Cistecephalus Assemblage Zone

The Cistecephalus Assemblage Zone is a

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):

• Pareiasaurus beds
• Endothiodon beds
• Kistecephalus beds
• Lystrosaurus beds
• Procolophon beds
• Cynognathus beds

These biozone divisions were approved by paleontologists of the time and were left largely unchanged for several decades.^[8] The Cistecephalus Assemblage Zone was first coined by Robert Broom in 1906,^[9] where Broom referred to the rock layers Cistecephalus fossils were found in as the "Kistecephalus beds". The biozone was originally assigned to a far more broader stratigraphic range between the uppermost occurrence of Endothiodon and the lowermost of Lystrosaurus.^[10] Decades later, James Kitching revised the biostratigraphic ranges of the Beaufort Group.^[11][12] Kitching observed that Cistecephalus fossils were most abundant in a narrow band at the very top of the biozone. He named this area the "Cistecephalus acme zone" or "Cistecephalus Band". Later, researchers sought to redefine the biozone again as Cistecephalus fossils are extremely rare apart from the uppermost portion.^[13] It was suggested that, due to the biozone containing the first appearance of Aulacephalodon and its fossils being found throughout, the biozone should be renamed to the Aulacephalodon Assemblage Zone. The biozone was then, for a brief time, renamed the Aulacephalodon-Cistecephalus Assemblage Zone.^[14] However, due to the biozone first being named after Cistecephalus, the initial nomenclature of the biozone later overruled its renaming on historical grounds.^[15][16]

Lithology

The Cistecephalus Assemblage Zone is located in the lower portion of the Teekloof Formation west of 24°E, in the upper Middleton Formation and lower Balfour Formation east of 24°E. These formations all fall within the Adelaide Subgroup of the Beaufort Group, sediments of which were formed in a large retroarc foreland basin in southwestern Gondwana.^[17] The sedimentary rocks are mainly sandstone, mudrock layers containing mudstone, siltstone, and fine sandstone. The sandstones are thought to have been deposited in broad alluvial plains where low-sinuosity streams flowed, while the mudrock accumulated on the floodplains that flanked these streams.^[18][19]

The majority of the