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. The thickest outcrops, reaching approximately , 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.
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.
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. 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, 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. Broom proposed the following biozones (from oldest to youngest): Pareiasaurus beds Endothiodon beds Kistecephalus beds Lystrosaurus beds Procolophon beds Cynognathus beds
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 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. 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.
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.