Cynognathus Assemblage Zone

The "Cynognathus" Assemblage Zone is a tetrapod assemblage zone or biozone which correlates to the Burgersdorp Formation of the Beaufort Group, a fossiliferous and geologically important geological Group of the Karoo Supergroup in the Karoo Basin of South Africa. The thickest outcrops of this biozone, reaching approximately , occur between Queenstown and Lady Frere in the Eastern Cape. Outcrops then thin out to between around Aliwal North, Burgersdorp, Steynsburg, and Rouxville. Thin outcrops are also found in areas in the Free State that border Lesotho. The "Cynognathus" Assemblage Zone is the eighth and youngest of the eight biozones found in the Beaufort Group, and is considered to be early Middle Triassic (around 247 Ma).
The name of the biozone refers to "Cynognathus crateronotus", a large and carnivorous cynodont therapsid. The biozone is characterized by the first appearance of "Cynognathus", "Diademondon tetragonus", and "Trirachodon".
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 "Cynognathus" Assemblage Zone was first named by Harry Govier Seeley in 1892 when he made the first attempt to organise the geological sequences of the Karoo Basin. In 1906, Robert Broom reorganised the Karoo geological sequences into six biozones. James Kitching kept the name of the "Cynognathus z"one in his revision of the biozones in the 1970s and 1980s.
From the late 1970s onwards, arguments were made to change the name of the biozone either to the "Kannemeyeria" Assemblage Zone or "Kannemeyeria-Diademodon" Assemblage Zone.
However, more recent analyses found that "Kannemeyeria" fossils were absent from Subzone A, and while "Diademodon" was found throughout the biozone it had been usurped as a dominant taxon by larger triracodontids by Subzone C. This meant that these species were not good index taxa for the biozone. As "Cynognathus" fossils are found consistently throughout, the current name for the biozone was retained.
Lithology.
The "Cynognathus" Assemblage Zone correlates with the Burgersdorp Formation in the upper Tarkastad Subgroup of the Beaufort Group. In 2013, it was subdivided into three separate Subzones - Subzones A, B, and C - with Subzone B being the largest of the three. The biozone encompasses the boundary between the late Early Triassic from the bottom of Subzone A and early Middle Triassic through to the uppermost Subzone C.The "Cynognathus" Assemblage Zone contains argillaceous mudstone successions varying from maroon to reddish, blueish-green, and greyish-green in colour. The mudstones are interbedded with lenticular and feldspathic sandstones which appear greenish-grey when fresh and brownish-yellow when eroded out.
Clay-rich pebble conglomerates are also observed in some areas. Complete, articulated fossils are frequently found encased in calcareous nodules within the mudstone layers while complete skulls are mainly found in the sandstone. The mudstones were likely deposited in low-energy, meandering fluvial environments alongside sand-rich river channels. The environment during time of deposition was semi-arid, but with seasonal rain and flooding due to the presence of crevasse splays in the mudstone layers. A lack of well developed channel sandstones provides evidence for more lacustrine areas in the more northerly occurring outcrops of the biozone.
Paleontology.
The "Cynognathus" Assemblage Zone holds a rich diversity of fossil species, of which it is most renowned for its cynodont fossils. Cynognathids, diademodontids, and trirachodontids are found throughout. In Subzone B the diademodontid "Diademodon" dominated over trirachodontids, however, by the Subzone C contact trirachodontids had become the dominant taxa. "Cynognathus" is the common denominator with its fossils being found throughout Subzones A – C, confirming its place as the index taxon of the biozone.
The biozone is likewise rich in other fossil fauna which include procolophonid parareptiles, the crocopod archosauromorph Rhynchosaur "Howesia browni", and archosauriformes "Erythrosuchus africanus" and "Euparkeria capensis.". In addition, the dicynodont species "Kannemeyeria" is found from Subzones B – C, anomodonts, and several theracephalian species are found throughout. Plant fossils such as "Dicroidium", "Dadoxylon", and "Schizoneura" are likewise found. Finally, numerous temnospondyl amphibian species, fishes, rare occurrences of molluscs, and ichnofossils of arthropod trackways and vertebrate burrows have been discovered.
Many of the cynodont species that are found in the "Cynognathus" Assemblage Zone have been found in formations in different countries which correlate in age. Most notably "Diademodon" fossils have been found in the Middle Triassic-aged Río Seco de la Quebrada Formation in Mendoza Province, Argentina. Likewise kannemeyerid and trirachodontid species found in the "Cynognathus" Assemblage Zone have also been found in Triassic-aged Omingonde Formation of Namibia and in the Manda Formation of Tanzania. It is currently accepted that Subzone C of the "Cynognathus" Assemblage Zone correlates in age to the Manda Formation as well as to the upper Denwa Formation in India.
Age and correlations.
The "Cynognathus" Assemblage Zone dates to approximately 249 to 244 Ma, and correlates with the Petropavlovskaya, Gostevskaya and Kzylsaiakaya Formations of Russia, and with the Ermaying and Heshanggou Formations of China, the Knocklofty Formation of Australia (lower "Cynognathus" Assemblage Zone), and the Yerrapalli Formation (upper "Cynognathus" Assemblage Zone) of India.