NEWS AND VIEWS

Tiarajudens: a significant mammal-like reptile

Anusuya Chinsamy-Turan

Department of Zoology, University of Cape Town, Cape Town, South Africa

Postal address

Popularly known as the 'mammal-like reptiles', the therapsids are well documented in the fossil record. Their abundance in the Karoo basin has made South Africa world renowned for the diversity of Permian and Triassic therapsids. The earliest known therapsid is Raranimus from China's Middle Permian Dashankou Formation,¹ and besides several poorly known taxa (mainly from Russia) six therapsid lineages are well recognised, although there is still controversy concerning their interrelationships^2,3: the heavily built, large-bodied Dinocephalia; the Biarmosuchia, which retain many similarities to the sphenacodontian pelycosaurs (better known as the sail-backed reptiles); the carnivorous Therocephalia and Gorgonopsia; the herbivorous Anomodontia; and the Cynodontia (to which mammals and their ancestors belong). Tiarajudens sheds light on the evolution of the Anomodontia.

In March 2011, the 260-million-year-old Tiarajudens eccentricus from Brazil was announced to the world in the journal Science by a team consisting of Brazilian and South African scientists⁴ (Figures 1 and 2). Tiarajudens consists of a partial skull with an ~120 mm long, laterally compressed canine, 5 leaf-shaped incisors and 13 expanded palatal teeth that formed a grinding surface. Although the skull is rather fragmentary, Juan Cisneros and his colleagues⁴ were able to recognise its resemblance to Anomocephalus, a basal anomodont recovered in 1999 from the Tapinocephalus assemblage zone of the Beaufort Group of the South African Karoo beds, near Williston in the Northern Cape.⁵ The Anomodontia were the most successful Permo-Triassic herbivorous therapsids, which included amongst them the widely distributed dicynodonts (which characteristically lose their teeth and develop a keratinised beak for processing herbaceous material).

The large transversely expanded palatal teeth in Tiarajudens can be explained as having formed a grinding or occlusal surface with that of the lower jaw (which regrettably, was not found) to process high-fibre plant matter. However, the function of the canines in this herbivore poses an enigma. Considering the presence of canines in extant animals such as the antlerless water deer and the Asian musk deer, Cisneros et al.⁴ propose that perhaps they were used in interspecific interactions (e.g. to ward off predators) or intraspecific interactions such as display and aggression.

Although Tiarajudens has similar cranial proportions to the oldest known anomodont, China's Biseridens,¹ it is more closely allied to Anomocephalus⁵ from South Africa - so much so, that rigorous cladistic analysis by the researchers suggests that there was a Gondwanan radiation of fairly robust, large basal anomodonts, the Anomocephaloidea.⁴ The similarity between Tiarajudens and Anomocephalus suggests that in the Permian period similar basal anomodonts roamed South America and South Africa. There are many extinct animals that are known from both these continents, but, until now, Brazil and South Africa have had only dinocephalians^6,7 and a small filter-feeding, aquatic reptile called Mesosaurus, in common. Although basal anomodonts are known from China, Russia and South Africa, Tiarajudens represents the first basal anomodont from South America and its discovery increases our understanding of the biogeographical distribution of the Anomodontia and the early radiation of the Therapsida.

References

1. Liu J, Rubidge BS, Li J. A new specimen of Biseridens qilianicus indicates its phylogenetic position as the most basal anomodont. Proc R Soc B. 2010;277:285-292.         [ Links ]

2. Kemp TS. The origin and evolution of mammals. Oxford: Oxford University Press; 2005.         [ Links ]

3. Kemp TS. The origin and radiation of Therapsids. In: Chinsamy-Turan A, editor. Forerunners of mammals: Radiation, histology, biology. Bloomington: Indiana University Press, in press.         [ Links ]

4. Cisneros JC, Abdala F, Rubidge BS, Dentzien-Dias PC, Oliveira Bueno de A. Dental occlusion in a 260-million-year-old therapsid with saber canines from the Permian of Brazil. Science. 2011;331:1603-1605.         [ Links ]

5. Modesto SP, Rubidge BS, Welman J. The most basal anomodont therapsid and the primacy of Gondwana in the evolution of the anomodonts. Proc R Soc London Ser B. 1999;266:331-337.         [ Links ]

6. Langer MC. The first record of dinocephalians in South America: Late Permian (Rio do Rasto Formation) of the Parana Basin. Neues Jahrbuch für Geologie und Paläontologie Abhandlungen. 2000;215:69-95.         [ Links ]

7. Smith RS, Rubidge BS, Van der Walt M. Therapsid biodiversity patterns and paleoenvironments of the Karoo Basin, South Africa. In: Chinsamy-Turan A, editor. Forerunners of mammals: Radiation, histology, biology. Bloomington: Indiana University Press, in press.         [ Links ]

Postal address:
Private Bag X3, Rhodes Gift
Rondebosch 7700
South Africa
Email: Anusuya.Chinsamy-Turan@uct.ac.za

© 2011. The Authors. Licensee: OpenJournals Publishing. This work is licensed under the Creative Commons Attribution License.