Endothiodon Assemblage Zone

In 1889, British palaeontologist Harry Seeley travelled to South Africa to conduct fieldwork in the Karoo Supergroup. He recognized that, as one traveled up the Nuweveld Escarpment, the outcrops demonstrated noticeably distinct zones; after discussing the pareiasaur fossils in the lower levels in a lecture delivered that year, he noted: This faunal transition Seeley observed is now recognized as one of the most distinct biotic changes in the succession of the Beaufort Group, marking the end of what is now the Tapinocephalus Assemblage Zone (Guadalupian fauna) and the younger and stratigraphically higher Lopingian faunas. In an 1892 discussion on the geological horizons (levels of rock outcrops) is South Africa, Seeley recognized the division of these layers into biozones based on their vertebrate content, and referred to these higher assemblages as the "zone of Dicynodonts" in a scheme comprising five distinct zones, three of which are part of the Beaufort Group. After Scottish-born palaeontologist Robert Broom observed some of the South African fossils Seeley brought to England, he recognized their biological significance and sailed to South Africa to begin collecting and studying fossils in the Karoo region. In 1906, he proposed an updated, more precise biostratigraphic division scheme for the Beaufort Group. Herein, Broom split Seeley's "zone of Dicynodonts" into a lower and upper section, regarded as the "Endothiodon" and "Kistecephalus" beds, respectively. Broom subsequently tabulated the relative abundance of fossil taxa in his proposed biozones in a 1909 occurrence list. Two years later, English zoologist D. M. S. Watson visited the South African outcrops. In 1914, he claimed that no pariasaurian or dinocephalian remains had been found in the Endothiodon Beds, but that abundant therocephalians and gorgonopsids were present in addition to large 'endothiodon' and very small 'dicynodon' specimens. This conflicted with Broom's list, which noted the presence of large 'dicynodons' and the pareiasaur "Propappus" (now regarded as a synonym of Pareiasaurus). Watson characterized the Endothiodon based on the presence of this genus in addition to "other large Endothiodonts and peculiar Gorgonopsids, and by the total absence of large Dicynodons". the Endothiodon Zone has a striking absence of large-bodied animals. However, the fossils Kitching collected were almost all from the upper part of the range of Endothiodon, while its lower range has less widely exposed outcrops. In a 1995 volume discussing each Beaufort Group assemblage zone separately, published in association with the South African Committee for Stratigraphy (S.A.C.S.), these two zones were formally recognized as the Pristerognathus and Tropidostoma assemblage zones. After 1995, these two assemblage zones were implemented widely in place of the historic "Endothiodon Zone". This name has been adopted by researchers in the scientific literature since then. == Lithology and description ==

map of Earth during the Wuchiapingian age The Endothiodon Assemblage Zone comprises two distinct subzones characterized by differences in faunal composition and lithology. The Subzone is stratigraphically lower (older in age), and is roughly equivalent to the upper two-thirds of the former "Pristerognathus Assemblage Zone". The lower third of the Pristerognathus AZ was transferred to the uppermost Tapinocephalus AZ. Lycosuchus - Eunotosaurus Subzone This subzone is named and defined based on the co-occurrence of the namesake Endothiodon with the distinctive taxa Lycosuchus (a therocephalian) and Eunotosaurus (an early reptile). Both of these genera are also found in the lower Tapinocephalus AZ, but they do not exist with Endothiodon here. In the southwest region of the Karoo Basin, the Lycosuchus - Eunotosaurus Subzone correlates with the upper two-thirds of the Poortjie Member of the Teekloof Formation, and its upper boundary (that contacts the Tropidostoma - Gorgonops Subzone) lines up with the base of the overlying Hoedemaker Member of this formation. In the southeast part of the Karoo Basin, this subzone correlates with the lower part of the Middleton Formation. The base of this subzone is defined based on the first occurrence of the genus Endothiodon, matching the definition for the onset of the assemblage zone as a whole. The age of the base is well-constrained based on radiometric dates from the Poortjie Member, at . The upper boundary is defined based on the first occurrence of Tropidostoma (which defines the upper subzone). Its age is less well-constrained, but is most likely between (latest Capitanian to earliest Wuchiapingian). It can likely be correlated with the Lycosuchus - Eunotosaurus Subzone of the EAZ. == Palaeontology ==

The Endothiodon Assemblage Zone preserves a diverse biota. While vertebrates are known from the greatest diversity, invertebrates and plants have also been described. Some specimens, such as articulated skulls and lower jaws of Diictodon and complete, robust rib cages of Eunotosaurus are commonly found in isolation, likely representing the heaviest regions of disarticulated carcasses that are less likely to have been dispersed due to flooding. Besides the most common genera and those that give the subzone its name, other vertebrate taxa include a single biarmosuchian (Lobalopex mordax), anomodontians (including dicynodonts), various gorgonopsians, and other therocephalians. Multiple instances of "Diictodon graveyards" are also known in this subzone, which preserve dense clusters of the skeletons and skulls of this genus in layers of alluvium (loose sediment deposited by running water). These scenarios may be the result of these animals becoming mired or predated at the margin of a distal floodplain lake. Burrows from digging dicynodonts are rare in the subzone's lower section, and have a straight or slightly sinuous morphology; they are not present in the upper section. In regions where Diictodon specimens are abundant, bone-bearing coprolites from therapsids have been found. Diamonelix helical burrows and the ichnotaxa Karoopes gansfonteinenesis (gorgonopsid tracks) and cf. Capitosauroides (therocephalian tracks) are known only from the Lycosuchus - Eunotosaurus Subzone. Invertebrate trace fossils in both subzones comprise the ichnogenera Diplichnites (two parallel rows of arthropod tracks), Planolites (worm-like feeding traces), Skolithos (vertical cylindrical burrow), and Undichna (fin trail). == References ==