Daptocephalus Assemblage Zone

The first fossils to be found in the Beaufort Group rocks that encompass the current eight biozones were discovered by Andrew Geddes Bain in 1856.[7] However, it was not until 1892 that it was observed that the geological strata of the Beaufort Group could be differentiated based on their fossil taxa. The initial undertaking was done by Harry Govier Seeley who subdivided the Beaufort Group into three biozones,[8] which he named (from oldest to youngest):

• Zone of “Pareiasaurians”
• Zone of “Dicynodonts”
• Zone of “highly specialized group of theriodonts”

These proposed biozones Seeley named were subdivided further by Robert Broom between 1906 and 1909.[9] Broom proposed the following biozones (from oldest to youngest):

• Pareiasaurus beds
• Endothiodon beds
• Kistecephalus beds
• Lystrosaurus beds
• Procolophon beds
• Cynognathus beds

These biozone divisions were approved by paleontologists of the time and were left largely unchanged for several decades.[10] The Daptocephalus Assemblage Zone was first named after the dicynodont Daptocephlaus leoniceps by James Kitching due to the high prevalence, and localized appearance, of this species.[11][12] However, the name of the biozone was changed to the Dicynodon Assemblage Zone as it was argued that Daptocephalus was in fact a juvenile of Dicynodon.[13][14][15] In addition it was thought that Dicynodon was a more suitable taxon for global biostratigraphic correlation of other Karoo-aged basins, namely to those in Zambia, Tanzania, China, and Russia. In recent years, with new paleontological finds and updated logging of geological formations and biostratigraphy of the Karoo Basin, it was found that there has been some taxonomic confusion with Dicynodon fossils.[16] In addition, recent taxonomic studies have shown that Daptocephalus is indeed its own species, and is only found within the confines of the biozone.[3] Subsequently it was renamed the Daptocephalus Assemblage Zone.

The Daptocephalus Assemblage Zone is located only in the upper Balfour formation and lower Normandien formation east of 24°E. These formations all fall within the Adelaide Subgroup of the Beaufort Group, sediments of which were formed in a large retroarc foreland basin in south-western Gondwana.[5] The biozone is subdivided into the lower Daptocephalus-Theriognathus subzone and the upper Lystrosaurus-Moschorhinus subzone. This satisfies the appearance of Lystrosaurus maccaigi below the Lystrosaurus Assemblage Zone boundary,[2] and the disappearance of the lower subzone taxa Theriognathus, Dicynodon, and Procynosuchus delaharpeae.

At the time of sedimentary deposition, the Karoo retroarc foreland system was in an overfilled phase, and purely terrestrial sediments occupied the Karoo Basin at this time. Two fining-upward sequences are observed in the sedimentary stratigraphy of Daptocephalus biozone, bounded by a second-order subaerial unconformity. These sedimentary rocks comprise greenish-grey mudrock with siltstone lenses in the lower to middle sections of the biozone,[1][5][3] which are indicative of a low-energy fluvial environments with meandering rivers. It is in these rocks that fossils are most commonly found. As the biozone reaches its termination, its contact with the overlying Katberg Formation marks a sharp change in the lithology. The sedimentary rocks change to being sandstone-rich with these sandstone bodies commonly appearing within outcrops of brownish-red shale and reddish mudstone.[17] The appearance and proliferation of these sandstone deposits marks the change to braided river systems where sands were only deposited[17] with the finer sediments being washed further downstream.[5]

Fossil abundance and preservation quality does not change across the boundary, however, species abundance does due to the extinction event, and also the preservational style of the fossils. More isolated skulls are found in the Permian, whereas in the earliest Triassic, curled up complete skeletons are more common as are bonebeds.[18][2] These lithological and taphonomic changes are used as evidence for the sudden drying of the climate associated with the Permian-Triassic extinction event.[5][19][20][18]

The lower to middle zones of the Daptocephlaus Assemblage Zone display a great diversity of vertebrate fauna. These include its current type species Daptocephalus and other dicynodonts such as the ubiquitous Diictodon, Dinanomodon, and Pristerodon.[1][6][21] Various species of burnetiamorph Biarmosuchians and Rubidgenae Gorgonopsians are also found.[1][22][23][24] Therocephalian species such as Moschorhinus kitchingi and the earliest cynodont Charassognathus gracilis also appear. Parareptile species, namely captorhinids,[25][26] the Younginiforme Youngina,[27] and a variety of temnospondyl amphibians,[13] fishes, and plant fossils such as Glossopteris are likewise found.[28] The upper zones as the Permian-Triassic boundary approaches, there is a marked drop in species diversity as the Permian-Triassic extinction event began to take its course.[29][30][31]

The Daptocephalus Assemblage Zone dates to approximately 254.5 to 251 Ma, and correlates with the Kutuluksaya and Kulchomovskaya Formations of Russia,[32] and with the lower Buena Vista Formation of the Paraná Basin, South America.[33]

• List of synapsids