Early Stages of Diapsid Evolution - Part II (Younginiformes)

In a past blog post, I have explained in detail the rise of the diapsids throughout the Late Carboniferous and Early Permian periods. Unfortunately, we have a rather vague knowledge about what occurred throughout the rest of the Permian because the fossil record of diapsids is very patchy between the Kungurian Stage (when Araeoscelis gracilis last lived) and the latest Permian (when a considerable amount of diapsid fossils reappear at a few localities across the world).

Figure 1: The skull of Lanthanolania ivakhnenko in left lateral view (drawn from Modesto et al., 2003).

Throughout the whole Guadalupian epoch (Mid Permian), the diapsids comprise what palaeontologists describe as a “ghost lineage” - this is because we only know that they were around back then because they are known to have lived before and after the Mid Permian, although no fossils have ever been discovered dating to that time. However, not quite: a partial skull found in the Mezen River basin in the Mezen District of Arkhangel'sk province in Russia has been described as Lanthanolania ivakhnenko, the genus name which translates as “forgotten ripper” (Modesto et al., 2003). Of all the hundreds of amniote fossils found at the Mezen River basin, the only diapsid is Lanthanolania and was initially misidentified as a specimen of Mesenosaurus, a synapsid of the varanopid type. Lanthanolania was a miniature neodiapsid, which may have been no bigger than 30 cm (1 ft) in length as estimated from the tiny 30 mm long skull.

At the time, diapsids must have been very uncommon, existing in the relative shadows of much larger anapsids, synapsids and amphibians which were a lot more successful back during the Permian period. The partial skull of Lanthanolania has been dated to the ICS Wordian stage, or the uppermost Kazanian in eastern Europe, somewhere between approximately 266 and 257 million years ago. Despite the discovery of Lanthanolania inducing a bit of excitement amongst those experts trying to fill the voids in our knowledge, it still didn’t explain much about the evolutionary history of diapsids during the Middle Permian because the remains were so incomplete. One thing we do know for certain is that between the late Early Permian and the Late Permian, the araeoscelids had already gone extinct whilst the neodiapsids managed to survive and somewhat diversify.

By the Late Permian, a mixed assortment of neodiapsids collectively known as eosuchians (meaning ‘early crocodiles’, although this may be misleading as it didn’t include crocodilians at all) had arrived on the scene. However, the Eosuchia is something of a wastebasket taxon and the phylogenetic relationships and taxonomic classification of eosuchians is rather unclear. It was initially assumed that eosuchians ought to be divided into the ancestors of archosaurs (e.g. dinosaurs, birds, pterosaurs, crocodilians) and the ancestors of lepidosaurs (e.g. lizards and snakes), but current thinking is that their is an even more complex evolutionary history than this. One recently proposed cladogram (Reisz et al., 2011) demonstrates a supposedly paraphyletic group with ‘younginiformes’ being the basalmost eosuchians (comprising both aquatic and terrestrial types), followed by the marine Claudiosaurus germaini (originally thought of as an early sauropterygian), then by some representatives of a family of terrestrial lizard-like types known as ‘paliguanids’, and lastly the gliding weigeltisaurids. They all perished by the Early Triassic, leaving no surviving descendants.

Figure 2: The skull of Youngina capensis in left lateral view (drawn from Carroll, 1981).

I shall now be covering the diversity of ‘younginiformes’ in greater detail, commencing with the terrestrial forms. Youngina capensis (Broom, 1914) was a miniscule lizard-like animal, originally described on the basis of a fragmentary skull discovered in the Daptocephalus Assemblage Zone of the Karoo Supergroup in South Africa, dated to the latest Permian at ~ 250 million years old. A particular characteristic present in Youngina is one simple row of osteoderms running midway along the back. Youngina was part of an ecosystem consisting of a diverse array of fauna such as synapsids (e.g. biarmosuchians, gorgonopsians, dicynodonts etc.) and anapsids, in an environment with a semi-arid climate. One amazing find is an assemblage of five intact, articulated juvenile skeletons, suggesting that Youngina may have been living in dens (Smith & Evans, 1996). In the subsequent years following its initial description, a few other specimens of Youngina were later discovered, predominantly skulls, each being given distinct names (such as Youngopsis and Youngoides) all of which are now believed to be synonyms of Youngina.

Figure 3: My reconstruction of Youngina capensis.

The following taxa are also classified as younginids, but with uncertainty: Heleosuchus griesbachi, first described as a species of Saurosternon by Richard Owen in 1876,  is known from just one specimen - an incomplete postcranial skeleton with posterior fragments of the skull. It was first discovered in South Africa from an obscure horizon, being dated to the Early Triassic or Late Permian. The specimen was assumed to have been lost until it was moved to the Natural History Museum of Vienna, Austria (Carrol, 1987).

Galesphyrus capensis (also described by Broom in 1914) is known from an incomplete postcranial skeleton, discovered at the bottom of the Cistecephalus Assemblage Zone of the Karoo Supergroup in South Africa, so Youngina is therefore younger.

Kenyasaurus mariakanensis (Harris & Carroll, 1977) is one species of neodiapsid discovered in the Maji-Ya-Chumvi Formation from Kenya, dated to the Early Triassic. Its phylogenetic relations are difficult to interpret because only one specimen is known, consisting of very incomplete forelimbs and pectoral girdle material as well as a missing skull, but may have been closely related to Lanthanolania. The remains of Kenyasaurus were also discovered in marine beds, but lacks adaptations for an aquatic lifestyle, just like Thadeosaurus. Thus, it was a small, terrestrial lizard-like form measuring roughly 0.5 m long. In addition, Kenyasaurus was recently found to have not been a member of the younginiformes. With the absence of well-preserved skull material, knowing the exact phylogenetic relations of all these taxa has proved to be very challenging.

Figure 4: My reconstruction of Thadeosaurus colcanapi.

Thadeosaurus colcanapi (Carroll, 1981) is a better-known neodiapsid first discovered in the Lower Sakamena Formation of southern Madagascar, known from two partial skeletons that are almost complete, but are lacking the distal (lower) segments of the limbs as well as the skull. Thadeosaurus was initially and mistakenly presumed to belong to Datheosaurus from Europe and in turn, Datheosaurus was believed to be a synonym of Haptodus (a genus of pelycosaur and therefore, a synapsid) but is now considered a basal caseid within the synapsids. Also bear in mind that two other taxa from the Permian, Apsisaurus witteri and Heleosaurus scholtzi, previously thought of as younginiformes, are now believed to have been synapsids of the varanopid family. It appears that a lot of these primitive diapsids from the Permian were misidentified as synapsids at first! One other interesting thing about the genus Thadeosaurus is that it’s merely an anagram of the genus Datheosaurus, the only anagram of a scientific name in an animal that I currently know of.

Figure 5: My reconstruction of the pelycosaurian synapsid Haptodus baylei.

Thadeosaurus was a rather small animal, estimated to be approximately 60 cm in length, and was very lizard-like in appearance, being distinguished by its particularly elongated tail. Several specimens of Thadeosaurus are known, which include juveniles (the only known skull is from a juvenile too), most of them originally being misinterpreted as Tangasaurus mennelli from northeastern Tanzania (Currie & Carrol, 1984). Thadeosaurus shows no clear specialisations for swimming despite being discovered in beds deposited in a marine environment, suggesting that it may have been a terrestrial neodiapsid living along the coast.

 

Figure 6: My reconstruction of Tangasaurus mennelli, a neodiapsid that must have lived an aquatic lifestyle as indicated by its long, powerful flattened tail. 

One intriguing aspect about the Lower Sakamena Formation is that the marine deposits consist of an extraordinarily large quantity of diapsid reptiles in comparison to all the other Late Permian formations elsewhere around the world. The age of the deposits has been determined by the correlation of vertebrate fossils with those from South Africa as well as by palynological analysis, being dated from the Capitanian stage of the late Middle Permian to the Wuchiapingian stage of the early Late Permian. Among the useful index fossils is the procolophonoid parareptile Barasaurus besairiei, similar in appearance to Owenetta rubidgei. However, Owenetta was subsequently found in Early Triassic beds too.

References: 

Broom, R. 1914. A new thecodont reptile. Proceedings of the Zoological Society of London B, 84 (4), 1072–1077.

Carroll, R. L. 1981. Plesiosaur ancestors from the Upper Permian of Madagascar. Philosophical Transactions of the Royal Society of London B, 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. J. 1982. The osteology and relationships of Tangasaurus mennelli Haughton (Reptilia, Eosuchia). Annals of the South African Museum, 86 (8), 247–265.

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

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

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

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

Reisz, R. R., Scott, D. 2002. Owenetta kitchingorum, sp. nov., a small parareptile (Procolophonia: Owenettidae) from the Lower Triassic of South Africa. Journal of Vertebrate Palaeontology, 22 (2), 244-255.

Reisz, R. R., Modesto, S. P., 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. E. 1996. New material of Youngina: evidence of juvenile aggregation in Permian diapsid reptiles. Palaeontology, 39 (2), 289-303.