Vertebrate burrow complexes from, the Early Triassic Cynognathus Zone (Driekoppen Formation, Beaufort Group) of the Karoo Basin, South Africa

A scratch-marked burrow complex with multiple branching tunnels and termina l chambers was excavated from the Lower Triassic Driekoppen Formation, nort heastern Free State, South Africa. The burrow complex is attributed to the therapsid Trirachodon, based on disarticulated but fairly complete skulls a nd skeletons of at least 20 individuals recovered from a nearby less well-p reserved system. The entrance shaft slopes gently downward and is characterized by a bilobat e floor and vaulted roof: The bilobate floor has a central flat-topped scra tch-marked ridge flanked by two smooth grooves, each approximately the widt h of the occupant. At deeper levels, tunnels display tighter lateral curvat ures, progressively decreasing burrow diameters, variable orientations, and some right angle branches. Burrow floors at these levels are vaguely bilob ate. Distally, burrows flatten dorso-ventrally, becoming wedge-shaped befor e they terminate. These complexes are interpreted as colonial dwelling structures. Numerous b ranching tunnels and terminal chambers, as well as an enlarged entrance con stitute an unrealistically high expenditure of energy for a single occupant . Furthermore, the bilobate floor is atypical of a single occupant system. Absence of scratch marks in the depressions reflects the regular locomotory activity of Trirachodon. In single-occupant burrows, the center of the str ucture is worn. The bilobate floor with preferential wear along the flanks suggests routine travel on one side of the tunnel or the other, as multiple occupants moved past one another during daily activity. The presence of numerous fossilized Trirachodon individuals in terminal cha mbers at the second locality supports multiple cohabitation There, the unif orm sedimentary fill and vertebrate taphonomy suggest that the occupants we re drowned in a flash flood. Vertebrate remains are absent at the first loc ality where sediment influx was incremental, permitting the escape of burro w occupants. This constitutes the earliest record of multiple cohabitation of a burrow c omplex by tetrapods. The Trirachodon (Cynodontia) of the Early Triassic Cyn ognathus Zone probably displayed complex social behaviors previously regard ed to be restricted to the mammals of the Cenozoic. This fossorial behavior may have been for thermoregulation, protection from predators, sites of re production, and the rearing of young.