Tropidostoma Assemblage Zone

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Tropidostoma Assemblage Zone
Karoo Basin
Type section
Named forTropidostoma microtrema
Named byKeyster & Smith
Year defined1978-79

The Tropidostoma Assemblage Zone is a

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]

The name of the

dicynodont species, Endothiodon uniseries
.

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.[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):

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.

Lithology

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 gypsum “desert 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

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]

Paleontology

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

therapsid species, and coprolites[31]
have been recovered.

References

  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. ISSN 1464-343X
    .
  3. 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. S2CID 128602890
    .
  6. JSTOR 91793
    .
  7. S2CID 129265956
    .
  8. ISSN 0078-8554.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )
  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. 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. ISSN 0031-0182
    .
  14. ISSN 1464-343X
    .
  15. ISSN 1464-343X
    .
  16. 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. ISSN 0031-0182
    .
  19. JSTOR 3515221
    .
  20. S2CID 86173425
    .
  21. 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. {{cite journal}}: Cite journal requires |journal= (help)
  23. ISSN 0031-0239
    .
  24. S2CID 85594758
    .
  25. S2CID 84987497
    .
  26. PMID 28168104
    .
  27. 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. ISSN 1342-937X
    .
  30. ISSN 0024-4082
    .
  31. ISSN 0031-0182
    .

See also
  • List of synapsids
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