Earlier we looked at the Ruta et al. (2013) family tree of the Anomodontia and noted their placement of Galeops (Fig. 1, AMNH 5536) as the transitional taxon linking more basal dromasaurs to more derived dicynodonts. Liu et al (2009) had the same results.
Indeed, the short high face and toothless grin of Galeops does remind one of dicynodonts. But was that by convergence?
Dromasaurs and Dicynodonts.
According to the results of the large reptile tree (Fig. 2), the Anomodontia (dicynodonts + dromasaurs) have ancestors going back to a primitive short-faced therapsid, Stenocybus, a taxon ignored by Ruta et al. (2013).
According to the results of the large reptile tree (Fig. 2) Galeops finds its closest sister in Galechirus, another small dromasaur with tiny teeth, a long tail and was a likely tree-dweller. Their purported sisters, according to Ruta et al. (2013), were dicynodonts like Eodicynodon. Arguing against this, dicynodonts were not tree-dwellers, but had short toes, a short tail and a large body. According to the large reptile tree, dromasaurs were closer to the smaller less tubby ancestors of dicynodonts, not the derived forms, like Eodicynodon.
Yesterday we looked at Microurania, a rarely studied ancestor of dicynodonts and their phylogenetic predecessors. That’s the taxon missing from the Ruta et al. (2013) tree that would probably upset their topology, as it does here (Fig. 2).
No teeth were found with Galeops (Fig. 1), but small root impressions remain in the both jaws, all the same size.
Galeops had a shorter, taller face than other dromasaurs. The jaw “joint” permitted the jaws to slide back and forth relative to each other, a trait dromasaurs shared with dicynodonts.
The clavicles were larger in Galeops than in other dromasaurs studied.
Different than other therapsids?
The current basal therapsid family tree (Fig. 2) indicates the Anomodontia had a different origin than the rest of the Therapsida, including mammals. Add in a few taxa like Stenocybus and Microurania and the traditional topology changes to the heretical one.
So is the Therapsida diphyletic? Perhaps so… another heretical result produced by expanding the taxon list.
Addendum: Giving credit where credit is due, Olson 1962 remarked that therapsids might have had a dual origin, with anomodonts arising from the edaphosaur pelycosaurs.
Is Galeops the sister to the dicynodonts? Apparently no, for the same reason.
Brinkman D 1981. The Structure and Relationships of the Dromasaurs (Reptilia: Therapsida) Breviora 465:34 pp. online here.
Broom, R. 1912. On some New Fossil Reptiles from the Permian and Triassic Beds of South Africa, Proc. zool. Soc. London 1912:859—876. online here.
Liu J, Rubidge B and Li J 2009. A new specimen of Biseridens qilianicus indicates its phylogenetic position as the most basal anomodont. Proceedings of the Royal Society B 277 (1679): 285–292.
Olson EC 1962. Late Permian terrestrial vertebrates, USA and USSR. Transactions of the American Philatelci Society N.S> 25: 1-225.
Ruta M, Angielczyk KD, Fröbisch J and Benton MJ 2013. Decoupling of morphological disparity and taxic diversity during the adaptive radiation of anomodont therapsids. Proceedings of the Royal Society B (Biological Sciences) online here. Supp material here.]