Therocephalian Synapsids

Anatomy & Systematics of Therocephalian Therapsids (Tetrapoda: Synapsida)

The non-mammalian ‘therapsids’ are a largely extinct group of tetrapods that dominated terrestrial ecosystems some 270-235 million years ago, and eventually gave rise to mammals during the Mesozoic age of dinosaurs. As such, our mammalian lineage is the sole survivor of the therapsids’ legacy, making their fossils critically important for understanding the origins of mammalian (and, by extension, human) structure and physiology.

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Two specimens of the tiny therocephalian Scaloposaurus from the earliest Triassic of South Africa (Iziko: South African Museum).

Although therocephalian therapsids (Greek for “beast-heads”) were diverse and widespread during the Permian-Triassic transition (ca. 252 million years ago), significant interest in their diversity and systematics has only arisen very recently. Their fossils are most abundant in Permian-Triassic rocks of southern Africa, but they have also been found in East Africa, Eastern Europe, Asia, and even Antarctica where I conducted fieldwork as a graduate student (2010-2011). Importantly, we also know that some therocephalians displayed a mosaic of mammal-like features, including a mammalian hand and foot structure, structural changes in bones associated with the jaw suspension, expansion of the back of the skull and greater ossification of the braincase, a secondary bony palate, and multicusped teeth for chewing. Consequently, a major question in therocephalian research is whether these traits represent the origins of classic “mammalian” skeletal structures, or structures that evolved in parallel with the mammalian lineage given convergent ecologies and a similar developmental genetic toolkit (as in other therapsid subgroups; e.g., dicynodonts, cynognathian cynodonts). Detailed revisions of their anatomy are underway using high-resolution x-ray computed tomography (HRXCT), and allowing greater precision in our systematics and phylogenetic hypotheses that will ultimately help to answer some of these questions. We are also interested in what their bone microstructure may reveal about their ecology, growth patterns, and their phylogenetic relationships (see Paleohistology Projects).

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HRXCT volume of the skull of the Triassic therocephalian Tetracynodon darti. Information from this specimen has revealed the internal structure of the brain case of early therocephalians and the characteristics they shared with that of non-mammals. (Courtesy T. Sigurdsen and UTCT, Austin)

Selected Publications:

Huttenlocker, A. K., and C. A. Sidor. 2016. The first karenitid (Therapsida, Therocephalia) from the upper Permian of Gondwana and the biogeography of Permo-Triassic therocephalians. Journal of Vertebrate Paleontology 36:e1111897. doi:10.1080/02724634.2016.1111897

Huttenlocker, A. K., and F. Abdala. 2015. Revision of the first therocephalian, Theriognathus Owen (Therapsida: Whatsiidae), and implications for cranial ontogeny and allometry in nonmammaliaform eutheriodonts. Journal of Paleontology 89:645–664. doi:10.1017/jpa.2015.32

Huttenlocker, A. K., C. A. Sidor, and K. D. Angielczyk. 2015. A new eutherocephalian from the upper Madumabisa Mudstone Formation (Upper Permian) of Zambia. Journal of Vertebrate Paleontology 35:e969400. doi:10.1080/02724634.2015.969400

Sidor, C. A., R. M. H. Smith, A. K. Huttenlocker, and B. R. Peecook. 2014. New Middle Triassic tetrapods from the upper Fremouw Formation of Antarctica and their depositional setting. Journal of Vertebrate Paleontology 34:793–801. doi:10.1080/02724634.2014.837472

Huttenlocker, A. K., and J. Botha-Brink. 2014. Bone microstructure and the evolution of growth patterns in Permo-Triassic therocephalians (Amniota, Therapsida) of South Africa. PeerJ 2:e325. doi:10.7717/peerj.325

Huttenlocker, A. K. 2014. Body size reductions in nonmammalian eutheriodont therapsids (Synapsida) during the end-Permian mass extinction. PLOS ONE 9:e87553. doi: 10.1371/journal.pone.0087553

Huttenlocker, A. K., and J. Botha-Brink. 2013. Body size and growth patterns in the therocephalian Moschorhinus (Therapsida) before and after the end-Permian extinction in South Africa. Paleobiology 39:253–277. doi:10.1666/12020

Sigurdsen, T., A. K. Huttenlocker, S. P. Modesto, T. Rowe, and R. Damiani. 2012. Reassessment of the morphology and paleobiology of the therocephalian Tetracynodon darti (Therapsida), and the phylogenetic relationships of Baurioidea. Journal of Vertebrate Paleontology 32:1113–1134. doi:10.1080/02724634.2012.688693

Huttenlocker, A. K., & C. A. Sidor. 2012. Taxonomic revision of therocephalians (Therapsida, Theriodontia) from the Lower Triassic of Antarctica. American Museum Novitates no. 3738:1-19.

Huttenlocker, A. K., C. A. Sidor, & R. M. H. Smith. 2011. A new specimen of Promoschorhynchus (Therapsida: Therocephalia: Akidnognathidae) from the lowermost Triassic of South Africa and its implications for therocephalian survival across the Permo-Triassic boundary. Journal of Vertebrate Paleontology 31:405-421.

Huttenlocker, A. K. 2009. An investigation into the cladistic relationships and monophyly of therocephalian therapsids (Amniota: Synapsida). Zoological Journal of the Linnean Society 157:865-891.