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.
See also…
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