Rough chronology of basal tetrapods and basal reptiles

Today we’ll look at WHEN
we find fossils of basal tetrapods and basal reptiles. According to the large reptile tree (959 taxa, LRT, subset shown in Fig. 1), oftentimes we find late survivors of earlier radiations in higher strata. The origin of Reptilia (amphibian-like amniotes) extends back to the Devonian and Early Carboniferous now, not the Late Carboniferous as Wikipedia reports and as the Tree of Life project reports.

Figure 1. Color coded chronology of basal tetrapods and reptiles.We're lucky to know these few taxa out of a time span of several tens of millions of years.

Figure 1. Color coded chronology of basal tetrapods and reptiles.We’re lucky to know these few taxa out of a time span of several tens of millions of years. Click to enlarge.

The Late Devonian 390–360 mya
Here we find late survivors of an earlier radiation: Cheirolepis, a basal member of the Actinopterygii (ray-fin fish) together with Eusthenopteron and other members of the Sarcopterygii (lobe-fin fish). Coeval are basal tetrapods, like Acanthostega and basal reptiles, like Tulerpeton. These last two launch the radiations we find in the next period. The presence of Tulerpeton in the Late Devonian tells us that basal Seymouriamorpha and Reptilomorpha are waiting to be found in Devonian strata. We’ve already found basal Whatcheeriidae in the Late Devonian taxa Ichthyostega and Ventastega.

Early Carboniferous 360–322 mya
Here we find the first radiations of basal reptilomorphs, basal reptiles, basal temnospondyls,  basal lepospondyls and microsaurs, lacking only basal seymouriamorphs unless Eucritta is counted among them. It nests outside that clade in the LRT.

Late Carboniferous 322–300 mya
Here we find more temnospondyls, lepospondyls and phylogenetically miniaturized archosauromorphs, likely avoiding the larger predators and/or finding new niches. Note the first prodiapsids, like Erpetonyx and Archaeovenator, appear in this period, indicating that predecessor taxa like Protorothyris and Vaughnictis had an older, Late Carboniferous, origin. Not shown are the large basal lepidosauromorphs, Limnoscelis and Eocasea and the small archosauromorphs, Petrolacosaurus and Spinoaequalis.

Early Permian 300–280 mya
Here we find the first fossil Seymouriamorpha and the last of the lepospondyls other than those that give rise to extant amphibians, like Rana, the frog. Here are further radiations of basal Lepidosauromorpha, basal Archosauromorpha (including small prodiapsids), along with the first radiations of large synapsids.

Late Permian 280–252 mya
Here we find the next radiation of large and small synapsids, the last seymouriamorphs, and derived taxa not shown in the present LRT subset.

Early/Mid Triassic 252 mya–235 mya
Among the remaining basal taxa few have their origins here other than therapsids close to mammals. Afterwards, the last few basal taxa  listed here, principally among the Synapsida, occur later in the Late Triassic, the Jurassic and into the Recent. Other taxa are listed at the LRT.

What you should glean from this graphic
Taxa are found in only the few strata where fossilization occurred. So fossils are incredibly rare and somewhat randomly discovered. The origin of a taxa must often be inferred from phylogenetic bracketing. And that’s okay. This chart acts like a BINGO card, nesting known taxa while leaving spaces for taxa we all hope will someday fill out our card.




Cacops: Temnospondyl or Lepospondyl?

In order to understand
the interrelationships of reptiles, one needs to known where to begin and what came before the beginning. Earlier the large reptile tree (LRT) recovered the Viséan Silvanerpeton and the Late Carboniferous Gephyrostegus bohemicus at the base of the Amniota (= Reptilia) with origins in the early Viséan or earlier (340+mya).

Reptiles were derived from the clade Seymouriamorpha, 
close to Utegenia, which also nests at the base of the Lepospondyli, + Seymouria + Kotlassia. These, in turn, were derived from the reptilomorphs, Proterogyrinus and Eoherpeton.

Reptilomorphs, in turn, were derived from Temnospondyls,
at present, Eryops (unfortunately too few taxa to be more specific at present), and temnospondyls, in turn, were derived from basal tetrapods, like Pederpes.

Figure 1. Cacops and its sisters.

Figure 1. Cacops and its sisters in the LRT.

A recent objection
by Dr. David Marjanovic suggested that the basal tetrapod, Cacops, was not a lepospondyl, but actually a temnospondyl.

Figure 1. Sclerocephalus in situ and reconstructed. This taxon nests with Eryops among the temnospondyls.

Figure 1. Sclerocephalus in situ and reconstructed. To no surprise, this taxon nests with Eryops among the temnospondyls. Note the expanded ribs.

That’s worth checking out.
So I added taxa: Sclerocephalus and Broiliellus (Fig. 2). The former nested with Eryops as a temnospondyl. The latter nested with Cacops and the lepospondyls. The new taxa did not change the topology. So… either the present topology is correct, or I’ll need some taxon suggestions to make the shift happen.

Figure 1. Broiliellus skull. This taxon nests with Cacops among the lepospondyls, derived from a sister to the Seymouriamorph, Utegenia.

Figure 1. Broiliellus skull. This taxon nests with Cacops among the lepospondyls, derived from a sister to the Seymouriamorph, Utegenia. Note the ‘new’ bone between the lacrimal and jugal. That’s a surface appearance of the palatine!


The large reptile tree tells us
that reptiles and lepospondyls are all seymouriamorphs with Utegenia at the last base of the lepospondyls, but known only form late-surviving taxa at present. Lepospondyls continue to include Cacops and Broiliellus, along with extant amphibians and microsaurs, which mimic basal reptiles. Most of these taxa should be found someday in Romer’s Gap prior to the Viséan in the earliest Carboniferous or late Devonian.

Wikipedia reports, “[Seymouriamorpha] have long been considered reptiliomorphs, and most paleontologists may still accept this point of view, but some analyses suggest that seymouriamorphs are stem-tetrapods (not more closely related to Amniota than to Lissamphibia) aquatic larvae bearing external gills and grooves from the lateral line system have been found, making them unquestionably amphibians. The adults were terrestrial.

The LRT finds
seymouriamorphs basal to reptiles + lepospondyls. The latter includes lissamphibians (all extant amphibians , their last common ancestor and all of its descendants) and several other clades, including Microsauria, Nectridea, and several very elongate taxa.

Wikipedia reports, “It has been suggested that the Dissorophidae may be close to the ancestry of modern amphibians (Lissamphibia), as it is closely related to another family called Amphibamidae that is often considered ancestral to this group, although it could also be on the tetrapod stem. The large reptile tree also recovers this relationship. Cacops and Broiliellus are both considered dissorophids.

Lewis GE and Vaughn PP 1965. Early Permian vertebrates from the Cutler Formation of the Placerville area, Colorado, with a section on Footprints from the Cutler Formation by Donald Baird: U.S. Geol. Survey Prof. Paper 503-C, p. 1-50.
Moodie RL 1909. A contribution to a monograph of the extinct Amphibia of North America. New forms from the Carboniferous. Journal of Geology 17:38–82.
Reisz RR, Schoch RR and Anderson JS 2009. The armoured dissorophid Cacops from the Early Permian of Oklahoma and the exploitation of the terrestrial realm by amphibians. Naturwissenschaften (2009) 96:789–796. DOI 10.1007/s00114-009-0533-x
Williston SW 1910. Cacops, Desmospondylus: new genera of Permian vertebrates. Bull. Geol. Soc. Amer. XXI 249-284, pls. vi-xvii.
Williston SW 1911. Broiliellus, a new genus of amphibians from the Permian of Texas. The Journal of Geology 22(1):49-56.


Ianthodon: a basal edaphosaur without tall neural spines

A new paper
by Spindler, Scott and Reisz (2015) brings us new data on the basal pelycosaur Ianthodon schultzei (Fig. 1; Garnet locality, Missourian Age; 305-306 mya, Middle Pennsylvanian, Late Carboniferous). The authors reported that Ianthodon represented a more basal sphenacodontid than Haptodus. In the large reptile tree Ianthodon was derived from a sister to Haptodus and nested at the base of Edaphosaurus + Ianthasaurus + Glaucosaurus, all edaphosaurids.

Figure 1. Ianthodon schultzei was considered a basal pelycosaur, and it is, but here nests as a basal edaphosaur. And it has no tall neural spines. So pelycosaur sails were convergent, not homologous.

Figure 1. Ianthodon schultzei (image modified from Spindler, Scott and Reisz 2015) was considered a basal pelycosaur, and it is, but here nests as a basal edaphosaur. And it has no tall neural spines. So pelycosaur sails were convergent, not homologous. Spindler, Scott and Reisz considered this specimen a juvenile due to its incomplete ossification.

Notably Ianthodon does not have tall neural spines. Earlier we wondered whether the tall neural spines of Edaphosaurus and Dimetrodon were convergent or homologous. Now it is clear, via Ianthodon, and Sphenacodon (sorry I did not notice this yesterday) that the tall neural spines of Edaphosaurus and Dimetrodon were convergent.

Most well-known pelycosaurs
were Early Permian in age. Ianthodon demonstrates an earlier origin for their carnivore/ herbivore split. And it retains carnivore teeth! Therapsids likewise originated in the Late Carboniferous according to this new data.

Phylogenetic history
Spindler, Scott and Reisz (2015) report, “In the original description and phylogenetic analysis of Kissel and Reisz (2004), Ianthodon was found to nest surprisingly high within Sphenacodontia, as a sister taxon to the clade that included Pantelosaurus, Cutleria and sphenacodontids. In a subsequent, large-scale analysis, Ianthodon was found to be more basal, near the edaphosaurid–sphenacodont node (Benson, 2012), but its exact position remained poorly resolved. In the latter analysis, Benson (2012) extensively revised the character list and included all known “pelycosaur” grade synapsids, while Kissel and Reisz (2004) used data and taxa derived from Laurin (1993), which mainly followed Reisz et al. (1992). Another recent analysis of sphenacodont synapsids by Fröbisch et al. (2011), as part of a description of a new taxon, recovered Ianthodon, Palaeohatteria and Pantelosaurus in an unresolved polygamy.”

The Spindler, Scott and Reisz (2015) analysis
used 122 characters (vs. 228 in the large reptile tree). Their tree shows 12 taxa, 4 of which are suprageneric. In their tree Ianthodon nested between Edaphosauridae and Haptodus. (So close, but no cigar.) Their tree also nested two therapsid taxa (Biarmosuchus and Dinocephalia) with Cutleria, Sphenacodon, Ctenospondylus and Dimetrodon. Thus Spindler, Scott and Reisz appear to be excluding several key taxa and their tree topology differs significantly from the large reptile tree at the base of the Therapsida, with or without Ianthodon.

Spindler F, Scott D. and Reisz RR 2015. New information on the cranial and postcranial anatomy of the early synapsid Ianthodon schultzei (Sphenacomorpha: Sphenacodontia), and its evolutionary significance. Fossil Record 18:17–30.

Purbicella, a basal scleroglossan from the Purbeck Limestone

Figure 1. Purbicella in situ (palatal view) and traced using DGS, then reconstructed using those tracings. Gray areas are unknown. If you think this looks like a generalized, plesiomorphic scleroglossan, you're right! Here colorizing the bones helps identify sutures and paired elements. That's a right pterygoid covering much of the paired frontals. The teeth are blunt.

Figure 1. Purbicella in situ (palatal view) and traced using DGS, then reconstructed using those tracings. Gray areas are unknown. If you think this looks like a generalized, plesiomorphic scleroglossan, you’re right! Here colorizing the bones helps identify sutures and paired elements. That’s a right pterygoid covering much of the paired frontals. The teeth are blunt.

A few years ago
a rather complete lizard skull (BGS GSb581) was described (Evans et al. 2012) from the Purbeck Limestone (Late Jurassic to Early Cretaceous) of England. It was originally excavated more than a century ago and assigned to the genus, Paramacellodus. Evans et al. renamed it Purbicella. Their cladistic analysis nested Purbicella with Lacertoidea: (Lacertidae (including Acanthodactylus), Teiidae (including Tupinambus), Gymnophthalmidae (including Gymnophthalmus), and the burrowing Amphisbaenia (including Amphisbaena)), not Paramacellodus, which nested with skinks. Evans et al. based their nesting on a partial data matrix of Conrad (2008).

The large reptile tree nested Purbicella between Acanthodactylus and Liushusaurus. The large reptile tree recovered the above listed ‘lacertoid’ taxa as members of a paraphyletic clade, some preceding Purbicella in various clades and others succeeding it.

While Purbicella is Late Jurassic/Early Cretaceous, it must have had its origins much earlier, in the Late Carboniferous, because a descendant taxon, the TA1045 specimen, is Early Permian.

Conrad JL 2008. Phylogeny and systematics of Squamata (Reptilia) based on  morphology. Bulletin of the American Museum of Natural History 310:1–182.
Evans SE, Jones MEH and Matsumto R 2012. A new lizard skull from the Purbeck Limestone Group (Lower Cretaceous) of England. Bull. Soc. géol. France, 2012, t. 183(6):517-524.


News at the base of the Amniota, part 8: The list of Viséan amniotes has grown.

Earlier (in seven prior blog posts) we looked at the new basalmost amniotes and how they evolved.

With the news
that the Amniota (and amniote eggs) extended back to the Viséan (326-345 mya) let’s take a look at those basalmost amniotes along with the genera that may have been more primitive, but survived more than 30 million years later, into the Westphalian (303-311 mya) and beyond to the Early and Late Permian. That’s a stretch of 80 million years for the most successful taxa. And what made them so successful? Or were they all just as successful, just not found yet in higher strata?

Figure 1. Basal amniotes to scale colorized according to the time strata in which their fossils were found. Visean, yellow; Namurian, pink; Westphalian, blue; Permain, tan.

Figure 1. Click to enlarge. Basal amniotes to scale colorized according to the time strata in which their fossils were found. Visean, yellow; Namurian, pink; Westphalian, blue; Permian, tan.

It’s well worth remembering
at this point that in similar fashion, basal primates, like lemurs, also co-exist today with derived primates, like apes and humans. So that happens. At least some of these basalmost amniotes (the Permian forms, like Utegenia, Fig. 1) developed successful traits so well matched to their own niche they survived for tens of millions of years thereafter.

Also remember
that fossilization is a rare event. Even more rare is the discovery of rare fossils. So we’re very lucky to have even single examples of these taxa. They were probably more widespread both across the globe and through time.

Two important points
1) We don’t find amniotes prior to the Viséan. So these Viséan amniotes  (Fig. 1) are likely the earliest representatives of their kind.

2) The Viséan is a short 15 to 35 million years after the very first tetrapods developed limbs from fins some 360 million years ago in the Late Devonian. So evolution was rapid during those first 15 million years. Not so rapid for the next 80 million years, at least for certain taxa.

That’s exciting to think about.

A Chronology of Basal Reptilia

The dual origin of the Reptilia (following Cephalerpeton) was blogged earlier. Here we’ll take a look at the chronology of basal reptiles during the Carboniferous and Permian.

A chronology of the basal Reptilia.

Figure 1. Click to enlarge. A chronology of the basal Reptilia.

The Most Primitive Known Reptile Was Not the Oldest
Cephalerpeton nested as the most basal reptile, the one closest to the nearest outgroup taxon, Gephyrostegus. Unfortunately the fossil record of both succeed the earliest known reptiles, Westlothiana and Casineria, by some 40 million years. That means the first appearance of Cephalerpeton had to precede its first (and only) appearance in the fossil record by a similar time span. Thus Cephalerpeton was a long-lived taxon. Supporting this hypothesis, the appearance of the proximal sister to Gephyrostegus, Silvanerpeton, first appeared in the fossil record alongside Casineria.

The first appearance of Cephalerpeton during the Kasimovian, some 305 million years ago, also followed the first appearances of several other reptilian taxa, including Hylonomus, Paleothyris and Solenodonsaurus. The first appearance of Cephalerpeton also coincided with the first appearances of Haptodus and Archaeothyris. Such timing demonstrates long ghost lineages in which one can expect to find more Cephalerpeton specimens back to the basal Visean, 345 million years ago.

Protorothyris and Limnoscelis Ghost Lineages
Protorothyris, an outgroup taxon to the Synapsida and Protodiapsida, first appeared in the fossil record about 290 million years ago. That succeeded the first appearances of phylogenetic descendants by some 15 million years. So, Protorothyris was also a long-lived taxon with an earlier origin.

Limnoscelis, a basal diadectomorph, succeeded its phylogenetic successor, Solenodonsaurus.

Turtle Ancestry
The earliest know turtles, Odontochelys and Proganochelys, first appeared in the Late Triassic, 225 million years ago. Their phylogenetic predecessor, Stephanospondylus, appeared 290 million years ago. That gives turtles 65 million years (nearly the entire Permian and Triassic) to develop their unique morphologies from their closest known sister taxon at the base of the Permian.

Therapsid Ancestry
Basal therapsids, like Nikkasaurus and Biarmosuchus, first appeared some 250-255 million years ago. Their closest outgroup sisters, Ophiacodon and Archaeothyris, lived some 50 million years earlier.

Heleosaurus and Milleretta Ghost Lineage
Heleosaurus appears in the fossil record approximately 270 mya, but its phylogenetic successors appeared 305 mya, 35 million years earlier. Thus Heleosaurus was a long-lived taxon.

Milleretta appears in the fossil record approximately 255 mya, but its phylogenetic successors, including Bolosaurus, appeared 300 mya, 45 million years earlier. Thus Milleretta was also a long-lived taxon.

Most of the rest of the taxa are chronologically ordered with regard to their phylogenetic order. Sphenodon, the living Tuatara, is a living example of a long-lived taxon.

Morphological Stasis and Rapid Radiation
The chronological tree (Fig. 1) illustrates the twin topics of morphological stasis and rapid radiation. The Tournaisian (Early Carboniferous) was a time of rapid change in our reptilian predecessors. Most of the rest of the Early Carboniferous was a time of stasis with a rapid radiation in the Pennsylvanian, producing all of the major reptilian clades. The Permian is where we find most of the basal reptile fossils. Here we find some basal taxa (presumably earliest Permian) surviving to the end of the Permian, a case of stasis. The Triassic, as I need not remind anyone, was a time of rapid radiation following the Permian extinction event.

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.

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 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.

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.


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

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.

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.

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.

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.