A new cynodont from the base of the Cynognathus Assemblage Zone (Lower Triassic) of the Karoo Basin: wrong teeth or wrong skull

ABDALA, F.; NEVELING, J.; RUBIDGE, B.S.

Mendoza

Conferencia; Gondwana 12; 2005

The Beaufort Group of the South African Karoo Basin documents a series of continental faunas ranging from Middle Permian to Middle Triassic (Rubidge, 2005). The younger faunas of this group are the Lower Triassic Lystrosaurus Assemblage Zone (AZ) and the Lower to Middle Triassic Cynognathus AZ. An informal subdivision is currently held for the latter assemblage zone, with the subzone A representing an Olenekian interval and subzone B and C being interpreted as early and late Anisian respectively (Hancox, 2000; Neveling, 2004). Nonmammaliaform cynodonts are well represented in the South African Karoo where a minimum of 18 different taxa have been recognised, most of them recorded in the Beaufort Group (Abdala, 2004; Abdala et al., 2005). Four cynodonts are represented in the Lystrosaurus AZ, while seven taxa, the highest diversity of cynodont in any Karoo fauna, are known from the Cynognathus AZ. Notably, there are no shared cynodont taxa between the Lystrosaurus and Cynognathus AZs, and in addition, the latter show the first record of gomphodont cynodonts (i.e., cynodont featuring bucco-lingually expanded postcanines; Hopson, 1984). This group, one of the most diverse of nonmammaliaform cynodonts, is first represented in the Karoo Basin by trirachodontids from the Subzone A (Neveling, 2004). We present here a new cynodont from the Subzone A of the Cynognathus AZ with similar morphology in the temporal region than trirachodontid cynodonts but with sectorial postcanines. The estimated skull length of the new taxon is 92 mm, being similar in size to that of most specimens of trirachodontids from the Subzone A and B of the Cynognathus AZ. As in trirachodontids, it shows a quadrangular temporal opening, limited by the zygoma which is subparallel to the longitudinal axis of the skull. In addition, the morphology of the zygomatic arch and the descendant process of the jugal are also similar to that of trirachodontids. Eight postcanines are present on the left side of the specimen, with the third postcanine showing a partially preserved main cusp and anterior and posterior accessory cusps. This tooth was freed from the skull and it shows no indication of any lingual cingulum. The fifth postcanine depict a recurved main cusp and a posterior accessory cusp. The latter tooth resembles the pattern of Galesaurus, a cynodont from the Lystrosaurus AZ. The presence of sectorial postcanines in gomphodont cynodonts is not a novelty. Diademodon has posterior sectorial postcanines during the entire life of the animal (Hopson, 1971); while in some traversodontids the last postcanines in juveniles are sectorial, and replaced by gomphodonts teeth in adults (Goñi, 1986; Sues and Olsen, 1990). Many trirachodontid specimens show posterior sectorial postcanines, but their correlation with young age is not entirely probed. The innovation in the new species here described is the mixture of trirachodontid skull morphology with an entirely sectorial postcanine tooth row with a pattern more similar to that of galesaurids. The record of new cynodonts, particularly from the Cynognathus AZ of the Karoo Basin reflects a steady increase in the diversity of this group in the Late Olenekian and Early Anisian, a period of time in which an explosive diversification of morphologies occurs in Gondwanan cynodonts. Abdala, F., 2004, Abundance and diversity of non-mammaliaform cynodonts in the South African Karoo: Geosciences Africa 2004, University of the Witwatersrand, Johannesburgh. Abstracts Volume 1, p. 1. Abdala F., Hancox P.J., and Neveling, J., 2005, Cynodonts from the uppermost Burgersdorp Formation, South Africa, and their bearing on the biostratigraphy and correlation of the Triassic Cynognathus Assemblage Zone: Journal of Vertebrate Paleontology, v. 25, p. 192-199. Goñi, R.G., 1986, Reemplazo de dientes postcaninos en Andescynodon mendozensis Bonaparte (Cynodontia, Traversodontidae): Actas del IV Congreso Argentino de Paleontología y Bioestratigrafia, v. 2, p. 7-14. Hancox, P.J., 2000, The Continental Triassic of South Africa. Zentralblatt für Geologie und Paläontologie, Teil I, Heft 11-12 1998, p. 1285-1324. Hopson, J.A., 1971, Postcanine replacement in the gomphodont cynodont Diademodon: Zoological Journal of the Linnean Society, v. 50, p. 1-21. Hopson, J.A., 1984, Late Triassic traversodont cynodonts from Nova Scotia and southern Africa: Palaeontologia Africana, v. 25, p. 181-201. Neveling, J., 2004, Stratigraphic and sedimentological investigation of the contact between the Lystrosaurus and Cynognathus Assemblage Zones (Beaufort Group: Karoo Supergroup): Bulletin of the Council for Geosciences, South Africa, v. 137, 165 p. Rubidge, B.S., 2005, Re-uniting lost continents- Fossils reptiles from the ancient Karoo and their wanderlust: South African Journal of Geology, v. 108, p. 135-172. Sues, H.-D., and Olsen, P.E., 1990: Triassic vertebrates of Gondwanan aspect from the Richmond Basin of Virginia: Science, v. 249, p. 1020-1023