Middle Permian continental biodiversity changes as reflected in the Beaufort Group of South Africa: a bio-and lithostratigraphic review of the Eodicynodon, Tapinocephalus and Pristerognathus assemblage zones
Date
2014-03-04
Authors
Day, Michael Oliver
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Abstract
The fluvio-lacustrine rocks of the Beaufort Group, South Africa have long been
known for their tetrapod fossil record, which is the richest and most complete
Middle Permian to Middle Triassic record for any terrestrial sequence in the world.
The abundance of fossil material has enabled the Beaufort Group to be
biostratigraphically subdivided into between 8 and 10 tetrapod assemblage zones, of
which the lowest three (Eodicynodon, Tapinocephalus and Pristerognathus) are attributed to
the Middle Permian. These lower assemblage zones record the earliest therapsiddominated
faunas and, because they were recorded during a largely uninterrupted
period of deposition, make the Beaufort Group the only place in the world where
biodiversity change through the terrestrial Middle Permian can be effectively studied.
In the last two decades, much interest has focused on an extinction of marine
invertebrates at or close to the end of the Middle Permian (Guadalupian epoch), but
the existence of a concurrent extinction in the terrestrial realm is contentious. The
Beaufort Group is already well known to record the Permo-Triassic Mass Extinction
but it also records an earlier extinction at the top of the Tapinocephalus Assemblage
Zone (AZ). This extinction is very poorly understood but recent radiometric dates
for many Permian assemblage zones of the Beaufort Group have confirmed a Middle
Permian age for Eodicynodon, Tapinocephalus and Pristerognathus assemblage zones and
suggest that the end-Tapinocephalus AZ extinction may coincide with the marine
extinctions.
A recently produced GIS database that accommodates all Beaufort Group fossil
material curated in South Africa formed the basis on which the stratigraphic range of
individual specimens was calculated. To put the fossil localities in a stratigraphic
context, lithostratigraphic information was retrieved from the literature and extensive
fieldwork was conducted, which measured stratigraphic sections in key areas and
traced the surface outcrop of lithostratigraphic units. In order to compensate for
lateral variations in lithostratigraphy, the basin was split into sectors, each
represented by a stratigraphic section. The stratigraphic ranges of fossil specimens
and, subsequently, of genera and families could then be calculated and a workable
biostratigraphic subdivision of the Middle Permian Beaufort Group proposed.
The Abrahamskraal Formation, which forms the majority of the Middle Permian
Beaufort sequence, can be divided into six lithostratigraphic members based on the
occurrence of sandstone ‘packages’. These members were traced laterally across the
Basin and their correspondence with fining-upwards cycles was refined and
correlated with the newly defined biostratigraphic units. This refined two-pronged
stratigraphic subdivision allowed the recognition of a waning period of subsidence in
the proximal sector of the Karoo Basin during the Middle Permian.
Stratigraphic ranges of individual genera were found to be far more heterogeneous
than previously recognised. Dicynodont genera are useful biostratigraphic indicators
due to their relative abundance and well-defined stratigraphic ranges, while
dinocephalians and pareiasaurs are clustered in the upper part of the Abrahamskraal
Formation. The stratigraphic range of Eodicynodon extends further up in the
Abrahamskraal Formation than was previously recognised. The Tapinocephalus AZ is
restricted to approximately the upper fifth of the Abrahamskraal Formation and is
characterised by advanced tapinocephalid dinocephalians and the pareiasaur
Bradysaurus. Between these two biozones is a stratigraphic interval dubbed the mid-
Abrahamskraal Formation Transition Zone, where both Eodicynodon and advanced
tapinocephalids coexisted.
A 75 % loss of generic diversity occurred between the upper Tapinocephalus AZ and
the base of the Pristerognathus AZ, which corresponds to a stratigraphic interval
between the mid-Karelskraal Member of the Abrahamskraal Formation and the mid-
Poortjie Member of the Teekloof Formation. Several taxa that survive the end-
Tapinocephalus AZ extinction, and are relatively common in the overlying
Pristerognathus AZ (scylacosaurid therocephalians, the dicynodont genus Eosimops and
the parareptile Eunotosaurus), all became extinct in the upper Poortjie Member at a
time when generic originations are increasing. This suggests a second wave of
extinctions in a similar fashion to that recorded at the Permo-Triassic boundary.