Saturday, July 27, 2013

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.

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