Microsaurs. Amphibians? Reptiles? Or Both?

Microsaurs (literally “little lizards”) have traditionally been nested with amphibians, like nectrideans (think of the boomerang-headers Diplocaulus and Diploceraspis). However a recent large survey nested two traditional microsaurs, Tuditanus and Utaherpeton, within the Reptilia, close to Anthracodromeus and Westlothiana.

That’s the problem I’m working on at present. I’ve added more microsaurs and finding loss of resolution. Results will follow when clarified. May miss a day or two in the meantime.

The Truth About “Toothless” Pterosaurs

While basal pterosaurs had lots of teeth, and certain derived pterosaurs, like SoS 2179 and Pterodaustro had dozens to hundreds of teeth, certain pteroaurs appear to have been toothless – or so they say…

I’ll just cut to the chase.
Apparently “toothless” pterosaurs, like Pteranodon, Nyctosaurus and Tapejara actually had one tooth at the tip of the premaxilla and one tooth at the tip of the dentary. That’s how the tips were able to become and remain so sharp. Like the wing ungual and manual digit V, these single teeth have been overlooked by all prior pterosaur workers. The evolution of these single anteriorly-directed teetth can be documented in predecessor taxa among the germanodactylids, but it is still not clear whether one tooth became smaller or the two anterior teeth fused to become one. It seems reasonable that these teeth would be replaced with new teeth at the root, but this has not yet been documented. In the images below, if there was a “next” tooth, it is not apparent.

Figure 1. Click to enlarge. The tooth at the tip the mandible of KUVP 66130, a nyctosaurid.

Figure 2. Click to enlarge. The tooth at the tip of the rostrum of KUVP 66130, a nyctosaurid.

In the nyctosaurids above the dentary and premaxillary tooth tips are shown.

Figure 3. Click to enlarge. The tooth at the tip of the rostrum of Tapejara.

The Evolution of “Toothless” Pterosaurs
In several pterosaurs the medial or first premaxillary tooth was procumbent. It angled forward as well as downward. In B St 1967 I 276 (No. 6 of Wellnhofer 1970), the tiniest pterosaur, the anterior premaxillary tooth was procumbent.

Figure 4. The rostrum of No. 6 in which the teeth at the tip were procumbent, but not anteriorly oriented.

In the specimen from the Senckenberg-Museum Frankfurt a. M. No. 4072, (No. 12 of Wellnhofer 1970) the anterior tooth was oriented further anteriorly and may have been a single tooth.

Figure 5. The jaw tips of No. 12

In Germanodactylus the SMNK-PAL 6592 specimen, the anterior premaxillary and dentary teeth were fully anterior in orientation.

Figure 6. Germanodactylus skull with anteriorly-oriented jaw tips

Figure 7. Germanodactylus SMNK skull drawing showing anteriorly-oriented teeth at jaw tips.

You’ll find anteriorly oriented teeth in all germanodactylids and their sharp-snouted descendants including dsungaripeterids, shenzhoupterids, tapejarids, nyctosaurids, eopteranodontids and pteranodontids. I have not been able to closely examine the anterior jaws of azhdarchids, but photographic examination appears to show tiny (< 1 mm) teeth lining the jaws of Quetzalcoatlus sp.

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.

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 Cosesaurus, Sharovipteryx 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 Cosesaurus, Sharovipteryx 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.

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.

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.

References:
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.

wiki/Longisquama

The Origin and Evolution of Ichthyosaurs

Wikipedia reports: “Ichthyosaurs thrived during much of the Mesozoic era; based on fossil evidence, they first appeared approximately 245 million years ago (mya) and disappeared about 90 million years ago, about 25 million years before the dinosaurs became extinct. During the middle Triassic Period, ichthyosaurs evolved from as-yet unidentified land reptiles that moved back into the water, in a development parallel to that of the ancestors of modern-day dolphins and whales.”

Well, here we’re going to identify those “unidentified” land animals as the ancestors of ichthyosaurs: They’re mesosaurs.

It’s not difficult to figure out which reptiles were the closest to ichthyosaurs. All you have to do is include representatives from all the major groups and run their characters through phylogenetic analysis. The ones most like ichthyosaurs will nest on a tree closest to ichthyosaurs. Such a study has never been undertaken before. Typically a few suprageneric (above the level of genus) taxa are chosen. This gives no opportunity for individual genera within a larger clade to step up to the plate and nest as sister taxa. This also allows cheating, the ability to cherry pick traits from several taxa within a clade to create a chimaera taxon.

Here is the tree I used (and it keeps growing and growing):

Figure 2. The Reptile Family Tree and the Evolution of Ichthyosaurs. Here you can trace the evolution of ichthyosaurs (represented by Utatsusaurus) all the way back to Ichthyostega.

Past Mistakes Based on Using Suprageneric Taxa
Motani (1998) linked ichthyosaurs to Younginiforms  or to Coelurosauravus (a gliding reptile) through Saurosternon. Unfortunately, Motani used suprageneric taxa and did not include mesosaurs.

Maisch (2010) nested ichthyosaurs either with mesosaurs within the diapsida (which is generally correct) or with Procolophon, an anapsid turtle sister. The connection was never spelled out, perhaps because of his use of suprageneric taxa. In fact, Maisch threw up his hands when he reported, “In the case of the ichthyosaurs we know that they are amniotes, and that they are not synapsids… Whether ichthyosaurs are diapsids, and if so, where exactly they have to be placed within the Diapsida, or whether they are parareptiles, and if so, whether they are related to mesosaurs or not, these are questions that remain as unresolved as one hundred years ago.”

 

Figure 2. Click to enlarge. The origin of ichthyosaurs and thalattosaurs from basal diapsids and basal mesosaurs. Relationships are rather apparent when seen in this context.

Here is the Ancestral Lineage of Ichthyosaurs
Starting with the basal diapsid, Petrolacosaurus and moving toward the basal enaliosaur, Claudiosaurus, the skull size was reduced (but probably not to the extents seen in Claudiosaurus) and the limbs become smaller and probably webbed. Hovasaurus was a sister to Claudiosaurus. Small-skulled Pachypleurosaurus was another sister to Claudiosaurus that nested at the base of the Sauropterygia.

Stereosternum is a somewhat forgotten taxon, but it represents a basal mesosaur, before all the temporal fenestra were closed off. The skull was not so small, the rostrum was elongated and the ribs became thicker. Mesosaurus was a derived sister, but too specialized in a different direction.

Wumengosaurus was considered an odd pachypleurosaur (Wu et al. 2011), but Wumengosaurus nests closer to the base of the ichthyosauria including Hupehsuchus. The quadratojugal was greatly reduced, eliminating the lower temporal bar, as in sauropterygians and ichthyosaurs.

Askeptosaurus was a thalattosaur, a sister taxa to the ichthyosaurs. Overall it retained the shape of a mesosaur, but the skull openings were reduced on top and expanded to the sides.

Hupehsuchus is a basal ichthyosauriform specialized without teeth. Even so it is the closest sister taxon known to basal ichthyosaurs represented by Utatsusaurus, a basal ichthyosaur.

In Summary
The long term trend in the evolution of ichthyosaurs was to enlarge the body overall, elongate the rostrum, shorten the neck, deepen the torso, reduce the thickness of the ribs, shorten the tail and give it a dorsal kink, reduce the limbs and turn them into paddles.

There is no more parsimonious solution among the tested reptiles. And the resemblances between each sister taxon are in line with the small steps that evolution takes. You can read more details starting here.

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

References
Carroll RL and Dong Z-M 1991. Hupehsuchus, an enigmatic aquatic reptile from the Triassic of China, and the problem of establishing relationships. Philosophical Transactions of the Royal Society London B 28 331:131-153.
Cope ED 1886. A contribution to the vertebrate paleontology of Brazil. Stereosternum tumidum, gen. et sp. nov. Proceedings of the American Philosophical Society 23 (121):1-21.
Gervais P 1865. Du Mesosaurus tenuidens, reptile fossile de l’Afrique australe. Comptes Rendus de l’Académie de Sciences 60:950–955.
Jiang D-Y, Rieppel O, Motani R, Hao W-C, Sun Y-I, Schmitz L and Sun Z-Y. 2008. A new middle Triassic eosauropterygian (Reptilia, Sauropterygia) from southwestern China. Journal of Vertebrate Paleontology 28:1055–1062.
Maisch MW 2010. Phylogeny, systematics, and origin of the Ichthyosauria – the state of the art. Palaeodiversity 3: 151-214.
Modesto SP 1999. Observations on the structure of the Early Permian reptile.
Modesto SP 2006. The cranial skeleton of the Early Permian aquatic reptile Mesosaurus tenuidens: implications for relationships and palaeobiology. Zoological Journal of the Linnean Society 146 (3): 345–368. doi:10.1111/j.1096-3642.2006.00205.x. Modesto SP 2010. The postcranial skeleton of the aquatic parareptile Mesosaurus tenuidensfrom the Gondwanan Permian. Journal of Vertebrate Paleontology 30 (5): 1378–1395. doi:10.1080/02724634.2010.501443.
Motani R, You H and McGowan C 1997. Eel-like swimming in the earliest ichthyosaurs. Nature, 382:347–348.
Motani R, Minoura N and Ando T 1998. Ichthyosaur relationships illuminated by new primitive skeletons from Japan. Nature, 393:255–257.
Shikama T, Kamei T and Murata M 1978. Early Triassic ichthyosaurs, Utatsusaurus hataiigen. et sp. nov., from the Kitakami Massif, Northwest Japan. Science Report of the Tohoku University Sendai, Japan, second series (Geology), 48: 77–97.
Vieira PC, Mezzalira S, Ferreira FJF 1991. Mesossaurídeo (Stereosternum Tumidum) e crustáceo (Liocaris Huenei) no Membro Assistência da Formação Irati (P) nos municípios
de Jataí e Montevidiu, estado de Goiás. Revista Brasileira de Geociências, 21:224-235.
Wiman C 1929. Eine neue Reptilien-Ordnung aus der Trias Spitzbergens. Bulletin of the Geological Institutions of the University of Upsala 22: 183–196.
Wu X-C, Cheng Y-N, Li C, Zhao L-J and Sato T 2011. New Information on Wumengosaurus delicatomandibularis Jiang et al., 2008, (Diapsida: Sauropterygia), with a Revision of the Osteology and Phylogeny of the Taxon. Journal of Vertebrate Paleontology 31(1):70–83.
Young C-C and Dong Z-M 1972. On the aquatic reptiles of the Triassic in China. Vertebrate Paleontology Memoirs. 9-1-34.

wiki/Utatsusaurus

Where to Nest Mesosaurus?

Figure 1. Mesosaurus up to 1 meter in length, was long considered an anapsid, but the temporal fenestrae were secondarily infilled from a basal diapsid configuration, as in Claudiosaurus.

Mesosaurus is a Problem for Paleontologists
Here’s the paradigm: Despite its many derived traits, Mesosaurus has long been nested with various other basal reptiles. This long-snouted, needle-toothed, aquatic Permian reptile apparently had no temporal fenestrae, according to Modesto (2006, 2010) and others. Therefore Gauthier (1988) and Modesto (1999) nested Mesosauridae with other such taxa, members of the Captorhinidae and Millerttidae, two basal herbivores without an aquatic niche. Laurin and Reisz (1995) nested Mesosauridae between synapsids and turtles. Laurin and Reisz (2004) nested Mesosaurus with Acleistorhinus (Figure 2), another herbivore sister to Milleretta. None of these proposed taxa even vaguely resemble mesosaurs. None of these studies included Claudiosaurus, ichthyosaurs and thalattosaurs.

Figure 2. Mesosaurus (left) and Acleistorhinus (right) were nested as sister taxa by Laurin and Reisz (2004) despite their many differences. Claudiosaurus and other enaliosaurs were not included in that study.

Not an Anapsid, but a Diapsid
A reconstruction of Mesosaurus (Figure 1) appears to retain at least a sliver of a lateral temporal fenestra. A reconstruction of the more primitive and typically ignored Stereosternum, another mesosaur, appears to retain a complete diapsid configuration. A reconstruction of yet another mesosaur, Wumengosaurus, retains a diapsid configuration, but with the loss of the lower temporal bar by reduction of the quadratojugal.

The Cleithrum
Mesosaurus had a tiny cleithrum, a sliver of bone dorsal to the clavicle on the leading edge of the scapula. It shares this trait with Petrolacosaurus and Claudiosaurus.

The Limbs
The structure of the  ankles made walking on land impossible, thus relatives should be looked for among aquatic taxa, not terrestrial herbivores, like Acleistorhinus

It Takes a Larger Study
A larger study of reptiles, and the largest one so far, nests Stereosternum within the basal aquatic diapsids, between Claudiosaurus, and Wumengosaurus, which was basal to ichthyosaurs and thalattosaurs. Other studies did not offer mesosaurs the opportunity to nest elsewhere. When you expand the inclusion list, as was done here, the opportunity for a correct nesting increases.

Figure 2. The nesting of the mesosaur, Stereosternum, in the Reptile Family tree at the base of the aquatic clade, the Enaliosauria.

Below are skeletal images of the sisters of Mesosaurus in phylogenetic order. The similarities are obvious and follow a gradual evolutionary sequence. The reduction and closure of the temporal fenestrae are also found in sister taxa including Araeoscelis and Pachypleurosaurus (not shown here).

Figure 3. The sisters of Mesosaurus from the basal diapsid, Petrolacosaurus to the basal ichthyosaur, Utatsusaurus.

Check out the various taxon names in www.reptileevolution.com for more details. 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.

References:
Gervais P 1865. Du Mesosaurus tenuidens, reptile fossile de l’Afrique australe. Comptes Rendus de l’Académie de Sciences 60:950–955.
Laurin M and Reisz RR 1995. A reevaluation of early amniote phylogeny. Zoological Journal of the Linnean Society 113:165-223.
Modesto SP 1999.Observations on the structure of the Early Permian reptile.
Modesto SP 2006. The cranial skeleton of the Early Permian aquatic reptile Mesosaurus tenuidens: implications for relationships and palaeobiology. Zoological Journal of the Linnean Society 146 (3): 345–368. doi:10.1111/j.1096-3642.2006.00205.x.
Modesto SP 2010. The postcranial skeleton of the aquatic parareptile Mesosaurus tenuidensfrom the Gondwanan Permian. Journal of Vertebrate Paleontology 30 (5): 1378–1395. doi:10.1080/02724634.2010.501443.

wiki/Mesosaurus

The Family Tree of the Pterosauria 19 – The Ornithocheiridae part 3 of 3

In part 1 of the Ornithocheiridae we looked at the base of this large clade of long-winged soaring pterosaurs. In part 2 we looked at Coloborhynchus, Istiodactylus and their kin. Here in part 3 look at more derived taxa such as Anhanguera and Liaoningopterus.

Figure 1. The Ornithocheiridae. Click to enlarge and expand.

We’ll Continue with Brasileodactylus
Brasileodactylus araripensis AMNH 24444 (Kellner, 1984; Veldmeijer 2003b) was originally described from just the anterior jaws and later a complete skull and other elements were found. It was derived from a sister to Coloborhynchus  and Haopterus (see part 1), skipping the istiodactylid clade (part 2). Distinct from Coloborhynchus, the skull of Brasileodactylus had no crest. The posterior premaxillary teeth were quite long. So were the matching dentary teeth. The squamosal had a dorsal process that gave the lateral temporal fenestra the appearance of a human ear. The lacrimal protruded into the orbit. The jugal was expanded anteriorly into the antorbital fenestra. The antorbital fenestra was shorter.

Barbosania 
Barbosania gracilirostris (Elgin and Frey 2011) was considered close to Brasileodactylus and was similar in size. The original report stated, “While elements of the cranium appear to suture very early in ontogeny (Kellner and Tomida 2000) all ornithocheiroids recovered from the Romualdo Member of the Santana Formation are considered to be ontogenetically immature based on the lack of fusion in the postcranial skeleton.” Actually this is a phylogenetic signal. As derived lizards, pterosaurs did not follow archosaur fusion patterns.

Figure 2. Click to enlarge. Ludodactylus.

Ludodactylus
Ludodactylus sibbicki SMNK PAL 3828 (Frey, Martill and Buchy 2003) is known from a skull with the unusual combination of a cranial crest and teeth. Distinct from Brasileodactylus, the skull of Ludodactylus was shorter overall with a parietal (cranial) crest with a frontal leading edge. The jugal was not expanded into the antorbital fenestra. The orbit was narrower. The postorbital was more robust. The mandible was more robust and was upturned anteriorly with smaller teeth posteriorly.

Cearadactylus
Cearadactylus atrox formerly: SMNK PAL 3828 and CB-PV-F-O93, now: UFRJ MN 7019-V (Leonardi and Borgomanero 1985) Cenomanian, Early Cretaceous, ~90 mya, ~57 cm skull length is known from a skull with an unusual history. Originally it was put together with the premaxilla and anterior dentary switched. Distinct from Brasileodactylus, the skull of Cearadactylus had a wide spoonbill or rosette tip from which erupted giant teeth. The maxillary teeth were tiny. The mandible was deeper, but flatter anteriorly.

Cearadactylus ligabuei CCSRL 12692/12713 (Dalla Vecchia 1993) was similar but had a distinctly shorter rostrum and smaller teeth with an upturned premaxilla. The tip was not a spoonbill, but the middle of the rostrum was narrowed or pinched in dorsal view. The jugal was more gracile.

Figure 3. Click to enlarge. Liaoningopterus

Liaoningopterus
Liaoningopterus gui IVPP V 13291 (Wang and Zhou 2003) Cenomanian, Early Cretaceous, ~90 mya, ~61 cm skull length. Distinct from Cearadactylus, the skull of Liaoningopterus was low anteriorly and very tall posteriorly. A very low crest surmounted the snout tip. Only one premaxillary tooth was enlarged to fang status. It is the largest tooth known for any pterosaur. The anterior dentary was expanded.

Figure 4. Click to enlarge. Anhanguera.

Anhanguera
Anhanguera piscator IVPP V 13291 (A. bittersdorffi No. 40 Pz-DBAV-UERJ Campos & Kellner, 1985; A. santanae AMNH 22555 Wellnhofer 1985; A. piscator, Kellner and Tomida 2000) Aptian, Early Cretaceous ~110 mya, ~60 cm skull length. Distinct from Liaoningopterus, the skull of Anhaguera had a longer premaxillary crest and smaller teeth. The anterior dentary formed a keel. The squamosal did not rise to form an “ear” shape of the lateral temporal fenestra. The tail was robust and had elongated vertebrae distally. Distinct from Brasileodactylus, manual 4.1 extended to the elbow when folded. Postcranially Anhanguera was most similar to SMNS PAL 1136, but without such a deep sternal complex keel and deep torso. The foot had very short metatarsals and elongated phalanges.

In Summary
The Ornithocheiridae is one of the few pterosaur clades without tiny members. Then again, from Yixianopterus at the base to Anhanguera as the most derived taxon, the morphology of this clade did not go through major changes. Trends toward the development and loss of a snout tip crest, more robust forelimbs and more gracile hindlimbs, an increase in the size of the antorbital fenestra in istiodactylids are all apparent. From the wing/leg ratios it seems apparent that this clade spent more time on the wing and less on the ground. Take-off was likely into the wind with a minimum take-off run from locations near steady and constant ocean breezes. A lack of skeletal fusion (sacrals, scapula/coracoid) permeates this clade, with some of the largest specimens lacking fusion. Fusion did affect some members, but the pattern was phylogenetic, not ontogenetic. The warped deltopectoral crest exhibited by some ornithocheirids has linked them to Pteranodon, but the morphology is distinct and the development was by convergence.

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.

References
Campos, D de A and Kellner AW 1985. Un novo exemplar de Anhanguera blittersdorffi(Reptilia, Pterosauria) da formaçao Santana, Cretaceo Inferior do Nordeste do Brasil.” In Congresso Brasileiro de Paleontologia, Rio de Janeiro, Resumos, p. 13.
Dalla Vecchia FM 1993. Cearadactylus? ligabuei, nov. sp., a new Early Cretaceous (Aptian) pterosaur from Chapada do Araripe (Northeastern Brazil)”, Bolletini della Societa Paleontologica Italiano, 32: 401-409.
Elgin RA and Frey E 2011. A new ornithocheirid, Barbosania gracilirostris gen. et sp. nov. (Pterosauria, Pterodactyloidea) from the Santana Formation (Cretaceous) of NE Brazil. Swiss Journal of Palaeontology. DOI 10.1007/s13358-011-0017-4.
Frey E, Martill DM and Buchy M-C 2003. A new crested ornithocheirid from the Lower Cretaceous of northeastern Brazil and the unusual death of an unusual pterosaur: In: Buffetaut, E., and J.-M. Mazin, Eds. Evolution and Palaeobiology of Pterosaurs. – London, Geological Society Special Publication 217: p. 55-63.
Kellner AWA 1984. Ocorrência de uma mandibula de pterosauria (Brasileodactylus araripensis, nov. gen.; nov. sp.) na Formação Santana, Cretáceo da Chapada do Araripe, Ceará-Brasil. Anais XXXIII Cong. Brasil. de Geol, 578–590. Rio de Janeiro
Kellner AWA and Tomida Y 2000. Description of a new species of Anhangueridae (Pterodactyloidea) with comments on the pterosaur fauna from the Santana Formation (Aptian–Albian), Northeastern Brazil. National Science Museum Monographs, 17:1–135.
Leonardi G and Borgomanero G 1985. Cearadactylus atrox nov. gen., nov. sp.: novo Pterosauria (Pterodactyloidea) da Chapada do Araripe, Ceara, Brasil. Resumos dos communicaçoes VIII Congresso bras. de Paleontologia e Stratigrafia, 27: 75–80.
Unwin DM 2002. On the systematic relationships of Cearadactylus atrox, an enigmatic Early Cretaceous pterosaur from the Santana Formation of Brazil. Mitteilungen Museum für Naturkunde Berlin, Geowissenschaftlichen Reihe 5: 1239–263.
Veldmeijer AJ 2003b. Preliminary description of a skull and wing of a Brazilian Cretaceous (Santana Formation; Aptian-Albian) pterosaur (Pterodactyloidea) in the collection of the AMNH. PalArch, series vertebrate palaeontology 1: 1-13.
Veldmeijer AJ, Meijer HJM and SignoreM 2009. Description of Pterosaurian (Pterodactyloidea: Anhangueridae, Brasileodactylus) remains from the Lower Cretaceous of Brazil, DEINSEA 13: 9-40
Vila Nova BC, Kellner AWA, Sayão JM 2010. Short Note on the Phylogenetic Position of Cearadactylus Atrox, and Comments Regarding Its Relationships to Other Pterosaurs. Acta Geoscientica Sinica 31 Supp.1: 73-75.
Wellnhofer P 1985. Neue Pterosaurier aus der Santana-Formation (Apt) der Chapada do Araripe, Brasilien. Paläontographica A 187: 105–182.

The Family Tree of the Pterosauria 18 – The Ornithocheiridae part 2 of 3

In part 1 of the Ornithocheiridae we looked at the base of this large clade of long-winged soaring pterosaurs. Here in part 2 we’ll look at Coloborhynchus, Istiodactylus and their kin. These taxa form a clade of their own, a little off to the side. In part 3 we will start again where we ended in part 1 and examine more derived taxa such as Anhanguera and Liaoningopterus.

Figure 1. The Ornithocheiridae. Click to enlarge and expand.

We’ll Continue with Coloborhynchus
Coloborhynchus spielbergi (Owen 1874, = Ornithocheirus clavirostris; C. spielbergi Veldmeijer 2003) RGM 401 880, Early Cretaceous was originally lumped with Ornithocheirus and much later was considered congeneric with Anhanguera by several workers (Kellner 2006). Distinct from Haopterus, the skull of Coloborhynchus had an anterior crest both above the snout and below the chin. The pre-antorbital fenestra region was longer. The orbit was narrower and raised higher over the antorbital fenestra. The neural spines were taller. A notarium was formed by several fused dorsals into which the scapula was articulated. The sacrals were interlocked if not fused. The sternal complex was rather deep. The scapulocoracoid was fused. The humerus was much more gracile. The ulna and radius were also thinner, but the distal ends were expanded. The pelvis was more robust with a more ossified ischium and a raised posterior ilium.

Criorhynchus 
Criorhynchus mesembrinus (Owen 1874, = Ornithocheirus clavirostris; = Tropeognathus mesembrinus, Fastnacht 2001) BSp 1987 I 46 from the Early Cretaceous was a sister to Coloborhynchus and may be congeneric with it. Distinct from Coloborhynchus, the skull of Criorhynchus was longer, lower and wider. The palatal elements were more robust. The ischium was narrower.

Nurhachius, a Basal Istiodactylid
Nurhachius ignaciobritoi (Wang, Kellner, Zhou & Campos 2005) IVPP V-13288, Early Cretaceous, skull length ~30 cm, ~2.5 m wingspan. Distinct from Criorhynchus the skull of Nurhachius further extended the rostrum and increased the size of the antorbital fenestra. If predecessors had a crest, it was greatly reduced or underdeveloped in Nurhachius. The orbit was very narrow and posteriorly slanted with a tiny sclerotic ring at the top. The upper temporal fenestra was completely above the orbit. Distinct from Coloborhynchus, the sternal keel was very deep (if that is the keel). The humerus was shorter. Fingers 1-3 were smaller. The femur was shorter. The metatarsals were robust and the pedal digits were slender, as in Zhenyuanopterus.

Figure 2. Click to enlarge. The unnamed largest ornithocheirid, SMNK PAL 1136

One of the Biggest Ornithocheirids Still Has No Name
SMNK PAL 1136 (not yet described, figured by Frey and Marill 1994) ~80 cm skull length, Aptian, Early Cretaceous ~130 mya, was originally considered an Anhanguera sp. Larger and distinct from Istiodactylus, the skull of SMNK PAL 1136 had the antorbital fenestra extend into the anterior rostrum just posterior to the premaxillary crest. The orbit was high and small. It was located just aft of the mandibular articulation. The jugal + lacrimal were reduced to slender rods oriented dorsoposteriorly. The sternal complex had a large keel and a reduced sternal plate. The scapulocoracoid was gracile. The gracile humerus expanded distally. Manual 4.1 extended to the elbow when folded. The pelvis was relatively smaller than in Coloborhynchus and the prepubis is tiny. The femur was considerably shorter than the tibia.

Istiodactylus latidens
Istiodactylus latidens BMNH R 3877 (Hooley 1913, Ornithodemus” latidens; Howse, Milner and Martill 2001) ~56 cm skull length, Aptian, Early Cretaceous ~130 mya was an unusual ornithocheirid known from a partial skeleton. Distinct from SMNK PAL 1136, the skull of Istiodactylus was a quarter smaller than SMNK 1136 PAL and similar in size to Coloborhynchus. The long gracile skull was dominated by an antorbital fenestra comprising 63 per cent of its estimated length. The anterior margin of the antorbital fenestra was posterior to all teeth, which fill only the anterior fourth of the jaws. Both narial openings (per side) were dorsal to the teeth. The long quadrates were so inclined that the orbit was positioned even further posteriorly than in PAL 1136. The teeth were lancet-shaped, closely spaced, and interlocked like a bear trap. A central dentary tooth filled the gap left by the medial premaxilla teeth, which were diminutive. The teeth increased in size posterolaterally. Two posterior dentary teeth fit into slots in the premaxilla. The dorsal vertebral transverse processes were nearly vertical. A notarium of six vertebrae was present. Asymmetrical coracoidal articulations on the anterior edge of the deep sternal complex keel continued on the lateral surface. The reconstructed wing/torso ratio was estimated at ~9:1. The deltopectoral crest was warped into a spiral. The ulna had a ridge that supported the radius. The antebrachium was relatively longer than in PAL 1136. What Hooley (1913) identfied as an ischium is identified here as the pubis and ischium.

Figure 3. Click to enlarge. Istiodactylus

Istiodactylus sinensis
Istiodactylus sinensis NGMC 99-07-011 (Andres and Ji 2006) Aptian, Early Cretaceous ~125 mya, ~35 cm skull length, appears to be more primitive in that the deltopectoral crest was not so curved and the skull was not as gracile. As in Nurhachius, when the wing was folded the elbow was closest to the middle of m4.2. The three free fingers each had only one phalanx, probably via fusion because there is no dimunition of the finger lengths. The resulting proximal phalanges are all subequal, approaching the configuration in Coloborhynchus, which had no phalanx fusion. Only pedal digit I and IV are known and they follow the pattern of larger medial digits seen in sister taxa.

In summary
This clade originated with a big crest on the rostrum tip and a small antorbital fenestra. As taxa evolved the crest slowly disappeared while the antorbital fenestra elongated anteriorly and pushed the orbit higher and smaller posteriorly.

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.

References
Andres B and Ji Q 2006. A new species of Istiodactylus (Pterosauria, Pterodactyloidea) from the Lower Cretaceous of Liaoning, China. Journal of Vertebrate Paleontology, 26: 70-78.
Bowerbank JS 1846. On a new species of pterodactyl found in the Upper Chalk of Kent P. giganteus). Quarterly Journal of the Geological Society 2: 7–9.
Bowerbank JS 1851. On the pterodactyles of the Chalk Formation. Proceedings of the Zoological Society, London, pp. 14–20 and Annals of the Magazine of Natural History (2) 10: 372–378.
Bowerbank JS 1852. On the pterodactyles of the Chalk Formation. Reports from the British Association for the Advancement of Science (1851): 55.
Fastnacht M 2001. First record of Coloborhynchus (Pterosauria) from the Santana Formation (Lower Cretaceous) of the Chapada do Araripe, Brazil. – Palaontologische Zeitschrift 75(1): 23-36.
Frey E and Martill DM 1994. A new Pterosaur from the Crato Formation (Lower Cretaceous, Aptian) of Brazil. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen 194: 379–412.
Hooley RW 1913. On the skeleton of Ornithodesmus latidens. An ornithosaur from the Wealden shales of Atherfield (Isle of Wight)”, Quarterly Journal of the Geological Society, 69: 372-421
Howse SCB, Milner AR and Martill DM 2001. Pterosaurs. Pp. 324-335 in: Martill, D. M. and Naish, D., eds. Dinosaurs of the Isle of Wight, The Palaeontological Association
Owen R 1861. Monograph on the fossil Reptilia of the Cretaceous Formations. Supplement III. Pterosauria (Pterodactylus). The Palaeontographical Society, London. (volume for 1858; pp. 1–19 & pls 1–4)
Owen R 1874. A Monograph on the Fossil Reptilia of the Mesozoic Formations. 1. Pterosauria. The Palaeontographical Society, London. pp. 1–14 & pls 1–2.
Wang X, Kellner AWA, Zhou Z and Campos DA 2005. Pterosaur diversity and faunal turnover in Cretaceous terrestrial ecosystems in China. Nature 437 (7060): 875–879. doi:10.1038/nature03982. PMID 16208369.

The Family Tree of the Pterosauria 17 – The Ornithocheiridae part 1 of 3

We just looked at one branch of descendants from Scaphognathus (No. 110) and Gmu 10157 that ultimately produced Cycnorhamphus and Feilongus. Here we look at the other branch of Scaphognathus No. 110 descendants, the Ornithocheiridae in three parts. Part 1 (below) will look at basal taxa. Part 2 will look at Coloborhynchus, Istiodactylus and their kin. Part 3 will look at more derived taxa such as Anhanguera and Liaoningopterus.

Figure 1. The Ornithocheiridae. Click to enlarge and expand.

We’ll start with Yixianopterus
Yixianopterus jingangshanensis JZMP-V12 (Lü et al. 2006) ~20 cm skull length, Barremian/Aptian Early Cretaceous ~125 mya, was overall 8x larger than and distinct from it tiny phylogenetic predecessor, Gmu-10157. The skull of Yixianopterus was longer judging by the pre-antorbital fenestra portion and the mandible. The teeth were more widely spaced. The caudals were shorter. Fingers 1-3 were smaller, but the wing finger was much more robust. Manual 4.1 approached the elbow when folded and the wingtip was higher than the skull when quadrupedal. The pelvis and tibia were more robust.

Figure 2. Click to enlarge DGS tracings. The JZMP ornithocheirid embryo, in situ and reconstructed.

We Haven’t Met the Adult Yet, But We Know This Embryo
The JZMP pterosaur embryo JZMP-03-03-2 (Ji et al. 2004) was found inside an eggshell, so we know it’s age precisely: zero. Considering the size of its pelvic opening one can estimate the size of the adult at 8x larger, which is consistent with Pterodaustro and its embryo/hatchling. The hypothetical adult size is also consistent with sister taxa. The embryo was originally compared to Beipiaopterus. Distinct from Yixianopterus, the skull of JZMP-03-03-2 was deeper anteriorly with an upturned premaxilla in which all of the premaxillary teeth were oriented chiefly anteriorly. The dentary was downturned at the tip. The antorbital fenestra was larger.The cervicals were longer posteriorly and shorter anteriorly. The sacrals were as long as the dorsals. The sternal complex was a wide rectangle with a transverse leading edge and a short cristospine. The scapula and coracoid were robust and oriented more laterally. The humerus was relatively smaller. The metacarpus was subequal to the ulna. The wing finger was robust proximally, but less so distally. Both m4.2 and m4.3 were longer than m4.1. The anterior ilium was much longer than the posterior process. The femur was shorter and the tibia was relatively longer. The pes was similar in size to that of Yixianopterus.

Note the long rostrum and small eye, as in the embryo of Pterodaustro. All of the small pedal bones were ossified. These facts falsify the hypothesis of pterosaur allometric growth (Wellnhofer 1970, Bennett 1991, 1992, 1994, 2001) and support the isometric hypothesis in which embryos and juveniles were almost identical to adults in morphology.

From Lebanon, a Nameless Pterosaur
The Lebanon ornithocheirid MSNM V 3881 (Dalla Vecchia, Adruini & Kellner 2001) A small, robust wing from Lebanon has a radius less than half the diameter of the ulna and manual digit 2 is subequal to 3. At present there is little else to distinguish it from Haopterus, except that it had a longer metacarpus relative to the ulna. The humerus, although incomplete, was small, as in the JZMP embryo.

The First Classic Ornithocheirid
Boreopterus cuiae JZMP 04-07-3 (Lü and Ji 2005) Distinct from the JZMP embryo, the skull of Boreopterus had at least 27 teeth in each upper jaw. They were long, slender and closely spaced. The rostrum was relatively longer and lower with a larger portion anterior to the antorbital fenestra. The postorbital portion was reduced with a posteriorly leaning orbit, as in Istiodactylus. The suborbital skull descended and the quadrate leaned posteriorly. The cervicals were longer with higher neural spines. The sacrals were shorter by more than half. The caudals were more robust. The humerus was larger, extending nearly to the acetabulum. The ulna and radius were also larger relative to the metacarpus. Fingers I-III were smaller. When folded the wing tip was no taller than the skull. The distal wing phalanges were shorter. The pelvis was tiny. The hind limb was more gracile, inluding a tiny foot.

Figure 3. Click to enlarge. Haopterus, the smallest ornithocheirid

Haopterus
Haopterus gracilis IVPP V11726 (Wang and Lü 2001) was overall smaller than and distinct from Boreopterus, the skull of Haopterus was shorter and relatively taller. The cervicals, dorsal, sacrals and caudals were all shorter. The scapula and coracoid were shorter. The humerus was extremely roubst with a deltopectoral crest extending for ~33% of the length. As in the Lebanon ornithocheirid, the radius and ulna were relatively short. Manual 4.1 approached the elbow. The relatively longer wing would have extended far above the head when folded. The pelvis was gracile and smaller. The femur was shorter. The metatarsals were shorter. Ornithocheirids, like Haopterus, were evidently spending more time in the air and less on the ground, judging by their wing/leg proportions.

Figure 4. Click to enlarge. Zhenyuanopterus

Zhenyuanopterus
Zhenyuanopterus longirostris (Lü et alk. 2010) GLGMV 0001 Early Cretaceous. Distinct from Boreopterus, the skull of Zhenyuanopterus was longer, especially in the pre-antorobital fenestra region. The teeth were more widely spaced and continued erupting closer to the orbit, which was smaller. A squarish crest surmounted the mid rostrum. The cranium was crest-like, probably for muscle attachments. The cervicals were more robust. The torso was smaller and shallower, as in Haopterus. The caudals were more robust. The sternal complex did not have lateral ‘wings’. The scapula and coracoid were fused. The coracoids were laterally oriented. The humerus was as long as the torso. The ulna and radius were more robust and relatively shorter. The hind limbs were longer, as in Haopterus. The feet were extremely tiny with robust metatarsals and slender digits.

 

Figure 5. Click to enlarge. Arthurdactylus dorsal view.

Arthurdactylus from South America
Arthurdactylus conandoylei (Frey and Martill 1994) SMNK 1132 PAL Early Cretaceous. Distinct from Zhenyuanopterus, the torso of Arthurdactylus was deeper, as in Boreopterus. The sacrals were all unfused. The caudals were vestigial. The coracoids were much longer than the scapula, producing a very high shoulder joint. The ulna was massive. Manual 4.1 approached the elbow when folded. The short pubis was directly beneath the actebulum. The ischium was slender. The foot was slightly larger than in Zhenyuanopterus with more slender metatarsals and longer digits.

In summary
Taxa at the base of the Ornithocheiridae are those closest to cycnorhamphids in their morphology. Yixianopterus is at the base followed by the JZMP embryo. Due to isometric growth in pterosaurs we can enlarge it eight times to gauge what the adult was like. A trend toward a longer snout, more and longer teeth, larger wings and smaller feet is apparent.

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.

References:
Bennett SC 1991. Morphology of the Late Cretaceous Pterosaur Pteranodon and Systematics of the Pterodactyloidea. [Volumes I & II]. Ph.D. thesis, University of Kansas, University Microfilms International/ProQuest.
Bennett SC 1992. Sexual dimorphism of Pteranodon and other pterosaurs, with comments on cranial crests. Journal of Vertebrate Paleontology 12: 422–434.
Bennett SC 1994. Taxonomy and systematics of the Late Cretaceous pterosaur Pteranodon (Pterosauria, Pterodactyloidea). Occassional Papers of the Natural History Museum University of Kansas 169: 1–70.
Bennett SC 2001. The osteology and functional morphology of the Late Cretaceous pterosaur Pteranodon. Part I. General description of osteology. Palaeontographica, Abteilung A, 260: 1–112. Part II. Functional morphology. Palaeontographica, Abteilung A, 260: 113–153
Dalla Vecchia FM, Arduin P and Kellner AWA 2001. The first pterosaur from the Cenomanian (Late Cretaceous) Lagerstätten of Lebanon. Cretaceous Research 22: 219-225.
Frey E and Martill DM 1994. A new Pterosaur from the Crato Formation (Lower Cretaceous, Aptian) of Brazil. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen 194: 379–412.
Ji Q, Ji S-A, Cheng Y-N, You HL, Lü J-C, Liu Y-Q and Yuan CX 2004. Pterosaur egg with leathery shell. Nature 432:572.
Lü J 2010. A new boreopterid pterodactyloid pterosaur from the Early Cretaceous Yixian Formation of Liaoning Province, northeastern China. Acta Geologica Sinica 24: 241–246.
Lü J and Ji Q 2005. A new ornithocheirid from the Early Cretaceous of Liaoning Province, China. Acta Geologica Sinica 79 (2): 157–163.
Lü J, Ji S, Yuan C, Gao Y, Sun Z and Ji Q 2006. New pterodactyloid pterosaur from the Lower Cretaceous Yixian Formation of Western Liaoning. In J. Lü, Y. Kobayashi, D. Huang, Y.-N. Lee (eds.), Papers from the 2005 Heyuan International Dinosaur Symposium. Geological Publishing House, Beijing 195-203.
Wang X and Lü J 2001. Discovery of a pterodactylid pterosaur from the Yixian Formation of western Liaoning, China. Chinese Science Bulletin 46(13):1-6.

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.

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.

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.

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).

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.

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.

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.

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 OceansofKansas.com 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.

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.

References:
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

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).

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.

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.

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.

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
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.

References:
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.