Daptocephalus Assemblage Zone

The "Daptocephalus" Assemblage Zone is a tetrapod assemblage zone or biozone found in the Adelaide Subgroup of the Beaufort Group, a majorly fossiliferous and geologically important geological Group of the Karoo Supergroup in South Africa. This biozone has outcrops located in the upper Teekloof Formation west of 24°E, the majority of the Balfour Formation east of 24°E, and the Normandien Formation in the north. It has numerous localities which are spread out from Colesberg in the Northern Cape, Graaff-Reniet to Mthatha in the Eastern Cape, and from Bloemfontein to Harrismith in the Free State. The "Daptocephalus" Assemblage Zone is one of eight biozones found in the Beaufort Group and is considered Late Permian (Lopingian) in age. Its contact with the overlying "Lystrosaurus" Assemblage Zone marks the Permian-Triassic boundary.
Previously known as the "Dicynodon" Assemblage Zone, the name of the biozone refers to "Daptocephalus", a medium-sized dicynodont therapsid. It is characterized by the presence of this species in co-occurrence with "Theriognathus microps", and "Dicynodon lacerticeps."
History.
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. 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, which he named (from oldest to youngest):
These proposed biozones Seeley named were subdivided further by Robert Broom between 1906 and 1909. Broom proposed the following biozones (from oldest to youngest):
These biozone divisions were approved by paleontologists of the time and were left largely unchanged for several decades. 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. 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". 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. In addition, recent taxonomic studies have shown that "Daptocephalus" is indeed its own species, and is only found within the confines of the biozone. Subsequently it was renamed the "Daptocephalus" Assemblage Zone.
Lithology.
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. 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, 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, 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. The appearance and proliferation of these sandstone deposits marks the change to braided river systems where sands were only deposited with the finer sediments being washed further downstream.
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. These lithological and taphonomic changes are used as evidence for the sudden drying of the climate associated with the Permian-Triassic extinction event.
Paleontology.
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". Various species of burnetiamorph Biarmosuchians and Rubidgenae Gorgonopsians are also found. Therocephalian species such as "Moschorhinus kitchingi" and the earliest cynodont "Charassognathus gracilis" also appear. Parareptile species, namely captorhinids, the Younginiforme "Youngina", and a variety of temnospondyl amphibians, fishes, and plant fossils such as "Glossopteris" are likewise found. 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.
Age and correlations.
The "Daptocephalus" Assemblage Zone dates to approximately 254.5 to 251 Ma, and correlates with the Kutuluksaya and Kulchomovskaya Formations of Russia, and with the lower Buena Vista Formation of the Paraná Basin, South America.