Charassognathus gracilis, Botha & Abdala & Smith, 2007, Botha & Abdala & Smith, 2007

Botha, J., Abdala, F. & Smith, R., 2007, The oldest cynodont: new clues on the origin and early diversification of the Cynodontia, Zoological Journal of the Linnean Society 149 (3), pp. 477-492 : 478-480

CHARASSOGNATHUS GRACILIS GEN. ET SP. NOV.

Diagnosis: This new cynodont is distinguished by the presence of a small notch in the base of the coronoid process, located in a similar position to the base of the masseteric fossa in Dvinia and Procynosuchus . The angle of the dentary is prominent, appearing more developed than in Procynosuchus and Dvinia , but less so than in Nanictosaurus . The dental formula is?4·1· 8/3·1·?, and it is distinguished from Procynosuchus and Dvinia by the absence of maxillary precanine teeth. The anterior postcanines have a single high cusp in labial view, whereas the posterior postcanines exhibit a main cusp directed slightly backward, and tiny anterior and posterior accessory cusps, with the anterior accessory cusps positioned slightly higher than the posterior accessory cusps.

Comments: The combined presence of the following characters indicates that C. gracilis is a cynodont: multicuspidated postcanines; groove in the squamosal for the quadratojugal; two occipital condyles; elongated ascending process of the epipterygoid; frontal excluded from the orbital margin; prominent angle of the dentary; reflected laminar of the angular with small lateral crests and femoral head set off dorsally from the shaft (see Description for more details).

Holotype: SAM-PK-K10369 consists of a complete skull with lower jaw in occlusion, the axis, third cervical vertebra,?cervical ribs and a few indeterminate bones. Associated material includes a right articulated femur, tibia and fibula.

Etymology: Greek, charasso, prefix meaning ‘notch’ and gnathus, ‘jaw’, referring to the characteristic notch on the dentary. The specific epithet, gracilis, Latin , meaning ‘slender’, refers to the delicate nature of the specimen.

Locality and horizon: The specimen was found in a roadside cutting of highway R 353 in Teekloof Pass, between the towns Leeu Gamka and Fraserburg in the Beaufort West District, Western Cape Province, South Africa. The locality lies on the farm Willowdene, a portion of Beato 238 ( Fig. 1 ); exact coordinates are given in the Iziko South African Museum. It is stratigraphically positioned in the upper half of a thick mudrockdominated succession of fluvio-lacustrine strata, known locally as the Hoedemaker Member of the Teekloof Formation ( Rubidge, 1995; Figs 1A , 2A ), which is considered to be early Late Permian (Wuchiapingian) in age ( Catuneanu et al., 2005). The few laterally extensive sandstone bodies in the Hoedemaker Member ( Fig. 2B ) are interpreted as having been deposited as amalgamated point bars within high-sinuosity Mississippi-sized meandering rivers ( Smith, 1987). Most of the vertebrate fossils are found in the thick greenish-grey massively bedded siltstone with minor mudstone intercalations that occur between the main channel sandstones. These sediments occur in 5– 10-m-thick coarsening-upward sequences interpreted as prograding crevasse splay sequences. They were laid down by repeated overbank flood events emanating from the channel banks and ponding in the lowland flood basins. The new cynodont specimen was recovered from a 0.25-m-thick lens of fissile, dark purple, mudstone on a parting between two 1-m-thick beds of massive grey siltstone with purple mottles. The massive siltstones contain scattered oblate pedogenic carbonate nodules and are interpreted as proximal floodplain deposits laid down at the base of the meanderbelt slope. The fissile mudstone interlayer in which the study specimen was found is interpreted as a small pond deposit within the splay sequence.

Numerous skulls and skeletons of Diictodon galeops , along with other dicynodonts, namely Oudenodon baini , Emydops minor and Pristerodon mackayi , and, most significantly, the biozone indicator fossil Tropidostoma microtrema were collected from nearby outcrops (see Fig. 1B and Table 1). The gorgonopsians Lycaenops ornatus and Cyonosaurus longiceps , the therocephalian Ictidosuchoides longiceps and a new burnetiamorph comprise the carnivore fossil fauna collected in the vicinity. The presence of Tropidostoma as well as the abundant Diictodon galeops , and the absences of Pristerognathus , which characterizes the underlying zone, and Cistecephalus and Aulacephalodon from the succeeding biozone, biostratigraphically positions this specimen in the upper half of the 180-m-thick Tropidostoma Assemblage Zone ( Rubidge, 1995) .

Figures

Figure 1. A, map of South Africa showing the extent of the Hoedemaker Member in the Beaufort Group; B, detailed map showing fossiliferous localities of the Tropidostoma Assemblage Zone, including the locality where Charassognathus gracilis gen. et sp. nov., SAM-PK-K10369, was found (star, locality 16; for additional information on material found in these localities see Table 1).

Figure 2. A, chronostratigraphic, lithostratigraphic and biostratigraphic chart of the Permian section of the Beaufort Group; B, sedimentological section of the locality showing the occurrence of Charassognathus gracilis gen. et sp. nov., SAM-PK-K10369 (arrow). Abbreviations: AZ, Assemblage Zone; BIO., biostratigraphy; CHRONO., chronostratigraphy; Chsn, Changhsingian; Ciste., Cistecephalus; FM., Formation; LITHO., lithostratigraphy; MEM., Member; Steenkamp., Steenkampsberg; Tapino., Tapinocephalus; Tropid., Tropidostoma.