure 4. Wing evolution in pterosaurs. Click to enlarge and animate.

Considering their original and ultimate orientation, the medial carpal likely emerged first, turned the corner, and was followed by the lateral carpal, which moved distally to the anterior face of the first distal carpal. Relieved of the intervening centralia, the distal carpals articulated with the proximal carpals as they do in pterosaurs. The migration time appears to be at the Lacertulus Huehuecuetzpalli stage in which the entire carpus was poorly ossified.

Why Did The Pteroid and Preaxial Carpal Migrate?
At the Lacertulus / Huehuecuetzpalli stage the ancestors of pterosaurs were practicing bipedal locomotion (Carroll and Thompson 1982, Peters 2000) in a manner similar to living lizards capable of bipedal locomotion (Snyder 1954). Peters (2000) provided ample evidence for this. In addition, between Huehuecuetzpalli and Cosesaurus the coracoid fenestrations expanded leaving only the posterior rim as a strut. That meant the former sliding coracoid disc upon the interclavicle and clavicle was reduced to an immovable stem inserted into a cup, as in pterosaurs and birds. Since pterosaurs and birds flap their forelimbs, we can imagine that Cosesaurus, provided with the same type of coracoid, did so as well. Cosesaurus also developed precursor wing membranes in the form of trailing aktinofibrils (Peters 2009), which supports the flapping hypothesis. Since the pteroid was intimately associated with the propatagium in pterosaurs, it is safe to assume that some sort of minor propatagium was present in Cosesaurus, too (Figure 6). A propatagium is a passive restraint on elbow overextension and increases the wing area. Perhaps Cosesaurus gained stability and thrust from flapping while running with these new dermal extensions. Or perhaps these were secondary sexual traits, or both!

Evidence from Fenestrasaur Manus Tracks
The manus tracks pointed directly anteriorly in Rotodactylus and other lepidosaurs, while in pterosaurs they pointed laterally. By this evidence, Cosesaurus, a likely trackmaker of Rotodactylus and a pterosaur precursor, was still able to pronate its forarms. Pterosaurs could not. The loss of the intermedium and the straightening of the radius and ulna reduced the ability of the forearm to suppinate and pronate in pterosaurs (contra Bennett 2008). This continued straightening and lack of pronation was a product of bipedal locomotion together with flight restraints. For the same reasons neither bats nor birds can pronate or supinate their forearms.

Rotodactylus tracks

Figure 5. Click to enlarge. Rotodactylus tracks. These digitigrade tracks match to Cosesaurus (Peters 2000). Digit 5 is posterior to the other four digits in both the manus and pes. The hands imprinted very close to the center line of the track.

How Did the Centralia Migrate?
The two centralia of Sphenodon, were neatly and inconspicuously tucked in at the same level with the rest of the carpals. When they migrated to a medial position on the wrist both were elevated above the traditional wrist contours, acting like twin violin bridges to pull the extensor tendon taut whenever the elbow was extended for flight. Originally the centralia were not insertion points for any muscles or tendons. Rather all the hand muscles and tendons passed over them and at right angles to their long axis (Figure 3).  In pterosaurs the extensor tendons pass over the pteroid parallel to the long axis.

The migration of the centralia occurred when the carpus was poorly ossified. The centralia migrated little by little, generation after generation, while the manus was elevated in a bipedal configuration and while it was not being used for traditional quadrupedal lizard-like locomotion. In living lizards capable of bipedal locomotion, the hands do nothing. That was likely the case in the Late Permian/Early Triassic prior to the appearance of Cosesaurus. By then the hands were flapping, based on the morphology of the Cosesaurus coracoid (see above).

The Propatagium and Overextension of the Elbow
It is a common mistake in reconstructing pterosaur wings to overextend the elbow and extend the propatagium to the neck (Frey et al. 2006; Bennett 2007,  2008; Prondvai and Hone 2009). Bats and birds rarely extend the elbow more than 20 degrees beyond a right angle and pterosaurs would have been the same (Peters 2002). All available evidence indicates that the propatagium stretched between the pteroid and deltopectoral crest, not the neck.

Bennett’s (2008) Incorrect Reconstruction
Bennett (2008) proposed a strange reconstruction of pterosaur wing myology (musculature) by reversing the extensors and flexors. He reported, “…the range of motion of the metacarpophalangeal joint of digit IV migrated posteriorly so that flexor muscles spread the wing and extensor muscles folded it.” Bennett (2008, fig. 3) insisted that the pterosaur forearm was suppinated (radius over ulna in dorsal view) but illustrated the wing in the neutral position (radius anterior). His reconstruction failed to inserte and originate no muscles or tendosns on the dorsal or palmar sides of the distal carpus, and only two minor slips on the proximal carpus, contrary to the pattern of all other tetrapods.

Prondvai and Hone’s (2009) Big Hypothetical Tendon
Prondvai and Hone (2009) attempted to model a hypothetical biomechanical automatism to keep the wings open in flight in order to save energy, as in bats and birds. They proposed a propatagial ligament or ligamentous system which, as a result of the elbow extension, automatically performed and maintained the extension of the wing finger during flight and prohibited the hyperextension of the elbow. Unfortunately they overextended the elbow in their pterosaur and bat reconstructions, they avoided involving the pteroid in their ligament and they offered no evolutionary pathway to produce their hypothetical ligament. Instead, their imagined ligament ran like a rope through the middle of the propatagium. Nothing like this has ever been recovered in the soft tissue fossil record. Actually the biomechanical automatism was the entire propatagium, a likely homolog to the m. extensor digitorum longus of lizards and Sphenodon.

Sphenodon hand muscles

Figure 6. Sphenodon hand muscles

Extensor Muscles and Tendons
In lepidosaurus, such as Sphenodon (Haines 1939) and Polychrus (Moro and Abdala 2004), the m. extensor digitorum longus originates on the distal humerus, splits four ways distally and inserts on the proximolateral side of metacarpals 1-4, passing over both centralia (Figure 6). M. extensor digitorum longus does not extend to the base of the first phalanx as Bennett (2008) proposed and Prondvai and Hone (2009) supported in Sphenodon, but it may have done so in pterosaurs to facilitate automatic wing extension with elbow extension Figure 7). The short digital extensors of digits 1-3 originate on the intermedium (in Sphenodon) and insert on the distal metacarpals of digits 1-3 with a tendon extending to the ungual. They short extensors also pass over the lateral centrale. The short digital extensors of digit IV originate on the ulnare and distal ulna. The short digital extensor of digit 5 originates on the ulnare and pisiform.

Figure 7. Pterosaur wing flexed and extended, compared to Cosesaurus and demonstrating the hypothetical movement of the loosely articulated pteroid and preaxial carpal.

In fenestrasaurs, including pterosaurs, the intermedium fused to the radiale which served to straighten out the extensors of digits 1-3 in line with metacarpals 1-3. If the long extensor from the humerus did not send a slip to the wing finger, then the extensor digitorum brevis of digit 4 that originated on the distal ulna extended the wing finger. This tiny muscle seems unworthy due to the mass, moment and drag the giant wing finger would exert on it. Rather the expanded long extensor, now called the propatagium likely pulled the pteroid during elbow extension. The pteroid, in turn, pulled the wing finger open.

Sesamoids are defined as skeletal elements that develop within a continuous band of regular dense connective tissue (tendon or ligament) adjacent to an articulation or joint (Jerez et al. 2009). Lepidosaurs don’t have sesamoids on their centralia. They would have developed on the preaxial carpal at the point of stress.

In summary
According to the evidence, the pteroid and preaxial carpal were former centralia that had migrated to the medial wrist during early experiments with a bipedal configuration. The pteroid pointed medially and was a passive bone, unable to change its orientation except within the confines of a stretched or relaxed propatagium. During elbow extension, the propatagium pulled the pteroid medially, which extended the wing finger via a new set of tendons not seen in Sphenodon because its centralia were not involved in any tendons or muscles.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Bennett SC 2007. Articulation and Function of the Pteroid Bone of Pterosaurs. Journal of Vertebrate Paleontology 27(4):881–891.
Bennett SC 2008. Morphological evolution of the wing of pterosaurs: myology and function. Zitteliana B 28 127-141.
Carroll and Thompson 1982. A bipedal lizardlike reptile fro the Karroo. Journal of Palaeontology 56:1-10.
Frey E and Riess J 1981. A new reconstruction of the pterosaur wing.Neues Jahrbuch für Geologie und Paläontologie Abhandlungen 161:1–27.
Goldfuss GA 1831. Beiträge zur Kenntnis verschiedener Reptilien der Vorwelt. Nova acta Academiae Caesareae Leopoldino−Carolinae Germanicae Naturae Curiosorum 15: 61–128.
Haines RW 1939. A revision of the extensor muscles of the forearm in tetrapods. Journal of Anatomy 80(1):211-233.
Hankin EH and Watson DSM 1914. On the flight of pterodactyls. The Aeronautical Journal 18: 324–335.
Jerez A, Mangione S and Abdala V 2009. Occurrence and distribution of sesamoid bones in squamates: a comparative approach. Acta Zoologica (Stockholm) doi: 10.1111/j.1463-6395.2009.00408.x
Marsh OC 1882. The wings of pterodactyles. American Journal of Science, 23, 251-256.
Moro S and Abdala V 2004. Análisis descrptivo de la miología flexora y extensora del miembro anterior de Polychrus acutirostris (Squamata, Polychrotidae). Papéis Avulsos de Zoologia 44(5):81-89.
Padian K 1983.
Osteology and functional morphology of Dimorphodon macronyx (Buckland) (Pterosauria: Rhamphorhynchoidea) based on new material in the Yeal Peabody Museum. Postilla, 189.
Peters D 2000. Description and Interpretation of Interphalangeal Lines in Tetrapods.  Ichnos 7:11-41.
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. – Historical Biology 15: 277–301.
Peters D 2009. 
A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29:1327-1330.
Prondvai E and Hone DWE 2009. New models for the wing extension in pterosaurs’,Historical Biology 20 (4):237-254.
Snyder RC 1954. The anatomy and function of the pelvic girdle and hind limb in lizard locomotion. American Journal of Anatomy 95:1-46.
Unwin DM, Frey E, Martill DM, Clarke JB and Riess J 1996. On the nature of the pteroid in pterosaurs. Proceedings of the Royal Society of London B 263:45–52.
Wilkinson MT, Unwin DM and  Ellington CP 2006. High lift function of the pteroid bone and forewing of pterosaurs. Proceedings of the Royal Society of London B 273:119–126.

The Tiniest Pterosaur: No. 6


Figure 1. Click to enlarge. Scaphognathus and its shrinking descendants through No. 6 (the tiny one in the middle) at the base of the increasingly larger Germanodactylus clade.

A Little Background
Earlier we looked at the tiny pterosaurs that phylogenetically succeeded the smaller specimens of Scaphognathus (Figure 1). Analysis (see below) indicates that these tiny taxa, not Darwinopterus, were the true transitional taxa appearing at the bases of every major pterosaur clade. While many of these tiny pterosaurs have been traditionally labeled “Pterodactylus,” not all of them actually nested with Pterodactylus, as noted earlier. While virtually all tiny pterosaurs were considered juveniles (Wellnhofer 1970, Bennett 2006), phylogenetic analysis indicates they were not.

In preparation for our look at the Germanodactylus clade (coming soon and previewed in Figure 1), we’ll examine the pipsqueak at its base. Wellnhofer (1970) considered this diminutive specimen a hatchling Pterodactylus prior to the discovery of embryo pterosaurs in their eggs that disprove the hypothesis of allometric growth in pterosaurs.

Meet B St 1967 I 276 (or No. 6 in the Wellnhofer 1970 catalog).
No. 6 is the single tiniest of all pterosaurs. It stands no taller than the shoulders of other tiny pterosaurs including several ready-to-fly embryos (Figure 2). No. 6 is about the size of the smallest bird, the bee hummingbird (Figure 2). Both are several times larger than the smallest living lizard, Sphaerodactylus ariasa (Hedges and Thomas 2001).  If No. 6 is not an adult, we know from the examples of embryo pterosaurs, that it is a good copy of an adult. It’s sisters are only slightly larger than No. 6, so it unlikely to be a hatchling of an eight times larger taxon. Perhaps No. 6 would have become an adult upon reaching the size of the other tiny pterosaurs that were its sisters. No. 6 is smaller than the four pterosaur embryos now known (Figure 2).

The smallest known pterosaur, bird and lizard.

Figure 2. From left to right, the smallest known bird (a bee hummingbird), the smallest known pterosaur (No. 6) and and the IVPP pterosaur embryo all to scale. Below them is the smallest known lizard, a living gecko, Sphaerodactylus ariasa.

No. 6 in Detail
Other than its size, there was nothing particularly “juvenile” about No. 6. Yes, it had large eyes, but these were inherited from its ancestors, sisters to Scaphognathus and Ornithocephalus.  The anterior teeth were procumbent (tilted forward), with the two medial teeth merging to become one (or else losing one tooth in the process).  The quadrate was set at a low angle, perhaps as a buttress to some sort of pecking motion. Descendant taxa, like No. 12, No. 23 and Germanodactylus rhamphastinus (Figure 1) successively developed a longer, sharper set of jaws, ideal for spearing or pecking. Distinct from its phylogenetic predecessor, No. 31, the cervicals and torso were longer. The deltopectoral crest was squared off. The humerus was straight. The metacarpus and ulna were longer. The hind limb was more robust. The unguals were shorter. Pedal digit 5 was straight and shorter. PILs (parallel interphalangeal lines) indicate a digitigrade pes was more likely.

The smallest pterosaur. No. 6

Figure 1. The smallest of all adult pterosaurs, B St 1967 I 276 or No. 6 in the Wellnhofer (1970) catalog. At left is the foot plantigrade and with metatarsals slightly raised, which simplifies and aligns the PILs (parallel interphalangeal lines). The gray oval is a hypothetical egg based on the pelvic opening. The sternal complex is also shown separated from the lateral view reconstruction.

The Problem With Being So Small
Recent studies of the world’s smallest lizards have revealed a problem with desiccation due to their high surface/volume ratio. With a snout/vent length under 18mm, Sphaerodactylus specimens dry up and die when removed from their moist leaf litter environment (Hedges and Thomas 2001). The younger, smaller juveniles of No. 6 would have been likewise faced with desiccation — only more so with their wings and uropatagia outstretched. With a snout/vent length around 5 mm, No. 6 hatchlings would have been smalller than house flies. It appears likely that such tiny pterosaurs were not ready to fly when hatched unless they had a novel method for conserving moisture with their wings out and flapping. Rather, they may have found their food by walking on all fours through moist leaf litter until large enough to take wing.

Pterosaur family tree

Figure 3. Click to enlarge. The family tree of the Pterosauria. No. 6 appears midway down the right column.

Why Did Pterosaurs Shrink?
We can look at the family tree of pterosaurs to see why pterosaur transitional taxa were much smaller than their predecessors and successors. After the appearance of the tiny pterosaurs, the larger predecessors became extinct. Evidently there were environmental pressures that were not favorable to larger pterosaurs. Evidently a faster maturation at a smaller size became a survival advantage.


Ginko leaf

Figure 4. Ginko leaf and the smallest pterosaur and its hatchling to scale.

How Did Pterosaurs Shrink?
Chinsamy et al. (2008) reported that Pterodaustro specimens were sexually mature at half their full size. If eggs were proportional to the pelvic passageway, then such young mothers would have produced smaller eggs and smaller embryos. This provides a method for rapid size reduction and generational turnover after just a few generations. The ultimate disappearance of pterosaurs may be blamed on their inability to shrink enough after achieving their greatest sizes by the end of the Cretaceous. Note there was no pterosaur genus that existed unchanged throughout the Mesozoic. Rather a succession of taxa, both larger and smaller, survived through that era.

Hone and Benton’s Three Mistakes
Hone and Benton (2006) reported, “The remarkable extinct flying reptiles, the pterosaurs, show increasing body size over 100 million years of the Late Jurassic and Cretaceous, and this seems to be a rare example of a driven trend to large size (Cope’s Rule).” They arrived at this “result” by drawing a straight line from early pterosaurs, like Anurognathus, to the Late Cretaceous pterosaur, Quetzalcoatlus over time and by deleting all purported juveniles. They did not realize that 1) there were four pterodactyloid-grade lineages; 2) the purported juveniles were actually adults; and 3) any sort of a roller-coaster effect of size increase/decrease/increase/decrease over time would be negated by drawing a straight line.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Bennett SC 2006. Juvenile specimens of the pterosaur Germanodactylus cristatus, with a review of the genus. Journal of Vertebrate Paleontology 26:872–878.SMNS
Chinsamy A, Codorniú L and Chiappe LM 2008. Developmental growth patterns of the filter-feeder pterosaur, Pterodaustro guinazui. Biology Letters, 4: 282-285.
Hedges SB and Thomas R 2001. At the Lower Size Limit in Amniote Vertebrates: A New Diminutive Lizard from the West Indies. Caribbean Journal of Science 37:168–173.
Hone and Benton 2006. Cope’s Rule in the Pterosauria, and differing perceptions of Cope’s Rule at different taxonomic levels. Journal of Evolutionary Biology 20(3): 1164–1170. doi: 10.1111/j.1420-9101.2006.01284.x
Wellnhofer P 1970. Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Abhandlungen der Bayerischen Akademie der Wissenschaften, N.F., Munich 141: 1-133.

No Wingtip Claw in Pterosaurs? Look Again!

No Wing Tip Claw?
It has long been a paradigm of pterosaur studies that the wing (manual digit 4) lacked an ungual or claw (Bennett 2008). In most pterosaurs, even well articulated ones, there is no obvious wing claw. There is only a trochlear joint a the tip of the wing. Such a joint typically marks the “knuckle” between phalanges, in this case at the base of the missing fifth phalanx, the ungual. The presence of a trochlear joint at the wing tip, even on Late Cretaceous pterosaurs like Pteranodon (Figure 1), indicates the likely presence of another phalanx, the ungual, now missing (displaced) or invisible due to poor ossification.

Pteranodon wing tip

Figure 1. Pteranodon wing tip from the Fick Museum. No wing claw here, only a trochlear (pulley-like) joint.

When Did the Wing Ungual Disappear?
Pterosaur predecessors, including Cosesaurus, Sharovipteryx and Longisquama all retained a curved claw on digit 4, the digit which elongated to become the wing in pterosaurs. A basal bat retained unguals on all the wing digits, but other bats don’t.

A wing claw might be as useful, like a bumper on a car, to protect the wingtip from damage. With a keratin sheath, any sustained damage would sooner or later be shed during nail growth.

Bennett’s Vision
Dr. Chris Bennett (2008) envisioned the origin of the pterosaur wing with the palmar side of the wing finger facing anteriorly, with former flexors becoming wing extensors and vice versa. In Bennett’s (2008) hypothesis the wing claw point would have been oriented anteriorly originally. But that orientation would have tended to hook on passing obstacles in Bennett’s view, so it disappeared. Only those pterosaurs who failed to develop a wing ungual survived, because they would not have hooked themselves on tree trunks. Unfortunately, Bennett (2008) did not present any real ancestors, just hypothetical and rather cartoonish line drawings. So his hypothesis is based solely on imagination and its execution presents many evolutionary hurdles.

Evidence for a Wing Claw
Here you’ll see evidence of articulated wing unguals in several pterosaurs. On pterosaurs that apparently lack wing unguals you’ll see the ungual displaced from the tip, overlooked nearby.

pterosaur wing ungual

Figure 2. Pterosaur wing ungual almost articulated to manual 4.4 from an unidentified Chinese pterosaur.

Here’s a Great Example
In his blog, Dr. David Hone published the above image of the distal phalanx of an unidentified pterosaur wing on June 29, 2010. Dr. Hone reported phalanx 4 tapers to a point, which it does not (unless one considers the claw tip the point). Actually phalanx 4 tapers to a trochlear interphalangeal joint. Dr. Hone wrote, “Finally it often has a slight pathology (as seen here) of the tip curving to point slightly (or occasionally, very) posteriorly. [And more here]”

Dr. Hone did not realize that his purported “pathology” was actually just a very healthy fifth phalanx, the ungual or claw, which is traditionally considered missing in pterosaurs. As you can see, it is very much present here. It’s not big. It’s not dangerous. It’s just there. Often it is disarticulated, hence its apparent absence.

Other examples of wing claws can be seen here using mouse rollovers to see both the specimen and the interpretation. Below is a Shenzhoupterus wingtip with ungual. If you can’t find it, a guide can be found here.

find the wing ungual

Figure 3. See if you can find the wing ungual on this photo of Shenzhoupterus. It is in the vicinity of the wing tip, but disarticulated. 

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Bennett SC 2008. Morphological evolution of the forelimb of pterosaurs: myology and function. Pp. 127–141 in E Buffetaut and DWE Hone eds., Flugsaurier: pterosaur papers in honour of Peter Wellnhofer. Zitteliana, B28.

Manual Digit 5 in Pterosaurs: It’s Still in There!

Figure 1. Manual digit 5 in pterosaurs. Click to access a webpage of several examples.

Time to Bust Some More Paradigms
There are three traditional hypotheses that go back over 200 years. The experts tell us all pterosaurs lack: (1) manual digit 5, (2) the wing ungual and (3) the pedal digit 5 ungual. We can see all three on predecessor taxa, such as Huehuecuetzpalli, Cosesaurus, Sharovipteryx and Longisquama, so it’s a wonder why at least some vestige isn’t preserved in Triassic pterosaurs, at the very least. Some snakes and whales retain vestiges of “lost” limbs. Some birds retain wing claws. So it seemed worthwhile to look more closely at pterosaurs to see if some vestiges of these three traits were retained.

Here we’ll take a look for them one at a time.

pterosaur wings

Figure 2. Click to enlarge. The origin of the pterosaur wing and whatever became of manual digit 5. A. Sphendon. B. Huehuecuetzpalli. C. Cosesaurus. D. Sharovipteryx. E. Longisquama. F-H. MPUM 6009, a basal pterosaur. Manual digit 5 is in green. Metacarpal 5 is a darker green. Carpal 5 is yellow. The reduction to a vestige began with Huehuecuetzpalli. Digit 5 was no longer than metacarpal 4 in Cosesaurus and Longisquama. It became half that size in basal pterosaurs and relatively much smaller in derived pterosaurs with an elongated metacarpus.

Manual Digit V
The pterosaur wing is a former hand that has been modified for flight (Figure 2), but in a different way than those of birds and bats. There was once some controversy over the identity and homology of the pterosaur fingers and pteroid. Goldfuss (1831) interpreted the pteroid as digit 1 and the wing finger as digit 5. However, Owen (1870) interpreted the wing as digit 4 and argued the pteroid was not a digit, an idea that is now universally accepted. That leaves us with a digit 5 that apparently disappeared sometime in the ancestry of pterosaurs (according to Bennett 2008) because no one had ever seen digit 5 in any pterosaur. After all, why look for something that isn’t supposed to be there? You’d have to be a heretic to test such a tradition.

Manual Digit 5 was Retained as a Vestige in Every Pterosaur
In the chaos of broken bone at the base of the metacarpal 4 in every pterosaur you’ll usually find the tiny remnants of digit 5, its metacarpal and carpal if you look hard enough. As in basal fenestrasaurs (Figure 2), digit 5 had three phalanges, including a sharp, curved ungual. The ungual helps to identify the rest of the finger. Typically the finger elements were curled one way or the other during taphonomy. Several samples can be seen here.

There was likely no use for this finger. Whether it was covered in skin or remained visible cannot be determined. It appears, as you might expect, on the former posterior margin of the metacarpal 4. In all pterosaurs metacarpal 4 has been rotated 90 degrees to facilitate wing folding in the plane of the wing. That’s why digit 5 now appears on the present dorsal surface of metacarpal 4 (contra Bennett 2008). The presence of digit 5 on the dorsal surface indicates that the entire metacarpal 4 rotated on its axis, rather than twisting only at its distal end.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Bennett SC 2008. Morphological evolution of the forelimb of pterosaurs: myology and function. In: Buffetaut E, Hone DWE, editors. Zitteliana Series B, 28 (Special Volume: Flugsaurier: pterosaur papers in honour of Peter Wellnhofer). p. 127–141.
Goldfuss GA 1831. Beitränge zur Kenntnis verschiedener Reptilien der Vorwelt. Nova acta Academiae Caesareae Leopoldino−Carolinae Germanicae Naturae Curiosorum 15: 61–128.
Owen R 1870. A monograph on the fossil Reptilia of the Liassic Formations. III. Part II: Order Pterosauria. – Monograph of the Palaeontological Society, London: 41-81
Unwin DM, Frey E, Martill DM, Clarke JB, and Riess J. 1996.
On the nature of the pteroid in pterosaurs. Proceedings of the Royal Society of London B 263:45–52.

Jeholopterus, the Vampire Pterosaur

Just another anurognathid?
Jeholopterus – 
IVPP V12705 (Wang et al. 2003) Late Jurassic/Early Cretaceous was a highly derived anurognathid with excellent preservation of bones, wings and other soft tissues, such as hair (pycnofibres). The enormous curved claws (like surgeon’s needles), the robust limbs, plus the powerful feet (like can openers) were easy to see and distinct from those of most anurognathid pterosaurs.

Figure 1. Click to enlarge. Comparing data gathering results using first-hand observation with the DGS method on the skull of Jeholopterus.. The digital outlines were then transferred into the reconstruction.

DGS solves the mystery of the skull. 
First-hand observations of the black, crushed skull of Jeholopterus resulted in a disappointing broad outline offering no possibility of a reconstruction (Figure 1 inset). On the other hand, the DGS (Digital Graphic Segregation) method was successful in segregating the chaotic jumble into separate bones (Figure 1). These could be digitally reconstructed into a skull (Figure 2) in which all the parts had symmetrical counterparts and all the parts fit precisely. DGS also enabled tracing of the tail from beneath various soft tissues. Note the palate bones had slid to the left toward the wrist and digit 1 of the right hand (in orange-red) was dislocated on top of the skull.

Jeholopterus in lateral view. This image supersedes others in having the coracoids extending laterally and other minor modifications.

Figure 2. Jeholopterus in lateral view. This image supersedes others in having the coracoids extending laterally and other minor modifications.

The reconstructed skull had an architecture that followed the patterns of other anurognathids (such as Anurognathus and Batrachognathus),  but with several important distinctions. Added together these traits  create  what appears to be our best candidate for a vampire pterosaur (Peters 2003). By all appearances, Jeholopterus was built to latch on to a dinosaur and sink its fangs between the tough scales and beneath the outer skin, probably at the most vulnerable corners and crevices. Here’s how the vampire hypothesis was put together, trait by trait.

The highly derived skull.
While the skull of Jeholopterus followed the pattern of other anurognathid pterosaurs, it also was quite specialized for its unique diet.  Jeholopterus had lost almost all of its upper teeth. Only a few stubby premaxillary teeth were present, but these were spanned by two large curved maxillary fangs. Moreover, these fangs were mounted on an upwardly curving jaw line, at right angles to the (atypical for pterosaurs) anteriorly leaning jaw joint. That jawline curve created the possibility of an enormous gape, similar in arc and shape to that of a rattlesnake. And we all know how a rattlesnake bites.The lower jaws rose up anteriorly to match that upper curve. The dentary teeth were no larger than the “teeth” on a pair of pliers, unable to pierce prey.


Figure 3. Click to enlarge. The skull of Jeholopterus. The palate bones are identified: pmx = premaxilla, mx = maxilla, q = quadrate,v = vomer, p = ectopalatine, pt = pterygoid, mxs = palatal process of the maxilla.

The vampire skull in action.
Anurognathid skulls were very fragile (less bone and more air than a typical pterosaur) and Jeholopterus was no different. Unusual for anurognathids, Jeholopterus had a robust palate reinforcing the roots of its twin fangs (Figure 2). The palate bones (vomers, ectopalatines (=fused ectopterygoid and palatine) and pterygoids) were shaped and arranged to distribute the forces of impact  from the front of the jaws to the sides and rear. Such an architecture tells us that Jeholopterus was banging its fangs on prey, probably to penetrate tough hide. The enormous gape permitted the lower jaw to get out of the way to maximize penetration. After the stabbing, the skull could roll forward, locking the fangs horizontally beneath the skin for maximum adhesion. With the jaw joint now elevated, the mandibles could close down on a  rise of skin behind the wound to “milk” the blood out. Remember, the lower teeth were incapable of penetration.

Surgeon’s needles for claws.
The robust limbs and extra-large claws could have been used to hold on to the bucking victim without getting shaken off. Adding the fangs made a total of five points of adhesion. The lengthening of metatarsal 5 provided more leverage for digit 5 to press against the victim’s skin, enabling toe claws 1-4 to flex and dig in deeper like an old-fashioned church key can opener. What more could a vampire pterosaur want?

How to deal with the inevitable flies. 
Since Jeholopterus would have been immobilized by its fangs and claws while feeding, it would have been defenseless against biting insects also attracted to the blood. To keep insects away, the pycnofibres (pterosaur hairs) were extra long and the tail could have been whipped around, like a horse’s tail to keep flies from landing.

The origin and evolution of blood-sucking in anurognathids.
The origin of vampirism in anurognathids was probably not much different than its origin in bats and vampire birds, including tickbirds. Anurognathids are widely considered to have been airborne insect-eaters due to their wide gaps and fragile skulls. Various insects, like flies and their maggots, are attracted to wounds on large mammals and we can presume that dinosaurs also carried bloody wounds at times. Anurognathids would have been attracted to such accumulations of blood-loving insects. Some anurognathids might have been attracted to the blood itself. A few, such as Jeholopterus, apparently skipped all the preliminaries and created its own wounds on dinosaur prey.

For those who don’t like technology
While the DGS/Photoshop technique has attracted a fair number of naysayers, the results should speak for themselves. The tracings of the bone in Jeholopterus revealed matching paired elements. Those were digitally transferred to the reconstruction and every bone fit. The bone shapes were similar to those in sister taxa. This is contra the results of Bennett (2007) who was unable to identify several bones and misidentified others using a camera lucida on a private anurognathid specimen. He reconstructed a monster skull unlike that of any other anurognathid.

Response to a criticism
Kellner et al. (2009) wrote: “Right after the description of J. ningchengensis by Wang et al. (2002), Peters (2002) argued that the wing membrane in the Chinese taxon did not reach the ankle but extended only to the elbow. However, despite the fact that no trailing edge of the posterior portion of the plagiopatagium is clearly discernible, an extensive portion of soft tissue that is attributable to the wing membrane is closely associated with the hind limbs, particularly with the tibiae (figure 3a). Apparently, Peters (2002), who based his studies on photographs, has only identified the limits of the actinopatagium that indeed terminate at the articulation of the humerus with radius and ulna, but the tenopatagium extends up to the ankle.” Below is their figure 3a, along with the complete plate. Yes, there is fossilized soft tissue there, as elsewhere around the bones (green areas). A closer look at the area shows the fibers near the left tibia are layed out in several directions, not just in the direction of the wing. Moreover, contra Kellner et al. (2009) the trailing edge of the wing is clearly defined (in yellow) all the way to the femur and identical to the wings of other distantly related pterosaurs, like Pterodactylus. Kellner et al. (2010) merely labeled areas within the fossil without defining borders of their hypothetical membrane, making sure that their closeup image did not include the trailing edge of the wing membrane that Peters (2002) had identified.

The purported tenopatagium of Jeholopterus

Figure 4. Click to enlarge. The purported tenopatagium of Jeholopterus. Note Kellner et al. 2010 cropped out the key portion of the trailing edge of the wing membrane that documents how narrow the wing becomes aft of the elbow. 

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Bennett SC 2007. A second specimen of the pterosaur Anurognathus ammoni. Paläontologische Zeitschrift 81(4):376-398.
Kellner AWA, Wang X, Tischlinger H, Campos DA, Hone DWE and Meng X 2010. The soft tissue of Jeholopterus (Pterosauria, Anurognathidae, Batrachognathinae) and the structure of the pterosaur wing membrane. Proc Royal Soc B 277: 321–329.
Peters D 2003. The Chinese vampire and other overlooked pterosaur ptreasures. Journal of Vertebrate Paleontology 23(3): 87A.
Wang X, Zhou Z, Zhang F and Xu X 2002. A nearly completely articulated rhamphorhynchoid pterosaur with exceptionally well-preserved wing membranes and “hairs” from Inner Mongolia, northeast China. Chinese Science Bulletin 47(3): 226-230.


Seven problems with the pterosaur wing launch hypothesis

How did pterosaurs take off?
Unlike bats and birds, pterosaurs, the first flying vertebrates, are all extinct. We can only imagine what it was like to see them fly, land and take off. It used to be thought that pterosaurs hung by their toes from cliff faces, like bats, before becoming airborne. Illustrations have shown pterosaurs clinging inverted to tree branches ready to drop off. The idea of bipedal pterosaur running downhill into a headwind was floated by Chatterjee and Templin (2004), but had few takers. Bennett (1997) imagined the first pterosaurs leaped with their hind limbs into flight. Padian (1981) imagined they ran first.

Vaulting pterosaur

Figure 1. Click to see video of the Habib/Molnar quadrupedal pterosaur launch sequence.

Recent studies by PhD candidate, Mike Habib (2008 pdf), on the dynamics of the pterosaur launch proposed a novel launch method (Figures 1 and 2.) based on the technique of Desmodus rotundus, the vampire bat (Schutt et al. 1999). The vampire launches itself quadrupedally, principally with its muscular forelimbs, opening its wings at the peak of its launch. Habib documented the great disparity in the comparative sizes and strengths of pterosaur forelimbs vs. the hindlimb and how these compared to bird limbs. Vampire bats are similar. In birds, Habib reported, the hind limbs provide the primary source of propulsion for launch, either by running or leaping, immediately before the wings take over. Habib reported that pterosaurs have stronger forelimbs than hindlimbs, just the opposite of birds. Thus, Habib reasoned, since all pterosaurs were quadrupeds (this is questionable, see below), they must have employed their strongest limbs, their wings  (this is also questionable, see below), to take off. Habib reported, “…expected flapping forces alone seem insufficient, at present, to explain the structural strength ratios of pterosaurs; it is simply implausible that pterosaurian lift coefficients would be several times higher than in birds…”

Dr. David Hone agreed with this hypothesis, “If pterosaurs actually launched like birds, with the hindlimbs producing most of the force, then the ratio of their bone strength values should be about the same as birds, and big pterosaurs should have slender wing bone elements (especially the proximal elements, like the humerus) and big, robust hindlimb bones.  Well, it turns out that the opposite is actually the case: pterosaurs have whopping huge forelimb bones and very slender hindlimb bones, and this gets more exaggerated in big pterosaurs.” Dr. Mark Witton is in agreement here. Julia Molnar created a video to demonstrate the Habib hypothesis here. Other images can be seen here and here. The forelimb launch seems to have become the widely accepted view.

Of course, evidence for either the Habib (2008) hypothesis or the heretical bird-style take-off would have to come in the form of take-off tracks. Unfortunately we don’t know of any such tracks. We only know of one landing trace (Mazin et al. 2009, Figure 3), which came in feet first. So let’s examine each of the precepts of Habib’s hypothesis and see if any problems arise.

Animation of a landing pterosaur matched to tracks.

Figure 3. Click to animate. Animation of a landing pterosaur matched to tracks based on Mazin et al. (2009).

Problem 1. Did all pterosaurs have stronger forelimbs than hindlimbs? While most pterosaurs had a larger diameter humerus than femur, not all did. Basal pterosaurs up to, but not including, Carniadactylus and Eudimorphodon had a humerus no thicker than the femur. The same held true for a few Rhamphorhynchus specimens, Sordes, some Pterodactylus specimens and at least one germanodactylid. The flightless pterosaur had a tiny humerus. Some pterosaurs, such as Zhejiangopterus and Fenghuangopterus had a much shorter humerus than femur. Thus the femur in these taxa would have had much more leverage, travel and associated muscles to launch the pterosaur further and higher. Why did some pterosaurs have a large diameter or longer humerus? I don’t know. I can’t see a clear phylogenetic pattern yet.

Kangaroo skeleton compared to a leaping pterosaur skeleton

Figure 4. Click to enlarge. Here a kangaroo skeleton is compared to that of a quadrupedally leaping pterosaur. The red arrows on the kangaroo indicate the major muscle pulls employed during each leap. Even with all those muscles, tensioned tendons and bone levers, kangaroo leaps only lift the toes to the height of the ankles. The pterosaur is less well-equipped with only a single major lever, the elbow as it straightens a very short distance, bound by the propatagium in front _preventing_ overextension of the elbow.

Problem 2. Were the proximal forelimb elements long enough and strong enough to produce the required height and airspeed during a forelimb launch to deploy the very long folded wings and initiate the first flap?
Habib (2008) reports the forelimb bones were more than strong enough. Were they long enough? The answer seems doubtful if one compares the skeleton of our greatest living leaper, the kangaroo with that of a pterosaur (Figure 4). Despite having five muscle groups from pelvis to toe contracting in a coordinated series, kangaroo initial leaps raise the toes only to the heights of the ankles. By contrast, pterosaurs had only their elbows and wrists to extend and they could extend their elbows a relatively shorter distance, not counting the effect of the propatagium, which in birds and bats prevents exactly this sort of overextension of the elbow. Even the vampire bat leaves 15-20 degrees of flex at the elbow during takeoff.

Problem 3. The evolutionary pathway to forelimb launching.
Pterosaur predecessors were bipeds that ran and/or launched themselves with their hind limbs. Basalmost pterosaurs were also bipeds, unable to touch the substrate with their hands. Nevertheless, the vast majority of pterosaurs had forelimbs long enough to touch the substrate without bending over. Thus quadrupedal locomotion was secondarily derived. That’s why pterosaur finger tracks point laterally and posteriorly instead of anteriorly, as in other tetrapods. At this point the forelimb launch pattern could have evolved. Meanwhile, the forelimbs/wings of pterosaurs were used for flapping, flying, clinging to tree trunks and, in beachcombing genera forelimbs supported the anterior torso while walking quadrupedally. However, in beachcombers, the forelimbs produced no forward propulsion vectors. The planted hands were never behind the elbows and shoulders. Pterosaur forelimbs acted more like crutches than traditional walking forelimbs (see walking pterosaur movie here).

So were does the impetus for a forelimb launch come from? There doesn’t appear to be an evolutionary sequence demonstrating the gradual acquisition of a rapid extension of the forearm at the magnitude needed to launch a pterosaur high enough to extend the wings and initiate flapping. Nor does there appear to be any change in pterosaur shoulder and elbow morphology that would signal such a change in behaviour. Moreover, flexion of the elbow would be constrained by the long pteroid in a few taxa and extension would be constrained by the propatagium in all taxa. The question of why most pterosaurs have a thicker, stronger humerus than femur remains unanswered. No skeletal correlates have been documented yet to gauge a large humerus or a large antebrachium with large claws, a large head or any other skeletal character. Such characters change one way or the other within genera such as Pteranodon and Rhamphorhynchus.

Problem 4. The margin of error.
Avoiding the narrow margin of error and calamity a pterosaur experiences every time it would have to snap open its (sometimes huge) wings during a forelimb launch (thousands of times during a lifetime) without ever striking the ground seems to tempt the odds. Moreover, the power and coordination needed to complete the vault and extend the wings in this fly-or-crash scenario gives little room for a learning curve in younger weaker pterosaurs or in the evolution of such a maneuver from predecessors that were already adept at performing the standard hind limb leap.

Here are three animations of Pteranodon (Figures 5-7) in the process of taking off. Two portray the widely accepted wing launch model (one successful and one not successful) and one portrays the heretical bird-style launch from a standing pose. Click each one to see the animation.

Successful Pteranodon wing launch based on work by Habib (2008).

Figure 5. Click to animate. Successful Pteranodon wing launch based on work by Habib (2008). Here the initial propulsion is strong enough to provide sufficient thrust and height for Pteranodon to unfold its wings and get off one downstroke. The timing of this launch has been slowed down from its hypothetical speed to help show what happens.

Unsuccessful Pteranodon wing launch based on Habib (2008).

Figure 6. Click to animate. Unsuccessful Pteranodon wing launch based on Habib (2008) in which the initial propulsion was not enough to permit wing unfolding and the first downstroke.

Successful heretical bird-style Pteranodon wing launch

Figure 7. Click to animate. Successful heretical bird-style Pteranodon wing launch from a standing start into a headwind  in which the hind limbs produce the initial thrust and the first downstroke takes over immediately in the manner of birds. Without a headwind a running start, like an albatross, would probably be required (see below). Note three wing beats take place in the same space that only one wing beat takes place in the Habib/Molnar model.

Problem 5. Opening time for the big wing finger.The vampire bat is able to snap open its wings in an instant at the acme of its leap because the finger bones are mere splints with little mass and therefore little momentum and drag. Moreover they can curl like human fingers and extend at least as quickly. Not so pterosaur wing fingers. They are long, stiff and massive by comparison. In many pterosaurs, the wing phalanges are the largest bones in the specimen, and some can exceed the skull in length. That means, relative to the vampire bat, the largest pterosaur wings would have to take longer to accelerate to opening speed, and then promptly decelerate to a stop at the acme of the leap. The individual phalanges didn’t bend much at all, so the entire wing finger acted like a very long rod or pipe. Imagine swinging a 1-2 meter length of PVC pipe through 180 degrees and stopping it precisely. That’s what a big pterosaur wing would have to do if it snapped open in the moments after lift off in the Habib/Molnar model. By contrast, in the bird-style model, a less snappy pterosaur can take as much time as a bird does to open its wings.

Nyctosaurus in lateral view

Figure 8. With such a short humerus it appears unlikely that this Nyctosaurus could achieve a forelimb launch of sufficient height and length to deploy those long wing fingers before crashing.

Problem 6. Incorrect reconstruction.
The Habib/Molnar reconstruction of the Anhanguera manus (Figure 9) had a few inaccuracies. The three free fingers were way to short. All pterosaurs have subequal metacarpals 3 and 4. Thus finger three (and usuallly two and one) always extended beyond the big wing metacarpolphalangeal joint. Ichnites confirm this. Only digits 1-3 ever appear in fossil handprints. The folded knuckle of finger 4 never made an impression. If knuckle 4 never touched the substrate then it could not be used to pinch the extensor tendon during the vault/preload phase of the quadrupedal launch, which was supposed to store energy prior to a sudden release of impulse power. Also, the metacarpals of all pterosaurs actually lined up anteriorly, as in all tetrapods, including lizards and fenestrasaurs. The Habib/Molnar reconstruction incorrectly placed metacarpals 1-3 on the dorsal (in flight) surface of metacarpal 4.

Errors in the Habib/Molnar reconstruction of the pterosaur manus

Figure 9a. Click to enlarge. Errors in the Habib/Molnar reconstruction of the pterosaur manus. The fingers were too small and incorrectly placed. The extensor tendon should not run over the extensor tendon process. It should split and run on either side of the knuckle.

The so-called catapult mechanism in pterosaurs

Figure 9b. Left: The so-called catapult mechanism in pterosaurs. Right. The actual design of pterosaur (in this case Anhanguera/Santandactylus) fingers.

The Habib/Molnar model illustrated the extensor tendon running over the extensor tendon process (which would have enabled it to be pinched during launch. However, if pterosaurs followed lizards and other tetrapods (as they probably did), the extensor tendon would have split at the knuckle, inserting on both sides of the first phalanx a short distance away from the knuckle. Such a split would have prevented the sort of tendon pinching called for in the Habib/Molnar model. Flexors were similar and a more distal insertion point permitted complete wing folding (shown in gray below, and more on that subject later) than would have been impossible with the Habib/Molnar model with insertion at the flexor process tip. That sort of engineering would only have permitted folding the wing at right angles before running out of pull.

Problem 7. Did pterosaurs really have weak hind limbs?
On Habib reported, “For all of the large pterosaurs, insufficient strength was present in the hindlimbs to initiative a bird-like launch. In addition, for most species, a bipedal launch position would have placed the wings at an inappropriately high angle of attack.” Of course, modern stilts, flamingoes and storks do very well in their bird-style launches, even with their “stilt”-like leg bones. Bending forward at the hip would have positioned the wings at the appropriate angle of attack (see below).

Bipedal lizard video marker

Figure x. Click to play video. Just how fast can quadrupedal/bipedal lizards run? This video documents 11 meters/second in a Callisaurus at the Bruce Jayne lab.

As descendants of bipedal lizards with sprawling femora (see below), pterosaurs would have run like highly improved lizards (see video), likely during take-off without a headwind. The Bruce Jayne lab in Cincinnati, Ohio, documented a zebra-tailed lizard, Callisaurus draconoides, running quadrupedally and later in the video, bipedally (Irschick and Jayne 1999). Note, the heels of the lizard never touch the ground. Speeds reach 5 m/sec or 11 mph for a 10 cm (snout/vent) length. Note that footfalls occur every three body lengths. That’s fast! What could a better-equipped pterosaur do? We’ll have to wait for running/takeoff footprints to find out, but here is an animation of a running Quetzalcoatlus to fire-up your imagination (Figure 10). Successive footfalls occur at a snout to vent length, which is quite a distance!

Quetzalcoatlus running like a lizard prior to takeoff.

Figure 10. Quetzalcoatlus running like a lizard prior to takeoff. Click to animate.

Albatross video showing take-off

Figure 11. Click for video. Albatross take-off in a no headwind situation. Running and flapping provides sufficient airspeed to provide lift.

It is important to note that bird legs and pterosaur legs had one fundamental difference: bird femora are not only tucked close to the body they are constrained by torso skin from moving much. So bird strides really begin at the knee. Birds elongate their metatarsals to produce yet another flexible leg section (the so-called “backwards knee”). By contrast, pterosaur femora swing from the hip and they all had relatively short metatarsals. Thus any mathematical comparisons, like those performed by Habib (2008) between the two types of flyers are going to be affected by this basic difference.

We end with a video (Figure 11) of several albatross taking off in a no headwind situation. With headwind an albatross need only unfold and lift its wings to become airborne.

I respect Mike Habib’s mathematical studies on pterosaurs, but I remain his loyal opposition for the reasons listed above. We have corresponded on this problem without swaying each other. Look for more studies from him in the near future. We’re both looking for those elusive pterosaur take-off tracks that will prove our hypotheses!

Every week since it was originally posted, this report on pterosaur wing launching has been a very popular post. There’s more to this story in a more recent blog here.

Bennett SC 1997.
The arboreal leaping theory of the origin of pterosaur flight. Historical Biology 123(4): 265-290.
Chatterjee S and Templin RJ 2004.
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