Cynognathus 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.[5] 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,[6] 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.[7] 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.[8] 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[9]. In 1906, Robert Broom reorganised the Karoo geological sequences into six biozones.[10] James Kitching kept the name of the Cynognathus zone in his revision of the biozones in the 1970s and 1980s.[11][12][13]

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.[14][15]

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.[16][17][18] 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.

The Cynognathus Assemblage Zone correlates with the Burgersdorp Formation in the upper Tarkastad Subgroup of the Beaufort Group.[19] 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.[20] The mudstones are interbedded with lenticular and feldspathic sandstones which appear greenish-grey when fresh and brownish-yellow when eroded out.[21][22]

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.[23]

The Cynognathus Assemblage Zone holds a rich diversity of fossil species, of which it is most renowned for its cynodont fossils.[24][25] 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,[26] and archosauriformes Erythrosuchus africanus[27] and Euparkeria capensis..[28] In addition, the dicynodont species Kannemeyeria[29] 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.[30]

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.[31] Most notably Diademodon fossils have been found in the Middle Triassic-aged Río Seco de la Quebrada Formation in Mendoza Province, Argentina.[32] 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.[33][34][35] 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.[36]

The Cynognathus Assemblage Zone dates to approximately 249 to 244 Ma, and correlates with the Petropavlovskaya, Gostevskaya and Kzylsaiakaya Formations of Russia,[37] 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.[38]

• Geology of Lesotho
• Geology of Namibia
• Geology of South Africa
• Santa Maria Formation