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.

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.

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


Moving Diadectomorphs Into the Reptilia

The Traditional View: Reptile-like Amphibians
Diadectomorphs are widely considered to be reptile-like amphibians that lived during the Late Carboniferous and Early Permian. However, no diadectomorph tadpoles are known and these taxa lack a long list of amphibian characters (see below). These often big (2-3 m long), bulky (wider than tall torsos) taxa include herbivores and carnivores, all were slow-moving and cold-blooded.

Traditionally diadectomorphs included these taxa: Diadectes, Orobates, Stephanospondylus, Tseajaia, Limnoscelis.

Basal Diadectomorpha

Figure 1. Basal Diadectomorpha

The Heretical View
The larger study found diadectomorphs to nest within the Reptilia and within the Lepidosauromorpha branch. So tadpoles will never be found. Additions to the diadectomorphs include Solenodonsaurus, Lanthanosuchus,  chroniosuchids, Tetraceratops and Procolophon, which nests as a sister to Diadectes. Pareiasaurs, like Anthodon and turtles are also basal diadectomorphs. All were derived from earlier precursor sisters to OedaleopsRomeria primus and Concordia. Successors within this monophyletic clade branching off Lanthanosuchus  and Nyctiphruretus include lizards, snakes, pterosaurs and their kin.

Reptile-like Amphibians???
There are no other “amphibians” that even vaguely resemble this group of bulky Early Permian reptiles — especially those close to basal reptiles like Cephalerpeton, Casineria and Westlothiana. Calling diadectomorphs “reptile-like amphibians” was a mismatch from the beginning.

The Procolophon Missed Connection
The resemblance between the recognized reptile Procolophon and Diadectes was completely overlooked. The resemblance between pareiasaurs and diadectids was also overlooked. None of these taxa have labyrinthodont teeth. None have palatal fangs. None have an intermedium (a bone in the temple of pre-reptile amphibians).

The Otic Notch
Diadectomorphs did have a classic amphibian trait: an otic notch, which is a concave embayment at the back of the skull, roofed over by an overhang of skull roof. Presumably it framed a large eardrum or tympanum. Trouble is, these well-established reptiles also had an otic notch: Concordia, Oedaleops, Procolophon, Odontochelys, Proganochelys, Lanthanosuchus and Macroleter and Sauropareion. They’re all sisters to the diadectidomorphs.

The Age of Bulk – The Early Permian in Pangaea
It’s odd to consider that reptiles as fragile and aerial as pterosaurs and kuehneosaurs could have evolved from bulky diadectids and flattened lanthanosuchids, but the family tree indicates exactly such a lineage. Diadectes and Limnoscelis were formerly considered dead-ends. Now they are key taxa. So, what was happening in the Early Permian to encourage such bulking up?

The continents were locked together into a supercontinent known as Pangaea, with the east coast of North America blended into western Europe and north Africa. The Appalachian and Atlas mountains were virtually continuous and equatorial. From Texas to Germany the climate was tropical. This is the zone that produced most of the known basal diadectomorphs in vast coal forests. Large carnivores, like Dimetrodon, were on the rise. Dimetrodon warmed up faster and was able to become more active earlier aided by its large dorsal-sail solar collector. The bulk of a large Diadectes or Anthodon stored heat better due to a smaller surface-to-volume ratio. Retaining a portion of yesterday’s heat within a bulky body is considered inertial homeothermy. Larger plant eaters are better able to defend themselves due to their bulk and the risk the predator takes trying to attack larger prey.

It’s too bad that traditional paradigms continue to hamper working palaeontologists when a large gamut study is available that more parsimoniously nests several misplaced and enigmatic taxa and clades. Hopefully this blog will jog others to create trees with a similar large gamut of taxa to test and refine the present one.

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.

Berman, DS et al. 2004. A new diadectid (Diadectomorpha), Orobates pabsti, from the Early Permian of Central Germany. Bulletin of Carnegie Museum of Natural History 35 :1-36. doi: 10.2992/0145-9058(2004)35[1:ANDDOP]2.0.CO;2
Berman DS, Sumida SS, and Lombard RE 1992. Reinterpretation of the temporal and occipital regions in Diadectes and the relationship of diadectomorphs. Journal of Paleontology 66:481-499.
Berman DS, Sumida SS and Martens T 1998Diadectes  (Diadectomorpha:  Diadectidae) from the Early Permian of central Germany, with description of a new species. Annals of Carnegie Museum 67:53-93.
Berman DS Reisz RR and Scott D 2010. Redescription of the skull of Limmoscelis paludis Williston (Diadectomorpha: Limnoscelidae) from the Pennsylvanian of Canon del Cobre, northern New Mexico: In: Carboniferous-Permian Transition in Canon del Cobre, Northern New Mexico, edited by Lucas, S. G., Schneider, J. W., and Spielmann, New Mexico Museum of Natural History & Science, Bulletin 49, p. 185-210.
Cope ED 1878a. Descriptions of extinct Batrachia and Reptilia from the Permian formation of Texas. Proceedings of the American Philosophical Society 17:505-530.
Cope ED 1878b. A new Diadectes. The American Naturalist 12:565.
Kissel R 2010. Morphology, Phylogeny, and Evolution of Diadectidae (Cotylosauria: Diadectomorpha). Thesis (Graduate Department of Ecology & Evolutionary Biology University of Toronto).
Moss JL 1972. The Morphology and phylogenetic relationship of the Lower Permian tetrapodTseajaia campi Vaughn (Amphibia: Seymouriamorpha): University of California Publications in Geological Sciences 98:1-72.
Romer AS 1946. The primitive reptile Limnoscelis restudied American Journal of Science, Vol. 244:149-188
Vaughn PP 1964. Vertebrates from the Organ Rock Shale of the Cutler Group, Permian of Monument Valley and Vicinity, Utah and Arizona: Journal of Paleontology 38:567-583.
Williston SW 1911.
 A new family of reptiles from the Permian of New Mexico: American Journal of Science, Series 4, 31:378-398.

HMNH link to Diadectes

Icarosaurus, Kuehneosaurus and the So-Called “Rib” Gliders

An Introduction
While pterosaurs were experimenting with flapping flight in the Late Triassic, several arboreal lepidosauriforms were gliding with hyper-elongated, rib-like, dermal extensions anchored to their reduced and modified ribs. Welcome to the world of the Triassic gliders, their Permian precursors and their one and only known successor in the Early Cretaceous, Xianglong.

Coelurosauravus reconstructions

Figure 1. Coelurosauravus reconstructions from Carroll, Frey et al and Peters.

Traditional and Published Views
Carroll (1978, 1988) separated Coelurosauravus from Icarosaurus + Kuehneosaurus. The former was considered a primitive diapsid and the latter two were considered lizards. Both were reported to extend lateral gliding membranes framed by elongated ribs, as in the modern gliding lizard, Draco. Like Draco, no transverse processes were reported in Coelurosauravus (Figure 1), but large transverse processes were reported in Icarosaurus + Kuehneosaurus. Then Frey et al. (2007, Figure 1) found short ribs in Coelurosauravus, which meant the gliding membrane extensors were ossified dermal rods. They reported, “The rods are independent of the ribcage and arranged in distinct bundles to form a cambered wing.” Finally, the Early Cretaceous glider, Xianglong, was reported (Li et al. 2007) to be an agamid lizard, like Draco.

The Triassic gliders and their non-gliding precursors.

Figure 1. Click to enlarge. The Triassic gliders and their non-gliding precursors.

The Heretical View
Here sets of anterior dermal rods of Coelurosauravus were bundled and anchored to the tips of the anterior two ribs while the posterior rods were associated one-to-one with individual dorsal ribs. Here the purported transverse processes of Icarosaurus and Kuehneosaurus are short, straight ribs fused to their centra and the purported “ribs” are dermal rods, as in Coelurosauravus. Here Coelurosauravus is a sister to Icarosaurus + Kuehneosaurus and all three are non-lepidosaur lepidosauriforms. Finally, Xianglong also had short, straight ribs fused to their centra and so was related to Icarosaurus + Kuehneosaurus, not Draco.

Traditional Origins
There are as many origins and nesting for the “rib” gliders as there are studies that include them. Laurin 1991 nested Coelurosauravus between the diapsid Petrolacosaurus and the synapsid Apsisaurus. Evans 1988 nested Coelurosauravus between Mesenosaurus and Claudiosaurus. Kuehneosaurs nested in two places, between Choristodera and rhynchosaurs and also between Saurosternon and Gephyrosaurus + Squamata. Evans 2003 nested kuehneosaurs between archosauromorpha (prolacertiforms, rhynchosaurs, archosauriforms) and Marmoretta. Motani (1998) neste kuehneosaurs between lizards and sauropterygians. Müller (2003) nested kuehneosaurs and Coelurosauravus together between Claudiosaurus and Ichthyosaurs + thalattosaurs. The latter seems especially unlikely, nesting aerial reptiles with marine taxa.

Nesting Within the Larger Study
The larger study nested the gliders together with Saurosternon and Palaegama as outgroup taxa.

Let’s Begin with Palaegama
Palaegama was a Late Permian lepidosauriform blessed with elongated arms and legs. These would have been useful living in trees, or perhaps sprinting on the ground bipedally. Palaegama has been recognized as a basal lepidosauriform along with Saurosternon and Paliguana.  Estes, Pregill and Camp (1988 ) reported, “they share more features of modern lizards than do any other reptiles of the lat Paleozoic and early Mesozoic.” Yet they were not lizards. They were lizard predecessors. In particular, the skull shape and naris placement of Palaegama indicate a close relationship with Coelurosauravus.

(Latest Permian/Earliest Triassic) Saurosternon was smaller, but with relatively larger feet. Twin sternae appear posterior to the coracoids. These likely indicated an increase in humerus adduction, as in tree clinging. The shorter body shape indicates a closer relationship to Icarosaurus than to Coelurosauravus.

(Late Permian) Coelurosauravus was longer, leaner, with a more exotic skull, shorter ribs and more gracile limbs. Elongated dermal ossicles anchored on the rib tips, were able to fold and extend huge lateral membranes, probably for gliding, but also useful as secondary sexual characters (again, check out that skull for exotic extremes).

(Late Triassic) Mecistotrachelos was a coelurosauravian with a longer neck, shorter tail and a much more slender (almost stick-like) torso in which the ribs were fused to the centra, making them appear to be transverse processes. Fewer dermal “pseudo-ribs” were used to frame the gliding membrane. The cranial crest remained, but was reduced.

(Late Permian) only the skull has been published (Modesto and Reisz 2003), and it was originally considered an enigma, but its affinities are with Icarosaurus and the gliders. In a recent abstract, Reisz and Modesto (2011) reported, “The skeletal anatomy of Lanthanolania provides evidence of limb proportions that suggest that this small reptile is the oldest known bipedal diapsid.” Unfortunately, Lanthanolania was not a diapsid. Nor was it as old as Eudibamus, another diapsid biped. Apparently it also does not have extended pseudoribs, otherwise, they would have been mentioned.

(Late Triassic) Icarosaurus transformed the short ribs of Saurosternon into short “transverse processes” fused to the centra. This transformation has been overlooked by other paleontologists, who report that Icarosaurus had extended ribs, like Draco, the living rib glider. The problem is, no sister taxa have transverse processes, Draco doesn’t have transverse processes, several unfused ribs appear between the scapulae in Icarosaurus and the phylogenetic precursors have not been identified as they are here. In any case, a short tail, deep pelvis and short torso characterize this genus.

(Late Triassic) The biggest of the gliders, Kuehneosaurus was most similar to Icarosaurus but had feet much larger than the hands. Certain posterior (fused) ribs angled anteriorly.

(Early Cretaceous) Xianglong was considered an agamid lizard by Li et al. (2007), but it clearly had short “transverse processes” (actually ribs fused to centra) not found in agamids like Draco. Xianglong demonstrates the survival of the PermoTriassic gliders into the Cretaceous. A poorly ossified carpus may indicate immaturity in the one known specimen.

The PermoCretaceous gliders reduced the dorsal ribs, fused these to the centra and developed elongated dermal extensions to extend lateral gliding membranes. Coelurosauravus and its membranes were considered distinct and convergent, but here they were homologous with those of kuehneosaurids. Xianglong was a late-surviving non- lepidosaur lepidosauriform.

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.

Colbert, Edwin H. (1966). A gliding reptile from the Triassic of New Jersey. American Museum Novitates 2246: 1–23. online pdf
Evans SE 1982. Gliding reptiles of the Late Permian. Zoological Journal of the Linnean Society, 76:97–123.
Evans SE and Haubold H 1987.
A review of the Upper Permian genera  CoelurosauravusWeigeltisaurus and Gracilisaurus (Reptilia: Diapsida). Zool J Linn Soc, 90:275–303.
Fraser NC, Olsen PE, Dooley AC Jr and Ryan TR 2007. 
A new gliding tetrapod (Diapsida: ?Archosauromorpha) from the Upper Triassic (Carnian) of Virginia. Journal of Vertebrate Paleontology 27 (2): 261–265. doi:10.1671/0272-4634(2007)27[261:ANGTDA]2.0.CO;2.
Frey E, Sues H-D and Munk W 1997. 
Gliding Mechanism in the Late Permian Reptile Coelurosauravus. Science Vol. 275. no. 5305, pp. 1450 – 1452
DOI: 10.1126/science.275.5305.1450
Li P-P, Gao K-Q, Hou L-H and Xu X. 2007. A gliding lizard from the Early Cretaceous of China. PNAS 104(13): 5507-5509. doi: 10.1073/pnas.0609552104 online pdf
Modesto SP and Reisz RR 2003. An enigmatic new diapsid reptile from the Upper Permian of Eastern Europe. Journal of Vertebrate Paleontology 22 (4): 851-855.
Modesto SP and Reisz RR 2011. The neodiapsid Lanthanolania ivakhnenkoi from the Middle Permian of Russia, and the initial diversification of diapsid reptiles. SVPCA abstract.
Robinson PL 1962. Gliding lizards from the Upper Keuper of Great Britain. Proceedings of the Geological Society London 1601:137–146.
Stein K, Palmer C, Gill PG and Benton MJ 2008. The aerodynamics of the British Late Triassic Kuehneosauridae. Palaeontology, 51(4): 967-981. DOI: 10.1111/j.1475-4983.2008.00783.x
Piveteau J 1926. Paleontologie de Madagascar, XIII. Amphibiens et reptiles permiens: Annales de Paleontologie, v. 15, p. 53-128.


Why Lizards Are Not Diapsids

The Paradigm
Reptiles were originally divided and sorted according to the number of openings in the temple and cheek regions of the skull. Some reptiles had one in the temple. Others had one in the cheek. Some had none. Reptiles with openings in the temple and cheek were considered diapsids, which described the “two arches” between and below the two openings.

Recent Studies Supporting the Paradigm
Traditional computer-assisted phylogenies, beginning with Gauthier et al. (1988) and Laurin (1991), placed the araeoscelidans (Petrolacosaurus and Araeoscelis) and the younginids (Youngina, Thadeosaurus and Orovenator) at the base of both the lepidosaurs (squamates + sphenodontians) and the archosauromorphs (prolacertiformes + rhynchosauria + archosauriforms). Paleothyris was identified as a precursor to the araeoscelidans. In this scenario diapsid openings appeared apart from the synapsid opening. The lower temporal arch, beneath the lateral temporal fenestra, was retained in Sphenodon and disappeared in squamates.

But is this true?

The Heretical View Supported by a Larger Dataset
The present large study, employing many more taxa, nested one Youngina at the base of the Prolacertiformes, which includes the Archosauriformes. Other Youngina nested within the Archosauriformes. Lepidosaurs nested elsewhere, far from Petrolacosaurus and Youngina. According to the present large study, lepidosaur precursors, including Owenetta, Paliguana, Gephyrosaurus and Meyasaurus, did not have a lower temporal arch, at least, not a complete one. Some lepidosaurs (such as Sphenodon, rhynchosaurs, pterosaurs and certain basal scleroglossans, such as Tianyusaurus) developed a lower temporal arch on their own, often independently of one another).

Reptile family tree

Figure 1. Click to enlarge. Here lepidosaurs nest far from Petrolacosaurus and the rest of the Diapsida.

Thus lepidosaurs can, at best, be considered quasi-diapsids. True diapsids, those related to and descending from Petrolacosaurus include the araeoscelids, enaliosaurs (Claudiosaurus, Mesosaurus and a host of marine reptiles) and younginiforms (Thadeosaurus through the Archosauriformes).

True Diapsid Precursors Were Derived from Basal Synapsids
The true diapsids were derived from a lineage of synapsids apart from the lineage of therapsids and eupelycosaurs beginning with Heleosaurus, which was originally described as a diapsid, ironically enough. In this scenario the lateral temporal fenestra was already present when the upper temporal opening appeared in Spinoaequalis, Eudibamus and Petrolacosaurus. Changes to the diapsid appearance occurred almost immediately with Milleropsis, Mesosaurus and Araeoscelis, but a more conservative branch that retained the diapsid configuration ultimately led to Youngina and the archosaur diapsids living today, the birds and crocs.

The large study provides genus-based precursors and successors from the first tetrapods to living taxa and all sister taxa resemble one another in a gradual spectrum of morphologies, echoing the evolutionary process. Other prior studies did not go into this depth, nor did other studies include the present gamut of taxa.

Definitions in Need of Revision
Laurin (1991) defined the Diapsida as the most recent common ancestor of araeoscelidians, lepidosaurs and archosaurs and all its descendants. According to the present results, the definition is now redundant with the Amniota and Reptilia.

Benton (1985) defined Neodiapsida as Youngina and all species more closely related to it than to Petrolacosaurus. According to Benton (1985) this definition likewise is in need of revision because it too contains lepidosaurs.

Gauthier, Kluge & Rowe (1988) defined Sauria as the most recent common ancestor of Lepidosauria and Archosauria and all of its descendants. Now that definition is redundant with Reptilia.

Gauthier (1994) defined Sauropsida as “Reptiles plus all other amniotes more closely related to them than they are to mammals,” based on traditional cladograms that indicated a basal split between the Synapsida and Sauropsida. Here the basal split was between archosauromorphs (which included synapsids) and lepidosauromorphs, so this definition defines a paraphyletic assemblage.

Many more definitions are no longer valid based on the new nestings and branchings recovered in the new tree. We’ll discuss these in future blogs.

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.

Benton MJ 1985. Classification and phylogeny of diapsid reptiles. Zoological Journal of the Linnean Society 84: 97-164.
Callaway JM 1997.
 Ichthyosauria: Introduction, in JM Callaway & EL Nicholls (eds.), Ancient Marine Reptiles. Academic Press, pp. 3–16.
Gauthier J, Kluge AG and Rowe T 1988. The early evolution of the Amniota. In Michael J. Benton (ed.) The phylogeny and classification of the tetrapods, Volume 1: amphibians, reptiles, birds: 103-155. Oxford: Clarendon Press.
Gauthier J, Estes R and DeQueiroz K 1988. A phylogenetic analysis of Lepidosauria; pp. 15-98 in R. Estes and G. Pregill (eds.), Phylogenetic Relationships of the Lizard Families. Stanford University Press, Stanford, California.
Laurin M 1991. The osteology of a Lower Permian eosuchian from Texas and a review of diapsid phylogeny. Zoological Journal of the Linnean Society 101:59-95.
Laurin M and Reisz R 1995. A reevaluation of early amniote phylogeny. Zoological Journal of the Linnean Society, 113: 165–223.
Modesto SP and Anderson JS 2004.
The Phylogenetic Definition of Reptilia. Systematic Biology 53(5):815-821.
Reisz RR, Modesto SP and Scot DMT 2011. A new Early Permian reptile and its significance in early diapsid evolution. Proceedings of the Royal Society, London B doi:10.1098/rspb.2011.0439

Moving Rhynchosaurs and Trilophosaurs Back into the Rhynchocephalia (Sphenodontia)

Rhynchosaurs are among the strangest reptiles that ever lived.
Characterized by a weird wide skull and protruding toothless, beak-like premaxillae, rhynchosaurs had rows of crushing teeth and giant jaw muscles for grinding food before swallowing to hasten digestion. Although the body was nothing special, no other reptile had such a skull. And evidently THAT is the cause of the current lack of a solid phylogenetic nesting.

Pre-rhynchosaurs, like Mesosuchus, appeared in the late Early Triassic to early Middle Triassic. All rhynchosaurs, including Hyperodapedon, disappeared in the Late Triassic.

Hyperodapedon in various views.

Figure 1. Hyperodapedon in various views. Note the extreme width of the skull and multiple rows of grinding teeth.

Where Did Rhynchosaurs Fit In?
Romer (1956) considered rhynchosaurs and sphenodontians to be related, but Cruickshank (1972), Benton (1983), Carroll (1988) and Dilkes (1998) split them apart, perhaps by placing too much emphasis on the lack of fusion in the tarsus and lack of acrodont teeth (see below). Caroll (1988) placed Trilophosaurus and rhynchosaurs with Prolacerta, Tanystropheus, Proterosuchus and Euparkeria. Unfortunately, no phylogenetic analysis has yet tested this nesting against a large gamut of reptiles, other than the large study.


Priosphenodon and its sphenodontid sisters, including Trilophosaurus and the rhynchosaur Hyperodapedo

Let’s Look at the Candidates
Above a selection of several rhynchocephalians is compared to three candidate diapsids. Overall Hyperodapedon, Mesosuchus and Trilophosaurus share more traits with Brachyrhinodon than with Youngina, Prolacerta or Proterosuchus. This is also demonstrated by several hundred characters and taxa in the large reptile tree. No other terrestrial reptile had such a wide skull, but Mesosuchus comes close. Brachyrhinodon and Priosphenodon come close to MesosuchusProlacerta and Proterosuchus were known for their narrow skulls filled with sharp teeth.

Here’s the Hump We Have to Get Over
According to the textbooks, lepidosaurs all have a fused astragalus and calcaneum and derived characters of bone growth with epiphyses. The problem is Trilophosaurus and rhynchosaurs don’t fuse those proximal ankle bones.

Benton (1983) reported, “Rhynchosaurs have no special relationship with the sphenodontids. The supposed shared characters are either primitive (e.g. complete lower temporal bar, quadratojugal, akinetic skull, inner ear structure, 25 presacral vertebrae, vertebral shape, certain character of limbs and girdles) or incorrect (e.g. rhynchosaurs do not have acrodont teeth, the ‘beak-like’ premaxilla of both groups is quite different in appearance, the ‘tooth plate’ is wholly on the maxilla in rhynchosaurs but on maxilla and palatine in sphenodontids).”

These nits and picks are important, but taken as a whole (which is what we must do) currently there are no taxa more closely related to rhynchosaurs than rhynchocephalians (sphenodontians) and the trilophosaurs. Granted, all other rhynchocephalians had fused ankle bones, but having an unfused ankle is simply a matter of not fusing those bones, which develop separately in embryos. Acrodont teeth also form with fusion. Again, this would be a simple matter of switching off a gene.

Some of the Strangest Teeth You’ll Ever See
Benton (1983) discussed the placement of teeth wholly on the maxilla in rhynchosaurs. Let’s see what that looks like. The palatine (in orange) is the key bone in this controversy. In Mesosuchus the palatine is reduced and has lost its teeth. In Hyperodapedon the palatine retains teeth and extends lateral to the choanae to contact the premaxilla. In Howesia the palatine fuses to the maxillary tooth plate. In Trilophosaurus the palatine likewise fused to the maxillary tooth plate and the palatine teeth fused to the maxillary teeth, creating laterally elongated teeth with three lateral cusps. Click here for enlargement.

The palates of several rhynchocephalians, including rhynchosaurs

Figure 3. The palates of several rhynchocephalians, including rhynchosaurs. Click to enlarge.

Romer was right. Rhynchosaurs are closer to rhynchocephalians (sphenodontians). The differences noted by Benton (1983) are insufficient to outweigh a larger suite of characters that nest these taxa together.

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.

Benton MJ 1983. The Triassic reptile Hyperodapedon from Elgin, functional morphology and relationships. Philosophical Transactions of the Royal Society of London, Series B, 302, 605-717.
Benton MJ 1990. The Species of Rhynchosaurus, A Rhynchosaur (Reptilia, Diapsida) from the Middle Triassic of England. Philosophical transactions of the Royal Society, London B 328:213-306. online paper
Benton MJ 1985. Classification and phylogeny of diapsid reptiles. Zoological Journal of the Linnean Society 84: 97-164.
Carroll RL 1988. Vertebrate Paleontology and Evolution. WH Freeman and Company.
Case EC 1928. A cotylosaur from the Upper Triassic of western Texas: Journal of Washington Academy of Science 18:177-178.
Cruickshank ARI 1972. 
The proterosuchian thecodonts. In Studies in Vertebrate Evolution (ed. Jenkins KA and Kemp TS) 89-119. Edinburgh: Oliver and Boyd.
Dutuit J-M 1972. Découverte d’un Dinosaure ornithischien dans le Trias supérieur de l’Atlas occidental marocain. Comptes Rendus de l’Académie des Sciences à Paris, Série D 275:2841-2844. 
Flynn JJ, Nesbitt, SJ, Parrish JM, Ranivoharimanana L and Wyss AR 2010. 
A new species of Azendohsaurus (Diapsida: Archosauromorpha) from the Triassic Isalo Group of southwestern Madagascar: cranium and mandible”. Palaeontology 53 (3): 669–688. doi:10.1111/j.1475-4983.2010.00954.x
Fraser NC and Benton MJ 1989.
The Triassic reptiles Brachyrhinodon and  Polysphenodon and the relationships of the sphenodontids. Zoological Journal of the Linnean Society 96:413-445.
Gregory JT 1945. Osteology and relationships of Trilophosaurus: University of Texas, Publication 4401:273-359.
Heckert AB et al. 2006. Revision of the archosauromorph reptile Trilophosaurus, with a description of the first skull of Trilophosaurus jacobsi, from the upper Triassic Chinle Group, West Texas, USA: Palaeontology 4(3):1-20.
Huxley TH 1859.
 Postscript to, R. I. Murchinson. On the sandstones of Morayshire (Elgin & c.) containing reptile remains; and their relations to the Old Red Sandstone of that country. Quarterly Journal of the Geological Society, London, 15, 138-152.
Huxley TH 1869. On Hyperodapedon. Quarterly Journal of the Geological Society, London, 25, 138-152.
Huxley TH 1887. Further observations upon Hyperodapedon gordoni. Quarterly Journal of the Geological Society, London, 43, 675-694. Parks P 1969. Cranial anatomy and mastication of the Triassic reptile,  Trilophosaurus [M.S. thesis]: University of Texas, 100 pp.
Romer AS 1956. Osteology of the Reptiles. University of Chicago Press, Chicago.


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.


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.

 (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.

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.


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.

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.

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.