Wikipedia

Lystrosaurus Assemblage Zone

The Lystrosaurus Assemblage Zone is a tetrapod assemblage zone or biozone which correlates to the upper Adelaide and lower Tarkastad Subgroups of the Beaufort Group, a fossiliferous and geologically important geological Group of the Karoo Supergroup in South Africa. This biozone has outcrops in the south central Eastern Cape (Middelburg, Queenstown, Aliwal North, Nieu-Bethesda) and in the southern and northeastern Free State (Bethulie, Gariep Dam, Mthatha, Harrismith). The Lystrosaurus Assemblage Zone is one of eight biozones found in the Beaufort Group, and is considered to be Early Triassic in age.[1][2]

Lystrosaurus Assemblage Zone
Stratigraphic range: Early Triassic
~251–249 Ma
↓
Lystrosaurus murrayi
TypeBiozone
Unit ofBeaufort Group within Adelaide Subgroup
Sub-unitsUpper Balfour Formation west of 24°E
Entire Katberg Formation east of 24°E
UnderliesCynognathus Assemblage Zone
OverliesDaptocephalus Assemblage Zone
Thicknessup to 2,723.1 feet (830 m)
Location
LocationKaroo Basin
RegionEastern Cape, Free State
Country South Africa
ExtentKaroo Basin
Type section
Named forLystrosaurus
Named byRobert Broom
Year defined1906, 1909

The name of the biozone refers to Lystrosaurus, a small to medium-sized dicynodont therapsid. It is characterized by the appearance of further Lystrosaurus subspecies which are confined to this biozone.[3] Lystrosaurus maccaigi and Lystrosaurus curvatus are the only two species found outside the Lystrosaurus Assemblage Zone in Upper Permian deposits of the underlying Daptocephalus Assemblage Zone.[4][5]

History edit

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.[6] 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,[7] which he named (from oldest to youngest):

These proposed biozones Seeley named were subdivided further by Robert Broom between 1906 and 1909.[8] 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.[9] The Lystrosaurus Assemblage Zone was first named by Robert Broom in 1906.[10] Initially Broom had subdivided the existing Lystrosaurus zone into the Lystrosaurus and Procolophon Assemblage Zones respectively. The biozone was later revised in 1976 by James Kitching where Kitching assimilated the Procolophon zone into the Lystrosaurus zone due to discovering that fossils of the small parareptile Procolophon were likewise found throughout the Lystrosaurus zone.[11][12]

Lithology edit

The Lystrosaurus Assemblage Zone ranges from the Palingkloof Member of the upper Balfour Formation west of 24°E. It comprises the entire Katberg Formation and the first third of the Burgersdorp Formation east of 24°E.[13] The Balfour Formation is located within the Adelaide Subgroup, while the Katberg and Burgersdorp are within the Tarkastad Subgroup of the Beaufort Group. Its contact with the underlying Daptocephalus Assemblage Zone marks the Permian-Triassic boundary.[14][15]

The boundary is defined by a change in the sedimentary rock types. The changing rock types across the boundary reveal a change in the fluvial environment, from meandering high sinuosity river channels composed of greenish-grey siltstones and mudstones found in the underlying Daptocephalus Assemblage Zone.[16][17][18] From the start of the Palingkloof Member the predominant presence of mudstone and siltstone show that meandering river channels were present, however, in arid and warmer conditions due to change in colour of the rocks to reddish-brown and maroon.[19][20] These are inter-spaced with claystones, olive to grey fine-grained sandstone, and reddish-brown to maroon shales. In the overlying Katberg Formation, alluvial fans containing braided low sinuosity river channels comprising mainly coarse-grained sandstone appear. These sandstones form either single and multi-storey channel sandstones and crevasse-splay sandstones.[21] The dominance of sandstones is diagnostic of the Katberg Formation. The sandstones are interspersed by reddish-brown siltstones and mudstones which were deposited as silt sediments washed down the braided channels further down the Karoo Basin.[22] The mudstones here often contain cracks which are infilled with sandstone.[23] The domination of sandstone in the Lystrosaurus Assemblage Zone shows that the climate at the time had become more arid, where rainfall was unpredictable and the shallow, braided rivers would seasonally dry up.[24] When the rivers flowed again after the rains, due to being shallow, they would frequently flood their banks, hence the presence of crevasse-splays. Conglomerates are also found in eastern outcrops, which are indicative of erosion occurring due to die-offs of plant ecosystems. There is in addition a notable gap in coal deposits at this time as a result. Siltstone and mudstone outcrops are less common, with the majority of outcrops of these being found in the lower sections within the Palingkloof Member and in its uppermost section within the Burgersdorp Formation. Nodule conglomerates comprising pedogenic nodules and intrabasinal clasts are also found.[25][26]

Paleontology edit

There is a marked drop in species abundance in the Lystrosaurus Assemblage Zone due to ecological crises which followed the Permian-Triassic extinction event.[27][28] However, this has not affected the abundance of vertebrate fossils found within this biozone. The most ubiquitous fossils found are different species of Lystrosaurus, the most commonly occurring being Lystrosaurus murrayi and Lystrosaurus declivis. Lystrosaurus maccaigi is the only species of Lystrosaurus not found in the biozone of its namesake.[29][30] Lystrosaurus curvatus does appear in the lowermost section of the biozone, although it disappears at the contact between the Palingkloof Member of the Balfour Formation and the Katberg Formation. For this reason L. curvatus is used as an index fossil for outcrops of the Permian-Triassic boundary. In the lower Katberg Formation, complete and sometimes mummified articulated skeletons of L. murrayi and L. declivus are found in bonebeds containing several individuals.[31] The bonebeds are almost always overlain by mudrock infilled with sandstone and capped by other coarse-grained sediments. This provides substantial geological and taphonomical evidence that these Lystrosaurus died near to dried up river channels, mummified in the arid climate before their remains were buried by floods.

The Permian-Triassic extinction event caused the extinction of all gorgonopsians and almost all dicynodont species[32] except for Lystrosaurus and a select few other species such as Myosaurus gracilis.[33] Therocephalian species experienced a Lilliput Effect where only smaller species survived and thrived after the extinction event. Moschorhinus, for example, was one of the larger therocephalians to survive the initial extinction event, however, fossil occurrences of this species cease above the lower Katberg Formation.[34] This is also true of Lystrosaurus where only the smaller species thrived in the Triassic. By the upper sections of the biozone, ecological niches began to recover as evidenced by the appearance of new species.[35] Cynodonts experienced the greatest diversification with species such as Thrinaxodon liorhinus and Galesaurus planiceps being found.[36][37][38] In the upper Katberg and the lower Burgersdorp Formations more derived cynodonts, the Eucynodontia, make an appearance.[39] Small procolonphonoid parareptiles such as Owenetta kitchingorum and Procolophon trigoniceps, and temnospondyl amphibians are also found.[40][41][42][43] In addition, the earliest ancestors of Archosauria appear in the Lystrosaurus zone. These species are known as archosauromorphs and archosauriformes. Examples of these are Prolacerta broomi and Proterosuchus fergusi respectively found in the lower Lystrosaurus Assemblage Zone.[44][45][46] A variety of ichnofossils are also found, especially burrow casts left by Lystrosaurus.[47][48][49] The deep-bodied ray-finned fish Caruichthys[50] was also found in layers of this biozone.

Age and correlations edit

The Lystrosaurus Assemblage Zone dates to approximately 251 to 249 Ma, and correlates with the Kopanskaya and Staritskaya Formations of Russia,[51] and with the Jiucaiyuan Formation of China, the Knocklofty and Arcadia Formations of Australia, the Buena Vista Formation of the Paraná Basin, South America, and the Panchet Formation of India.[52]

See also edit

References edit

  1. ^ Keyser, A. W., & 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.
  2. ^ Rubidge, B. S. (ed.) 1995b. Biostratigraphy of the Beaufort Group (Karoo Supergroup). South African Committee of Stratigraphy. Biostratigraphic Series 1. Pretoria, Council for Geoscience.
  3. ^ Lucas, Spencer G. (2009-11-02). "Timing and magnitude of tetrapod extinctions across the Permo-Triassic boundary". Journal of Asian Earth Sciences. 36 (6): 491–502. Bibcode:2009JAESc..36..491L. doi:10.1016/j.jseaes.2008.11.016. ISSN 1367-9120.
  4. ^ Smith, R.M.H. (1995-08-01). "Changing fluvial environments across the Permian-Triassic boundary in the Karoo Basin, South Africa and possible causes of tetrapod extinctions". Palaeogeography, Palaeoclimatology, Palaeoecology. 117 (1–2): 81–104. Bibcode:1995PPP...117...81S. doi:10.1016/0031-0182(94)00119-S. ISSN 0031-0182.
  5. ^ BOTHA, JENNIFER; SMITH, ROGER M. H. (2007-04-08). "Lystrosaurus species composition across the Permo-Triassic boundary in the Karoo Basin of South Africa". Lethaia. 40 (2): 125–137. doi:10.1111/j.1502-3931.2007.00011.x. ISSN 0024-1164.
  6. ^ 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. S2CID 128602890.
  7. ^ 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. Bibcode:1895RSPTB.186....1S. doi:10.1098/rstb.1895.0001. JSTOR 91793.
  8. ^ Broom, R. (January 1906). "V.—On the Permian and Triassic Faunas of South Africa". Geological Magazine. 3 (1): 29–30. Bibcode:1906GeoM....3...29B. doi:10.1017/S001675680012271X. ISSN 1469-5081. S2CID 129265956.
  9. ^ Watson, D. M. S. (May 1914). "II.—The Zones of the Beaufort Beds of the Karroo System in South Africa". Geological Magazine. 1 (5): 203–208. Bibcode:1914GeoM....1..203W. doi:10.1017/S001675680019675X. ISSN 1469-5081. S2CID 130747924.
  10. ^ Broom, R. (1906). V.—On the Permian and Triassic Faunas of South Africa. Geological Magazine, 3(1), 29-30.
  11. ^ Kitching, J. W. (1977). The distribution of the karroo vertebrate fauna: with special reference to certain genera and the bearing of this distribution on the zoning of the Beaufort Beds, Bernard Price Institute for Palaeontological Research, University of the Witwatersrand.
  12. ^ Kitching, J. W. (1984). A reassessment of the biozonation of the Beaufort Group. Paleo News, 4(1), 12-13.
  13. ^ Rubidge, B. S. (ed.) 1995b. Biostratigraphy of the Beaufort Group (Karoo Supergroup). South African Committee of Stratigraphy. Biostratigraphic Series 1. Pretoria, Council for Geoscience.
  14. ^ Merrill, van der Walt; Michael, Day; Bruce, Rubidge; K., Cooper, Antony; Inge, Netterberg (December 2010). "A new GIS-based biozone map of the Beaufort Group (Karoo Supergroup), South Africa". Palaeontologia Africana. ISSN 0078-8554.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  15. ^ Viglietti, Pia; Rubidge, Bruce; Malcom Harris Smith, Roger (2017-03-01). "Revised lithostratigraphy of the upper Permian Balfour and Teekloof formations of the main Karoo Basin, South Africa". South African Journal of Geology. 120: 45–60. doi:10.25131/gssajg.120.1.45.
  16. ^ 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. Bibcode:2016JAfES.113..153V. doi:10.1016/j.jafrearsci.2015.10.011. ISSN 1464-343X.
  17. ^ Viglietti, Pia A.; Smith, Roger M.H.; Rubidge, Bruce S. (2018-02-01). "Changing palaeoenvironments and tetrapod populations in the Daptocephalus Assemblage Zone (Karoo Basin, South Africa) indicate early onset of the Permo-Triassic mass extinction". Journal of African Earth Sciences. 138: 102–111. Bibcode:2018JAfES.138..102V. doi:10.1016/j.jafrearsci.2017.11.010. ISSN 1464-343X.
  18. ^ Smith, R.M.H. (1995-08-01). "Changing fluvial environments across the Permian-Triassic boundary in the Karoo Basin, South Africa and possible causes of tetrapod extinctions". Palaeogeography, Palaeoclimatology, Palaeoecology. 117 (1–2): 81–104. Bibcode:1995PPP...117...81S. doi:10.1016/0031-0182(94)00119-S. ISSN 0031-0182.
  19. ^ Hiller, Norton; Stavrakis, Nicholas (1984-02-01). "Permo-Triassic fluvial systems in the southeastern Karoo Basin, South Africa". Palaeogeography, Palaeoclimatology, Palaeoecology. 45 (1): 1–21. Bibcode:1984PPP....45....1H. doi:10.1016/0031-0182(84)90106-8. ISSN 0031-0182.
  20. ^ Viglietti, Pia A.; Smith, Roger M.H.; Compton, John S. (2013-12-15). "Origin and palaeoenvironmental significance of Lystrosaurus bonebeds in the earliest Triassic Karoo Basin, South Africa". Palaeogeography, Palaeoclimatology, Palaeoecology. 392: 9–21. Bibcode:2013PPP...392....9V. doi:10.1016/j.palaeo.2013.08.015. ISSN 0031-0182.
  21. ^ Viglietti, Pia A.; Smith, Roger M.H.; Rubidge, Bruce S. (2018-02-01). "Changing palaeoenvironments and tetrapod populations in the Daptocephalus Assemblage Zone (Karoo Basin, South Africa) indicate early onset of the Permo-Triassic mass extinction". Journal of African Earth Sciences. 138: 102–111. Bibcode:2018JAfES.138..102V. doi:10.1016/j.jafrearsci.2017.11.010. ISSN 1464-343X.
  22. ^ Smith, R., Rubidge, B., & Van der Walt, M. (2012). Therapsid biodiversity patterns and paleoenvironments of the Karoo Basin, South Africa. Forerunners of Mammals: Radiation, Histology, Biology. Indiana University Press, Indianapolis, Indiana, 30-62.
  23. ^ Viglietti, Pia A.; Smith, Roger M.H.; Compton, John S. (2013-12-15). "Origin and palaeoenvironmental significance of Lystrosaurus bonebeds in the earliest Triassic Karoo Basin, South Africa". Palaeogeography, Palaeoclimatology, Palaeoecology. 392: 9–21. Bibcode:2013PPP...392....9V. doi:10.1016/j.palaeo.2013.08.015. ISSN 0031-0182.
  24. ^ Smith, Roger M.H.; Botha-Brink, Jennifer (2014-02-15). "Anatomy of a mass extinction: Sedimentological and taphonomic evidence for drought-induced die-offs at the Permo-Triassic boundary in the main Karoo Basin, South Africa". Palaeogeography, Palaeoclimatology, Palaeoecology. 396: 99–118. doi:10.1016/j.palaeo.2014.01.002. ISSN 0031-0182.
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  28. ^ Viglietti, Pia A.; Smith, Roger M.H.; Rubidge, Bruce S. (2018-02-01). "Changing palaeoenvironments and tetrapod populations in the Daptocephalus Assemblage Zone (Karoo Basin, South Africa) indicate early onset of the Permo-Triassic mass extinction". Journal of African Earth Sciences. 138: 102–111. Bibcode:2018JAfES.138..102V. doi:10.1016/j.jafrearsci.2017.11.010. ISSN 1464-343X.
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  31. ^ Viglietti, Pia A.; Smith, Roger M.H.; Compton, John S. (2013-12-15). "Origin and palaeoenvironmental significance of Lystrosaurus bonebeds in the earliest Triassic Karoo Basin, South Africa". Palaeogeography, Palaeoclimatology, Palaeoecology. 392: 9–21. Bibcode:2013PPP...392....9V. doi:10.1016/j.palaeo.2013.08.015. ISSN 0031-0182.
  32. ^ Smith, Roger M.H.; Botha-Brink, Jennifer (2014-02-15). "Anatomy of a mass extinction: Sedimentological and taphonomic evidence for drought-induced die-offs at the Permo-Triassic boundary in the main Karoo Basin, South Africa". Palaeogeography, Palaeoclimatology, Palaeoecology. 396: 99–118. doi:10.1016/j.palaeo.2014.01.002. ISSN 0031-0182.
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  40. ^ Carlos, Cisneros, Juan (April 2008). "Taxonomic status of the reptile genus Procolophon from the Gondwanan Triassic". Palaeontologia Africana. ISSN 0078-8554.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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Bibliography edit

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