The Big Kahuna: The Reptilia is Diphyletic

The Traditional Reptilia
Paleontologists have traditionally  assumed that all the animals we commonly refer to as “reptiles” (lizards, snakes, turtles, crocs, the tuatara and all their prehistoric ancestors) were monophyletic, descending from a single ancestor and forming a single clade, the Reptilia (Modesto and Anderson 2004). Birds were added to this group, having descended from the dinosaurian sisters of crocodile ancestors. In this hypothesis the mammals and their ancestors (collectively the Synapsida) were not considered reptiles because they were thought to have branched off the family tree earlier.

The Traditional Amniota
Traditionally, mammals (and their ancestors) joined birds, crocs, lizards, turtles (and their ancestors) in a monophyletic clade, the AmniotaAll these taxa share a trait derived from a common ancestor, an amnion, an embryonic membrane that protects the embryo during development whether an egg shell is present or not.

Purported outgroup taxa
Various diadectids and microsaurs were said to nest just outside the Amniota. According to tradition, these “reptile-like amphibians” must have laid their eggs in water and produced tadpoles because they were thought to precede the development of the amnion. It didn’t seem to matter what sort of evolutionary mismatches resulted. Diadectids and microsaurs certainly do not share many traits with each other.

Now these traditions have been changed, according to the results of a very large cladistic analysis, unprecedented in scope.

Just like a larger telescope brings greater resolution to astronomical images, a larger cladistic analysis brings greater resolution to family trees. No one had ever created a cladistic analysis that included basal representatives from the gamut of the Amniota until now. All prior analyses used smaller inclusion sets based on assumption and tradition. Many recovered poorly resolved trees with poorly matched purported sisters sharing few traits.

The present analysis recovered a single tree from over (the number continues to grow) 235 specific and generic taxa with all reptiles (including synapsids) descending from the “pre-reptile” Gephyrostegus (see below). All sister taxa share many traits and greatly resemble one another. The tree solves many prior mysteries and nests several former enigmas.

A Big Surprise
The new tree produced two major reptilian branches before the advent of any known reptile fossils. Thus, there was not a single basal reptile (defined here as “without a discrete intertemporal bone”). These two major branches go by old names: the Lepidosauromorpha and the Archosauromorpha because one branch includes lepidosaurs and the other branch includes archosaurs. The second branch also includes mammals and their ancestors.

Tiny Origins
As Carroll (1970) predicted, the most basal known reptiles, Cephalerpeton and Casineria, were indeed tiny, but not as tiny as the last of the pre-reptiles (one of which would have been the sister to the last common ancestor of Cephalerpeton and Casineria, and thus would have been the first reptile/amniote).

Goodbye Amniota
Now several reptiles (including Casineria, the microsaurs, and at least four protosynapsids) precede the branching of the Synapsida. That means the Reptilia = the Amniota. Since the former term precedes the latter, the Amniota has now become redundant, no longer distinct from the Reptilia.

These results shift taxa around like branches on a fake Christmas tree.
Diadectids and microsaurs join the Reptilia. Caseids leave the synapsids. Mesosaurs join the ichthyosaurs. We have a new basal dinosaur family tree. We’ll talk about other details in future blogs, or you can read them for yourself now at reptileevolution.com.

What does this have to do with pterosaurs?
Everything. This is the study that nested pterosaurs with lizards, specifically with a new, previously unidentified third squamate clade, the Tritosauria, originating with Lacertulus in the Late Permian. Breaking paradigms left and right, this new tree invalidates such clades as the Ornithodirathe Avemetatarsalia and several others that included pterosaurs with dinosaurs in their definitions.

More later.

The new, diphyletic tree of the Reptilia (= the Amniota)

Figure 1. Click to enlarge. The new, diphyletic tree of the Reptilia (= the Amniota)

As always, I encourage readers to see the 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:
Brough MC and Brough J 1967. The Genus Gephyrostegus. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 252 (776): 147–165. doi:10.1098/rstb.1967.0006 
Carroll RL 1970. 
The Ancestry of Reptiles. Philosophical Transactions of the Royal Society London B 257:267–308. online pdf
Jaeckel O 1902. Über Gephyrostegus bohemicus n.g. n.sp. Zeitschrift der Deutschen Geologischen Gesellschaft 54:127–132.
Modesto SP and Anderson JS 2004. The Phylogenetic Definition of Reptilia. Systematic Biology 53(5):815–821. DOI: 10.1080/10635150490503026

 

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The Family Tree of the Pterosauria 4 – Basal Eudimorphodontia

We’ve just looked at the base of the pterosauriabasal Dimorphodontia and derived Dimorphodontia (aka Anurognathidae). Now we shift our gaze to the base of ALL other pterosaurs – the Eudimorphodontia. This base includes the pointy-nosed pterosaurs shown in Figure 1 and begins with its namesake, Eudimorphodon. More details and references are found at reptileevolution.com.

Basal members of the Eudimorphodontia.

Figure 1. Click to enlarge. Basal members of the Eudimorphodontia.

Eudimorphodon ranzii
Eudimorphodon ranzii (Zambelli 1973, Wild 1978) was a large and robust Triassic pterosaur with multicusped teeth in the back of its jaws. When other Triassic pterosaurs were found that shared this trait this genus became a sort of wastebasket for many of them. Earlier workers did not realize that this was a trait retained from non-pterosaur fenestrasaurs such as Sharovipteryx and Longisquama. Distinct form MPUM 6009, Eudimorphodon was more than twice as large, with a longer snout and longer jaws. The skull and torso appear to be robust, with shorter hind limbs (unknown from the top of the shins down) and a larger sternal complex. The humerus was more robust, but the pelvis was reduced. The wing length is unknown. The femoral head was not set at right angles, so this clade retained sprawling femora.

Eudimorphodon cromptonellus
This tiny Eudimorphodon sister species (Jenkins et al. 1999), a third the size of the holotype (see above), is known from scattered bits and pieces. If it is an adult, it represents yet another size reduction between the major groups. This transition led to Campylognathoides, which does not have multicusped teeth and to Bsp 1994, which does. Here the foot and lower leg can be reconstructed with the foot slightly larger than the tibia and the tibia subequal to the femur, which is not typical of pterosaurs.

Campylognathoides
Several specimens are known of Campylognathoides and they differ significantly from one another, but in every one manual 4.1 reaches the elbow when the wing is folded. The most primitive of the Campylognathoides species is C. zitteli, the Paris specimen MNHN, Paris HLZ 50, or C4 in the Wild 1975 catalog).  It shares more traits with Eudimorphodon ranzii than any other Campy.

The  Cz specimen, SMNS 11879, Cz in the Wild (1975) was the giant of the clade with a larger head and a relatively shorter torso.

The C5 specimen SMNS 9787, C5 in the Wild (1975) catalog was similar but overall much smaller with distinct proportions.

Nesodactylus, a partial specimen from Cuba (Colbert 1969) AMNH FR 2000 , nests within this clade, so actually it should be called Campylognathoides hesperius. The sternum was not so wide, but it had a deep keel. The metatarsals were not bound together.

The C3 specimen or C. liasicus (Wellnhofer 1974) CM 11424, is the most derived Campylognathoides and the one sharing the most traits with Rhamphorhynchus.

Figure 2. Click to enlarge. The family tree of Rhamphorhynchus.

Rhamphorhynchus
Several recent reports nested Rhamphorhynchus with Dorygnathus, but that nesting was not recovered here. Bennett (1995) tested Rhamphorhynchus using statistics and decided that all known specimens were growth stages of one species. Bennett’s hypothesis was tested using cladistic analysis and found to be false. A lineage of increasingly derived Rhamphorhynchus was found, and besides, now we know from embryos that pterosaurs do not change shapes during maturity, something Bennett was not aware of at the time.

Rhamphorhynchus intermedius (St/Ei 8209, no. 28 in the Wellnhofer 1975 catalog) was half the size of the C3 specimen of Campylognathoides. In addition, it had a relatively shorter tail, shorter legs and shorter wings. R. intermedius represents the most primitive Rhamphorhynchus and the one closest to Campylognathoides.

An unnumbered and unpublished BMM specimen attributed to Rhamphorhynchus was half the size of R. intermedius, continuing the size reduction between genera. The jaws were shorter.

R. longicaudus B St 1959 I 400, no. 10 of Wellnhofer 1975 was a tiny specimen with a slightly longer snoutt.

R. longicaudus  BSP 1938 I 503a,  no. 11 of Wellnhofer 1975 had increasingly longer jaws without becoming very much larger overall.

Ironically, the largest Rhamphorhynuchus, R. longiceps (Smith-Woodward 1902) BMNH 37002, no. 81 in Wellnhofer 1975, follows the smallest specimens. I will have to dive back into some specimens to see if transitional specimens are known. Currently I know of none, but I have not looked at that many.

The medium-sized R. muensteri follows. The “darkwing” specimen nests here. Several other dozen specimens of R. muensteri would nest here if tested.

Several specimens of the larger R. gemmingi follow. No. 74, No. 43, No. 38 and No. 52 are in phylogenetic order, with No. 52 as the most derived and the last of this lineage. These taxa share m4.1 extending beyond the elbow and the naris is increasingly dorsal over the antorbital fenestra, among other traits. The oddball of the bunch is No. 75 with its extremely elongated snout and mandible.

Three Big-Eyed “Common Brown Sparrows”
Most basal Eudimorphodontia had a huge and pentagonal sternal complex, but these three did not. They were the basal members of the Sordesidae.

Bsp 1994 is known from scraps (Figure 1). A small triangular sternal complex sets it apart from Campylognathoides, etc. and well on the lineage to Dorygnathus.

Changchengopterus was much smaller (Figure 1) and known from a mandible and post-crania. These are the “common brown sparrows” of the pterosaur family tree. Not much to get excited about here, except the tail was much shorter and without the bony spars that stiffened it.

Sordes is the best known of this group, and the most misunderstood (Figure 1). Large curved teeth, big eyes, a relatively large skull and a torso about the same size mark Sordes as one to watch. From Sordes we get Dorygnathus which ultimately produces toothy ctenochasmatids and giant azhdarchids. We also get Pterorhynchus, Scaphognathus and all of the giant pterosaurs that descend from these.

Pterosaur family tree

Figure 3. The Eudimorphodontia within the pterosaur family tree.

Next time we look at the pterosaur family tree, Dorygnathus will be introduced, one of two TRUE transitional pterosaurs.

As always, I encourage readers to see the 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 1995. A statistical study of Rhamphorhynchus from the Solnhofen Limestone of Germany: Year-classes of a single large species. Journal of Paleontology 69: 569–580.
Jenkins FA Jr, Shubin NH, Gatesy SM and Padian K 1999. A primitive pterosaur of Late Triassic age from Greenland. Journal of the Society of Vertebrate Paleontology 19(3): 56A.
Jenkins FA Jr, Shubin NH, Gatesy SM and Padian K 1999. A diminutive pterosaur (Pterosauria: Eudimorphodontidae) from the Greenlandic Triassic. Bulletin of the Museum of Comparative Zoology, Harvard University 155(9): 487-506.
Colbert EH 1969. A Jurassic Pterosaur from Cuba. American Museum Novitates, New York, 2370: 1-26.
Padian K 2009. The Early Jurassic Pterosaur Dorygnathus banthenis (Theodori, 1830) and The Early Jurassic Pterosaur Campylognathoides Strand, 1928, Special Papers in Paleontology 80, Blackwell ISBN 9781405192248
Quenstedt FA 1858. Über Pterodactylus liasicus, Jahrbuch des Vereins vaterländischer Naturkundler in Württemberg, 14:299-336papers in palaeontology no. 80. The Palaenotological Association, London.
Smith-Woodward A 1902. On two skulls of the Ornithosaurian Rhamphorhynchus. Annals and Magazine of Natural History, London, (7) 9: 1-5.
Strand E 1928. Miscellanea nomenclatorica Zoologica et Palaeontologica, Archiv fur Naturgeschichte, 92: 30-75.
Wellnhofer P 1974. Campylognathoides liasicus (Quenstedt), an upper Liassic pterosaur from Holzmaden – the Pittsburgh specimen. Annals of the Carnegie Museum, Pittsburgh, 45: 5-34.
Wild R 1975. Ein Flugsaurier-Rest aus dem Lias Epsilon (Toarcium) von Erzingen (Schwäbisher Jura). -Stuttgarter Beiträge zur Naturkunde Serie B, Nr. 17: 1-16.
Wild R 1978. Die Flugsaurier (Reptilia, Pterosauria) aus der Oberen Trias von Cene bei Bergamo, Italien. Bolletino della Societa Paleontologica Italiana 17(2): 176–256.
Zambelli R 1973. Eudimorphodon ranzii gen.nov., sp.nov. Uno Pterosauro Triassico. Rendiconti Instituto Lombardo Accademia, (rend. sc.) 107: 27-32.

The Family Tree of the Pterosauria 3 – The Anurognathidae

You Can Call Them “Bubbleheads”
because there was more air than bone in their fragile round skulls than bone, but anurognathids (Figure 1) were important members of the pterosaur family tree (see below). Our last stop-off was the protoanurognathid, MCSNB 8950, which retained a robust tail.

The Anurognathidae to scale.

Figure 1. Click to enlarge. The Anurognathidae to scale.

The IVPP Embryo
The most primitive anurognathid, the IVPP embryo, is also the most controversial because it is an embryo the size of most of the other adult anurognathids. That means an adult would have been eight times larger (pretty standard growth to full size, based on egg size vs. pelvic opening). The IVPP embryo, IVPP V13758 (Wang and Zhou 2004), was originally considered a baby ornithocheirid. Further studies and an accurate reconstruction revealed its true identity. It is the most primitive anurognathid to have a vestigial tail.

Dimorphodon weintraubi made the cover of Nature

Figure 2. Click to enlarge. Dimorphodon weintraubi made the cover of Nature

Dimorphodon? weintraubi
The only other “giant” anurognathid is the mislabeled “Dimorphodon” weintraubi, (Clark et al. 1994, 1998) made famous on the cover of Nature (Figure 2), as the specimen that “proved” all pterosaurs were flat-footed because the metatarsophalangeal joint was a squared-off butt joint. Not true.  Peters (2000) matched the morphologically similar foot of Cosesaurus to Rotodactylus, fairly common digitigrade tracks in which digit 5 impressed far behind the other four toes and the proximal phalanges were elevated. So the butt joint kept the proximal phalanges elevated in fenestrasaurs, including basal pterosaurs. This is the only known western hemisphere anurognathid. Unfortunately it is only known from its limbs.

Dendrorhynchoides

Figure 3. Dendrorhynchoides. Click to enlarge.

Dendrorhynchoides curvidentatus
The most primitive of the small anurognathids with small tails, Dendrorhynchoides (Ji and Ji 1998) is a complete specimen with soft tissue impressions. A false tail (perhaps from a tiny dinosaur) was added to the holotype by the preparators. I bought into that fake tail because a robust tail would have been similar to the next most primitive pterosaur, MCSNB 8950. However, the IVPP embryo demonstrates reduction to a vestige, and the same would hold true for Dendrorhynchoides. I found it on my computer screen using the DGS method. The tiny tail curled around the left tibia before passing anteriorly over the left femur and ventral pelvis. Dendrorhynchoides had an extremely broad sternal complex, which made its torso twice as wide as its skull. The palate is a good precursor to the palate in Jeholopterus, but without the robust architecture needed by the vampire.

Hone and Lü 2010 reported on a new specimen of Dendrorhynchoides, GLGMV 0002, but a DGS tracing and reconstruction reveal it to be a non-anurognathid, perhaps a specimen close to Dimorphodon.

The Flathead Anurognathid

Figure 4. The Flathead Anurognathid. Click to enlarge.

The Flathead Anurognathus?
Controversy also surrounds the private (SMNS has casts and photos) specimen that Bennett (2007) attributed to Anurognathus. The “Flathead” specimen is complete and much smaller than the holotype Anurognathus. While the “Flathead” nests as a sister to the holotype, it had a different skull architecture, neck length, relative body size, a much smaller metacarpal 4, different femur/tibia ratios and the list goes on. Why would Bennett (2007) consider the “Flathead” an immature specimen if the skull was relatively smaller and the neck relatively longer? Paleontologists, as a rule, consider it inappropriate to publish on private specimens, but Bennett (2007) reported that the “Flathead” was merely a second specimen of an already established genus and species. Bennett (2007) also completely misunderstood the skull architecture, as documented here. Bennett (2007) mistakenly placed the eye in the antorbital fenestra after confusing a displaced maxilla subdivided with tooth roots with a “sclerotic ring” preserved on edge somehow, not noticing that his eye bones had tiny teeth. The “Flathead” specimen had a much wider than tall skull with extremely fragile bones connecting the skull roof to the jaw line. The palate bones were also extremely thin, with fragile Y-shaped extensions of the maxilla touching medially and extending posteriorly. Like Dendrorhynchoides, the torso was much wider than tall, but the sternum was no wider than the skull.

 

Figure 5. Anurognathus (the holotype). Click to enlarge.

Figure 5. Anurognathus (the holotype). Click to enlarge.

Anurognathus ammoni – the holotype
Anurognathus ammoni (Döderlein 1923) B St 1922 I 42, No. 111 of Wellnhofer 1975, was the first anurognathid discovered. Some of the bones were preserved. Others are only impressions. As in Dendrorhynchoides, the distal wing bones curled toward the body, leaving Bennett (2007) the impression that they were missing because he did not see them. The DGS method found the distal wing phalanges in these taxa. Moreover, derived anurognathids, like Batrachognathus and Jeholopterus, also have distal wing phalanges, so it would not be likely that they would redevelop after complete loss.  Anurognathus had a longer rostrum and shorter neck than other anurognathids. The sternal complex was small and pentagonal. The femur was about half the length of the tibia.

The CAGS specimen attributed to Jeholopterus

Figure 6. The CAGS specimen attributed to Jeholopterus. Click to enlarge.

Jeholopterus? the CAGS specimen
Jeholopterus? sp. (Lü et al., 2006) CAGS IG 02-81 was originally considered a Dendrorhynchoides then a Jeholopterus specimen. It was distinct from both. It had large eyes. Unlike Batrachognathus the eyes were oriented more laterally. The skull was muffin-shaped, wider than tall. The nares opened anteriorly and the premaxilla was largely transverse. The pelvis was quite small and the femur was relatively longer. The sternal complex was the width of the skull. Some excellent soft tissue here obscures some of the bones. Distinct morphology suggests this specimen deserves its own genus.

Jeholopterus ninchengensis
Jeholopterus ninchengensis
 (Wang, Zhou, Zhang and Xu 2002) IVPP V 12705  is the famous vampire pterosaur, covered in detail earlier. No other anurognathids are younger than Jeholopterus, so it may represent the last of this lineage.

Batrachognathus

Figure 7. Batrachognathus Click to enlarge. 

Batrachognathus volans
Batrachognathus volans (Rjabinin 1948) PIN 13 was the second anurognathid discovered and the last in this lineage. Narrow nasals and wide frontals gave this owl-eyed anurognathid binocular vision. The extremely robust hand, arm and wing distinguish Batrachognathus from the others. Pedal digit 5 was extremely long. Bristles originally reported around the mouth, but never traced, were probably hairs emanating from the nearby tail.

Did anurognathids become extinct during the Early Cretaceous?
There are so few anurognathid skeletons known, and each one is a distinct genus, so it is difficult to judge whether or not we’ll find more anurognathids in younger strata. So far, they are not known after the Early Cretaceous.

Next we’ll take a look at the base of the Eudimorphontia.

As always, I encourage readers to see the 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.

Pterosaur family tree

Figure 8. Click to enlarge. The pterosaur family tree. The Anurognathidae terminate the clade Dimorphodontia.

References:
Bennett SC 2007. A second specimen of the pterosaur Anurognathus ammoni. Paläontologische Zeitschrift 81(4):376-398.
Clark J, Montellano M, Hopson J and Fastovsky D. 1994. In: Fraser, N. & H.-D Sues, Eds. 1994. In the Shadows of Dinosaurs. New York, Cambridge: 295-302.
Clark J, Hopson J, Hernandez R, Fastovsk D and Montellano M. 1998. Foot posture in a primitive pterosaur. Nature 391:886-889.
Hone DWE and Lü J-C 2010. A New Specimen of Dendrorhynchoides (Pterosauria: Anurognathidae) with a Long Tail and the Evolution of the Pterosaurian Tail. Acta Geoscientica Sinica 31 (Supp. 1): 29-30.
Ji S-A and Ji Q 1998. A New Fossil Pterosaur (Rhamphorhynchoidea) from Liaoning. Jiangsu Geology 4: 199-206.
Peters D 2000. Description and Interpretation of Interphalangeal Lines in Tetrapods. – Ichnos 7(1): 11-41.
Peters D 2003. The Chinese vampire and other overlooked pterosaur ptreasures. Journal of Vertebrate Paleontology 23(3): 87A.
Rjabinin AN 1948. Remarks on a Flying Reptile from the Jurassic of Kara-Tau. Akademia Nauk, Paleontological Institute, Trudy 15(1): 86-93.
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.

Pterosaurs and Skimming

Pteranodon skimming

Charles Knight painting image courtesy of the Field Museum of Natural History

It has been a tradition to illustrate certain pterosaurs, like Pteranodon and Thalassodromeus (Kellner and Campos 2002) flying low over the waters, skimming for subsurface prey or floating carrion. This behavior would have been similar to living black skimmers (Rynchops niger), feeding while on the wing, occasionally dipping a mandible into the water to snare small fish. At first glance, some pterosaurs seem to have been perfectly suited for skimming. They sported extremely sharp jaw tips, either with a set of procumbent teeth, as in Rhamphorhynchus, or largely without, as in Nyctosaurus (Figure 1). Some pterosaurs were preserved with fish bones beneath their ribcages.

Figure 1. Click to enlarge. Nyctosaurus KUVP 66130 showing in two views the extremely sharp mandible tipped by a single tooth. This is the only known genus of pterosaur with a mandible that extended further than the rostrum.

The Anti-Skimming Hypothesis
A few years ago, Humphries et al. (2007) reported: “Our results refute the hypothesis that some pterosaurs commonly used skimming as a foraging method and illustrate the pitfalls involved in extrapolating from limited morphological convergence.” But they also conceded, “It also leaves the theoretical possibility that smaller (2-m wingspan) pterosaurs such as Rhamphorhynchus may have been able to skim-feed occasionally without particularly specialised jaw morphology due to their lower flight costs.” Good. Apparently someone reminded them there FISH inside certain pterosaurs, including Rhamphorhynchus.

To determine the amount of drag a dipped mandible would experience, Humphries et al. (2007) modeled several mandible shapes based on the black skimmer and several pterosaurs, including a jaw fragment they assigned to Thalassodromeus sethi (DGM 1476-M). Unfortunately this specimen had a robust upturned tip with a tear-drop shaped cross-section and so probably belonged to a dsungaripterid. Even so, that misidentification may not have been a deal killer. As a point of reference, a sister to Thalassodromeus, Tupuxuara, had a boat-hull-shaped mandible cross-section, as Humphries et al. (2007) illustrated in their figure 1. More on this shape below.

Let’s Look at the Anti-Skimming Methods and Assumptions
Humphries et al. (2007) employed a number of velocities (1.8 to 6.8 m/s or 4 to 15 mph) on the skimmer bill model and found that range produced a 14-fold increase in drag at the higher speeds. The larger pterosaur mandible model experienced a magnitude more drag at all speeds. Humphries et al. (2007) reported the typical speed of a black skimmer was 10 m/s (22 mph), or a third faster than their highest tested speed. Hmm. Wonder why they didn’t test the skimmer model at its observed speed?

After reporting, “The added costs of flight due to hydrodynamic drag range from 20% of total costs in Rynchops [the black skimmer], up to 68% in the low mass estimate for Pteranodon,” they wrote, “It is clear that in all species, hydrodynamic drag constitutes a major component of total flight costs.” Here, unfortunately, Humphries et al. (2007) assumed that a high-speed skimmer-like penetration of the surface was the only means by which a pterosaur might skim. But is this so?

Alternative Methods for Skimming
Black skimmers prefer windless conditions over still bodies of water. Humphries et al. (2007) apparently did not take into account that 1) a head wind produced by an oceanic breeze could have produced a much lower water (ground) speed and 2) the pterosaur skimming technique was likely different (see below). Pterosaurs may have employed a little or a lot of headwind (Figure 4) often present near larger bodies of water. This could have greatly reduced their hydrodynamic speed to a hover, obviating the need for a skimmer-like cut through the water with its attendant drag problems.

Humphries et al. (2007)  reported that skimmers have some 30 adaptations for skimming that pterosaur lacked, including an ever growing mandible tip. This alone indicates that if pterosaurs skimmed, they would have done so using a less damaging technique.

Heron damage from spearing a fish

Figure 2. Heron damage from spearing a fish. Perhaps this was the preferred technique for the sharp billed germanodactylids, including the nyctosaurs and pterandontids.

Not All Pterosaurs Would Have Fed by Skimming
Ctenochasmatids had too many fragile teeth at odd angles, hence they would have acted more like modern spoonbills, dipping their beak occasionally while slowly wading in shallows. Azhdarchids had bills like yardsticks. They would have acted more like storks and probably fed the same way, not on the wing. Pterodactylus was also a hunt-and-peck beachcomber. Germanodactylus had a sharp rostrum that was capable of stabbing prey like a heron does on occasion (Figure 2).

The Best Candidates for Skimming Pterosaurs appear to be Nyctosaurus (Figure 1) and Pteranodon (Figure 3)Like the black skimmer, Nyctosaurus had a mandible longer than the rostrum (Figure 1). The jaws were needle-sharp tipped by a single tooth (so nyctosaurs were not totally toothless). According to cladistic analysis, Nyctosaurus was derived from certain Germanodactylus and so may have retained a stabbing ability and behavior. Pteranodon was similar to Nyctosaurus in having a sharp rostrum and mandible, but in all cases the mandible was shorter.

Pteranodon-sharp-mandible

Figure 3. The shorter, but just as sharp mandible of the Triebold specimen of Pteranodon.

Black skimmers don’t have a sharp mandible. It was tipped with a keratinous sheath that occasionally suffered damage. While skimming placid waters with their open jaws partly submerged, skimmers bang into prey with the dorsal rim of their mandible (not the tip). The force pulls their head down and the rostrum snaps on the captured fish.

By contrast, Nyctosaurus and Pteranodon would have speared their prey, probably at a much lower speed and a much lower angle relative to the water. Perhaps that is why the cross section of the deep mandibles of Pteranodon and Tupuxuara look like boat hulls. Nyctosaurus (Figure 1) did not have such a deep mandible. In these three pterosaurs a quick flick of the mandible would have dislodged the fish and flipped it into the throat. So is this still skimming? Or is it dipping and stabbing?

Hovering albatross video

Figure 4. Click to Play. Hovering albatross video

A video of a hovering albatross (Figure 4) demonstrates the ability of ocean breezes to provide sufficient airspeed over the wings to permit flight at very low water speeds, down to zero.

The Black Skimmer (Rynchops niger) is Not a Good Analog
The skimming speeds attained by black skimmers over placid waters were not necessary in skimming pterosaurs over breezy oceanic waters, hence drag would not have been the limiting factor, as Humphries et al. (2007) proposed. Moreover, the technique for snaring fish was dissimilar in skimmers and pterosaurs due to beak shape. The extremely pointed mandibles of Nyctosaurus and Pteranodon were hydrodynamically streamlined, likely to reduce drag but also to increase prey penetration.

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:
Humphries S, Bonser RHC, Witton MP and Martill DM 2007.
Did pterosaurs feed by skimming? Physical modelling and anatomical evaluation of an unusual feeding method. PLoS Biol 5(8): e204. doi:10.1371/journal.pbio.0050204 online
Kellner AWA and Campos DA 2002. The function of the cranial crest and jaw of a unique pterosaur from the Early Cretaceous of Brazil. Science 297: 389–392.

The Family Tree of the Pterosauria 2 – basal Dimorphodontia

Following the origin of pterosaurs and our look at the most primitive member of the family tree, the Milan specimen (MPUM 6009), today we take a look at the Dimorphodontia, the first clade to branch off from the main lineage of all other pterosaurs. Click on the blue links to see the various pterosaurs described in greater detail and further referenced on reptileevolution.com.

The Dimorphodontia
This clade includes several recently discovered Triassic taxa with premaxillary crests, the crestless Dimorphodontidae and finally the typically small, snub-nosed, short-tailed Protoanurognathidae and Anurognathidae (covered in a future blog). All the rest of the pterosaurs, collectively known as the Eudimorphodontia, will be covered over several weeks in future blogs.

Raeticodactylus  The most primitve member of the Dimorphodontia was the Triassic pterosaur, Raeticodactylus. The similarities it shared with MPUM 6009 indicate Raeticodactylus was primitive. However, the many differences not shared provide evidence that the origin of pterosaurs occurred many millions of years prior to the Late Triassic. Those differences also hint at a tremendous variety of Triassic pterosaurs yet to be discovered.

Figure 1. Click to enlarge. Raeticodactylus in dorsal view, skull in lateral view. Hind limbs extended into the plane of the wings, which was unlikely due to the right angled femoral heads (arrow) which would not have permitted such a sprawl. 

Unique in many ways, Raeticodactylus had a knife-like horn on its snout with the naris displaced posterior to it. The skull was robust with a deep anterior mandible. An extra long retro articular process was present at the rear of the skull. The rest of this pterosaur was extraordinarily gracile, with the thinnest humerus known among pterosaurs. Importantly,Rhaeticodactylus and Eudimorphodon were the most primitive known pterosaurs with the ability to touch the ground without bending over from a bipedal stance.

Regarding the walking abilities of pterosaurs, Dr. David Hone wrote online, “On the ground the ‘rhamphorhynchoids’ were probably pretty poor… the shape of their hips and upper legs meant that could only really sprawl and not walk upright.” Well, this is clearly NOT the case with this very primitive ‘rhamphorhynchhoid’ because the femoral heads were set nearly at right angles to the femoral shafts, convergent with dinosaurs. Raeticodactylus had no trouble walking [really, no pterosaurs did and that myth is dispelled here], butRaeticodactylus might have had trouble sprawling the hind limbs during flight. Instead the femora could likely only manage an inverted “V” configuration while airborne.

The two specimens attributed to Austriadactylus.

Figure 2. The two specimens attributed to Austriadactylus.

Austriadactylus  Two specimens have been attributed to Austriadactylus. Although they do nest next to one another, the differences are substantial enough to deserve a new genus for the second of the two. Both initiate the expansion of the naris that reaches an acme with Dimorphodon. The larger holotype retained a long slender humerus, as in Raeticodactylus. The smaller referred specimen had a shorter, more typical humerus, metatarsal 4 was no longer than 3 and pedal digit 4 was no longer than metatarsal 4. Both retained the curved coracoid initially found in Cosesaurus and straightened out in most later pterosaurs. The size reduction between the two Austriadactylus specimens appears to mark the first time this pattern of size reduction occurs in pterosaurs. It is observable many times within the Pterosauria (contra Hone and Benton 2006).

The Dimorphodontidae

Preondactylus

Figure 3. Preondactylus

Preondactylus – One of the first Triassic pterosaurs to be discovered was Preondactylus, preserved as a natural mold with most of its bones washed away. Here Preondactylus serves as a transitional taxon between the Italian Austriadactylus and Dimorphodon. The very short cervicals also demonstrate a transition to the most primitive protoanurognathid, Peteinosaurus. The pectoral girdle was more robust with a straight coracoid. The pelvis and pes were longer with longer metatarsals than digits.

Dimorphodon

Figure 4. Dimorphodon

Dimorphodon – One of the first pterosaurs ever discovered, Dimorphodon had the largest skull fenestra of any pterosaur. The naris was larger than its enormous antorbital fenestra. This highly derived early Jurassic pterosaur had large clawed fingers, longer than its hand, which would have been ideal for tree trunk clinging.  The robust tail was twice the length of the body and head combined. A recent paper by Nesbitt and Hone (2010) claimed to identify a mandibular fenestra in Dimorphodon and so count pterosaurs in among the archosaurs. Unfortunately what they considered an angular defining the lower rim of that fenestra was actually a displaced pterygoid as documented here. The actual angular was displaced elsewhere (I haven’t found it yet) and the surangular had drifted toward the skull where it appeared to be a very deep and autapomorphic jugal plate.

The Protoanurognathidae

Peteinosaurus

Figure 5. Peteinosaurus

Peteinosaurus – The Late Triassic Peteinosaurus was at the base of a clade of snub-nosed pterosaurs with a smaller skull, a  much shorter neck and a greatly reduced tail, the Protoanurognathidae. Peteinosaurus retained a robust tail only half as long as in Dimorphodon. The terminal caudals were transformed into mere grains the size of most anurognathid caudals (unless this is an artifact of preservation). The sternum was twice as wide as long.

Carniadactylus

Figure 6. Carniadactylus

Carniadactylus – Another Late Triassic pterosaur, Carniadactylus, is known from a partial jaw, a pectoral girdle and limb bones. Here the sternum was as broad as deep.

The Bergamo specimen, MCSNB 8950

Figure 7. The Bergamo specimen, MCSNB 8950

MCSNB 8950 – From the Late Triassic, this tiny pterosaur was originally considered a juvenile Eudimorphodon. MCSNB 8950 was the first pterosaur to show that the sternal complex was composed of three discrete elements, the interclavicle, the sternum and two clavicles that wrapped around the leading edge of the sternum, as in Longisquama. The tail was robust. Unfortunately its length cannot be ascertained due to breakage.

Pterosaur family tree

Figure 8. Click to enlarge. The family tree of the Pterosauria.

Next time we look at the pterosaur family tree, members of the Anurognathidae will be introduced.

As always, I encourage readers to see the 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:
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
Nesbitt SJ and Hone DWE 2010.
 
An external mandibular fenestra and other archosauriform character states in basal pterosaurs. Palaeodiversity 3: 225–233

Other references for each taxon are found at www.reptileevolution.com


What Do Those Pterosaur Embryos Really Look Like?

In this blog you’re going to see the benefits of using DGS (Digital Graphic Segregation) a technique of tracing high resolution digital images on a computer monitor without the specimen at hand. This is widely considered to be inferior to first-hand observations using a microscope, pencil and camera lucida. However one method has not been tested against another, until now. The results speak for themselves.

I know of five pterosaur eggs with embryos inside. Four have been published. Three were reported to contain embryos. Let’s look at them one at a time. Image links will take you to individual taxon pages on reptileevolution.com. Embryo pterosaurs trapped inside their eggshells are exciting subjects because: 1) we can expect all of their bones to be present and 2) we know that each specimen was exactly zero years old.

1. The IVPP specimenIVPP V13758 (Wang and Zhou 2004, Figs. 1, 2) Early Cretaceous, ~125 mya, was the first pterosaur embryo to be published and it was originally considered to be a baby ornithocheirid, like the JZMP embryo. Using DGS to trace and reconfigure the bones into a standing reconstruction, the IVPP embryo turns out to be an anurognathid, but every bit as large as virtually all other adult anurognathids! Except one.

The IVPP embryo pterosaur

Figure 1. Click to enlarge DGS tracing. The IVPP embryo pterosaur (far left) as originally traced, (near left) as originally reconstructed as a baby ornithocheirid, (near right) traced using the DGS method, (far right) adult reconstructed at 8x the embryo size.

The only other anurognathid in the same size category as the IVPP hypothetical adult is a Mexican specimen mistakenly assigned to the genus Dimorphodon, but under the species D. weintraubi. Only its limbs are known and in cladistic analysis it is a sister to the IVPP embryo.

The IVPP embryo

Figure 2. The IVPP embryo scaled to an adult size (based on matching the egg to the pelvic opening diameter, along with various views of the skeleton and an egg and hatchling.

The IVPP embryo has a longer neck than most anurognathids and a longer metacarpus. The humerus is quite a bit shorter. Perhaps this will turn out to be a juvenile trait in this clade. The IVPP embryo is the most primitive clade member with a short tail, but it is also among the latest members to appear chronologically. Apparently it is a late relic. Learn more here.

The JZMP ornithocheirid embryo

Figure 3. Click to enlarge DGS tracings. The JZMP ornithocheirid embryo, in situ and reconstructed.

2.The JZMP embryoJZMP-03-03-2 (Ji et al. 2004, Fig. 3) was the second pterosaur embryo described from China. It was considered close to Beipiaopterus, but it is a basal ornithocheirid close to Haopterus. Here again the humerus was atypically small when compared to those of adult sister taxa, but otherwise the embryo had full size wings and adult proportions, including an elongated, tooth-filled rostrum and small eyes. Here the dorsal vertebrae were separated from the sacral vertebrae and the legs were disarticulated, so this egg was shaken or rolled before it was buried. Learn more here.

Pterodaustro embryo

Figure 3. Pterodaustro embryo. There certainly is no short snout/large eye here!

3. The Pterodaustro embryo MHIN-UNSL-GEO-V246 (Chiappe et al. 2004) was the first embryo found in association with adults and other juveniles of various sizes. The Pterodaustro egg was longer and narrower than the others and no wonder — it had to contain that elongated upturned rostrum! Distinct from the Chinese embryos, the Pterodaustro embryo had a relatively larger humerus and antebrachium (forearm), but it had a relatively smaller metacarpus. The sternal complex was also slightly larger. The details of the embryo in situ will be shown when it is published by its discoverers. Rather than estimating adult size from making comparisons to egg and pelvic opening, with Pterodaustro we have direct evidence for an 8x larger adult associated with an embryo. Learn more here.

Darwinopterus mother and premature embryo

Figure 4. Darwinopterus mother and premature embryo. Click to see in situ tracing.

4. The Darwinopterus egg/embryo – After several specimens of the germanodactylid, Darwinopterus, were published, one  (AMNH M8802) was reported (Lü et al. 2011) with an egg between its legs, evidently just expelled from the pelvis before or during burial. Originally no embryo was reported present in the egg, but the DGS method enabled the tracing of virtually all the articulated, but poorly ossified bones of the less than full term embryo. A reconstruction closely resembled the mother. Learn more here.

Ornithocephalus pterosaur egg.

Figure 5. Ornithocephalus pterosaur egg. Click to see in situ specimen.

5. The Ornithocephalus embryo – (Soemmerring 1812-1817) Pterodactylus micronyx von Meyer 1856, No. 29 in the Wellnhofer (1970) catalog). Ornithocephalus was the second pterosaur ever described. The tiny size of the specimen and its short snout immediately earned it juvenile status in the eyes of every paleontologist who saw it. Unfortunately, for that hypothesis, Ornithocephalus nests with other tiny pterosaurs in a transitional series from Scaphognathus to the bases of Pterodactylus and Germanodactylus (more on this in future blogs). Between the femora of this pterosaur is an elongated egg-like structure with an apparent embryo inside. This observation needs to be tested because this egg/embryo has not been published and confirmed. I happened upon it when I was tracing the bones of the specimen. Like Darwinopterus, the Ornithocephalus egg was apparently discharged while the mother lay motionless. Learn more here.

Immediately able to fly?
The size of pterosaur eggs is key to understanding the abilities of the hatchlings to fly–or not. The IVPP egg was ~50 mm in length. The JZMP egg was ~60 mm in length. The Pterodaustro egg was ~60 mm in length. The snout/vent length of each emerging hatchling would have been greater than these measurements because the pterosaur was tucked in with its snout down prior to hatching. These examples all fit in the category of: “hatchlings ready to fly” because the embryos equalled or exceeded the sizes of many tiny adult pterosaurs.

Please consider that living lizards with a snout/vent length under 18mm (Hedges and Thomas 2001) dry up and die when removed from their moist leaf litter environment. Now increase that surface area with wing membranes, crests and uropatagia and you have a real problem as a hatchling of a tiny pterosaur with a relatively greater surface/volume ratio will spend every flying moment evaporating precious moisture in a steady airstream.

At ~27 mm in length, the Darwinopterus egg produced a much smaller hatchling only about 35 mm tall. This was slightly shorter than the smallest known pterosaur not associated with an eggshell, B St 1967 I 276 (No. 6 in the Wellnhofer 1970 catalog), which stood at ~40mm tall with a similar snout/vent length. Thus the Darwinopterus hatchling was likely able to fly, but it was nearing what appears to be some sort of theoretical limit.

Now the real problem: Hatchlings of no. 6 would have been ~5 mm in snout/vent length, the size of house flies. Their high surface/volume ratios would have grounded such tiny hatchlings until they were able to grow up to the theoretical limit (unless they were somehow able to keep themselves hydrated some other way). The various problems tiny grounded pterosaurs likely encountered likely provided certain natural selection pressures for whatever traits followed, such as the reduction of the tail and the elongation of the metacarpus. These changes occurred at least four times, by convergence, according to the present fully resolved tree.

Archosaur Eggs or Lizard Eggs?
Archosaurs, like birds and crocs, lay their eggs shortly after fertilization and the embryos develop chiefly outside of the mother. Grellet et al. (2007) suggested that, as archosaurs,
pterosaurs would have buried their eggs for months at a time, forcing the tiny hatchlings to crawl through the sand, mud and rotten leaves to get to the surface to fly. Seems unlikely since any tear to the wing fabric would have been a death sentence, but that’s the traditional hypothesis.

Lizards, generally lay their eggs long after fertilization, sometimes at the moment of hatching or shortly before. Since pterosaurs are lizards, you should always look for embryos, even poorly ossified embryos, inside of pterosaur eggs. The extremely thin pterosaur eggshell is most comparable to the eggshells of living lizards that retain the egg until just before hatching. Finally, just think of the benefits for the embryo. Inside of its warm-blooded mother for most of its development, a pterosaur embryo could develop quickly and safely.

Isometric Growth is Proven with Pterosaur Embryos
Each of the pterosaur embryos had the proportions of an adult (with humerus length the chief exception). Their eyes were not larger and their beaks were not shorter than in adults. Pterosaur hatchlings did not have “cute” facial proportions like baby mammals, birds and crocodilians. That falsifies decades of earlier traditions supporting allometric development in pterosaurs by Wellnhofer (1970) and others, especially Bennett (1993a, b, 1995, 1996a, 2001a,b, 2006, 2007) who both wrongly considered pterosaurs to be archosaurs and tiny pterosaurs to be juveniles. Precise tracings and reconstructions of the embryos demonstrate that pterosaurs matured isometrically, like their precursor, Huehuecuetzpalli, the basal taxon in the Tritosauria. That strategy for growth supports the hypothesis that all the tiny pterosaurs listed on the pterosaur cladogram (except the juvenile Pterodaustro) represent distinct taxa and unique genera.

Everything about pterosaurs points to a lizard ancestry. The archosaur hypothesis cannot be defended except by excluding all lizard and fenestrasaur candidates, which is how it is so often done nowadays (Hone and Benton 2007, 2008, Nesbitt 2011).

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 1993a. The ontogeny of Pteranodon and other pterosaurs. Paleobiology 19, 92–106.
Bennett SC 1993b. Year classes of pterosaurs from the Solnhofen limestone of southern Germany. Journal of Vertebrate Paleontology. 13, 26A.
Bennett SC 1995. A statistical study of Rhamphorhynchus from the Solnhofen limestone of Germany: year classes of a single large species. Journal of Paleontology 69, 569–580.
Bennett SC 1996a. Year-classes of pterosaurs from the Solnhofen limestones of Germany: taxonomic and systematic implications. Journal of Vertebrate Paleontology 16:432–444.
Bennett SC 1996b. The phylogenetic position of the Pterosauria within the Archosauromorpha. Zoological Journal of the Linnean Society 118:261–309.
Bennett SC 2001a, b. 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.
Bennett SC 2006. Juvenile specimens of the pterosaur Germanodactylus cristatus, with a review of the genus. Journal of Vertebrate Paleontology 26:872–878.
Bennett SC 2007. A review of the pterosaur Ctenochasma: taxonomy and ontogeny. Neues Jahrbuch fur Geologie und Paläontologie, Abhandlungen 245:23–31.
Chiappe LM, Codorniú L, Grellet-Tinner G and Rivarola D. 2004. Argentinian unhatched pterosaur fossil. Nature, 432: 571.
Grellet-Tinner G, Wroe S, Thompson SB and Ji Q 2007. A note on pterosaur nesting behavior. Historical Biology 19:273–277.
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 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.
Hone DWE and Benton MJ 2008. Contrasting supertree and total evidence methods: the origin of the pterosaurs. Zitteliana B28:35–60.
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-C, Unwin DM, Deeming DC, Jin X, Liu Y and Ji Q 2011a. An egg-adult association, gender, and reproduction in pterosaurs. Science, 331(6015): 321-324. doi:10.1126/science.1197323
von Meyer CEH 1856.  Zur Fauna der Vorwelt. Saurier aus dem Kupferschiefer der Zechstein-Formation. Frankfurt-am-Main. vi + 28 pp., 9 pls.
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.
von Soemmering ST 1812. Über einen Ornithocephalus. – Denkschriften der Akademie der Wissenschaften München, Mathematischen-physikalischen Classe 3: 89-158.
von Soemmering ST 1817. Über einer Ornithocephalus brevirostris der Vorwelt. Denkschriften der Akademie der Wissenschaften München, Mathematischen-physikalischen Classe 6: 89-104.
Wang X-L and Zhou Z 2004. Palaeontology: pterosaur embryo from the Early Cretaceous. Nature 429: 623.
Wellnhofer P 1970. Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Abhandlungen der Bayerischen Akademie der Wissenschaften, N.F., Munich 141: 1-133.

Pterosaurs and turtles?? Say it ain’t so…

Serious topic: done to prove a point.
I have often argued (Peters 2000a, b, 2002, 2007, 2009, 2010, 2011) against the nesting of pterosaurs with archosaurs and archosauromorphs such as Scleromochlus, Lagerpeton, Parasuchus and Proterochampsa (contra Sereno 1991; Bennett 1998; Irmis et al. 2007; Nesbitt et al. 2009; Hone and Benton 2007, 2008; Brusatte et al. 2010; Nesbitt and Hone 2010; Nesbitt 2011 and others). The problem has always been taxon exclusion. If you delete or exclude the real ancestors and sisters of pterosaurs, they’ll nest in the weirdest places. In the following experiments, you’ll see pterosaurs nest almost anywhere except with archosaurs. They’ll even nest with turtles and pachypleurosaurs, rather than associate themselves with archosaurs!

Figure 1. Click to expand. Here's what happens when you exclude fenestrasaurs, squamates and all the other lepidosauromorphs, but leave the turtles, in a study on pterosaur origins. Note: pterosaurs do not nest with archosaurs.

From the large and fully resolved cladistic analysis in which pterosaurs nested with fenestrasaur squamates, I excluded every taxon from the Lepidosauromorpha except two turtles (Odontochelys and Proganochelys) and the pterosaur MPUM 6009 to see what would happen. Click to expand this image:

When Ichthyostega is the outgroup taxon turtles nest with it and pterosaurs nest at the base of the Sauropterygia.

When three specimens of Gephyrostegus form the outgroup, pterosaurs and turtles nest at the base of the Sauropterygia.

When all of the included non-reptiles form the outgroup taxa turtles nest with Eocaecelia while pterosaurs nest at the base of the Sauropterygia.

When no outgroups are employed pterosaurs nest with turtles as an outgroup to the Archosauromorpha. A shift of branches brings the Sauropterygia to the base of the tree.

Given the opportunity, pterosaurs nest with turtles and basal sea reptiles rather than archosaurs and archosauromorphs. Taxon exclusion was responsible for this. So why do paleontologists continue to include pterosaurs in archosaur studies?

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 1996. The phylogenetic position of the Pterosauria within the Archosauromorpha. Zoological Journal of the Linnean Society 118:261–309.
Brusatte SL , Benton MJ , Desojo JB and Langer MC 2010. The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida), Journal of Systematic Palaeontology, 8:1, 3-47.
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.
Hone DWE and Benton MJ 2008. Contrasting supertree and total evidence methods: the origin of the pterosaurs. Zitteliana B28:35–60.
Irmis RB, Nesbitt SJ, Padian K, Smith ND, Turner AH, Woody D and Downs A 2007. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science 317 (5836): 358–361. doi:10.1126/science.1143325. PMID 17641198.
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.
Nesbitt SJ, Irmis RB, Parker WG, Smith ND, Turner AH and Rowe T 2009. Hindlimb osteology and distribution of basal dinosauromorphs from the Late Triassic of North America. Journal of Vertebrate Paleontology 29 (2): 498–516. doi:10.1671/039.029.0218
Nesbitt SJ and Hone DWE 2010.An external mandibular fenestra and other archosauriform character states in basal pterosaurs. Palaeodiversity 3: 225–233.
Peters D 2000a Description and Interpretation of Interphalangeal Lines in Tetrapods. Ichnos, 7: 11-41
Peters D 2000b.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 2007. The origin and radiation of the Pterosauria. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29: 1327-1330.
Peters D 2010. In defence of parallel interphalangeal lines. Historical Biology iFirst article, 2010, 1–6 DOI: 10.1080/08912961003663500
Peters D 2011. A Catalog of Pterosaur Pedes for Trackmaker Identification
Ichnos 18(2):114-141.
Sereno PC 1991. Basal archosaurs: phylogenetic relationships and functional implications. Journal of Vertebrate Paleontology 11 (Supplement) Memoire 2: 1–53.