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The Tropidostoma Assemblage Zone is a tetrapod assemblage zone or biozone which correlates to the lower Teekloof Formation, Adelaide Subgroup of the Beaufort Group, a fossiliferous and geologically important geological Group of the Karoo Supergroup in South Africa.[1] The thickest outcrops, reaching approximately 240 metres (790 ft), occur from east of Sutherland through to Beaufort West and Victoria West, to areas south of Graaff-Reinet. Its northernmost exposures occur west/north-west of Colesberg. The Tropidostoma Assemblage Zone is the fourth biozone of the Beaufort Group.[2][3][4]

Tropidostoma Assemblage Zone
Stratigraphic range: Late Permian
Tropidostoma.png
Tropidostoma microtrema
TypeBiozone
Unit ofLower Teekloof Formation, Adelaide Subgroup of the Beaufort Group
UnderliesCistecephalus Assemblage Zone
OverliesPristerognathus Assemblage Zone
Thicknessup to 787.402 feet (240 m)
Location
RegionNorthern, Western & Eastern Cape
Country South Africa
Type section
Named forTropidostoma microtrema
Named byKeyster & Smith
Year defined1978-79

The name of the biozone refers to Tropidostoma microtrema, a herbivorous dicynodont therapsid. This biozone is characterized by the presence of this species in association with another dicynodont species, Endothiodon uniseries.

Contents

HistoryEdit

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):

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):

The current stratigraphic range of the Tropidostoma Assemblage Zone was previously included within the Endothiodon beds ascribed by Broom. Decades later between the 1970s and early 1990s, Keyser and Smith [8][9][10][11] collaborated on several field trips into known outcrop areas in order to re-evaluate the fossil biostratigraphy. After relevant fossil taxa data had been collected, the biozone was renamed the Tropidostoma Assemblage Zone due to the discovery that Tropidostoma fossils were only found within the confines of a certain area.

LithologyEdit

The Tropidostoma Assemblage Zone correlates with the lower Teekloof Formation, Adelaide Subgroup of the Beaufort Group. This biozone is considered to be early Late Permian in age. The rock composition of this biozone is dominated by mudstones and siltstones, ranging from greenish-grey, grey, and dark reddish brown in colour.[12] The mudstones and siltstones, measuring approximately 3 m thick in most outcrops and exposures, frequently contain calcareous or micritic nodule horizons and rhizocretions. Sheet-type single-storied channel sandstones are also found, the lower portion of the biozone often containing fining upward sequences. Calcium carbonate precipitation crusts and gypsumdesert rose” crystals have been uncovered in the sandstone layers.[13][14]

The preserved rocks show that the depositional environment of this biozone was formed in a lacustrine and overbank environment, which flowed northwards in long, meandering rivers. This low energy environment is conducive for preferential deposition of finer sediment types which formed the mudstone and siltstone deposits. However, the climate was seasonally dry due to the presence of calcium carbonate precipitation in the sandstones. Coarser clastic material was only deposited during seasonal floods. The meandering rivers flowed from a foreland basin that was being formed from the rising of the Gondwanide mountains in the south. The Gondwanides were the result of crustal uplift that had previously begun to take course due to tectonic activity. The pressure of the growing Gondwanides mountain chain caused the formation of the Karoo Basin where the deposits of the Tropidostoma Assemblage zone, and all other succeeding assemblage zone deposits, were deposited over tens of millions of years.[15][16]

PaleontologyEdit

Vertebrate fossils of the Tropidostoma Assemblage Zone are predominantly found in the thick mudstone and siltstone sequences.[17] Fossils are often encrusted within the calcareous or micritic nodules. The biozone is characterized by the presence of Tropidostoma in association with Endothiodon. However, the occurrence of these taxa are fairly rare in this biozone and their presence is characterized together with the more numerous fossils of the dicynodont species Pristerodon and Diictodon. Diictodon fossils are particularly ubiquitous in this biozone with isolated skull and post-cranial material being most commonly found. In the lower deposits, complete fossil skeletons of Diictodon are more commonly found, often in pairs and curled up together.[18] These intriguing paired Diictodon fossils are invariably recovered from the bottom of helical burrow casts, which have been attributed to Diictodon. The helical burrows - a generally accepted feature of the Tropidostoma Assemblage Zone - are vertical, spiralling tubes measuring just under 1 m in length. The burrows are found infilled with either fine sandstone or siltstone with calcareous concretions. The burrows were likely infilled during single flood events, suddenly trapping and burying the Diictodon pairs inside alive.[19] This provides an explanation for the near perfect preservation of the Diictodon pairs found in the burrow casts.

Other notable fossil material from the Tropidostoma Assemblage Zone includes the first appearance of larger gorgonopsid species such as Lycaenops ornatus, Cyonosaurus longiceps, and Gorgonops torvus.[20] It is noted that gorgonopsians take over from the more basal therocephalians as the top predators from this biozone through to the end of the Permian period. However, smaller therocephalian species such as Ictidosuchoides longiceps remained present.[21] The cynodont species Procynosuchus delaharpeae has been recovered from the upper sections of this biozone.[22] Dicynodont species diversified greatly in this biozone, a speciation trend that continued with this group until the end of the Permian period.[23][24][25][26] A pareiasaur species, Pareiasaurus serridens,[27] various subspecies of the temnospondyl amphibian Rhinesuchus,[28][29][30] the fishes Namaichthys and Atherstonia, and fossil plant material of Glossopteris and Dadoxylon have been found. Finally, vertebrate burrows left by Diictodon as previously discussed, vertebrate trackways left by various therapsid species, and coprolites[31] have been recovered.

ReferencesEdit

  1. ^ Rubidge, B. S. (ed.) 1995b. Biostratigraphy of the Beaufort Group (Karoo Supergroup). South African Committee of Stratigraphy. Biostratigraphic Series 1. Pretoria, Council for Geoscience.
  2. ^ Hancox, P.J; Rubidge, B.S (2001-01-01). "Breakthroughs in the biodiversity, biogeography, biostratigraphy, and basin analysis of the Beaufort group". Journal of African Earth Sciences. 33 (3–4): 563–577. doi:10.1016/S0899-5362(01)00081-1. ISSN 1464-343X.
  3. ^ Rubidge, Bruce S.; Day, Michael O.; Barbolini, Natasha; Hancox, P. John; Choiniere, Jonah N.; Bamford, Marion K.; Viglietti, Pia A.; McPhee, Blair W.; Jirah, Sifelani (2016), "Advances in Nonmarine Karoo Biostratigraphy: Significance for Understanding Basin Development", Origin and Evolution of the Cape Mountains and Karoo Basin, Springer International Publishing, pp. 141–149, doi:10.1007/978-3-319-40859-0_14, ISBN 9783319408583
  4. ^ van der Walt, Merrill; Day, Michael; Rubidge, Bruce; Cooper, Antony; Netterberg, Inge (2010-12-31). "A new GIS-based biozone map of the Beaufort Group (Karoo Supergroup), South Africa". Palaeontologia Africana. 45: 1–6.
  5. ^ Bain, Andrew Geddes (1845-02-01). "On the Discovery of the Fossil Remains of Bidental and other Reptiles in South Africa". Quarterly Journal of the Geological Society. 1 (1): 317–318. doi:10.1144/GSL.JGS.1845.001.01.72. hdl:2027/uc1.c034667778. ISSN 0370-291X.
  6. ^ Seeley, H. G. (1895). "Researches on the Structure, Organization, and Classification of the Fossil Reptilia. Part IX., Section 4. On the Gomphodontia". Philosophical Transactions of the Royal Society of London B. 186: 1–57. doi:10.1098/rstb.1895.0001. JSTOR 91793.
  7. ^ Broom, R. (January 1906). "V.—On the Permian and Triassic Faunas of South Africa". Geological Magazine. 3 (1): 29–30. doi:10.1017/S001675680012271X. ISSN 1469-5081.
  8. ^ W., Keyser, A. (1973). "A RE-EVALUATION OF THE GENUS TROPIDOSTOMA SEELEY". ISSN 0078-8554.
  9. ^ Keyser, A.W. and Smith, R.M.H., 1978. Vertebrate biozonation of the Beaufort Group with special reference to the western Karoo Basin. Geological Survey, Department of Mineral And Energy Affairs, Republic of South Africa.
  10. ^ Keyser, A.W., 1979. A review of the biostratigraphy of the Beaufort Group in the Karoo Basin of South Africa. Geocongress, Geological Society of South Africa, 2, pp.13-31.
  11. ^ Smith, Roger M. H. (1993). "Vertebrate Taphonomy of Late Permian Floodplain Deposits in the Southwestern Karoo Basin of South Africa". PALAIOS. 8 (1): 45–67. doi:10.2307/3515221. JSTOR 3515221.
  12. ^ Rubidge, B. S. (ed.) 1995b. Biostratigraphy of the Beaufort Group (Karoo Supergroup). South African Committee of Stratigraphy. Biostratigraphic Series 1. Pretoria, Council for Geoscience.
  13. ^ Smith, Roger M.H. (1987-01-01). "Helical burrow casts of therapsid origin from the Beaufort Group (Permian) of South Africa". Palaeogeography, Palaeoclimatology, Palaeoecology. 60: 155–169. doi:10.1016/0031-0182(87)90030-7. ISSN 0031-0182.
  14. ^ Viglietti, Pia A.; Smith, Roger M.H.; Angielczyk, Kenneth D.; Kammerer, Christian F.; Fröbisch, Jörg; Rubidge, Bruce S. (2016-01-01). "The Daptocephalus Assemblage Zone (Lopingian), South Africa: A proposed biostratigraphy based on a new compilation of stratigraphic ranges". Journal of African Earth Sciences. 113: 153–164. doi:10.1016/j.jafrearsci.2015.10.011. ISSN 1464-343X.
  15. ^ Hancox, P.J; Rubidge, B.S (2001-01-01). "Breakthroughs in the biodiversity, biogeography, biostratigraphy, and basin analysis of the Beaufort group". Journal of African Earth Sciences. 33 (3–4): 563–577. doi:10.1016/S0899-5362(01)00081-1. ISSN 1464-343X.
  16. ^ Rubidge, Bruce S.; Day, Michael O.; Barbolini, Natasha; Hancox, P. John; Choiniere, Jonah N.; Bamford, Marion K.; Viglietti, Pia A.; McPhee, Blair W.; Jirah, Sifelani (2016), "Advances in Nonmarine Karoo Biostratigraphy: Significance for Understanding Basin Development", Origin and Evolution of the Cape Mountains and Karoo Basin, Springer International Publishing, pp. 141–149, doi:10.1007/978-3-319-40859-0_14, ISBN 9783319408583
  17. ^ Rubidge, B. S. (ed.) 1995b. Biostratigraphy of the Beaufort Group (Karoo Supergroup). South African Committee of Stratigraphy. Biostratigraphic Series 1. Pretoria, Council for Geoscience.
  18. ^ Smith, Roger M.H. (1987-01-01). "Helical burrow casts of therapsid origin from the Beaufort Group (Permian) of South Africa". Palaeogeography, Palaeoclimatology, Palaeoecology. 60: 155–169. doi:10.1016/0031-0182(87)90030-7. ISSN 0031-0182.
  19. ^ Smith, Roger M. H. (1993). "Vertebrate Taphonomy of Late Permian Floodplain Deposits in the Southwestern Karoo Basin of South Africa". PALAIOS. 8 (1): 45–67. doi:10.2307/3515221. JSTOR 3515221.
  20. ^ Sidor, Christian A.; Smith, Roger M. H. (2007-06-12). "A second burnetiamorph therapsid from the Permian Teekloof Formation of South Africa and its associated fauna". Journal of Vertebrate Paleontology. 27 (2): 420–430. doi:10.1671/0272-4634(2007)27[420:asbtft]2.0.co;2. ISSN 0272-4634.
  21. ^ ABDALA, FERNANDO; RUBIDGE, BRUCE S.; van den HEEVER, JURI (2008-07-01). "The Oldest Therocephalians (Therapsida, Eutheriodontia) and the Early Diversification of Therapsida". Palaeontology. 51 (4): 1011–1024. doi:10.1111/j.1475-4983.2008.00784.x. ISSN 0031-0239.
  22. ^ Botha-Brink, Jennifer; Abdala, Fernando (2008-04-01). "A new cynodont record from the tropidostoma assemblage zone of the beaufort group: Implications for the early evolution of cynodonts in South Africa". 43: 1–6.
  23. ^ BOTHA, J.; ANGIELCZYK, K. D. (2007-09-01). "An Integrative Approach to Distinguishing the Late Permian Dicynodont Species Oudenodon Bainii and Tropidostoma Microtrema (Therapsida: Anomodontia)". Palaeontology. 50 (5): 1175–1209. doi:10.1111/j.1475-4983.2007.00697.x. ISSN 0031-0239.
  24. ^ Fröbisch, Jörg; Reisz, Robert R. (2008-09-12). "A new species ofEmydops(Synapsida, Anomodontia) and a discussion of dental variability and pathology in dicynodonts". Journal of Vertebrate Paleontology. 28 (3): 770–787. doi:10.1671/0272-4634(2008)28[770:ansoes]2.0.co;2. ISSN 0272-4634.
  25. ^ Kammerer, Christian F.; Angielczyk, Kenneth D.; Fröbisch, Jörg (2011-12-14). "A comprehensive taxonomic revision ofDicynodon(Therapsida, Anomodontia) and its implications for dicynodont phylogeny, biogeography, and biostratigraphy". Journal of Vertebrate Paleontology. 31 (sup1): 1–158. doi:10.1080/02724634.2011.627074. ISSN 0272-4634.
  26. ^ Kammerer, Christian F.; Smith, Roger M.H. (2017-01-31). "An early geikiid dicynodont from theTropidostomaAssemblage Zone (late Permian) of South Africa". PeerJ. 5: e2913. doi:10.7717/peerj.2913. ISSN 2167-8359. PMC 5289114. PMID 28168104.
  27. ^ LAURIN, MICHEL; REISZ, ROBERT R. (1995-02-01). "A reevaluation of early amniote phylogeny". Zoological Journal of the Linnean Society. 113 (2): 165–223. doi:10.1111/j.1096-3642.1995.tb00932.x. ISSN 0024-4082.
  28. ^ Kitching, J.W. (1978-01-01). "The stratigraphic distribution and occurrence of South African fossil Amphibia in the Beaufort Beds". Palaeontologia Africana. 21: 101–112.
  29. ^ Damiani, Ross J. (2004-01-01). "Temnospondyls from the Beaufort Group (Karoo Basin) of South Africa and Their Biostratigraphy". Gondwana Research. 7 (1): 165–173. doi:10.1016/S1342-937X(05)70315-4. ISSN 1342-937X.
  30. ^ Marsicano, Claudia A.; Latimer, Elizabeth; Rubidge, Bruce; Smith, Roger M.H. (2017-05-29). "The Rhinesuchidae and early history of the Stereospondyli (Amphibia: Temnospondyli) at the end of the Palaeozoic". Zoological Journal of the Linnean Society. 181 (2). doi:10.1093/zoolinnean/zlw032. ISSN 0024-4082.
  31. ^ Smith, Roger M.H.; Botha-Brink, Jennifer (2011-12-01). "Morphology and composition of bone-bearing coprolites from the Late Permian Beaufort Group, Karoo Basin, South Africa". Palaeogeography, Palaeoclimatology, Palaeoecology. 312 (1–2): 40–53. doi:10.1016/j.palaeo.2011.09.006. ISSN 0031-0182.

See alsoEdit