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 The Karoo Supergroup is the most widespread stratigraphic unit in Africa south of the Kalahari Desert. The supergroup consists of a sequence of units, mostly of nonmarine origin, deposited between the Late Carboniferous and Early Jurassic, a peri ...

South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring coun ...

.Rubidge, B. S. (1995). Biostratigraphy of the Beaufort Group (Karoo Supergroup). ''Biostratigraphic series''. 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 Mthatha , formerly Umtata, is the main city of the King Sabata Dalindyebo Local Municipality in Eastern Cape province of South Africa and the capital of OR Tambo District Municipality. The city has an airport, previously known as the K. D. Matan ...

in the Eastern Cape, and from Bloemfontein to

Harrismith Harrismith is a large town in the Free State province of South Africa. It was named for Sir Harry Smith, a 19th-century British governor and high commissioner of the Cape Colony. It is situated by the Wilge River, alongside the N3 highway, a ...

in the Free State. The ''Daptocephalus'' Assemblage Zone is one of eight biozones found in the Beaufort Group and is considered Late

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleo ...

(

Lopingian The Lopingian is the uppermost series/last epoch of the Permian. It is the last epoch of the Paleozoic. The Lopingian was preceded by the Guadalupian and followed by the Early Triassic. The Lopingian is often synonymous with the informal terms l ...

) 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 ''Daptocephalus'' is an extinct genus of non-mammalian synapsid anomodont dicynodont, it which was found in Late Permian strata, in a biozone known precisely for the presence of fossils of this dicynodont, the ''Daptocephalus'' Zone, in the Karoo ...

'', a medium-sized

dicynodont Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivorous animals with a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, typic ...

therapsid. It is characterized by the presence of this species in co-occurrence with '' Theriognathus microps'', and '' Dicynodon lacerticeps.''

History

The first

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

s 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

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): * Zone of " Pareiasaurians" * Zone of "

Dicynodont Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivorous animals with a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, typic ...

s" * Zone of "highly specialized group of theriodonts" These proposed biozones Seeley named were subdivided further by

Robert Broom Robert Broom FRS FRSE (30 November 1866 6 April 1951) was a British- South African doctor and palaeontologist. He qualified as a medical practitioner in 1895 and received his DSc in 1905 from the University of Glasgow. From 1903 to 1910, he ...

between 1906 and 1909. 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. The ''Daptocephalus'' Assemblage Zone was first named after the

'' 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 ''

'' was in fact a juvenile of ''

Dicynodon ''Dicynodon'' ("two dog-teeth") is a genus of dicynodont therapsid that flourished during the Upper Permian period. Like all dicynodonts, it was herbivorous animal. This reptile was toothless, except for prominent tusks, hence the name. It probab ...

''. In addition it was thought that ''

'' was a more suitable taxon for global biostratigraphic correlation of other Karoo-aged basins, namely to those in Zambia,

Tanzania Tanzania (; ), officially the United Republic of Tanzania ( sw, Jamhuri ya Muungano wa Tanzania), is a country in East Africa within the African Great Lakes region. It borders Uganda to the north; Kenya to the northeast; Comoro Islands ...

China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...

, and

Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-ei ...

. In recent years, with new paleontological finds and updated logging of geological formations and biostratigraphy of the

Karoo Basin The Karoo Supergroup is the most widespread stratigraphic unit in Africa south of the Kalahari Desert. The supergroup consists of a sequence of units, mostly of nonmarine origin, deposited between the Late Carboniferous and Early Jurassic, a perio ...

, it was found that there has been some taxonomic confusion with ''

'' fossils. In addition, recent taxonomic studies have shown that ''

'' 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 The back-arc region is the area behind a volcanic arc. In island volcanic arcs, it consists of back-arc basins of oceanic crust with abyssal depths, which may be separated by remnant arcs, similar to island arcs. In continental arcs, the back-ar ...

foreland basin in south-western Gondwana. The biozone is subdivided into the lower ''

Theriognathus ''Theriognathus'' (from Greek therion: beast, mammal, Greek, gnathos, “jaw,” +us, pronounced THEH-ree-OG-nah-thuss) is an extinct genus of therocephalian therapsid belonging to the family Whaitsiidae, known from fossils from South Africa, Za ...

'' subzone and the upper '' Lystrosaurus-

Moschorhinus ''Moschorhinus'' is an extinct genus of therocephalian in the family Akidnognathidae with only one species: ''M. kitchingi''. It was a carnivorous synapsid which has been found in the Late Permian to Early Triassic of the South African Karoo Sup ...

'' subzone. This satisfies the appearance of '' Lystrosaurus maccaigi'' below the ''Lystrosaurus'' Assemblage Zone boundary, and the disappearance of the lower subzone taxa ''

'', ''

'', and '' Procynosuchus delaharpeae''. At the time of sedimentary deposition, the Karoo

foreland system was in an overfilled phase, and purely terrestrial sediments occupied the

at this time. Two fining-upward sequences are observed in the sedimentary stratigraphy of ''

'' biozone, bounded by a second-order subaerial

unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

. These

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

s 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

s 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 ''

'' and other

s 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 ''Moschorhinus'' is an extinct genus of therocephalian in the family Akidnognathidae with only one species: ''M. kitchingi''. It was a carnivorous synapsid which has been found in the Late Permian to Early Triassic of the South African Karoo Sup ...

'' and the earliest cynodont '' Charassognathus gracilis'' also appear. Parareptile species, namely

captorhinids Captorhinidae (also known as cotylosaurs) is an extinct family of tetrapods, traditionally considered primitive reptiles, known from the late Carboniferous to the Late Permian. They had a cosmopolitan distribution across Pangea. Description Cap ...

, the Younginiforme '' Youngina'', and a variety of

temnospondyl Temnospondyli (from Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carb ...

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.Ezcurra, 2016, p.8

References

Bibliography

* {{cite journal , last=Ezcurra , first=Martín D , year=2016 , title=The phylogenetic relationships of basal archosauromorphs, with an emphasis on the systematics of proterosuchian archosauriforms , url=https://peerj.com/articles/1778.pdf , journal=

PeerJ ''PeerJ'' is an open access peer-reviewed scientific mega journal covering research in the biological and medical sciences. It is published by a company of the same name that was co-founded by CEO Jason Hoyt (formerly at Mendeley) and publisher ...