| Cynognathus Assemblage Zone | |
|---|---|
| Stratigraphic range:Early-Mid Triassic ~249–244 Ma | |
 Cynognathus crateronotus | |
| Type | Biozone |
| Unit of | Burgersdorp Formation (Beaufort Group) |
| Underlies | Molteno Formation (Stormberg Group) |
| Overlies | Lystrosaurus Assemblage Zone |
| Thickness | up to 1,968.5 feet (600 m) |
| Location | |
| Region | Eastern Cape,Free State |
| Country |  South Africa |
| Extent | Karoo Basin |
| Type section | |
| Named for | Cynognathus |
| Named by | Harry Govier Seeley (1892) Robert Broom (1906, 1909) |
TheCynognathus Assemblage Zone is atetrapodbiozone utilized in theKaroo Basin ofSouth Africa. It is equivalent to theBurgersdorp Formation, the youngestlithostratigraphicformation in theBeaufort Group, which is part of the fossiliferous and geologically importantKaroo Supergroup. TheCynognathus Assemblage Zone is the youngest of the eight biozones found in the Beaufort Group, and is considered to be lateEarly Triassic (Olenekian) to earlyMiddle Triassic (Anisian) in age (around 247 Ma). The name of the biozone refers toCynognathus crateronotus, a large and carnivorouscynodonttherapsid which occurs throughout the entire biozone.[1]
The firstfossils to be found in theBeaufort Group rocks that encompass the current eightbiozones were discovered byAndrew Geddes Bain in 1856.[2] However, it was not until 1892 that it was observed that the geological strata of theBeaufort Group could be differentiated based on theirfossiltaxa. The initial undertaking was done byHarry Govier Seeley who subdivided theBeaufort Group into threebiozones,[3][4] which he named (from oldest to youngest):
Under Seeley's system, the "highly specialized theriodonts" zone corresponds to the modernCynognathus Assemblage Zone. Seeley's proposed biozones were subdivided further byRobert Broom between 1906 and 1909.[5]Broom proposed the followingbiozones (from oldest to youngest):
Thesebiozone divisions were approved by paleontologists of the time and were left largely unchanged for several decades.[6]James Kitching retained the name of theCynognathus zone in his revision of the biozones in the 1970s and 1980s.[7][8][9] In 1995, he formalized the biozone under the name "Cynognathus Assemblage Zone".[1]
In the mid-1990s, new discoveries of additional outcrops presented the possibility that the Cynognathus Assemblage Zone could be subdivided further. A 1995 paper split it into three subzones: a lower subzone characterized byKestrosaurus, a middle subzone characterized by the "classic" zone fauna which was already well-established, and an upper zone with reduced diversity.[10] This informal three-part subdivision scheme was later labelled with letters (as subzones A, B, and C from oldest to youngest) until it was formalized with robustindex taxa andtype sections in 2020. Subzone A was formalized as theLangbergia-Garjainia Subzone, subzone B as theTrirachodon-Kannemeyeria Subzone, and subzone C as theCricodon-Ufudocyclops Subzone.[1]
From the late 1970s onwards, some authors argued thatCynognathus was less common than previously considered, so they instead renamed the biozone to theKannemeyeria Assemblage Zone orKannemeyeria-Diademodon Assemblage Zone.[11][12] However, later work found thatKannemeyeria fossils were absent from Subzone A, and whileDiademodon was found throughout the biozone it had been usurped as a dominant taxon by largertrirachodontids by Subzone C.[13][14][15] This meant that these species were not ideal index taxa for the biozone. AsCynognathus fossils are found consistently throughout, the current name for the biozone was retained.
TheCynognathus Assemblage Zone correlates with the Burgersdorp Formation in the upper Tarkastad Subgroup of theBeaufort Group.[13] TheCynognathus Assemblage Zone containsargillaceousmudstonesuccessions varying from maroon to reddish, blueish-green, and greyish-green in colour.[16] Themudstones areinterbedded withlenticular andfeldspathicsandstones which appear greenish-grey when fresh and brownish-yellow when eroded out.[17][18]Clay-richpebbleconglomerates are also observed in some areas. Complete, articulatedfossils are frequently found encased incalcareousnodules within themudstone layers while complete skulls are mainly found in thesandstone. Themudstones were likely deposited in low-energy,meanderingfluvial environments alongside sand-richriver channels. The environment during time of deposition wassemi-arid, but with seasonal rain and flooding due to the presence ofcrevasse splays in themudstone layers. A lack of well developed channelsandstones provides evidence for morelacustrine areas in the more northerly occurring outcrops of the biozone.[15]
The thickestoutcrops of theCynognathus Assemblage Zone, reaching approximately 600 metres (2,000 ft), occur betweenQueenstown andLady Frere in theEastern Cape. Outcrops then thin out to between 200 and 100 metres (660 and 330 ft) aroundAliwal North,Burgersdorp,Steynsburg, andRouxville. Thin outcrops are also found in areas in theFree State that borderLesotho.[17][19][20]
TheLangbergia-Garjainia Subzone (Subzone A) is most well-exposed and fossiliferous in northeast Free State, betweenSenekal andBethlehem. At thestratotype near Bethlehem, it reaches around 50 m in thickness. Exposures of this subzone rapidly decrease to the east, disappearing aroundBergville. Although the subzone thickens further south (to up to 100 m) towards Aliwal North, fossils diminish in abundance. Sediments of Subzone A are likely present in the Eastern Cape, though a lack of sufficient fossil material prevents it from being distinguished from younger strata.[1]
TheTrirachodon-Kannemeyeria Subzone (Subzone B) is the thickest and historically the most well-studied portion of the Assemblage Zone. It is primarily exposed in the Eastern Cape south of Lesotho. Subzone B reaches up to 500 m thick at the stratotype between Queenstown and Lady Frere, thinning northwards until it disappears east ofThaba 'Nchu. TheCricodon-Ufudocyclops Subzone (Subzone C) has limited exposures in the vicinity ofMolteno andSterkstroom in the Eastern Cape. It reaches up to 150 m at its stratotype in Sterkstroom.[1]
TheCynognathus Assemblage Zone holds a rich diversity offossilspecies, of which it is most renowned for itscynodont fossils.[13][21]Cynognathids,diademodontids, andtrirachodontids are found throughout the Assemblage Zone. In Subzone B, the diademodontidDiademodon is far more common than trirachodontids. However, by the contact with Subzone C, trirachodontids had become the dominant taxa.Cynognathus is the common denominator, with its fossils found throughout Subzones A – C, confirming its place as the index taxon of the biozone as a whole. The largedicynodontKannemeyeria simocephalus[22] appears in Subzone B alongside other anomodonts, andtherocephalian species can be found throughout the biozone. Apart fromsynapsids, the biozone is rich in otherfossilfauna, includingprocolophonidparareptiles and archosauromorph reptiles. Plant fossils such asDicroidium,Dadoxylon, andSchizoneura have been uncovered from limited areas corresponding to Subzones B - C. Aquatic life is well represented: numerous species oftemnospondylamphibian, fishes, rare occurrences ofmolluscs, andichnofossils ofarthropod trackways andvertebrateburrows have been discovered.[17]
TheLangbergia-Garjainia Subzone (Subzone A) is the oldest subzone in theCynognathus Assemblage Zone. Its base is defined by the first appearance ofCynognathus crateronotus, as well as the trirachodontid cynodontLangbergia modisei and theerythrosuchidarchosauriformGarjainia madiba. The temnospondyl amphibiansKestrosaurus andParotosuchus haughtoni are also common and distinctive fossils of theLangbergia-Garjainia Subzone. Dicynodonts are absent, unlike every other zone and subzone in the Beaufort Group.[1]
TheTrirachodon-Kannemeyeria Subzone (Subzone B) corresponds to traditional conceptions of theCynognathus Assemblage Zone. Alongside abundant fossils ofCynognathus crateronotus, the base of Subzone B sees the first appearance of fellow cynodontsDiademodon tetragonus andTrirachodon berryi. Dicynodonts reappear withKannemeyeria simocephalus, whileXenotosuchus africanus is the most common temnospondyl in this subzone. TheTrirachodon-Kannemeyeria Subzone also has the highest diversity of fish, plants, andarchosauromorph reptiles in the entire Assemblage Zone. The erythrosuchidErythrosuchus africanus is a common component of the fauna,[23] living alongside its smaller relativeEuparkeria capensis[24] and several species of earlyrhynchosaurs (Howesia browni,[25]Eohyosaurus wolvaardti,Mesosuchus browni).[1]
TheCricodon-Ufudocyclops Subzone (Subzone C) is the youngest subzone, and has the most restricted exposures and fossil content. Its base is defined by the first appearance of the trirachodontidCricodon metabolus and the dicynodontUfudocyclops mukanelai, the only species which are common in this subzone.Cynognathus andDiademodon still persist, though they are far more rare than in the previous subzone. Other species with utility for correlation include the temnospondylParacyclotosaurus morganorum and the dicynodontShansiodon sp., which co-occur in the lower part of the subzone.[1]
Color key
| Notes Uncertain or tentative taxa are insmall text; |
| Temnospondyls of the Burgersdorp Formation | ||||
|---|---|---|---|---|
| Genus / Taxon | Species | Subzone | Notes | Image |
| Bathignathus | B. poikilops | Langbergia-Garjainia Subzone (A) | Abrachyopid | |
| Batrachosuchus | B. browni | Trirachodon-Kannemeyeria Subzone (B) | A brachyopid |  |
| Jammerbergia | J. formops | Trirachodon-Kannemeyeria Subzone (B) | Amastodonsaurid |  |
| Kestrosaurus | K. dreyeri | Langbergia-Garjainia Subzone (A) | A mastodonsaurid | |
| K. kitchingi | Langbergia-Garjainia Subzone (A) | A mastodonsaurid | ||
| Laidleria | L. gracilis | Trirachodon-Kannemeyeria Subzone (B) | Alaidleriid |  |
| Microposaurus | M. casei | Trirachodon-Kannemeyeria Subzone (B) | Atrematosaurid |  |
| Paracyclotosaurus | P. morganorum | Cricodon-Ufudocyclops Subzone (C) | A mastodonsaurid |  |
| Parotosuchus | P. haughtoni | Langbergia-Garjainia Subzone (A) | A mastodonsaurid |  |
| Trematosuchus | T. sobeyi | Langbergia-Garjainia Subzone (A) | A trematosaurid |  |
| Vanastega | V. plurimidens | Trirachodon-Kannemeyeria Subzone (B) | A brachyopid | |
| Xenotosuchus | X. africanus | Trirachodon-Kannemeyeria Subzone (B) | A mastodonsaurid |  |
| Cynodonts of the Burgersdorp Formation | ||||
|---|---|---|---|---|
| Genus | Species | Subzone | Notes | Image |
| Bolotridon | B. frerensis | Trirachodon-Kannemeyeria Subzone (B) | Anepicynodontiancynodont |  |
| Cricodon | C. kannemeyeri | Trirachodon-Kannemeyeria Subzone (B) | Atrirachodontid cynodont |  |
| C. metabolus | Cricodon-Ufudocyclops Subzone (C) | A trirachodontid cynodont | ||
| Cynognathus | C. crateronotus | Langbergia-Garjainia Subzone (A),Trirachodon-Kannemeyeria Subzone (B),Cricodon-Ufudocyclops Subzone (C) | Abasalcynognathian cynodont | |
| Diademodon | D. tetragonus | Trirachodon-Kannemeyeria Subzone (B),Cricodon-Ufudocyclops Subzone (C) | Adiademodontid cynodont |  |
| Guttigomphus | G. avilionis | Cricodon-Ufudocyclops Subzone (C)[26] | A trirachodontid cynodont | |
| Impidens | I. hancoxi | Cricodon-Ufudocyclops Subzone (C) | A trirachodontid cynodont |  |
| Langbergia | L. modisei | Langbergia-Garjainia Subzone (A) | A trirachodontid cynodont |  |
| Lumkuia | L. fuzzi | Trirachodon-Kannemeyeria Subzone (B) | Aeucynodontian cynodont |  |
| Dicynodonts of the Burgersdorp Formation | ||||
|---|---|---|---|---|
| Genus | Species | Subzone | Notes | Image |
| Kannemeyeria | K. simocephalus | Trirachodon-Kannemeyeria Subzone (B) | Akannemeyeriiformdicynodont |  |
| Kombuisia | K. frerensis | Trirachodon-Kannemeyeria Subzone (B) | Akingoriid dicynodont |  |
| Shansiodon | S. sp. | Cricodon-Ufudocyclops Subzone (C) | Ashansiodontid dicynodont |  |
| Ufudocyclops | U. mukanelai | Cricodon-Ufudocyclops Subzone (C) | Astahleckeriid dicynodont |  |
| Therocephalians of the Burgersdorp Formation | ||||
|---|---|---|---|---|
| Genus | Species | Subzone | Notes | Image |
| Bauria | B. cynops | Trirachodon-Kannemeyeria Subzone (B) | Abauriidtherocephalian |  |
| Microgomphodon | M. oligocynus | Langbergia-Garjainia Subzone (A),Trirachodon-Kannemeyeria Subzone (B) | A bauriid therocephalian | _skull%2c_from_Abel%2c_O._(1919)._Die_St%25C3%25A4mme_der_Wirbeltiere.jpg) |
| Reptiles of the Burgersdorp Formation | ||||
|---|---|---|---|---|
| Genus / Taxon | Species | Subzone | Notes | Image |
| Eohyosaurus | E. wolvaardti | Trirachodon-Kannemeyeria Subzone (B) | Abasalrhynchosaur | |
| Erythrosuchus | E. africanus | Trirachodon-Kannemeyeria Subzone (B) | Anerythrosuchidarchosauriform |  |
| Euparkeria | E. capensis | Trirachodon-Kannemeyeria Subzone (B) | Aeuparkeriid archosauriform |  |
| Garjainia | G. madiba | Langbergia-Garjainia Subzone (A) | An erythrosuchid archosauriform |  |
| Howesia | H. browni | Trirachodon-Kannemeyeria Subzone (B) | A basal rhynchosaur |  |
| Mesosuchus | M. browni | Trirachodon-Kannemeyeria Subzone (B) | A basal rhynchosaur |  |
| Myocephalus | M. crassidens | Trirachodon-Kannemeyeria Subzone (B) | Aprocolophonidparareptile | |
| Palacrodon | P. browni | Langbergia-Garjainia Subzone (A),Trirachodon-Kannemeyeria Subzone (B) | An indeterminatediapsid | |
| Procolophonidae | indet. | Langbergia-Garjainia Subzone (A) | Indeterminate procolophonid parareptiles | |
| Teratophon | T. spinigenis | Trirachodon-Kannemeyeria Subzone (B) | Aprocolophonine procolophonid parareptile |  |
| Theledectes | T. perforates | Trirachodon-Kannemeyeria Subzone (B) | Atheledectine procolophonid parareptile | |
| Thelephon | T. contritus | Trirachodon-Kannemeyeria Subzone (B) | A procolophonine procolophonid parareptile | |
| Thelerpeton | T. oppressus | Trirachodon-Kannemeyeria Subzone (B) | A procolophonine procolophonid parareptile | |
Based on tentative biostratigraphic correlations, theCynognathus Assemblage Zone is considered to have been emplaced from approximately 249 to 244 Ma, in the later part of the Early Triassic and early part of the Middle Triassic. Based on the presence ofGarjainia andParotosuchus, theLangbergia-Garjainia Subzone is correlated with theYarenskian Gorizont ("Parotosuchus fauna") ofRussia. The "Parotosuchus fauna", exemplified by thePetropavlovskya Svita (a local lithological unit), is assigned to the lateOlenekian Stage (latest Early Triassic, about 249-247 Ma) based on correlation with nearby marine fauna.[1]
The classic faunal assemblage of theTrirachodon-Kannemeyeria Subzone is correlated with a wide variety of geological formations.[27] Similar cynodont and dicynodont species are known from the LowerOmingonde Formation ofNamibia,[28][29] the LowerNtawere Formation ofZambia,[30] and the lower Lifua Member of theTanzanianManda Beds.[31] These correlations can be extended beyond Africa, as far as theFremouw Formation ofAntarctica,Donguz Formation ("Eryosuchus fauna") of Russia, and LowerErmaying Formation of China.[1]Cynognathus andDiademodon fossils have even been found in theRío Seco de la Quebrada Formation ofMendoza Province,Argentina.[32] These formations are often considered early Anisian in age (earliest Middle Triassic, about 247 Ma).[1] However, there is some debate over their age; oneash bed below the Rio Seco de la Quebrada Formation wasradiometrically dated to around 236 Ma (early Carnian), much younger than previously suggested purely based on tetrapod biostratigraphy.[33] The classicCynognathus Assemblage Zone has been equated with theNonesianLand Vertebrate Faunachron, part of a heavily-debated global system of Triassic tetrapod biostratigraphy.[34]
TheCricodon-Ufudocyclops Subzone may be correlated with the upper parts of the Omingonde, Ntawere, and Manda Formations in Africa.[30]Paracyclotosaurus is also known from theYerrapalli Formation and UpperDenwa Formation ofIndia,[35] whileShansiodon is found in the Upper Ermaying Formation ("Sinokannemeyeria fauna") of China.[1]Shansiodon defines the base of thePerovkanLand Vertebrate Faunachron, which has been applied in a global context.[34] These formations may be late Anisian in age, a suggestion supported by radiometric dating which positions the Upper Ermaying Formation at around 244 Ma.[36]
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