Avicranium: a 3D drepanosaur skull

Pritchard and Nesbitt 2017
bring us a new Late Triassic drepanosaur, Avicranium renestoiAMNH FARB 30834, based on CT scanning a crushed skull and reconstructing it digitally (Fig. 1). I added a little distance between the anterior and posterior elements in order to get a rounder orbit. I also restored the missing ascending process of the premaxilla and nasal.

Figure 1. Avicranium from Pritchard and Nesbitt 2017, in situ, original reconstruction and revised with rostral restoration.

Figure 1. Avicranium from Pritchard and Nesbitt 2017, in situ, original reconstruction and revised with rostral restoration in accord with the plesiomorphic drepanosaur, Vallesaurus. Distinct from Vallesaurus, Avicranium has a concave maxilla and lacks teeth. That long convex squamosal is unique.

The new reconstructed skull
looks like a little oviraptorid (Fig. 2)— strictly by convergence.

Figure 3. Khaan, an oviraptorid that nests with Limusaurus in the large reptile tree AND the repaired Cau, Brougham and Naish tree.

Figure 2. Khaan, an oviraptorid, has a skull similar to that of Avicranium.

The closest sister taxon of Avicranium
in the large reptile tree (LRT, 1087 taxa) is the drepanosaur, Vallesaurus based on the skull alone. Pritchard and Nesbitt nested Avicranium as a drepanosaur based on the cervical vertebrae (Fig. 1) and noticed ‘striking similarities’ to birds. The difference includes the crooked jaw line, perhaps related to the absence of teeth in Avicranium.

Figure 2. Vallesaurus is a sister to Avicranium in the LRT.

Figure 2. Vallesaurus is a sister to Avicranium in the LRT.

Pritchard and Nesbitt nested
drepanosaurs uncertainly, reporting “A phylogenetic analysis of Permo-Triassic diapsids supports the hypothesis that drepanosaurs are an archaic lineage that originated in the Permian, far removed from crown group Reptilia.” They did not realize their clade ‘Diapsida’ was actually diphyletic, with lepidosauriformes arising convergently with archosauriformes. The authors did not include taxa that nested basal to drepanosaurs in the LRT including Jesairosaurus, Palaegama and Saurosternon even those these have been on the Internet for several years. In fact, their figure 4 cladogram shows no outgroups for the Drepanosauromorpha, a very dangerous phylogenetic proposition. By contrast the LRT provides certain and verified outgroups back to Devonian tetrapods.

Figure 4. Subset of the LRT focusing on drepanosaurs and Avicranium.

Figure 4. Subset of the LRT focusing on drepanosaurs and Avicranium.

So once again taxon exclusion obscures relationships.
All Pritchard and Nesbitt had to do was to go online for some taxonomic suggestions and their unsolved problem would have been quickly remedied using their own character list. Are paleo-workers trying to avoid taxa offered for testing by the LRT? It would seem so given the present circumstances. Would it tear down the walls if someone knowingly confirmed the present hypothesis of interrelationships?

Well, that’s not going to happen,
except, as we’ve seen, without citation.

References
Pritchard AC and Nesbitt SJ 2017. A bird-like skull in a Triassic diapsid reptile increases heterogeneity of the morphological and phylogenetic radiation of Diapsida. Royal Society Open Science DOI: 10.1098/rsos.170499

wiki/Avicranium

The hand and super-claw of Drepanosaurus

When Drepanosaurus (Pinna 1980, Figs. 1-3) was first discovered and described, this oddity, metaphorically from the land of Dr. Seuss, presented several never-before-seen morphologies including a hooked tail, humped shoulder, a giant olecranon sesamoid (earlier misidentified as a displaced ulna) and an odd hand with a super-claw on finger two — all on one headless body.

Figure 1. Drepanosaurus featuring fused finger phalanges and a super claw -- among several other odd traits.

Figure 1. Drepanosaurus featuring fused finger phalanges and a super claw — among several other odd traits. This image is updated from a prior attempt. Note: the tips of manual unguals 2-4 are aligned.

After fielding a question,
I told a reader that I would take another look at the Drepanosaurus hand. I’m glad I did. The prior tracing was not based on DGS techniques or high resolution images. This one (Figs. 1-3) is. Earlier I mistakenly reconstructed ungual 2 extending beyond the others. Now I find that the middle three unguals terminated at about the same line (Fig. 3).

Figure 1. Drepanosaurus hand with DGS (digital graphic segregation) techniques used to separate the fingers and discover the vestigial joints in the fused digits. The size and proportions of ungual 1 are guesstimated based on very vague outlines impressed from below on ungual 2. On digit 4 (gold on gold) the original tracing appears to have missed the penultimate phalanx (n dark gold).

Figure 2. In situ tracing using DGS (Photoshop layers) to segregate fingers from one another. The outline of digit 1 (purple) is tentative, based on general patterns and very slight impressions in ungual 2.

Yes
several of the phalanges are apparently fused together. Nevertheless their former joints are still visible and are traced here. The penultimate phalanges are very short, the opposite of most arboreal lizards. The cervicals are also quite short, the opposite of other drepanosaurs.

Why did the phalanges fuse?
Perhaps because that big claw prevented the typical flexion function among phalanx sets. Ungual 2 is so big that several former PILs (now fused phalanges) ran through it.

Figure 2. GIF animation of Drepanosaurus fingers reconstructed and imagined in dorsal view. Metacarpal outlines may not be reconstructed in dorsal view. They are typically arranged with mc4 the longest.

Figure 2. GIF animation of Drepanosaurus fingers reconstructed and imagined in dorsal view from data in figure 2. Metacarpal outlines may not be reconstructed in dorsal view. They are typically arranged with mc4 the longest.

Function?
Like almost all digits, the acted together for grasping. The large size of ungual 2 simply made up for the relative brevity of metacarpal 2 and the proximal phalanges, traits that are plesiomorphic for reptiles. That the extensor surface of the ungual is much larger than the flexor surface suggests that the claws were often held retracted, like cat claws. So these were more like paws, the tendril like arboreal lizard toes. Others have considered drepanosaurs slow movers. I agree.

Unlike earlier chameleon-like hypotheses
for Megalancosaurus, the manual digits of Drepanosaurus appear to have swung through parallel arcs, as in most tetrapods.

In situ
the tall narrow claws lie on their sides, as is typical of ungual preservation in crushed fossils. In figure 3, I imagined them in dorsal view, which is the typical presentation of a manus for other tetrapods. Atypically the ungual extends proximally over the penultimate phalanges in dorsal view. So the transparent colors help to visualize this. One can only imagine the size of the extensor tendons on those hands. The flexors were strong too. Don’t let one of these climb on your arm or hand. You might not ever be able to shake it off.

Phylogeny
Wikipedia reports that Drepanosaurus nests within the Protorosauria, a terrestrial clade or small to large archosauromorphs. In counterpoint, and with actual phylogenetic testing (not tradition), the large reptile tree nests Drepanosaurus and the drepanosaurs with Jesairosaurus and the Keuhneosaurs at the base of the Lepidosauriformes. This clade was arboreal.

References
Pinna G 1980. Drepanosaurus unguicaudatus, nuovo genere e nuova specie di Lepidosauro del trias alpino. atti Soc. It. Sc.Nat. 121:181-192.
Pinna G 1986. On Drepanosaurus unguicaudatus, an upper Triassic lepidosaurian from the Italian Alps. Journal of Paleontology 50(5):1127-1132.
Renesto S 1994. The shoulder girdle and anterior limb of Drepanosaurus unguicaudatus (Reptilia, Neodiapsida) from the upper Triassic (Norian of Northern Italy. Zoological Journal of the Linnean Society 111(3):247-264

wiki/Drepanosaurus

 

Jesairosaurus and the drepanosaurs leave the Tritosauria :-(

My earlier reconstruction
of the basal lepidosauriform, Jesairosaurus (Fig. 1; contra Jalil 1997, not a protorosaur/prolacertiform) included several errors based on attempting to create a chimaera of several specimens of various sizes based on scale bars. In this case, scale bars should not have been used. Rather fore and hind parts had to be scaled to common elements, like dorsal vertebrae, as shown below (Fig. 2). I think this version more accurately reflects the in vivo specimen, despite its chimeric origins. All of the partial skeletons assigned to this genus were discovered at the same Early to Middle Triassic sandstone site and two were touching one another. A larger skull, ZAR 7, shows the variation in size from the skull to shoulders remains of the ZAR 6 specimen.

Figure 1. New reconstruction of the basal lepidosauriform, Jesairosaurus (Jalil 1993).

Figure 1. New reconstruction of the basal lepidosauriform, Jesairosaurus (Jalil 1997). The wide and flat ribs are interesting traits for a likely arboreal reptile.

Mother of all drepanosaurs
The Drepanosauria is an odd clade of slow-moving arboreal reptiles that includes Hypuronector, Vallesaurus, Megalancosaurus and Drepanosaurus (Figs. 2, 3). Jesairosaurus was not a drepanosaur, but nested basal to this clade before the present revisions. It remains basal to the Drepanosauria now with revisions.

The revised reconstruction of Jesairosaurus 
shifts this clade away from Huehuecuetzpalli, Macrocnemus and the rest of the Tritosauria. Now Jesairosaurus and the drepanosaurs nests between Saurosternon, Palaegama and the so-called “rib” gliders, beginning with Coelurosauravus.

A short history of Jesairosaurus
Shortly after their discovery Lehman 1971 referred the several hematite encrusted specimens to the Procolophonida. Further preparation showed that they were referable to the Diapsida, according to Jalil (1990) and the, more specifically, to the Prolacertiformes (Jalil 1997) as a sister to Malerisaurus with Prolacerta as a common ancestral sister. Jalil did not include the closest sisters of Jesairosaurus as revealed by the present analysis.

With a much larger list of taxa,
the large reptile tree nests Malerisaurus between the Antarctica specimen assigned to Prolacerta (AMNH 9520) and the holotype of Prolacerta. Jesairosaurus, as mentioned above, nests with the basal lepidosauriformes. Any traits shared with protorosaurs are by convergence. Deletion of Jesairosaurus does not affect the nesting of the Drepanosauria as basal lepidosauriformes.

Figure 3. Drepanosaurs and their ancestor sisters, Jesairosaurus and Palaegama to scale.

Figure 3. Drepanosaurs and their ancestor sisters, Jesairosaurus and Palaegama to scale.

Arboreal
This new nesting shifts drepanosaurs closer to kuehneosaurs (Figs. 3, 4), another notably arboreal clade.

Figure 3. The new nesting for Jesairosaurus and the drepanosaurs as sisters to the Kuehneosaurs, several nodes away from Huehuecuetzpalli and the tritosaurs.

Figure 3. The new nesting for Jesairosaurus and the drepanosaurs as sisters to the Kuehneosaurs, several nodes away from Huehuecuetzpalli and the tritosaurs.

Certainly
there will someday be more taxa to fill in the current large morphological gaps in and around Jesairosaurus, but here’s what we have at present (Fig. 3) with regard to the origin of the so-called “rib” gliders (actually dermal rods, not ribs, as shown by Coelurosauravus) and the origin of the drepanosaurs.

Figure 4. Jesarosaurus to scale with sisters Palaegama and Coelurosauravus.

Figure 4. Jesairosaurus to scale with sisters Palaegama and Coelurosauravus.

The shifting of a clade
like Jesairosaurus + Drepanosauria occurred due to inaccurate reconstructions used for data. Science builds on earlier errors and inaccuracies. I let the computer figure out where taxa nest in a cladogram of 606 possible nesting sites, minimizing the negative effects of bias and tradition.

It’s sad
to see the drepanosaurs leaving the Tritosauria as it contains several oddly Dr. Seuss-ian variations on the tritosaur theme.

Also note the nesting
of the basal Rhynchocephalians, Megachirella and Pleurosaurus, between the palaegamids and the tritosaurs (Fig. 4). In the course of this study, both also received updates to their skull reconstructions. The former was difficult to interpret without knowing where it nested. What appeared to be an odd sort of a squamosal in Megachirella now appears to be a pair of displaced pleurosaur-like premaxillae. For Pleurosaurus I should not have trusted a prior line drawing by another worker. Here I used DGS to create what appears to be a more accurate skull without so many apparent autapomorphies.

References
Jalil N 1990. Sur deux cranes de petits Sauria (Amniota, Diapsida) du Trias moyen d’ Algerie. Comptes Rendus de I’ Academic des Sciences, Paris 311 :73 1- 736.
Jalil N-E 1997. A new prolacertiform diapsid from the Triassic of North Africa and the interrelationships of the Prolacertiformes. Journal of Vertebrate Paleontology 17(3), 506-525.
Lehman JP 1971. Nouveaux vertebres du Trias de la Serie de Zarzai’tine. Annales de Paleontologic (Vertebres) 57 :71-93.

 

 

A closer look at Jesairosaurus

Updated November 29, 2015 with a new reconstruction and nesting for Jesairosaurus.

Earlier we looked at Jesairosaurus (Jalil 1991) and the origin of the Drepanosauridae. Today we’ll take a closer look at Jesairosaurus.

Figure 1. New reconstruction of the basal lepidosauriform, Jesairosaurus (Jalil 1993).

Figure 1. New reconstruction of the basal lepidosauriform, Jesairosaurus (Jalil 1993).

Jesairosaurus was originally considered a procolophonid, then a prolacertiform
Sure it has a long neck, but in phylogenetic analysis, it doesn’t nest with Prolacerta or Marlerisaurus, but after the basal lepidosauriform Palaegama and prior to drepanosaurs and kuehneosaurs. Note the dorsal nares and large orbit. There’s a very tall scapula here, a precursor to the tall stem in drepanosaurs.

Distinct from most lepidosauriforms,
Jesairosaurus has fairly large thecodont teeth. Gastralia appear here for the first and last time in this lineage. The ventral pelvis isn’t fused, but the thyroid fenestra is gone.

Like most lepidosauriforms,
the scapulocoracoid was not fenestrated. And there’s a nice ossified sternum there.

On a side note…
As you know, I’m always attempting to improve the data here. Several months ago I mentioned to a detractor that most prior workers reported  forelimbs present in Sharovipteryx. Only Unwin et al. (2000) thought they were buried in the matrix. My detractor claimed the opposite, that I was the only one to see forelimbs. No word yet on this issue. Still waiting.

Another detractor claimed I had seen soft tissue on a Bavarian museum fossil pterosaur. When I asked which specimen, he refused to provide the number.

In a third case I asked to see a closeup of a pterosaur mandible tip that had been published. I wondered if the sharp tip might be a tooth, as it is in other sharp mandible pterosaurs. The offer was refused with the phrase, “trust us, it’s not there.” I replied “trust” is antithetical to Science. No reply and no closeup yet.

So, is it so hard to provide a museum number? A closeup of a photograph? Or a reply to a note on forelimbs? Should we trust other scientists? Or should we test and confirm or refute? There may be cooperation among other paleontologists. Or maybe they’re all very protective of their data. Evidently I also have a very bad reputation, and that may be the reason for the lack of cooperation. These things happen when paradigms are broken.

References
Jalil N-E 1997. A new prolacertiform diapsid from the Triassic of North Africa and the interrelationships of the Prolacertiformes. Journal of Vertebrate Paleontology 17(3), 506-525.
Unwin DM, Alifanov VR and Benton MJ 2000. Enigmatic small reptiles from the Middle-Late Triassic of Kyrgyzstan. In: Benton M.J., Shishkin M.A. & Unwin D.M. (Eds) The Age of Dinosaurs in Russia and Mongolia. Cambridge: Cambridge U. Press: 177-186.

wiki/Prolacertiformes

 

 

The Antorbital Fenestra of Drepanosaurs

The drepanosaurs are a rag-tag clade of slow-moving arboreal tritosaur lizards derived from Jesairosaurus in the large reptile tree. Drepanosaurs include Hypuronector, Vallesaurus, Megalancosaurus and Drepanosaurus in order of greater derivation.

The skull of Vallesaurus in situ.

Figure 1. The skull of Vallesaurus in situ. Difficult to interpret, to be sure, but I don’t see any large maxillary plates here. Perhaps palatal elements, though.

There has been some debate as to whether or not drepanosaurs had an antorbital fenestra. If so, this would have been the fifth (or perhaps the fourth since this clade is very close to Langobardisaurus and Cosesaurus) origin of the antorbital fenestra. Unfortunately only two drepanosaurs include skulls, Vallesaurus and Megalancosaurus. Here we’ll trace and reconstruct both. It’s no easy task.

The skull of Vallesaurus as interpreted by Renesto

Figure 2. The skull of Vallesaurus as interpreted by Renesto (2006). He found no evidence of an antorbital fenestra. The stem-like nasal and lacrimal are both present here. The question is, what, if anything, filled the space between them? Was it folded and flattened palatal bones? Renesto did not note any palatal elements here, yet they must have been present.

The palatal elements of Jesairosaurus,

Figure 3. The palatal elements of Jesairosaurus, the outgroup for the drepanosaurs. Fairly solid-looking. This contrasts with the lighter airier palates of fenestrasaurs and Megalancosaurus.

The palate
When skulls are crushed in lateral view, something has to happen to the transverse palatal elements. Typically they flip up or down, like playing cards, presenting their broadest surface to the matrix plane. It is difficult to discern the shape of the palatal elements in Vallesaurus, but if they were more solid, like the elements in Jesairosaurus, then this may account for the bone that Renesto found behind and through the rostrum (= antorbital fenestra).

In the more derived drepanosaur, Megalancosaurus (fig. 8) these solid elements were reduced to more gracile elements.

Figure 4. Interpretation of the skull elements of Vallesaurus. Note the slender ascending process of the maxilla in green.

Figure 4. Interpretation of the skull (sans palatal) elements of Vallesaurus. Note the slender ascending process of the maxilla in green and the slender lacrimal stem in magenta.

Interpretation of the skull of Vallesaurus based on figure 4.

Figure 5. Interpretation of the skull of Vallesaurus based on figure 4.

Vallesaurus
Talk about difficult skulls to trace (Figs. 1, 4), reconstruct and restore (Fig. 5)!! And remember, if we are looking at a light and airy skull, we’re looking through a light and airy skull to the right side elements obscured by intervening palatal elements. I did not attempt to outline the palatal elements in Vallesaurus, but considering the size of the eyes, the palate had to be nearly as wide as the height of the skull, as in Megalancosaurus.

Both part and counterpart of Megalancosaurus superimposed.

Figure 6. Both part and counterpart of Megalancosaurus superimposed using Photoshop.

Interpretation of figure 6, the skull of Megalancosaurus.

Figure 7. Interpretation of figure 6, the skull of Megalancosaurus. Struts of bone surround antorbital fenestra here. The slender ascending process of the left maxilla is convincing evidence for an antorbital fenestra in Megalancosaurus. The huge size of the naris is further evidence for skull lightening as a selective influence.

Megalancosaurus
The skull of Megalancosaurus was split in two, the part and counterpart (Figs. 6, 7). Reassembling them with software helps to realign the two crushed halves together for interpretation of the elements. What I see there is a slender ascending process of the maxilla. That can only occur if there is an antorbital fenestra. If that bone is another bone, then this will have to be reconsidered. At this point an antorbital fenestra seems likely. And I’m open to new data if anyone out there has some.

Megalancosaurus including the palate, the only palate ever figured for a drepanosaur.

Figure 8. Megalancosaurus including the palate, the only palate ever figured for a drepanosaur.

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
Calzavara M, Muscio G and Wild R 1980. Megalancosaurus preonensis n. gen. n. sp., a new reptile from the Norian of Friuli. Gortania 2: 59-64.
Feduccia A and Wild R 1993. Birdlike characters in the Triassic archosaur Megalancosaurus. Natur Wissenschaften 80:564–566.
Geist NR and Feduccia A 2000. Gravity-defying Behaviors: Identifying Models for Protoaves. American Zoologist 4):664-675. online pdf
Renesto S 1994. Megalancosaurus, a possibly arboreal archosauromorph (Reptilia) from the Upper Triassic of Northern Italy. Journal of Vertebrate Paleontology 14(1):38-52.
Renesto S 2000. Bird-like head on a chameleon body: new specimens of the enigmatic diapsid reptile Megalancosaurus from the Late Triassic of Northern Italy. Rivista Italiana di Paleontologia e Stratigrafia 106: 157–179.
Renesto S and Binelli G 2006. ’Vallesaurus Cenensis“’ Wild, 1991, A Drepanosurid (Reptilia, Diapsida): From the Late Triassic of Northern Italy”, Rivista Italiana di Paleontologia e Stratigrafia 112: 77–94, Milano.
Wild R 1990. Ein Flugsaurierrest (Reptilia, Pterosauria) aus der Unterkreide (Hauterive) von Hannover (Niedersachsen). Neues Jahrbuch für Geologie und Paläontologie. Abhandlung 181,241–254.

wiki/Vallesaurus
wiki/Megalancosaurus

Jesairosaurus and the origin of drepanosaurs

Updated November 29, 2015 with new data and a new nesting for Jesairosaurus and the drepanosaurs. 

The authors of Wikipedia
have no idea what drepanosaurs are other than reptiles. In the large reptile tree drepanosaurs were descended from Jesairosaurus and, more primitively, the basal tritosaur lepidosauriform, Palaegama. Here we’ll start with Palaegama (Fig. 1)the father of all drepanosaurs and kuehneosaurs.

Figure 3. Drepanosaurs and their ancestor sisters, Jesairosaurus and Palaegama to scale.

Figure 1 Drepanosaurs and their ancestor sisters, Jesairosaurus and Palaegama to scale.

Palaegama is a basal lizard-like lepidosauriform that shared few obvious traits with the highly derived drepanosaurs. It does not take a transitional taxon, like Jesairosaurus (Fig. 1) to make the case for a relationship here because deletion does nothing to tree topology.

Now we narrow our focus
to Jesairosaurus, the father of all drepanosaurs (Fig. 2). Essentially Jesairosaurus was a long torso palaegamid lepidosauriform. The skulls are readily comparable. The limbs are shorter in the derived taxon.

Figure 1. New reconstruction of the basal lepidosauriform, Jesairosaurus (Jalil 1993).

Figure 2. New reconstruction of the basal lepidosauriform, Jesairosaurus (Jalil 1993).

Jesairosaurus lehmani (Jalil 1997) Early to Middle Triassic ~240 mya was originally described as a prolacertiform and thus related to Prolacerta. All shared traits are by convergence here. By virtue of its nesting, Jesairosaurus is also basal to the kuiehneosaurs, like Coelurosauravus (Fig.  3).

 

Figure 4. Jesarosaurus to scale with sisters Palaegama and Coelurosauravus.

Figure 3. Jesarosaurus to scale with sisters Palaegama and Coelurosauravus.

With its bigger torso and smaller limbs,
Jesairosaurus was not a speedster. This was the first step in the evolution of the slow-moving, arboreal drepanosaurs. The high scapula of drepanosaurs finds an origin in Jesairosaurus.

It is unfortunate
that the hind feet and tail are not known for Jesairosaurus, because both of these body parts underwent a great transformation in early drepanosaurs.

As in Palaegama, the pelvis of Jesairosaurus was relatively tiny and included an anterior process of the ilium that developed further in drepanosaurs. While  Palaegama employed its expanded ilium to sprint, drepanosaurs were not sprinters, but arboreal climbers and clingers, like modern day chamaeleons.

 

References
Jalil N-E 1997. A new prolacertiform diapsid from the Triassic of North Africa and the interrelationships of the Prolacertiformes. Journal of Vertebrate Paleontology 17(3), 506-525.

wiki/Prolacertiformes

The Tritosauria – An Overlooked Third Clade of Lizards

Traditionally there have been just two lizard clades in the Squamata. The Iguania included Iguana, Draco, Phrynosoma and other similar lizards. The Scleroglossa included Tupinambis, Chalcides, Varanus, Heloderma and all the snakes and amphisbaenids. Squamate outgroups within the Lepidosauria included members of the Rhynchocephalia (such as Sphenodon) and the basal lepidosaur, Homoeosaurus, which probably appeared in the Permian, but is only known from the Late Jurassic.

Traditional Nesting
Wikipedia reports the following about the Squamata, “Squamates are a monophyletic  group that is a sister group to the tuatara. The squamates and tuatara together are a sister group to crocodiles and birds, the extant archosaurs.” This is the traditional concept, but testing this in a larger study finds that lizards and archosaurs are not closely related. Not by a long shot.

The Tritosauria, a new lizard clade that was previously overlooked.

Figure 1. Click to enlarge. The Tritosauria, a new lizard clade that was previously overlooked.

The New Heretical Tritosauria
The large study (Peters 2007) recovered a third clade of squamates just outside of the Squamata (Iguania + Scleroglossa), but inside the Lepidosauria (which includes Sphenodon and the other Rhynchocephalia). At the base of this third clade, called the Tritosauria (“third lizards”), are three very lizardy forms, none of which had fused proximal ankle bones, a trait shared by most squamates (at least those that have legs!). Lacertulus, Meyasaurus and Huehuecuetzpalli are known from crushed but articulated fossils. Lacertulus was considered a possible biped (Carroll and Thompson 1982) based on its long hind legs. It is likely that Huehuecuetzpalli (Reynoso 1998) was also a biped. All three were considered close to the base of the lepidosauria, not closely related to any living lizards.

The Tritosauria
A Clade of Misplaced and Enigmatic “Weird-Ohs”

Phylogenetically following Huehuecuetzpalli three distinct clades emerge within the Tritosauria. Some of these were formerly considered “prolacertiforms” (Peters 2000), but now we know that none are related to ProlacertaAll three subclades have some pretty weird members.

The Tanystropheidae
This clade was named by Dilkes (1998) to include “the most recent common ancestor of MacrocnemusTanystropheus and Langobardisaurus and all of its descendants.” Clade members include several long-necked taxa, some of which, like Dinocephalosaurus, preferred swimming to walking. Tanystropheus was the largest, attaining 4.5 meters in length.

The Jesairosauridae
This clade includes Jesairosaurus (Jalil 1991) and the drepanosaurs, from Hypuronector to Drepanosaurus.  This clade included several arboreal, hook-tailed taxa with short-toed feet that were able to grasp slender branches in their slow-motion quest for insects. All were rather small.

The Fenestrasauria
This clade was named by Peters (2000) to include “Cosesaurus, Preondactylus, their common ancestor and all of its descendants.” This clade started off with bipeds that flapped their arms, probably for display during mating rituals because some members, like Longisquama were exotically decorated with extradermal membranes and plumes. Powered gliding (as in Sharovipteryx) was followed by flapping flight in pterosaurs, the first flying vertebrates. Several pterosaurs secondarily developed a quadrupedal pace. Quetzalcoatlus was the largest tritosaur, attaining a wingspan of 10 meters.

Summary
Due to the wide gamut and large inclusion list of the present phylogenetic analysis, many former enigmas, mismatches and leftovers came together in a new clade of lepidosaurs that was previously overlooked. Together, the Tritosauria include some of the strangest and, at times largest, of all lizards. Hyper-elongated necks and hyper-elongated fingers, together with experiments in both a sedentary marine lifestyle (Dinocephalosaurus) and a homeothermic aerial lifestyle (Dimorphodon, for example) make this a truly dynamic and diverse clade. Some of these out-of-the-ordinary morphologies seem to have been kick-started by early experiments with bipedalism. While the arboreal niches of drepanosaurs and pterosaurs are relatively easy to identify, the long-necked tanystropheids may also have used bipedalism and a long neck to reach into tree boughs to snatch prey, creating their own arboreal niche.

Unfortunately, only pterosaurs and Huehuecuetzpalli survived the end of the Triassic and they did not survive the end of the Cretaceous. So tritosaurs are the only major clade of lizards that is extinct today.

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
Carroll and Thompson 1982. A bipedal lizardlike reptile fro the Karroo. Journal of Palaeontology 56:1-10.
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 2007. The origin and radiation of the Pterosauria. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.
Reynoso V-H 1998. Huehuecuetzpalli mixtecus gen. et sp. nov: a basal squamate (Reptilia) from the Early Cretaceous of Tepexi de Rodríguez, Central México. Philosophical Transactions of the Royal Society, London B 353:477-500.