Among the many, many groups I have yet to cover on Tet Zoo are stem-group synapsids: Synapsida is the tetrapod clade that includes mammals and all of their relatives, and there is a long tradition of referring to non-mammalian synapsids as ‘mammal-like reptiles’ (other names include protomammals and paramammals). Because synapsids are not part of Reptilia*, referring to them as ‘mammal-like reptiles’ is both technically incorrect and misleading, hence the push to use their proper name.
* Reptilia and Synapsida are sister-taxa within the tetrapod clade Amniota.
The photo here was provided by Matt Wedel and shows the mounted skeleton of the Early Permian non-mammalian synapsid Cotylorhynchus hancocki, as mounted at the Oklahoma Museum of Natural History. Because I haven’t had time to finish any proper articles lately, this provides me with a good excuse to talk briefly about Cotylorhynchus and its relatives. Or, not so briefly, as it turns out…
Cotylorhynchus is the best known representative of the basal synapsid clade Caseidae, a group that is odd for lots of reasons. While caseids are among the most basal of all synapsids, they don’t make their first appearance until relatively late in basal synapsid history: they debut in the Early Permian, whereas four of the six other basal synapsid clades (varanopids, ophiacodontids, edaphosaurids and sphenacodontids) were all present in the Carboniferous*. Caseids must therefore have a long ghost lineage that extends back to the Carboniferous (Reisz et al. 1998), and the very earliest, most primitive members of the group remain unknown.
* All of these basal synapsids were formerly grouped together as Pelycosauria: because this group is paraphyletic, ‘pelycosaur’ is only used informally today. Eupelycosauria is a formal clade name however – it’s the sister-group within Synapsida of Caseasauria [read on].
Caseids almost certainly descended from small carnivorous forms, but experts have disagreed as to whether their closest relatives were the eothyridids or varanopids. Reisz (1980) noted that caseids shared with eothyridids the same sort of tilted anterior margin of the premaxilla (where the dorsal surface of the snout tip extends further rostrally than the mouth), an elongated external nostril, and a shortened face where the maxilla contributed to the margin of the orbit (in other basal synapsids, the lacrimal and jugal keep the orbit and maxilla well apart). Consequently he grouped the two together in a clade, for which the name Caseasauria Williston, 1911 already exists. I think that eothyridids look like good ‘proto-caseids'; unlike caseids, they possessed enlarged caniniform teeth and were presumably predatory [adjacent image is life restoration of Cotylorhynchus].
Indeed, caseids are unusual among basal synapsids in that all known members of the group were herbivorous, as is demonstrated by their proportionally small skulls, spatulate teeth, absence of any sort of caniniform region in the dentition, and by their massive broad bodies* (Sues & Reisz 1998). So far as we can tell, these features were true of Oromycter dolesorum, the oldest and apparently most basal caseid (Reisz 2005). Probably c. 1 m long, it was much smaller than some of the later members of the group: Cotylorhynchus romeri (the presumed ancestor of the slightly larger, geologically younger C. hancocki) from the late Early Permian reached 3.5 m in length and had an estimated weight of 330 kg (Stovall et al. 1966). It was gigantic compared to many of its relatives and contemporaries, and in fact it and C. hancocki were the largest ‘pelycosaurs’ and largest terrestrial vertebrates of their time.
* Some caseids, such as Caseopsis agilis, were slimmer-bodied than the best known forms, but were still otherwise similar.
As you can see from the accompanying picture, Cotylorhynchus had a massive scapulocoracoid, enormous flaring ends on its humeri, stout forearm bones, and broad, robust hands with large claws. Large retractor processes on the ventral surfaces of the unguals show that caseids could flex their claws with a very powerful motion, and the articulatory surfaces of phalanges weren’t perpendicular to the bone’s long axis, but oblique to it, thereby providing a much larger surface area for flexor muscles. These features all suggest that ‘in life the animal did considerable digging for its food supply’ (Stovall et al. 1966, p. 23). I wonder if they constructed burrows. Articulated specimens confirm that the digits had a considerable range of motion: in a specimen kept at the Cleveland Museum of Natural History, the toes are folded right under the rest of the foot, with the claws pointing backwards towards the tail. One odd thing I’d like to know more about concerns the apparent variation that exists in the size of the hand claws of Cotylorhynchus: some specimens seem to have rather longer unguals than others (look at figs. 13 and 14 in Stovall et al. (1966)). Is this genuine variation? If so, it is sexual or ontogenetic or what?
Why is the early history of caseids so poorly known? Olson (1968) proposed that these animals spent the early part of their evolutionary history in areas that were well away from the lowland/deltaic environments that were best incorporated into the fossil record. This hypothesis is supported by the discovery of the new basal caseid Oromycter in an upland depositional setting, and specimens from upland environments at the rich Bromacker site in Germany provide further support for this (Reisz 2005). The inference is that it took caseids a considerable time to change from living in remote upland habitats to the better-sampled, lowland flood-plain environments where most of our Carboniferous and Permian fossils come from.
Despite the fact that they were so archaic within the synapsid family tree, caseids became the most abundant Early and Middle Permian herbivores in North America at least, out-living their herbivorous cousins the sail-backed edaphosaurids. Ennatosaurus from Russia shows that caseids survived to near the end of the Middle Permian to live alongside dinocephalians and other therapsid synapsids [image of Ennatosaurus above borrowed from here].
As always there’s much more to say, but that’ll have to do. This was only meant to be a picture of the day submission, but I suppose I got carried away. I must do more stem-group synapsids in the future.
Refs – –
Olson, E. C. 1968. The family Caseidae. Fieldiana: Geology 17, 223-349.
Reisz, R. R. 1980. The Pelycosauria: a review of phylogenetic relationships. In Panchen, A. L. (ed) The Terrestrial Environment and the Origin of Land Vertebrates. Academic Press (London/NY), pp. 553-592.
– . 2005. Oromycter, a new caseid from the Lower Permian of Oklahoma. Journal of Vertebrate Paleontology 25, 905-910.
– ., Dilkes, D. W. & Berman, D. S. 1998. Anatomy and relationships of Elliotsmithia longiceps Broom, a small synapsid (Eupelycosauria: Varanopseidae) from the Late Permian of South Africa. Journal of Vertebrate Paleontology 18, 602-611.
Stovall, J. W., Price, L. I. & Romer, A. S. 1966. The postcranial skeleton of the giant Permian pelycosaur Cotylorhynchus romeri. Bulletin of the Museum of Comparative Zoology 135, 1-30.
Sues, H.-D. & Reisz, R. R. 1998. Origins and early evolution of herbivory in tetrapods. Trends in Ecology & Evolution 13, 141-145.