Endothiodon Assemblage Zone

The Endothiodon Assemblage Zone is a tetrapod assemblage zone (also called a biozone) in the Adelaide Subgroup of the Beaufort Group, a fossiliferous and geologically important geological group of the Karoo Supergroup in South Africa. Outcrops of the lower Teekloof Formation and much of the Middleton Formation represent this assemblage zone, and other formations throughout Africa have also been correlated to it. The Endothiodon Assemblage Zone (EAZ) is the third oldest biozone of the Beaufort Group, dated to around 260 to 257 million years ago in the middle to late Permian period. It follows the older Tapinocephalus and Eodicynodon assemblage zones, and is named for the relative abundance and distinctiveness of the herbivorous dicynodont therapsid Endothiodon in this unit.

The Endothiodon Assemblage Zone is further divided into the stratigraphically lower (older) Lycosuchus - Eunotosaurus Subzone and higher (younger) Tropidostoma - Gorgonops Subzone. The thickest outcrops are located along the Nuweveld region of the Great Escarpment, and preserve the depositional environments of rivers and lakes. The EAZ overlaps with the Capitanian mass extinction event and, as such, preserves a record of ecosystem recovery immediately after this event. Many animals are represented by the EAZ, including diverse dicynodonts, gorgonopsians, therocephalians, biarmosuchians, cynodontians, early reptiles, and temnospondyls.

Like other assemblage zones in the Beaufort Group, the Endothiodon AZ has a convoluted history, having been referred to under several different names and definitions; from 1906 to 1970, the biozone was deemed either the "Endothiodon Beds" or "Endothiodon Zone". It was then lumped into the younger Cistecephalus Zone in 1970, before being split into two separate zones—the lower Pristerognathus AZ and upper Tropidostoma AZ—in 1978. In 2020, a revision of the Beaufort Group recombined these assemblage zones into a newly redefined Endothiodon AZ.