Sciency Thoughts: Dinocephalian Therapsids from the Middle Permian of the Karoo Basin, South Africa.

Tuesday, 31 March 2015

Dinocephalian Therapsids from the Middle Permian of the Karoo Basin, South Africa.

The Dinocephalians were a group of mostly large, herbivorous Therapsids (the group that also includes Dicnodonts and Mammals) known from the Middle Permian of Russia, Central Asia, China, Brazil, Tanzania, Zambia, Zimbabwe and South Africa, that briefly formed a dominant part of terrestrial faunas before becoming abruptly extinct. Their sudden rise to dominance and equally sudden disappearance has led to them being used to help define the fossil biozones (stratigraphic time periods defined from fossil assemblages – these are generally defined using pollen in later terrestrial settings, but no pollen was present in the Permian) used to date terrestrial sediments from South Africa, and since these South African rocks have a number of volcanic horizons used to obtain isotopic dates for these biozones, they in turn are considered a global reference point for dating events in early Tetrapod evolution. Dinocephalian Therapsids first appear in the Eodicynodon Biozone, then rise to become a diverse and dominant part of the fauna in the Tapinocephalus Biozone, but are absent from the overlying Pristerognathus Biozone; there disappearance is therefore one of the factors used to define the boundary between the Tapinocephalus and Pristerognathus Biozones.

In a paper published in the South African Journal of Science on 27 March 2015, Michael Day, Saniye Güven, Fernando Abdala, Sifelani Jirah and Bruce Rubidge of the Evolutionary Studies Institute at the School of Geosciences at the University of the Witwatersrand and John Almond of Natura Viva in Cape Town describe two new Dinocephalian specimens from the lower Poortjie Member of the Teekloof Formation in the Beaufort West District of Western Cape Province, South Africa.

Both of the new specimens comprise the rear portions of skulls, and both are assigned to the genus Criocephalosaurus, of which two species have previously been described. However since both of these species were described from cranial roofs only, with Criocephalosaurus vanderbyli described from a single weathered cranial roof and Criocephalosaurus gunyankaensis described from four cranial roofs, all of which are currently missing, the new specimens are not assigned to species level. They are notably smaller and more slender than other specimens assigned to the genus, but it is unclear based upon the available material whether they are members of a new species or juveniles of one of the previously described species.

Photos of the specimens (a–d) SAM-PK-K10888 and (e–g) BP/1/7214 showing (a,e) dorsal view, (b,f) left lateral view, (c,g) right lateral view and (d) occipital view. (h) View of sagittal plane on right posterior part of the skull of SAM-PK-K10888, showing the pineal canal orientated parallel to the occipital plane. (i, j) Idealised skull of Criocephalosaurus showing the portions preserved in (i) BP/1/7214 and (j) SAM-PK-K10888. Day et al. (2015).

The boundary between the Tapinocephalus and Pristerognathus Biozones has previously been placed in the upper part of the Abrahamskraal Formation, with the boundary between the Abrahamskraal and Teekloof Formations within the Pristerognathus Biozone. The discovery of specimens of Criocephalosaurus within the lower Poortjie Member of the Teekloof Formation suggests that this scenario is wrong, and that either the boundary between the Abrahamskraal and Teekloof Formations lies within the Tapinocephalus Biozone or Criocephalosaurus extends into the Pristerognathus Biozone. This adds to a growing problem defining the boundary betweem the two biozones, with specimens of the two fossil groups considered diagnostic of the Pristerognathus Biozone, Diictodon and Pristerognathus, having recently been discovered within the Tapinocephalus Biozone, and Day et al. suggest that these biostratigraphical units need reviewing in the near future.

(a) Extension of the Tapinocephalus Assemblage Zone (AZ) into the lower Teekloof Formation. Arrows indicate the extension of Tapinocephalus AZ into the Teekloof Formation. (b) Stratigraphic section measured at Beaufort West between the level of the SAM-PK-K10888 locality and the lower Hoedemaker Member. (c) Stratigraphic section on the farm Putfontein. Day et al. (2015).