First among the synapsids: the ophiacodonts

The synapsids constitute the second major group of reptiles after the diapsids. They are characterized by a single hole in the skull just behind each eye socket called temporal fenestrae. Synapsids include the mammals (thus including us), their reptilian ancestors (“mammal-like reptiles”) and  all their closest extinct relatives . The oldest known synapsid belongs to a group called Ophiacodontids which flourished during the Late Carboniferous and Early Permian periods. These were a rather peculiar lot with their unusually large head compared to their body size. The skull was very tall and narrow. The slightly recurved teeth were sharp, numerous and tightly packed inside the jaws, two being a bit larger than the other forming  the upper canines. These medium size animals were among the largest land carnivores of their time. The limbs were rather short, broad and bulky. 

Archaeothyris florensis

The ancestral ophiacodontid type can be seen in Archaeothyris florensis (Reisz, 1972) from the Late Carboniferous of Nova Scotia, Canada (Westphalian C; 308-311 MYA), which is by the way, the earliest known definite synapsid. It somewhat looked like a modern lizard but with already the group hallmark of a tall and elongated skull. Measuring about 50 cm, it was contemporary to other early reptiles such Palaeothyris and living in a swampy forest made of giant tree-like lycopsids (club mosses) and dominated by amphibians and giant arthropods.

Varanosaurus acutirostris (Broili, 1904) from the Early Permian of Texas, measured about 1.2 m in length and was an agile predator with a slender laterally compressed snout. It is known from decent material including the almost complete holotype skeleton. A second species, V. wichitaensis, has been described, based on isolated postcranial remains (Romer, 1937) and is virtually indistinguishable from V. acutirostris besides being smaller and geologically slightly older.

Varanosaurus acutirostris

Body proportions reached its extremes in Ophiacodon (Marsh, 1878; Cope, 1878; Case, 1907; Romer, 1925; Romer & Price, 1940) the best known and most studied ophiacodont. Many skeletons of this odd-looking animal are known from the Early Permian of Texas, New Mexico, Kansas, Oklahoma, Colorado, Utah and Ohio and it is unclear how many of the described species are actually valid. For the story, the type species, Ophiacodon mirus was first described by Othniel Marsh in 1878 based on a mandible and vertebrae in the midst of the so-called  “bone war” against his rival Edwin Cope. Marsh clearly intended to beat Cope, who had a paper in press, in the naming of this animal but his hastily written paper was so deficient that the name Ophiacodon was ignored by the paleontological community for over 30 years. In the meantime, Cope’s paper published just days after Marsh’s, described three species, Theropleura retroversa, T. uniformis and T. triangulata, based on isolated vertebrae. The type specimen of Ophiacodon was reinvestigated by Williston and a new complete skeleton described in the 1910s (Williston & Case, 1913)  and it is much later that Theropleura was found to be synonymous with Ophiacodon although the species names were retained (Romer & Price, 1940).  The following species are still recognized today: O. mirus, known from several skeletons including a nearly complete one from New Mexico and Oklahoma; O. retroversus, known from multiple materials from Texas and Oklahoma, including a near complete skelton; O. uniformis from several partial skeletons from Texas and Oklahoma; O. navajonicus frong fragmentary postcranial skeletons from New Mexico; O. hilli  known from a fragmentary skeleton from Kansas; O. major from fragmentary materials from Texas.   It appears that size difference might reflect different growth stage rather than species (Brinkman, 1988). Ophiacodon was originally thought to be a semi-aquatic animal but recent studies debunked all the supposed aquatic adaptation that the animal might have possessed and today (Felice & Angielczyk, 2014), Ophiacodon is viewed as a fully terrestrial predator. Specimens of Ophiacodon have sizes ranging from 1.5 m up to 3 m in length.

Ophiacodon mirus

Other ophiacodontids are known from rather fragmentary remains and include the Late Carboniferous genera Clepsydrops and Echinerpeton from North America and Stereorhachis from France, the Early Permian Stereophallodon, Baldwinonus from North America. Protoclepsydrops haplous (Carrol, 1964) from Nova Scotia might also have been an ophiacodontid predating Archaeothyris but the remains are so fragmentary that it is difficult to tell for sure.

Refrerences:

Brinkman, D. (1988). Size-independent criteria for estimating relative age in Ophiacodon and Dimetrodon (Reptilia, Pelycosauria) from the Admiral and lower Belle Plains formations of west-central Texas. Journal of Vertebrate Paleontology, 8(2), 172-180.

F. Broili. 1904. Permische Stegocephalen und Reptilien aus Texas. Palaeontographica 51:1-120

Carroll, Robert L. (1964). "The earliest reptiles". Journal of the Linnean Society of London, Zoology 45 (304): 61–83

E. C. Case. 1907. Revision of the Pelycosauria of North America. Carnegie Institution of Washington 55:3-176

E. D. Cope. 1878. Descriptions of extinct Batrachia and Reptilia from the Permian formations of Texas. Proceedings of the American Philosophical Society 17:505-530

Felice, R. N., & Angielczyk, K. D. (2014). Was Ophiacodon (Synapsida, Eupelycosauria) a swimmer? A test using vertebral dimensions. In Early evolutionary history of the Synapsida (pp. 25-51). Springer Netherlands.

R. R. Reisz. 1972. Pelycosaurian Reptiles from the Middle Pennsylvanian of North America. Bulletin of the Museum of Comparative Zoology 144(2):27-60

A. S. Romer. 1925. An ophiacodont reptile from the Permian of Kansas. Journal of Geology 33(2):173-182

A. S. Romer. 1937. New genera and species of pelycosaurian reptiles. Proceedings of the New England Zoölogical Club 16:89-95

A. S. Romer and L. I. Price. 1940. Review of the Pelycosauria. Geological Society of America Special Paper 28:1-538

Williston, S. W., & Case, E. C. (1913). Description of a nearly complete skeleton of Ophiacodon Marsh. Carnegie Institution of Washington Publication, 181, 37-59.

Meet the Chroniosuchians

Chroniosaurus dongusensis measured about 30 cm in length.

Chroniosuchians form a very obscure group of small, superficially croc-like four-legged animals that lived during the Late Permian period and survived until the Middle and perhaps Late Triassic periods. Most species are from the Late Permian (Late Tatarian regional stage) of Russia, but they are also known from China, Germany and Kyrgyzstan. They were aquatic or semi-aquatic fish-eating predators characterized by a distinct row of interconnected “butterfly-shaped” bony plates or scutes (osteoderms) covering their back and part of their tail, one for each vertebra. They are traditionally subdivided into two families, the more primitive Chroniosuchidae from the Late Permian with one representative in the Triassic and the Bystrowianidae from the Late Permian to the Middle Triassic. The exact affinities of the Chroniosuchians are quite uncertain. They are usually placed among non-amniote  reptiliomorphs but more recent studies make them “stem tetrapods”, so not reptiles and not quite amphibians but something a bit more primitive. The eel-like embolomeres appear to have been their closest relatives.

Madygenerpeton pustulatus from Kyrgyzstan.

Among the Chroniosuchids, Chroniosaurus dongusensis Tverdochlebova, 1972 from the Late Permian of the Orenburg province of Russia is probably the best known species, with fossils from many individuals including one near complete articulated specimen. Chroniosuchus paradoxus Vjuschkov, 1957 also from the Orenburg province but a bit younger, differs in the taller shape of its skull. Madygenerpeton pustulatus Schoch et al., 2010 from the Middle or Late Triassic of Kyrgyzstan appears to be the basalmost member of the group despite its late occurrence. The type specimen is a nearly complete skull showing a broad snout quite different from the two previous species. Jarilinus mirabilis (Vjuschkov, 1957) from the Novgorod province and Uralerpeton  tverdochlebovae Golubev, 1998 from the Vladimir Province were quite large species for the group, the reconstructed skull of the former measuring 20 cm and of the latter some 55 cm. The others three species, Chroniosaurus levis Golubev, 1908, Chroniosuchus lichaveri (Riabinin, 1962) and Suchonica vladimiri Golubev, 1999, all from the Vologda Province of Russia, are only known from fragmentary remains.

Chroniosuchus paradoxus.

The other family, the Bystrowianids, are much less known, their fossil remains consisting mainly on isolated armor scutes. Late permian species include Bystrowiana permira Vjuschkov, 1957 from the Vladimir Province of Russia, Bystrowiana sinica Young, 1979, Dromotectum largum Liu et al., 2014 and Jinyuanitectum flatum Liu et al., 2014, the last three from the Henan Province of China. Early Triassic species are Axitectum vjushkovi Shishkin & Novikov, 1992 from the Novgorod province, Axitectum georgi Novikov & Shishkin, 2000 from the Kirov province, Dromotectum spinosum Novikov & Shishkin, 1996 from the Orenburg Province. Middle Triassic forms are Synesuchus muravjevi Novikov & Shishkin, 2000 from the Komi Republic and Bystrowiella schumanni Witzmann et al., 2008 from Germany.

References:

Buchwitz, M., Foth, C., Kogan, I., & Voigt, S. (2012). On the use of osteoderm features in a phylogenetic approach on the internal relationships of the Chroniosuchia (Tetrapoda: Reptiliomorpha). Palaeontology, 55(3), 623–640.

Golubev, V. K. (1998). Narrow-armored Chroniosuchians from the Late Permian of Eastern Europe. Paleontological Journal, 32(3), 278–287.

Golubev, V. (1998). Revision of the Late Permian chroniosuchians (Amphibia, Anthracosauromorpha) from Eastern Europe. Paleontological Journal, 32(4), 390–401.

Golubev, V. K. (1999). A New Narrow-Armored Chroniosuchian from the Upper Permian of Eastern Europe. Paleontological Journal, 33(2), 166–173.

Klembara, J., Clack, J. a., & Čerňanský, A. (2010). The anatomy of palate of Chroniosaurus dongusensis (Chroniosuchia, Chroniosuchidae) from the Upper Permian of Russia. Palaeontology, 53(5), 1147–1153.

Liu, J., Xu, L., Jia, S.-H., Pu, H.-Y., & Liu, X.-L. (2014). The Jiyuan tetrapod fauna of the Upper Permian of China — 2 . stratigraphy , taxonomical review , and correlation. Vertebrata PalAsiatica, 52(3), 328–339.

Schoch, R., Voigt, S., & Buchwitz, M. (2010). A chroniosuchid from the Triassic of Kyrgyzstan and analysis of chroniosuchian relationships. Zoological Journal of the Linnean Society, 160, 515–530.

Witzmann, F., Schoch, R. R., & Maisch, M. W. (2008). A relict basal tetrapod from Germany: first evidence of a Triassic chroniosuchian outside Russia. Die Naturwissenschaften, 95(1), 67–72.

The Late Permian Diapsids, Part I

The Late Permian Diapsids, Part I

In a previous post, I have covered the emergence of the diapsids during the Late Carboniferous and Early Permian. We don't really know what happened next as the diapsidian fossil record is unfortunately very sketchy between the Kungurian when we left Araeoscelis gracilis and the very end of the Late Permian period when diapsids reappear as fossils in reasonable number at a handful of locations on the planet.

 Fig. 1.- Reconstructed skull of Lanthanolania ivakhnenko (after Modesto et al., 2003).

Through the entire Middle Permian, the diapsids constitute what scientists call a 'ghost lineage': we merely know they were there because their existence is attested before and after that time but no fossil has been found dating from that period. Well, almost... there is one single partial skull discovered in the Mezen River basin in the Arkhangel'sk province of Russia which was named Lanthanolania ivakhnenko (Modesto et al, 2003). This was a small neodiapsid, probably no more than 30 cm long judging from the 3 cm small skull. The fossil is the only diapsid found among hundreds of other amniote specimens in the Mezen river basin and was naturally overlooked, originally labeled as yet another specimen of the synapsid Mesenosaurus (the generic name Lanthanolania means 'forgotten ripper').  Diapsids were obviously a rare thing at that time, living in the shadows of the larger and more successful synapsids, anapsids and enormous amphibians. The specimen dates from the Uppermost Kazanian or the lowermost Tatarian around 260 MYA which translates into anything between the ICS Wordian (~265 MYA) and the ICS Wuchiapingian (~257 MYA) stages. Although the discovery of Lanthanolania created a little sensation among the specialists in filling the gap, it did not say much on the evolution of the group partly due to the quite fragmentary nature of the finds. All we know is that between the Early Permian and the Late Permian, the Araeoscelids were gone forever and that the terrestrial Neodiapsids survived and somehow diversified. 

Fig 2.- Reconstruction of Youngina capensis.

In the Late Permian, a motley crew of forms collectively and formerly called "Eosuchians" (which means “early crocs”) was present. The Eosuchians do not constitute a natural group and their classification and phylogenetic relations are a bit hazy. At first it appeared that the Eosuchians should be divided into the ancestors of the lepidosaurians (lizards, snakes and such) and the ancestors of the archosaurs (crocs, dinosaurs, etc...) but current understanding is that the story is more complicated than this. The most recent cladogram (Reisz et al., 2011) shows  a seemingly paraphyletic "younginiformes" group as the most basal eosuchians, with both terrestrial and aquatic forms, then the aquatic form Claudiosaurus (once thought to be a basal Sauropterygian), followed by some members of a family of terrestrial forms called "paliguanids", and finally the gliding coelurosauravids. None of them left any descendants and disappeared in the Early Triassic. Accompanying these "Eosuchians" in the Late Permian were the first members of the Archosauromorphs: the semi-aquatic protorosaurids.

Fig 3.- Skull of Youngina capensis.

Let's first go over the so-called "younginiformes", starting with the terrestrial ones. Youngina capensis was a small lizard-like creature that was first described from a partial skull (Broom, 1914) found in the Dicynodon assemblage zone of South Africa and dating from the latest Permian period. Youngina lived alongside a rich fauna of synapsids, including dicynodonts, gorgonopsids and biarmosuchians and anapsid reptiles, in what was a semi-arid environment. Since its original description, several fossils of Youngina, mostly skulls, were found, receiving different names (Youngoides, Youngopsis, etc...) that are all now considered to be synonymous to Youngina. One peculiar characteristics of Youngina is its single row of osteoderms on its back. A remarkable discovery consists of a set of 5 juvenile, fully articulated and complete individuals, indicating that those critters most certainly lived in a den (Smith &Evans, 1996).

The next two taxa are only tentatively placed among the younginids. Galesphyrus capensis, also described by Broom in 1914 is from the base of the Cistecephalus assemblage zone of South Africa (thus older than Youngina) and is known from a partial postcranial skeleton. Heleosuchus griesbachi, originally described by Owen (1876) as a species of Saurosternon, is known from a single specimen consisting of the posterior part of the skull and a partial postcranial skeleton. The fossil was thought to be lost until relocated in the Natural History Museum in Vienna, Austria (Carrol, 1987). The specimen is from South Africa from an unknown horizon and would date either from the Late Permian or the Early Triassic. Without good skull material, getting the exact affinities of these two animals is very difficult.

Fig 4.- Reconstruction of Thadeosaurus colcanapi.

From the Lower Sakamena formation of southern Madagascar originates Thadeosaurus colcanapi (Carroll, 1981) which is based on two nearly complete skeletons missing the skull and the lower parts of the limbs. This one was originally thought to belong to the European genus Datheosaurus, which is now known to be synonymous with the pelycosaur Haptodus (thus a Synapsid … it seems many of these early diapsids were first mistaken with a synapsid). As an intended pun, the name Thadeosaurus is simply an anagram of the name Datheosaurus, the only deliberate anagram of an animal scientific name that I am aware of. Thadeosaurus is known from several specimens, including juveniles, many being at first confused with the Tanzanian Tangasaurus (Currie & Carrol, 1984). It was a small lizard like creature measuring perhaps about 60 cm in length and characterized by a very long tail. Although found in marine strata, Thadeosaurus has no obvious adaptation for swimming indicating it was most probably a coastal fully terrestrial animal. The interesting thing about the Lower Sakamena formation is that this aquatic deposit contains an unusually large proportion of diapsid reptiles as compared to any other upper Permian deposits in the world. This is quite anomalous and the exact age of the deposits can be questioned. The age is based on palynological (fossil pollen) evidence and correlation of the vertebrate fauna with South Africa. One index fauna is the procolophonid Barasaurus which is similar to the South African Owenetta. But the latter was later also found in Early Triassic strata.

Let us conclude this tour of the terrestrial younginiformes with Kenyasaurus mariakanensis from the Early Triassic Maji-Ya-Chumvi formation of Kenya (Harris & Carroll, 1977). It is known from a single specimen lacking the skull and most of the pectoral girdle and forelimb, making its affinities hard to establish. This one was recently kicked out of the younginiformes. Like Thadeosaurus, it was found in marine strata but does not appear to have any specialization for aquatic environment. Therefore, it was probably a terrestrial form which measured about 50 cm in length.

Note also that two animals of that time, Heleosaurus and Apsisaurus, once considered being younginiformes turn out to be Synapsids of the varanopid sort.

Next, the marine “younginiformes”…

References:

Carroll, R. (1981). Plesiosaur ancestors from the Upper Permian of Madagascar. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, 293(1066), 315-383.

Carroll, R. (1987). Heleosuchus: An enigmatic diapsid reptile from the Late Permian or Early Triassic of southern Africa. Canadian Journal of Earth Sciences, 24, 664-667.

Currie, P., & Carroll, R. (1984). Ontogenetic changes in the eosuchian reptile Thadeosaurus. Journal of Vertebrate Paleontology, 4(1), 68–84.

Harris, J., & Carroll, R. (1977). Kenyasaurus, a new eosuchian reptile from the Early Triassic of Kenya. Journal of Paleontology, 51(1), 139-149.

Modesto, S., & Reisz, R. (2003). An enigmatic new diapsid reptile from the Upper Permian of Eastern Europe. Journal of Vertebrate Paleontology, 22(4), 851-855.

Olson, E. (1936). Notes on the skull of Youngina capensis Broom. The Journal of Geology, 44(4), 523-533.

Reisz, R. R., Modesto, S. P.  and Scott, D. M. (2011). A new Early Permian reptile and its significance in early diapsid evolution. Proceedings of the Royal Society B 278, 3731-3737

Smith, R., & Evans, S. (1996). New material of Youngina: evidence of juvenile aggregation in Permian diapsid reptiles. Palaeontology, 39(2), 289-303.

Humble beginnings for the mighty diapsids: the Araeoscelids and Orovenator

The amniotes (those initially four-legged creatures that produce "amniotic eggs", i.e. eggs adapted for land life) are traditionally divided into a few branches depending on some key characteristics of their skull, and the diapsids appear to be the most successful of all these branches. Virtually all living vertebrates that we commonly name "reptiles" are  diapsids: crocs, lizards, snakes, the whole lot of them... Diapsids also include the birds by way of their forebears, the dinosaurs. In contrast, mammals and their ancestors belong to the synapsid branch of the amniotes. The distinction between diapsid and synapsid lays in the number of holes (scientist called those "fenestrae" which means windows) in the skull just behind the eye socket. Diapsids typically have two, the supratemporal (or upper temporal) fenestra on top and the infratemporal (or lower temporal) fenestra below. Synapsids only have one,  the bottom infratemporal fenestra, simply called temporal fenestra. Both diapsids and synapsids evolved from more primitive reptiles with no opening behind the eye: the anapsids. The distinction between the different type of skulls are depicted in Figure 1. 

Fig 1.- Different type of skulls among reptiles. Top left: anapsid skull of Procolophon trigonoceps (after Romer, 1956) top right: diapsid skull of Petrolacosaurus kansensis (after Reisz, 1981), bottom: synapsid skull of Eothyris parkeri (after Reisz et al., 2009).

The distinction seems simple enough but only reflects the primitive initial condition. Evolution indeed loves playing tricks. The same way that we know that dolphins are really mammals and not some strange air-breathing fish, scientists figured that everybody's favorite marine mesozoic  reptiles, the plesiosaurs and ichthyosaurs, are really diapsids in disguise. Their skulls only have one opening behind the eye socket, the supratemporal fenestra, a condition which is called 'euryapsid'. The euryapsids used to be considered as a fourth branch of reptiles, but carefull examinations of the fossils show they evolved from diapsid ancestors and that the loss of the infratemporal fenestra is only a secondary characteristics. More tricky are the turtles. These do not have any opening behind the eye which classified them as "anapsids". However, nowadays, it seems quite firmly established that they too are highly modified diapsid reptiles, although it took scientists quite a while to figure this one out. Now look a the highly modified skulls of birds and snakes: you will have hard time recognizing any of the original temporal openings in them, but since their ancestors were diapsid reptiles, by way of basic phylogeny law (you shall belong to the same clade as your ancestors, ... i.e the monophyly principle), they too are diapsid reptiles. 

That being said, the diapsids then really are the most successful group of amniotes with some 18,000 species, including birds, alive today (compared to the 5700 species of mammals representing the only survivors of the synapsid branch). But what is their origin? They probably evolved during the Late Pennsylvanian from a group of anapsid ancestors to which such lizard-like creatures as Paleothyris and Hylonomus belong to.  But this is not at all very clear because of the scarcity of the fossil record. The first true diapsids are a family of small superficially lizard-like creatures called Araeoscelidia, to which Petrolacosaurus and Araeoscelis are the best known representatives. The fossil record of the Araeoscelids extends from the Late Carboniferous to the Early Permian. 

Fig 2.- a reconstruction of Petrolacosaurus kansensis.

The most ancient known diapsid is Petrolacosaurus kansensis from the Late Pennsylvanian (the North American 'Missourian' stage which corresponds to the ICS Kasimovian stage, ~305 MYA) of Kansas. The generic name means "rock lake reptile" in reference to the "Rock Lake Shale" in which the type specimen (a nearly complete hind limb) was found. Although originally described in 1945 as a pelycosaur (thus a synapsid), it was not until 1977 after new specimens including the skull with its two characteristic openings, were thoroughly described, that it was realized to be the earliest known diapsid, raising the little critter from relative obscurity to paleontological stardom. In a world dominated by giant arthropods and fearful amphibians,  Petrolacosaurus was indeed relatively small, measuring probably about 70 to 80 cm in length, accounting for the long tail.

Fig 3.- Reconstruction of Spinoaequalis schultzei.

Also from the Late Pennsylvanian of Kansas but a bit later (from the Calhouns Shale formation dated to the North American 'Virgilian' stage roughly corresponding to the ICS Gzhelian, ~ 300 MYA), comes Spinoaequalis schultzei. This early diapsid is a bit smaller (30 cm) and is only tentatively placed among the araeoascelids. The interesting note about this critter is the tail:  the tall and equal size neural and haemal spines of the caudal vertebra (thus the generic name) which gives the distinct tall and laterally compressed shape to the tail is viewed as an adaptation for swimming, a good indication that Spinoaequalis was an aquatic animal. This is supported by the fact that its fossil was discovered in freshwater deposits among remains of spiny sharks (acanthodians) and other fully aquatic animals. However, the long and slender limbs, are those of a terrestrial animal, an indication that it wasn't fully aquatic. In any case,  this makes Spinoaequalis the first amniote to have ever returned  to water since their epic conquest of the dry lands.

Fig 4.- Skulls of Araeoscelis gracilis (after Reisz et al., 1984).
 
Araeoscelis dates from the Early Permian and was originally described in 1910 as a lizard. It has the same body plan than Petrolacosaurus but the skull was more massive with strong teeth, ideal for crushing the heavy exoskeleton protecting some of the arthropods of that time. Araeoscelis was about the same size too, with an estimated length of 70-80 cm accounting for the unknown tail. As a probable adaptation of its specialized diet, the lower temporal fenestra has closed making the skull more robust. Araeoscelis, has therefore this 'euryapsid' condition that will be common to the large marine predators of the Mesozoic.  Two species have been  described, A. gracilis from the Arroyo Formation of Texas (Kungurian age, ~275 MYA), known from several fairly complete specimens, and A. casei from the Admiral Formation of Texas (Artinskian age, ~285 MYA), known from at least seven individuals. The two are virtually indistinguishable and the separation into distinct species seems only to be justified by their difference in age, A. casei being from slightly (~ 10 millions years) older rocks than A. gracilis. 

Fig 5.- Reconstruction of Araeoscelis gracilis.

The other Araeoscelids are poorly known and all date from the Early Permian. The 70 cm long Dictybolos tener from the Wellington formation of Oklahoma (~280 MYA) is known from isolated bones from numerous individuals. This one was presumably semi-aquatic and a fish eater. Zarcasaurus tanyderus from the Cutler Formation of New Mexico is known from a partial disarticulated skeleton. Characterized by its rather long neck vertebrae, it was a close relative of Araeoscelis. In Europe, the rather dubious Aphelosaurus lutevensis from the Tuilières formation of South Central France and first described in the 19th century,  is another possible Araeoscelid, though it is hard to tell without any knowledge of a crucial piece of fossil information: the skull. Similarly, Kadaliosaurus priscus from the Rotliegend of Germany is only known from a postcranial skeleton and its classification among the araeoscelids is only tentative.

Fig 6.- Reconstructed skull of Orovenator mayorum (after Reisz et al., 2011).

Besides the Araeoscelids, there is one additional diapsid dating from the Early Permian, Orovenator mayorum discovered in one of the fissure fills of the Richards Spur locality of Oklahoma, and known from two partial crushed skulls. Orovenator has the distinction of being the earliest known and most primitive of the Neodiapsids, a clade that contains all the known diapsids except for the araeaoscelids. This was a small animal with an elongated skull half the length of those of Petrolacosaurus and Araeoscelis. The Richards Spur locality with its distinct Early Permian fauna of some 30 taxa of fully terrestrial vertebrates, is thought to originate from an upland ecosystem, which would only fossilized in very exceptional cases. This is in sharp contrast to the Araeoscelids which were all found in lowland swampy habitats, with better chances for fossilization. The hypothesis is therefore that the initial split of the early diapsids into the Araeoscelids and the Neodiapsids is the result of adaptation to two different habitats, with the Araeoscelids in the lowlands and the Neodiapsids in the uplands.

In conclusion, the early diapsids show a surprising degree of diversity with some forms that became at least partially aquatic (Spinoaequalis, Dictybolos) while others adapted to the harsher conditions of the uplands (Orovenator).  There is also evidence of a quite specialized diet for some (Araeoscelis). However, the remains of these animals are quite rare and there is a rather long gap in the fossil record before we see them appear again in the Late Permian. Their number would not significantly increase before the Early Triassic. It is somewhat tempting to imagine that like the mammals of the Mesozoic dominated by the dinosaurs, the early diapsids lived in the relative shadow of the other reptiles for some 50 million years,  when the world was dominated by  larger synapsids, anapsids and  amphibians and that it will take the massive Permian-Triassic extinction event to see the diapsids finally take the upper hand.


References:

Brinkman, D., Berman, D., & Eberth, D. (1984). A new araeoscelid reptile, Zarcasaurus tanyderus from the Cutler Formation (Lower Permian) of north-central New Mexico. New Mexico Geology, 34–39. 

Debraga, M., & Reisz, R. (1995). A new diapsid reptile from the uppermost carboniferous (Stephanian) of Kansas. Palaeontology, 38(1), 199–212. 

Lane, H. (1945). New mid-Pennsylvanian reptiles from Kansas. Transactions of the Kansas Academy of Science (1903-), 47(3), 381–390. 

Olson, E. (1970). New and little known genera and species of vertebrates from the Lower Permian of Oklahoma. Fieldiana: Geology, 18(3), 359–434. 

Reisz, R. R. (1977). Petrolacosaurus, the oldest known diapsid reptile. Science, 196(4294), 1091–3. 

Reisz, R., Berman, D., & Scott, D. (1984). The anatomy and relationships of the Lower Permian reptile Araeoscelis. Journal of Vertebrate Paleontology, 4(1), 57–61. 

Reisz, R. R., Modesto, S. P., & Scott, D. M. (2011). A new Early Permian reptile and its significance in early diapsid evolution. Proceedings. Biological sciences / The Royal Society, 278(1725), 3731–7. 

Williston, S. (1910). New Permian reptiles: rhachitomous vertebrae. The Journal of Geology, 18(7), 585–600. 

Williston, S. (1913). The skulls of Araeoscelis and Casea, Permian reptiles. The Journal of Geology, 21(8), 743–747.

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