April 13, 2013 | 31
Continuing with the theme of the previous article on trematosauroid temnospondyls, I thought I may as well publish another randomly chosen chunk of my grand, super-long temnospondyl review. This time we look at the trimerorhachids. While there are diverse and often conflicting opinions on the phylogenetic affinities of the many temnospondyl lineages, it’s generally agreed that trimerorhachids are part of Dvinosauroidea (a clade easily recognized by their inflated, enlarged proximal end of the humerus, vaulted palate and other characters); in turn, dvinosauroids are generally (but not universally: see Ruta et al. 2007) agreed to be part of Limnarchia (Yates & Warren 2000, McHugh 2012), the great temnospondyl clade, best known for its Permian and Mesozoic representatives, that – as a generalization – can be imagined as including the mostly aquatic and amphibious forms with poorly ossified limbs.
A more inclusive group recognized by some authors for dvinosauroids and supposed relatives – termed Dvinosauria by Yates & Warren (2000) – was argued to be polyphyletic by McHugh (2012).
Anyway, trimerorhachids were dvinosauroids of the Carboniferous and Early Permian of North America, characterized by a distinctive palatal anatomy and an unusual vertical ilium where the dorsal part of the bone is strongly flared in the parasagittal plane. The best known member of the group is Trimerorhachis (of which two species are currently recognised; T. insignis and T. sandovalensis); other taxa include Lafonius, Nannospondylus, Doragnathus and Neldasaurus [photo of Trimerorhachis above by Ghegoghedo; swimming Neldasaurus image by Smokybjb]. Reaching about 1 m in total length, they were blunt-snouted and with closely set, dorsally facing orbits and a long skull table which, at the level of the occiput, was broader than the snout. Small and very small teeth line the jaw edges while several sets of substantially larger fangs are located around the edges of the palate; a row of small, conical teeth also decorate the edge of the palate in between the interpterygoid vacuities (remember, a key peculiarity of temnospondyls) and the lateral edges of the upper jaw. Branchial elements preserved in-situ at the back of the skull show that trimerorhachids were perennibranchiate [UPDATE: see comments below for clarification on this], just like dvinosaurids, and thus certainly fully aquatic. Their skulls are also covered by lateral line sulci.
Trimerorhachid bodies were long and their limbs small, and articulated specimens show that they were covered in multiple overlapping layers (between 3 and 20, and perhaps more) of small scales (Colbert 1955, Chase 1965, Olson 1979). Olson noted that these scales would have added considerably to the weight of the animal, and he estimated that, in a specimen weighing 5 kg, the scales alone would have added another 0.8-0.85 kg. Presumably this increased weight would have made these animals more negatively buoyant than they already were, and thus perhaps it was an adaptation to assist the animal in staying under water. A defensive function was also considered for the scales, and Olson (1979) also noted that it seems unlikely that the scales would have made the animal stiff-bodied.
One particularly bizarre discovery makes trimerorhachids really interesting: one specimen of Trimerorhachis, today held at the University of California, preserves a cluster of tiny Trimerorhachis bones in the same approximate area as its gills. The bones seem to belong to baby animals that would have been 12-15 cm long.
Olson (1979) considered two possible explanations for their presence; (1) that they were cannibalized, and had fortuitously found their way into the gill region from the mouth, throat or elsewhere; or (2) that Trimerorhachis was a pharyngeal brooder – an animal that held its babies inside special gill pouches. Regarding the latter idea, Olson had been inspired by the internal brooding of fry practiced by some fishes, and by the retention of some frog larvae within dermal pouches and throat sacs. It’s a fascinating, speculative area, and to my knowledge no further information has been published on it. As is so often the case, we need more and better fossils before we can say any more!
Sooo many other temnospondyl groups to cover, I might carry on like this for a while. For previous Tet Zoo articles on temnospondyls, see…
Refs – -
Chase, J. N. 1965. Neldasaurus wrightae, a new rhachitomous labyrinthodont from the Texas Lower Permian. Bulletin of the Museum of Comparative Zoology 133, 153-225.
Colbert, E. H. 1955. Scales in the Permian amphibian Trimerorhachis. American Museum Novitates 1740, 1-17.
McHugh, J. B. 2012. Temnospondyl ontogeny and phylogeny, a window into terrestrial ecosystems during the Permian-Triassic mass extinction. University of Iowa, dissertation (available at http://ir.uiowa.edu/etd/2942).
Olson, E. C. 1979. Aspects of the biology of Trimerorhachis (Amphibia, Temnospondyli). Journal of Paleontology 53, 1-17.
Ruta, M., Pisani, D., Lloyd, G. T. & Benton, M. J. 2007. A supertree of Temnospondyli: cladogenetic patterns in the most species-rich group of early tetrapods. Proceedings of the Royal Society of London B 274, 3087-3095.
Yates, A. M. & Warren, A. A. 2000. The phylogeny of the “higher” temnospondyls (Vertebrata: Choanata) and its implications for the monophyly and origins of the Stereospondyli. Zoological Journal of the Linnean Society 128, 77-121.