Pre-Diapsids. The Opening Act.

The Traditional View 
Diapsids were derived from the Protorothyridae (or Captorhinomorpha), close to Paleothyris (Carroll 1969). Petrolacosaurus is the earliest known diapsid. The diapsid configuration was not preceded by any temporal fenestration. Petrolacosaurus nests at the base of the Neodiapsida, which includes most other reptiles with temporal fenestration other than synapsids and bolosaurids.

The Heretical View
Diapsids were derived from basal synapsids close to Aerosaurus (a synapsid) and Protorothyris (a protosynapsid). Heleosaurus is the most primitive known protodiapsid (it nests outside the Synapsida). Eudibamus and Spinoaequalis are the most primitive known diapsids (two pairs of temporal openings). The diapsid configuration was preceded by lateral temporal fenestration. Click here to see the list of reptiles that succeeded  Eudibamus. Lepidosaurs and their sisters are not included on that list.

The Origin of the Protodiapsida
Petrolacosaurus has been the poster child for the origin of the Diapsida and it’s a good example. However, what evolved before the Diapsida has been largely ignored or overlooked.

Basal Protodiapsida to scale.

Figure 1. Basal Protodiapsida to scale. Diapsids in yellow. The synapsid Aerosaurus is grey.

Basal Archosauromorpha with a focus on the Protodiapsida

Figure 2. Click to enlarge. Basal Archosauromorpha with a focus on the Protodiapsida beginning with Heleosaurus.

Heleosaurus
Heleosaurus was considered an indeterminate diapsid by Broom (1907) and Carroll (1976), but Reisz and Modesto (2007) determined it was a varanopid synapsid. Here, with the benefit of new data on the skull (Botha, Brink and Modesto 2009) Heleosaurus nested just outside of the Synapsida at the base of a previously unrecognized clade, the Protodiapsida. Distinct from its predecessors, the skull was longer and the more cervicals were added. The suborbital jugal was more gracile. The pelvis was relatively larger. The limbs were longer. At 270 million years of age, the sole specimen of Heleosaurus is 40 million years younger than its phylogenetic descendants, indicating a long ghost lineage.

The Reduction of the Lateral Temporal Fenestra
The next two taxa, Archaeovenator (306 mya) and Mesenosaurus (266 mya) spanned that 40 million year gap. They had a smaller lateral temporal fenesatra, reduced by an advancing squamosal. Together with Heleosaurus, these two formed a clade.

The Milleropsis Detour
The temporal region of Milleropsis (Gow 1972, 290 mya) deviated from the basic skull pattern of its sisters. The lower temporal bar was absent, convergent with owenettids and squamates. The whip-like tail was incredibly long and the pelvis that anchored it was robust. Not enough is known of this taxon, but it appears able to run bipedally given available data.

Eudibamus
The most primitive diapsid may be Eudibamus (Berman et al. 2000, 290 mya), which was originally considered a sister to BolosaurusThe crushed skull does bear a strong resemblance. The teeth were blunt. The squamosal expanded further anteriorly to reduce the lateral temporal fenestra and the upper temporal fenestra first appeared. As in Milleropsis, the tail was whiplike. With an enlarged hind limb and short torso, Berman et al. (2000) considered Eudibamus an early biped. The proximal fingers and toes were greatly reduced, as in sister taxa, but more so.

Spinoaequalis.

Figure 3. Spinoaequalis. This reconstruction finds tiny upper temporal fenestrae.

Spinoaequalis 
Spinoaequalis (deBraga and Reisz 1995, Fig. 3) had small upper temporal fenestrae and a shorter temporal area. The skull is otherwise a good match for Petrolacosaurus. The tail of Spinoaequalis was distinct from all sisters in having high neural spines and deep chevrons. Spinoaequalis does not nest as a sister to Hovasaurus, a diapsid with a similar deep tail.

Petrolacosaurus
In Petrolacosaurus (Lane 1945, Reisz 1977) we find further reduction in the lateral temporal fenestra and further expansion of the upper temporal fenestra. The fingers and toes were asymmetrical, but less so than in Eudibamus and Spinoaequalis. The neck was further elongated. Interestingly, in Petrolacosaurus (Fig. 1) the two temporal fenestra are visible in lateral view.

Araeoscelis
Another araeoscelid, Araeoscelis (Williston 1910, Reisz, Berman and Scott 1984), completely infilled the lateral temporal fenestra but kept the upper temporal fenestra, which is unlike the vast majority of the other phylogenetic successors of Petrolacosaurus. On the other hand, Mesosaurus infilled the upper temporal fenestra and largely infilled the lower one, leaving only a small opening in some specimens. Restoration is difficult on many others due to crushing and scattering.

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
Botha-Brink J and Modesto SP 2009.Anatomy and Relationships of the Middle Permian Varanopid Heleosaurus scholtzi Based on a Social Aggregation from the Karoo Basin of South Africa. Journal of Vertebrate Paleontology 29(2):389-400.
Berman, DS, Reisz RR, Scott D, Henrici AC, Sumida SS and Martens T 2000. Early Permian bipedal reptile. Science 290: 969-972.
Broom R 1907. On some new fossil reptiles from the Karroo beds of Victoria West, South Africa. Transactions of the South African Philosophical Society 18:31–42.
Carroll R L 1969. A middle Pennsylvanian captorhinomorph, and the interrelationships of primitive reptiles: Journal of Paleontology, 43:1151-170.
Carroll RL 1976. Eosuchians and the origin of archosaurs; pp. 58–79 in C. S. Churcher (ed.), Athlon: Essays on Paleontology in Honour of Loris Shano Russell. Miscellaneous Publications of the Royal Ontario Museum, Toronto.
deBraga M and Reisz RR 1995. A new diapsid reptile from the uppermost Carboniferous (Stephanian) of Kansas. Palaeontology 38 (1): 199–212. palass-pub.pdf
Gow CE. 1972. The osteology and relationships of the Millerettidae (Reptilia: Cotylosauria). Journal of Zoology, London 167:219-264.
Lane HH 1945. New Mid-Pennsylvanian Reptiles from Kansas. Transactions of the Kansas Academy of Science 47(3):381-390.
Reisz RR 1977. Petrolacosaurus, the Oldest Known Diapsid Reptile. Science, 196:1091-1093. DOI: 10.1126/science.196.4294.1091
Reisz RR and Modesto SP 2007. Heleosaurus scholtzi from the Permian of South Africa: a varanopid synapsid, not a diapsid reptile.
Reisz RR, Berman DS and Scott D 1984. The Anatomy and Relationships of the lower Permian reptile Araeoscelis. Journal of Vertebrate Paleontology 4: 57-67.
Rieppel O and deBraga M 1996. Turtles as diapsid reptiles. Nature 384:453-454.
Vaughn PP 1955. The Permian reptile Araeoscelis re-studied. Harvard Museum of Comparative Zoology, Bulletin 113:305-467.

wiki/Araeoscelis
wiki/Archaeovenator
wiki/Eudibamus
wiki/Heleosaurus
wiki/Mesenosaurus
wiki/Milleropsis
wiki/Petrolacosaurus
wiki/Spinoaequalis

The Most Primitive Reptile: No longer Cephalerpeton

Cephalerpeton is no longer the most primitive reptile. As the large reptile tree keeps growing other taxa, like Gephyrostegus and Tulerpeton now nest at the most primitive node. Updated Dec. 6, 2017.

The Oldest vs. Most Primitive Reptile
The oldest known reptiles, Casineria and Westlothiana, were close to, but not THE most primitive known reptile. That title goes to Cephalerpeton (Moodie 1912, Figures 1, 2), the reptile phylogenetically closest to the non-reptile, Gephyrostegus. A single Cephalerpeton fossil was found in the Mazon Creek beds, dated to the Mid-Pennsylvanian about 300 mya. That’s 35-38 million years after Casineria and Westlothiana. So, the most primitive reptile was not the oldest reptile, testifying once again, to the longevity of many reptile taxa. Evidently Cephalerpeton had survival fitness, in the Darwinian sense.

Cephalerpeton, the most primitive known reptile

Figure 1. Cephalerpeton, the most primitive known reptile

A Tiny Gephyrostegid Able to Lay Eggs Protected with Membranes
Everyone knows that the one character that best unites all birds, mammals and reptiles of all sorts is the production of eggs protected by an amniotic membrane. By employing phylogenetic bracketing, we infer that the most primitive reptile also had this ability.

Cephalerpeton was originally considered to be an amphibian (Moodie 1912). Here it was derived from Gephyrostegus, a much larger “amphibian” that likewise survived for a long time on the planet. Fossils are known from 310 mya, at least 30 million years after the appearance of the oldest known fossil reptile, Westlothiana. Gephyrostegus bohemicus (Figure 2) had a snout-vent length of 20 cm, large enough to require two hands to hold it. Unlike most other “amphibians,” Gephyrostegus had a lizard-like build, well-suited to a terrestrial environment.

The Visionary Contribution of Robert Carroll (1970)
Dr. Robert Carroll (1970) got it right when he proposed that the first reptiles would be tiny, so their eggs would be tiny. Gephyrostegus watsoni (Brough and Brough 1967) was a tiny gephyrostegid (skull length ~1 cm, Figure 2) bearing an even closer resemblance to Cephalerpeton. The first eggs provided with an amniotic membrane were probably small and laid by small adult females who lived in and laid eggs in moist leaf litter, a transitional environment that stayed humid and protected both the adult and the egg from desiccation.

With a snout-vent length barely as long as just the skull of the holotype, tiny G. watsoni would have been a great candidate for “the most primitive reptile” based on its greatly reduced adult size. Unfortunately G. watsoni retained a discrete intertemporal bone, a trait no other reptile has. Cephalerpeton was twice its size. We’ll probably never know if G. watsoni laid eggs protected by an amnion, but I like the idea that it did.

 Cephalerpeton size comparisons

Figure 2. Cephalerpeton size comparisons

More About Cephalerpeton, the Most Primitive Known Reptile
Distinct from Gephyrostegus, the skull of Cephalerpeton was relatively large with a large orbit. Such a pattern is similar to that of Gephyrostegus watsoni or what would be expected in a juvenile gephyrostegid. A discrete intertemporal bone was absent. The quadrate was aligned vertically. The otic notch was greatly reduced with a squamosal that had a near vertical posterior rim. The maxillary teeth were enormous. The mandible was concave dorsally in order to accommodate the upper teeth. The postorbital portion of the skull was shorter and no longer concave posteriorly. The postfrontal extended over the postorbital to mid orbit. The maxilla was slightly raised to just above the lower rim of the orbit. The premaxillary teeth were longest medially and the deeper premaxilla tipped down. The palate was relatively shorter. The transverse process of the pterygoid was more developed and had a transverse row of teeth.

The cervicals were elongated and there were two more of them. The pleurocentra were greatly enlarged, crowding out the intercentra.

The scapula and coracoid were unfused and as tall as the neural spines. The humerus was slender and hourglass-shaped. The radius and ulna were likewise more slender and relatively longer. Of the hand, only the metacarpals were preserved and they appear more assymmetrical with #4 still the longest.

Thereafter
The earliest known reptiles from both reptilian branches were similar in size to Cephalerpeton (Figure 2). One branch, the lepidosauromorpha, were largely herbivores. The other branch, the archosauromorpha, were larger insectivores grading toward carnivores.

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
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 B 257: 267–308.
Carroll RL and Baird D 1972. Carboniferous Stem-Reptiles of the Family Romeriidae. Bulletin of the Museum of Comparative Zoology 143(5):321-363. online pdf
Gregory JT 1948. The structure of Cephalerpeton and affinities of the Microsauria. American Journal of Science, 246:550-568 doi:10.2475/ajs.246.9.550
Moodie RL 1912. The Pennsylvanic Amphibia of the Mazon Creek, Illinois, Shales. Kansas University Science Bulletin 6(2):232-259.

Lanthanosuchus: Weird Dead End? Or the Weird Precursor of Snakes and Pterosaurs?

With new data come revisions 01/05/12:
I added data on a sister taxon, Saurorictus. See below.

Lanthanosuchus was a flat-headed, aquatic, Late Permian tetrapod known chiefly from a skull. It was originally considered to be a reptilomorph (not a reptile, but in the reptile precursor lineage) by Efremov (1946). Later it was considered a sister to the millerettid, Acleistorhinus (deBraga and Reisz 1996, Cisneros et al. 2004, Lyson et al. 2010). Lee (1997) considered Lanthanosuchus a procolophomorph (along with Owenetta, Procolophon, Proganochelys and others.

The skull of Lanthanosuchus in several views and colorized.

Figure 1. The skull of Lanthanosuchus in several views and colorized.

Pretty Much All By Itself? Maybe Not.
Like pareiasaurs the skull is rather knobby and sculptured overall. Unlike any potential sister, the skull is fenestrated in the temple region. Since there is no other potential sister with such a flat skull Lanthanosuchus sort of stand alone. Most workers, I presume, view Lanthanosuchus as a sort of dead end taxon. That’s why I was more than surprised to see where it nested in the large study.

[The addition of Saurorictus and Macroleter make it clear that Lanthanosuchus was not derived from the pareiasaurs, but rather formed a clade with Saurorictus and Macroleter. This also clarifies the nesting of Nyctiphruretus, which shares more characters with the equally plesiomorphic Saurorictus.]

Lanthanosuchus nests with Macroleter and Saurorictus.

Figure 2. Lanthanosuchus nests with Macroleter and Saurorictus within the Diadectomorpha.

Weird Precursor
Lanthanosuchus nests within the Diadectomorpha, outside of the Pareiasauria. Surprisingly it nests at the base of the rest of the Lepidosauromorpha, basal to the precursors to the owenettids and Lepidosauriformes (living lizards, extinct pterosaurs, drepanosaurs, gliding reptiles, owenettids). It’s precursors include pareiasaurs and diadectids. In this regard, Lanthanosuchus was a small, flat diadectomorph (or procolophonomorph).

[The addition of Saurorictus clarifies the evolution of Lanthanosuchus from a Macroleter-like transitional form (Fig. 3)]

Saurorictus, Macroleter and Lanthanosuchus

Figure 3. Saurorictus, Macroleter and Lanthanosuchus demonstrating the evolution of one to another and another of these sister taxa. Here the lateral temporal fenestra of Lanthanosuchus was a derived character in a dead-end taxon, as many suspected all along.

The Evolution of Lanthanosuchus and the Lanthanosuchia
According to the results recovered by the large tree, a basal pareiasaur, such a Deltavjatia, was a sister to the precursor to Lanthanosuchus and the nyctephruretid, Nyctiphruretus, was a sister to the successor. Together they create a serial size reduction. All these specimens were found in Late Permian (~255 mya) sediments, which means the split and succession occurred earlier and these three taxa represent random branches of a much larger bush of taxa yet to be discovered. Although these three seem quite different from each other, there are no other more parsimonious sister candidates in the present list of taxa.

[The addition of Saurorictus creates a new predecessor taxon outside of the Pareiasauria. The general lumpiness pareiasaurs share with Lanthanosuchus was by convergence.]

What Happened Here?
Lanthanosuchus is the first in this lineage to have a lateral temporal fenestra at the intersection of all the temporal bones. This becomes a lateral temporal embayment with the reduction of the quadratojugal and its separation from the jugal.  The parietal shrank to the size of the frontal. The teeth were sharp. The postfrontal fused to the frontal, but that fusion was not found in Nyctiphruretus. Toward Nyctiphruretus, the orbit was increased, the postorbital half of the skull was reduced and skull ornamentation and sculpturing was also reduced, probably by precocial maturity and the retention of juvenile traits into a smaller adult size. In other words, the transition from a bulky, short-tailed herbivore to a small, lizardy insectivore happened during this transition. Successors all remained rather small until the advent of the giant snakes, mosasaurs and pterosaurs.

[As above, the addition of Saurorictus creates a new predecessor taxon outside of the Pareiasauria. General size increase, rather than reduction, gave us Lanthanosuchus with skull fenestrae, reinvented in certain specimens of Nyctiphruretus, but not others.]

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
Cisneros et al 2004. A procolophonid reptile with temporal fenestration from the Middle Triassic of Brazil. Proceedings of the Royal Society London B (2004) 271, 1541–1546 DOI 10.1098/rspb.2004.2748
deBraga M and Reisz RR 1996. The Early Permian reptile Acleistorhinus pteroticus and its phylogenetic position. Journal of Vertebrate Paleontology 16(3): 384–395. doi:10.1080/02724634.1996.10011328.
Efremov JA 1946. On the subclass Batrachosauria – an intermediary group between amphians and reptiles. USSR Academy of Sciences Bulletin, Biology series 1946:615-638.
Lee MSY 1997. Pareiasaur phylogeny and the origin of turtles. Zoological Journal of the Linnean Society 120: 197-280.

Batrachosauria web page
wiki/Lanthanosuchus

What is Eunotosaurus?

Eunotosaurus was a wide-bellied reptile with extremely broad ribs and eight dorsal vertebrae…which is how one could also describe a turtle. Several specimens are known. Smaller than a human hand, some were preserved curled up.

With Turtles. Then Not. Then With Again.
Watson (1914) originally considered Eunotosaurus a turtle ancestor.

Romer (1956) separated Eunotosaurus from turtles, with no other idea how Eunotosaurus nested.

Cox (1969) nested Eunotosaurus with Captorhinus and kin.

Cisneros et al. (2008) nested Eunotosaurus with Milleretta. (Bravo!!)

Currently Lyson et al. (2010) puts Eunotosaurus and turtles together again. Unfortunately the support values were chiefly below 0.66 and too few taxa were used compared to the large study found here. On the plus side, Eunotosaurus did nest with Acleistorhinus and Milleretta in that study.

Eunotosaurus and its sister taxa, Acleistorhinus and Milleretta RC14.

Figure 1. Eunotosaurus and its sister taxa, Acleistorhinus and Milleretta RC14.

The Closest Sister Taxa to Eunotosaurus
Here, in the large study, Acleistorhinus was the closest sister taxon to Eunotosaurus (Figure 1). Unfortunately, only the skull is known. The next closest sister taxon was the RC14 specimen of Milleretta, which shares the expanded ribs that characterize Eunotosaurus. More Eunotosaurus details can be found at reptileevolution.com

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
Watson DMS 1914. Eunotosaurus africanus (Seeley) and the ancestors of the Chelonia, Proceedings of the Zooogical Society of London 11:1011–1020.
Cisneros JC, Rubidge BS, Mason R and Dube C 2008. Analysis of millerettid parareptile relationships in the light of new material of Broomia perplexa Watson, 1914, from the Permian of South Africa. Journal of Systematic Palaeontology 2008 (6): 453–462. doi:10.1017/S147720190800254X
Cox CB 1969.
 The problematic Permian reptile Eunotosaurus. Bulletin of the British Museum of Natural History 18: 167–196.
Keyser AW and Gow CE 1981. First complete skull of the Permian reptile
Eunotosaurus africanus Seeley. South African Journal of Science 77: 417–420.
Gow CE 1997. A reassessment of Eunotosaurus africanus Seeley (Amniota: Parareptilia). Palaeontologia Africana, 34:33–42.
Gow CE and de Klerk B 1997. First record of Eunotosaurus (Amniota: Parareptilia) from the Eastern Cape. Palaeontologia Africana, 34: 27–31.
Lyson TR, Bever GS, Bhullar B-AS, Joyce WG and Gauthier JA 2010. Transitional fossils and the origin of turtles. Proceedings of the Royal Society B Available online 9 June 2010: 830. online pdf
Modesto SP 2000. Eunotosaurus africanus and the Gondwanan ancestry of anapsid reptiles. Palaeontologia Africana, 36:15–20.

wiki/Eunotosaurus

The Origin of the Amphisbaenia

Updated August 12, 2014. Moving Cryptolacerta to a closer relationship to Heloderma, elevating Sineoamphisbaena to its place between skinks and amphisbaenids.

Amphisbaena literally means, “goes both ways.” The name is that of the mythological “Mother of Ants,” an ant-eating serpent with a head on both ends. Amphisbaena is also a genus within the Amphisbaenia. Like its mythological namesake, Amphisbaena can back up as easily as it moves forward, despite having no legs.

Amphisbaenia are the worm lizards, typically (with one exception, Figure 1) legless squamates that burrow and have a superficial resemblance to earth worms, including having their scales arranged in rings (annuli). Their right lung is reduced to make more room for the left lung. (In snakes the left lung is reduced.) The eyes are reduced and deeply recessed and the tail resembles the head. Amphisbaenians are so different from other squamates that they have been considered a third suborder, after lizards and snakes.

The primitive Amphisbaenian, Bipes.

Figure 1. The primitive amphisbaenian, Bipes.

Bipes, a Primitive Extant Amphisbaenian
Bipes (Figure 1) is a living amphisbaenian with strong front legs. The hand is stout, like that of a mole, with digits 2 and 3 the longest, digit 1 absent and digit 5 vestigial. The vestigial hind limbs do extend beyond the body wall. By contrast, in typical lizards digit 4 is the longest.

Extinct Burrowers
Several fossil taxa have been linked to amphisbaenians. Tamaulipasaurus lived during the Early Jurassic. Sineoamphisbaena and Crythiosaurus lived during the Late Cretaceous.  Spathorhynchus lived during the early Oligocene. Most of these are known from skulls and partial skulls. No hands yet known in any of these taxa.

Amphisbaenian Origins – part 1 – Sineoamphisbaena
Wu et al. (1993), Wu et al. (1996) and Gao (1997) proposed and argued that a round-skulled Late Cretaceous squamate, Sineoamphisbaenea (Figure 2), was the oldest known amphisbaenian. Unfortunately, it didn’t look very much like most amphisbaenians (Figure 2) which made accurate nesting something of a problem in the eyes of many.

Amphisbaenian Origins – part 2 – Not Sineoamphisbaena
Kearney (2003) argued that Sineoamphisbaena nested closer to Macrocephalosaurus and that Amphisbaena nested with Dibamus and snakes, not far from Gekko and the legless geckos, the Pygopodidiae.

 

Figure 2. Cryptolacerta and kin, including Heloderma and the Amphisbaenia.

Figure 2. Cryptolacerta and kin, including Heloderma and the Amphisbaenia.

Amphisbaenian Origins – part 3 – Cryptolacerta
Müller et al. (2011) argued that a new Eocene lizard, Cryptolacerta  (Figure 2), was the sister to the Amphisbaenia and both were sisters to Sineoamphisbaena and the Teiioidea, a lizard taxon that includes the skinks, Gymnophthalmus and Chalcides. 

Where Do Amphisbaenians Nest in the Large Study?
Here Cryptolacerta does indeed nest close to skinks and amphisbaenians, but it nests closer to Heloderma, another burrowing lizard. Müller et al. (2011) reported that Cryptolacerta had sealed up its upper temporal fenestrae with expansion of the very large postfrontal bone. I was unable to duplicate that reconstruction. Instead I found upper temporal fenestrae in the specimen. A GIF movie and comparative reconstructions can be found here. In any case, amphisbaenians do not add bone to their skulls, they lose bone.

Amphisbaenians nest close to skinks with Sineoamphisbaena nesting close to the base of the other amphisbaenians. In consideration of Kearney (2003), I deleted all amphisbaenians, then all skinks and amphisbaenians from the large study, but those tests failed to dislodge Sineoamphisabaenia form its node, which kept it far from Macrocephalosaurus.

Summary
While amphisbaenians are distinct from most other lizards, they are closer to skinks and legless skinks than to any other lizard taxa. More legless taxa will be added to the large tree as time goes by and I will report on each one in turn.

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:
Cope ED 1894. On the genera and species of Euchirotidae. American Naturalist 28: 436-437.
Gao K 1997.
 
Sineoamphisbaena phylogenetic relationships discussed. Canadian Journal of Earth Sciences. 34: 886-889. online article
Kearney M 2003. The Phylogenetic Position of Sineoamphibaena hextabularis reexamined. Journal of Vertebrate Paleontology 23 (2), 394-403.
Müller J, Hipsley CA, Head JJ, Kardjilov N, Hilger A, Wuttke M and Reisz RR 2011. Eocene lizard from Germany reveals amphisbaenian origins. Nature 473 (7347): 364–367. doi:10.1038/nature09919
Wu XC., Brinkman DB, Russell AP, Dong Z, Currie PJ, Hou L, and Cui G 1993. Oldest known amphisbaenian from the Upper Cretaceous of Chinese Inner Mongolia. Nature (London), 366: 57 – 59.
Wu X-C Brinkman DB and Russell AP 1996. Sineoamphisbaena hexatabularis, an amphisbaenian (Diapsida: Squamata) from the Upper Cretaceous redbeds at Bayan Mandahu (Inner Mongolia, People’s Republic of China), and comments on the phylogenetic relationships of the Amphisbaenia. Canadian Journal of Earth Sciences, 33: 541-577.
Papenfuss TJ 1982. The Ecology and Systematics of the Amphisbaenian Genus Bipes. Occasional papers of the California Academy of Science 136: 1-42.

wiki/Cryptolacerta
wiki/Amphisbaenia
wiki/Bipes
wiki/Sineoamphisbaena

What are Choristoderes? (you know…Champsosaurus, Cteniogenys, Doswellia, etc.)

The Choristordera constitute a clade of elongated aquatic to semi-aquatic, lizard-like to croc-like diapsid reptiles. Traditional taxa include: Champsosaurus, Cteniogenys, Lazarrusuchus and Hyphalosaurus. The first two-headed fossil reptile came from this clade.

What Wiki Sez:
Cladists have placed [choristoderes] between basal diapsids and basal  archosauromorphs but the phylogenetic position of Choristodera is still uncertain. It has also been proposed that they represent basal lepidosauromorphs.”

So we have an enigma taxa, an ideal opportunity to use the large study to narrow down choristodere outgroup relations.

Several choristoderes

Figure 1. Several choristoderes (in white), their predecessor and sisters (in yellow).

Choristoderes are Pararchosauriformes
The large study nested choristoderes within the Archosauriformes and within the Pararchosauriform branch between Youngoides (the RC91 specimen) and Proterochampsa.

A section of the large study focusing on choristodere relations.

Figure 2. A section of the large study focusing on choristodere relations.

Doswellia was also a Choristodere
Doswellia (Weems 1980) has been considered an enigma taxon, different enough from all other known taxa to create more questions than answers. Dilkes and Sues (2009) proposed a nesting with Proterochampsa, which is confirmed here.

Parsimonly Rules
Side by side, the resemblance of several choristoderes to Youngina, Doswellia and parasuchians is clear and reasonable. In the present taxon list, there is no more parsimonious nesting to be found. Think of choristoderes as successors to Youngoides (RC91 specimen), a taxon that has never been tested with choristoderes before.

The Dorsal Naris
Most choristoderes have a dorsal naris, similar to Cerritosaurus, parasuchians and Proterochampsa. Champsosaurus has a naris at the tip of it snorkel like snout. This appears to be a reversal because the premaxilla has no ascending process.

Another Appearance of the Antorbital Fenestra
This nesting highlights an important taxonomic fact: the antorbital fenestra appeared in reptiles at least four times. Parasuchians and Cerritosaurus had an antorbital fenestra. Precursors, including choristoderes, did not. This means the antorbital fenestra in parasuchians and their kin developed independently of the antorbital fenestra in Euarchosauriformes, such as Proterosuchus and its successors.

The Longevity and Variety Within the Choristodera
Choristoderes appeared in the Late Triassic, but probably originated in the Late Permian, along with their sister taxa. Some survived into the Early Miocene. Despite the longevity of this clade, relatively few modifications to the basic body plan appeared. Oh, sure, the lateral temporal fenestra disappeared in Doswellia and Lazarussuchus. The rostrum elongated in Champsosaurus. The neck elongated in Hyphalosaurus. The unguals were enlarged in Lazarussuchus, which means it was probably more terrestrial than its aquatic sisters and may have climbed trees. Doswellia was the giant of the clade, reaching 1.6 m in length, or slightly larger than Champsosaurus at 1.5 m. No choristoderes developed an herbivorous diet, a mammal-like dentition, a bipedal stance or wings.

Summary
Traditional enigmas, choristoderes were a monophyletic clade that nested between Youngoides and Parasuchia + Proterochampsa, close to the base of the Archosauriformes. Relatively conservative in morphology, choristoderes were a relatively minor presence throughout the Mesozoic and into the Cenozoic.

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
Brown B 1905. The osteology of Champsosaurus Cope. Memoirs of the AMNH 9 (1):1-26. http://digitallibrary.amnh.org/dspace/handle/2246/63
Cope ED 1876. 
On some extinct reptiles and Batrachia from the Judith River and Fox Hills beds of Montana: Proceedings of the Academy of Natural Sciences, Philadelphia. 28, p. 340-359.
Dilkes D and Sues H-D 2009. 
Redescription and phylogenetic relationships of Doswellia kaltenbachi (Diapsida: Archosauriformes) from the Upper Triassic of Virginia. Journal of Vertebrate Paleontology 29(1):58-79
Evans SE and Hecht MK 1993.A history of an extinct reptilian clade, the Choristodera: longevity, Lazarus-Taxa, and the fossil record. Evolutionary Biology 27:323–338.
Foster JR and Trujillo KC 2000.
 
New occurrences of Cteniogenys (Reptilia, Choristodera) in the Late Jurassic of Wyoming and South Dakota. Brigham Young University Geology Studies 45:11-18.
Gao K-Q, Tang Z-L and Wang X-L 1999
A long-necked reptile from the Upper Jurassic/Lower Cretaceous of Liaoning Province, northeastern China. Vertebrata PalAsiatica 37:1–8.
Gilmore CW 1928. 
Fossil lizards of North America. Memoirs of the National Academy of Sciences 22(3):1-201.
Hecht MK 1992. A new choristodere (Reptilia, Diapsida) from the Oligocene of France: an example of the Lazarus effect. Geobios 25:115–131. doi:10.1016/S0016-6995(09)90041-9.
Matsumoto R and Evans SE 2010. Choristoderes and the freshwater assemblages of Laurasia. Journal of Iberain Geology 36(2):253-274. online pdf
Weems RE 1980. 
An unusual newly discovered archosaur from the Upper Triassic of Virginia, U.S.A. Transactions of the American Philosophical Society, New Series 70(7):1-53

wiki/Champsosaurus
wiki/Cteniogenys
wiki/Hyphalosaurus
wiki/Lazarussuchus

Ophiacodon and the Origin of the Therapsida

Nobody cares about Ophiacodon, but we should.
Ophiacodon is an overlooked key taxon in the evolution of synapsids, therapsids and by all accounts, mammals and humans.

Ophiacodon

Figure 1. Ophiacodon, large, squat and amphibious - not the perfect therapsid precursor... or is it?

Overlooked for Good Reason
Ophiacodon was large, low-slung, pretty darn ugly and apparently nothing like the lithe little mammals it would give rise to. (As an aside, let’s not forget that — way back — pterosaurs also arose from bulky diadectids and birds had their origins with equally bulky and amphibious erythrosuchids.) Various Ophiacodon species grew larger and more specialized throughout the Early Permian, so therapsids and sphenacodonts would have arisen from less specialized, smaller, earlier members.

 Biarmosuchus, the most basal therapsid.

Figure 2. Biarmosuchus, the most basal therapsid.

The Basal Therapsid
Most studies (other than those including Tetraceratops) place Biarmosuchus at the base of the Therapsida. Now all we have to do is find the pelycosaur that most parsimoniously matches Biarmosuchus.

Biarmosuchus vs. the Sphenacodonts
Traditional studies have always placed sphenacodonts like Haptodus, Sphenacodon and Dimetrodon (Figure 3) as predecessors to Biarmosuchus largely due to the presence of the reflected lamina as a shared trait. A reflected lamina is that thin, circular bony leaf peeling off the back of the mandible. In reptiles that mandible bone is called the angular. In mammals the angular and reflected lamina shrinks to frame the eardrum.

The reflected lamina is important, but overall Ophiacodon looks more like Biarmosuchus (Figure 3). However, it’s not good practice to rely on just one character, but a whole suite to make a most parsimonious nesting.

No doubt therapsids were derived from pelycosaurs, but the key sister taxon has not been found yet.

 

Ophiacodon and the Origin of the Therapsida

Figure 3. Ophiacodon and its phylogenetic successors, the pelycosaurs and the therapsids.

The Problem(s) with Sphenacodonts as Therapsid Ancestors
Traditionally the sphenacodonts, Haptodus and Dimetrodon have been considered the closest sisters to the Therapsida, but sphenacodonts have a relatively shorter, taller skull, a short premaxillary ascending process, a kink at the premaxilla/maxilla jawline, a shorter, taller rostrum and a deeply concave posterior jawline. Biarmosuchus has none of these traits. But Eotitanosuchus does.

Eotitanosuchus
Eotitanosuchus
 (Figure 3) has often been compared to Dimetrodon. Both share a convex rostral margin and both lose or greatly reduce the pre-canine maxillary teeth. However, taken as a whole we find that Eotitanosuchus nests between Biarmosuchus and various higher therapsids, especially gorgonopsids in the lineage of mammals. So the characters Eotitanosuchus seemed to share with Dimetrodon were convergent.

The Reptile Family Tree
Here Biarmosuchus nests closer to Ophiacodon. Haptodus and Dimetrodon  branch off as sisters to this node. However, if we consider all the clues together, the base of the Therapsida actually lies somewhere between Ophiacodon and Haptodus, with a lean toward Ophiacodon.

Biarmosuchus vs. Ophiacodon
Several Biarmosuchus traits shared with Ophiacodon are not found in HaptodusSphenacodonand Dimetrodon: 1) Premaxilla longer than naris; 2) Rostrum twice as long as tall; 3) Quadratojugal not reduced to anearly invisible nub; 4) Premaxilla rises anteriorly; 5) Transition from premaxilla and maxilla without a kink.

Biarmosuchus vs. Haptodus
Fewer Biarmosuchus traits shared with Haptodus are not found in Ophiacodon: 1) Reflected lamina. 2) Anterior dentary deep and ventral margin sharply angled. These traits would be expected to appear in the last common ancestor of the Therapsida originating between Ophiacodon and Haptodus.

In therapsids the nasal is relatively narrow, but in sphenacodonts it is broader. The purported septomaxilla in therapsids appears to be the anterior lacrimal beneath the ascending process of the maxilla, perhaps laminated over it. Check all these out on Figure 3. Finally, let’s take a look at the right hand of our candidates. Biarmosuchus had a robust manus, not as robust as Ophiacodon, but not nearly as gracile as Haptodus.

Comparing the right manus of Haptodus, Biarmosuchus and Ophiacodon.

Figure 4. Comparing the right manus of Haptodus, Biarmosuchus and Ophiacodon. Biarmosuchus is right in the middle, literally and morphologically. The reduction of those three disc-like phalanges in Biarmosuchus signals a more erect stride.

We’ll Keep Looking
Someday we’ll find a small, early ophiacodont with longer legs, a pretty big canine, a shorter postorbital region and a reflected lamina. Essentially I’ve just described Biarmosuchus, haven’t I?

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
Marsh OC 1878. Notice of new fossil reptiles: American Journal of Science, 3rd series, v. 15, p. 409-411.
Romer AS and Price LW 1940. Review of the Pelycosauria. Geological Society of America Special Papers 28: 1-538.
Tchudinov PK 1960. Diagnosen der Therapsida des oberen Perm von Ezhovo: Paleontologischeskii Zhural, 1960, n. 4, p. 81-94.

wiki/Biarmosuchus
wiki/Ophiacodon

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.

The Origin of the Dinosauria

Updated August 20, 2015 with a new figure 2 

Everyone is familiar with the many members of the Dinosauria (Heterodontosaurus, Coelophysis, Saturnalia, for instance), the preeminent denizens of the Jurassic and Cretaceous eras. Unfortunately, the origin of the Dinosauria has been left in the shadows, miscast and muddled with taxa that do not belong in the dinosaur lineage.

The Traditional Model
Here is the traditional lineage, as most recently recovered in Nesbitt (2011):

The traditional model of dinosaur origins

Figure 1. The traditional model of dinosaur origins, based on Nesbit (2011).

No problem with the three dinosaurs. No problem with the two dinosauriforms. A little deeper into the lineage problems arise. It’s easy to see how Euparkeria could give rise to Ornithosuchus, but Parasuchus, with its elongated rostrum and displaced nares, does not belong here. Following giant Ornithosuchus (with small lateral fingers) comes the relatively tiny pterosaur, Dimorphodon, with hyper-elongated finger #4. Pterosaurs don’t belong here either. This lack of a long finger #4 in the immediate sisters proposed for pterosaurs is a “red flag” discussed earlier hereLagerpeton does not belong here either. See below.

The Heretical Model
Here is the heretical model based on Peters (2007) and the large reptile tree:

Figure 2. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.

Figure 2. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.

The heretical model (Figure 2) employed many more taxa, which provided many more possible nesting sites. It recovered a family tree that had fewer untenable reversals and one in which all sister taxa more closely resemble one another. Here, the basal rauisuchians Vjushkovia and Decuriasuchus, were basal to both crocodylomorphs, like Pseudhesperosuchus, and basal dinosauromorphs, like Turfanosuchus and Trialestes). All these taxa more closely resemble one another and none have wings or displaced nares.

The Pterosaur Problem
Pterosaurs do not belong in the lineage of dinosaurs. All dinosaur sister taxa have reduced the lateral digits, #4 and #5 on the manus and #5 on the pes.  Pterosaurs have a hyper-elongated manual digit #4 and a long pedal digit #5, as in other fenestrasaurs. Pterosaurs do not have a mandibular fenestra and none have an antorbital fossa. Chevrons do not extend ventrally on pterosaurs, as they do in archosaurs. Extradermal membranes and an ossified sternum do not appear on archosaurs until the appearance of bird precursors. In Nesbitt (2011) pterosaurs nest close to parasuchians and proterochampsids because both share a displaced naris, but otherwise they share virtually no other traits in common, as is plainly obvious (Figure 1).

The Phytosaur/Proterochampsa Problem
Phytosaurs, like Parasuchus, and proterochampids, like Proterochampsa, Chanaresuchus, Tropidosuchus and Cerritosaurus were clearly sister taxa. However, neither clade shared very much in common with Euparkeria and Ornithosuchus, other than an antorbital fenestra, which was seen as a trait unique to one all encompassing clade, the Archosauriformes. Earlier we saw that the antorbital fenestra appeared in at least four separate reptilian clades. Phytosaurs and proterochampsids shared more traits in common with choristoderes and Doswellia, two taxa not typically employed in archosaur family trees, and all appeared here as pararchosauriforms, separate from the dinosaur lineage. Lagerpeton does not have the robust pedal digit 2 of Tropidosuchus, but other aspects of its foot and pelvis, for that matter, are nearly identical.

Lagerpeton compared to Tropidosuchus

Figure 3. Click to enlarge. Lagerpeton compared to other pararchosauriforms, including Tropidosuchus and Chanaresuchus. Note added Aug. 1, 2012: BPI 2871 now nests with other Youngina specimens at the base of the Pararchosauriformes, not far from Doswellia and the Choristodera.

The Lagerpeton Problem
Lagerpeton has been a traditionally included taxon in the lineage of dinosaurs, but a larger study indicates it does not belong there. It shares more traits with Tropidosuchus and ChanaresuchusLagerpeton, known only from the middle of the skeleton and hind limbs, had a longer pedal digit 4 than 3, but predecessor and successors all have a longer pedal digit 3 than 4. Tropidosuchus and Chanaresuchus share a long rostrum and displaced nares, like that of Parasuchus and Cerritosaurus.

The Rauisuchia Problem
Most rauisuchians, like most crocodylomorphs, were clearly distinct from the lineage of dinosaurs — but the basal members were not so far removed. The Nesbitt (2011) study suffered from not including several basal crocodylomorphs which were closest to the origin of dinosaurs, aetosaurs, Ticinosuchus and Qianosuchus, which may be the reason for the nesting of Postosuchus with Hesperosuchus, which otherwise do not closely resemble one another.

Some Positives Out of Nesbitt (2011)
Nesbitt (2011) reported several “novel” findings that were early found in the large study. These include: (1) proterochampsians are monophyletic (i.e. included chanaresuchids). Unfortunately by not including several Youngina and Youngoides specimens and choristoderes Nesbitt failed to recover a tree in which proterochampsians nested far from Euparkeria on a separate lineage along with phytosaurs; (2) Gracilisuchus is one of the most basal suchians. Unfortunately Nesbitt  did not include other basal crocs, including DecuriasuchusPseudhesperosuchus, ScleromochlusSaltopus and Saltopusuchus; (3) Rauisuchidae is recovered as monophyletic (but here the Rauisuchia also include the Archosauria); (4) ‘‘Silesaurs’’ are a monophyletic sister taxon to Dinosauria (actually, with the addition of Daemonosaurus, silesaurs are within the Dinosauria). Both studies nested Lewisuchus close to the base of the Archosauria and Marasuchus, a more complete biped at the base of the Dinosauria. In addition, Turfanosuchus nested with Gracilisuchus, Ticinosuchus and the Aetosauria in Nesbitt (2011)), not far from the base of the Dinosauria.

Some Negatives Out of Nesbitt (2011)
(1) Phytosaurs are the most proximal outgroup to Archosauria; (2) Hesperosuchus and similar taxa are the basalmost crocodylomorphs (but that was only because Nesbitt removed a long list of even more basal crocs); (3) Crocodylomorpha is the sister taxon to Rauisuchidae (but the problem is Nesbitt nested basal crocs close to derived rauisuchians like Postosuchus.

Other problems, like the nesting of Vancleavea with archosauriformes were discussed earlier.

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
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.

Drepanosaurs! The Strangest of the Strange

Some of the strangest, most Dr. Seussian reptiles of all time were the Late Triassic drepanosauromorphs: Hypuronector, Vallesaurus, Drepanosaurus and Megalancosaurus. Found world wide and described as chameleon-like due to their unique grasping feet, drepanosaurs almost always provide something special to gawk at, including a hooked tail tip. Due to their strangeness, drepanosaurs have been difficult to classify.

Prior Assignments
Although Drepanosaurus was first considered a lepidosaur, drepanosaurs have been difficult to nest because they have rarely been part of a large, comprehensive taxonomic inclusion set.  Prior studies mistakenly nested drepanosaurs with Coelurosauravus and Longisquama (Senter 2004), which are not related to each other. Renesto and Binelli (2004) found that the pterosaur, Eudimorphodon, nested with drepanosaurs, between archosauriforms and Prolacerta + Tanystropheidae (Tanystropheus + Macrocnemus + Langobardisaurus) which is pretty close. Unfortunately, no one attempted to include basal lizards, including Huehuecuetzpalli and Jesairosaurus, in prior studies.

Current Assignment
Here drepanosaurs were lizards and members of the previously overlooked third clade, the Tritosauria with HuehuecuetzpalliMeyasaurus and Jesairosaurus at its base. These lizards did not fuse the astragalus and calcaneum like other lizards did and do.  CosesaurusMacrocnemus and several other strange tritosaurs are sister taxa.

Jesairosaurus
Jesairosaurus
 was the first in this line to develop the higher dorsal spines over the fully erect and fully ossified scapula, perhaps most immediately comparable to that of Meyasaurus. The skull was relatively larger and may have had a slender antorbital fenestra, taller than wide. The neck was relatively longer and quite gracile. One can surmise that this was the first of the slow-moving types due to the lack of robust bones in the neck. The hind foot is unknown but probably did not have the great asymmetry of Huehuecuetzpalli, but the shorter toes of Hypuronector. The tail is also unknown.

Drepanosaurs

Figure 1. Several drepanosaurs and their ancestors, the tritosaurs Huehuecuetzpalli and Jesairosaurus.

Hypuronector
Well preserved, but lacking a skull, Hypuronector (Olsen 1979) was smaller, had a smaller mandible, shorter neck, taller neural spines, much deeper tail chevrons, narrower scapula, a taller pelvis and shorter toes.

Vallesaurus
The most primitive drepanosaur with a curled tail was Vallesaurus  (Wild 1990, Renesto and Binelli 2006). The naris and antorbital fenestra were much larger on a shorter rostrum. The tail chevrons were not so deep and the posterior tail was curled ventrally, likely a prehensile organ able to grasp branches. Pedal digit 1 was greatly enlarged, like a thumb and may have opposed the other four digits while grasping branches. The proximal ankle bones were fused.

Megalancosaurus
Originally considered a possible bird ancestor, when only the skull and neck were known, Megalancosaurus (Calvazara, Muscio and Wild 1980) had a curled tail tipped by a hook. The antorbital fenestra was smaller, the narrow snout enabled binocular vision, the neck was longer, the shoulder hump was larger, the torso and tail were deeper, the scapula was more slender, the ilium leaned anteriorly and the pedal digit 1 was smaller but still robust.

Drepanosaurus
The first drepanosaur to be described was Drepanosaurus (Pinna 1980), which was originally and correctly considered a lizard. Renesto (1994) figured out that the ulnare enlarged, replacing the ulna, and the ulna became a sort of an elbow bone. These changes were related to the hyper-enlargement of manual ungual 2, which could have been used for digging or slicing into bark, perhaps to extricate buried insects. Larger than the other drepanosaurs, headless drepanosaurus also had the shortest neck and deepest torso. The pedal “thumb” was as large as the longest digit, #2.

No drepanosaurs are known to have survived into the Jurassic.

Summary
The drepanosaurs were an arboreal clade of tritosaur lizards adapted to slow-motion, clinging to branches with four short-toed, grasping feet and a prehensile tail. Later forms either lurched out at prey with a cobra-like neck snap and binocular vision, or dug into branches with outsized claws.

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.
Colbert EH and Olsen PE 2001. A New and Unusual Aquatic Reptile from the Lockatong Formation of New Jersey (Late Triassic, Newark Supergroup) American Museum Novitates, 3334: 15pp.
Evans SE 1991. A new lizard−like reptile (Diapsida: Lepidosauromorpha) from the Middle Jurassic of Oxfordshire. Zoological Journal of the Linnean Society 103:391-412.
Evans, SE and Barbadillo, LJ 1996. The Early Cretaceous lizards of Montsec (Catalonia, Spain) Treb. Museo de Geol. Barcelona 5:5-13 online pdf
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
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.
Olsen PE 1979. A new aquatic eosuchian from the Newark Supergroup LateTriassic-Early Jurassic) of North Carolina and Virginia. Postilla 176: 1-14.
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.
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. 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 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.
Renesto S, Spielmann JA, Lucas SG, and Spagnoli GT 2010. The taxonomy and paleobiology of the Late Triassic (Carnian-Norian: Adamanian-Apachean) drepanosaurs (Diapsida: Archosauromorpha: Drepanosauromorpha). New Mexico Museum of Natural History and Science Bulletin. 46:1–81.
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Hairy Museum blog on Drepanosaurs