Sedimentology and paleoecology of a diverse Early Jurassic tetrapod tracksite in Lesotho, southern Africa

Abstract

An Early Jurassic tetrapod tracksite in the upper Elliot Formation at Moyeni, southern Lesotho, displays a variety of trackways attributed to large- and medium-sized theropod (Neotrisauropus-type) and ornithischian (Moyenisauropus-type) dinosaurs, basal crurotarsal archosaurs (chirotheroid-type), and a short-legged basal tetrapod (Episcopopus-type). The tracks are on a low-angle pointbar and are buried with loessic floodplain fine-grained sediment. Calcic paleosols indicate a warm semiarid climate. Many of the footprints were imprinted through an algal mat in a water-margin setting. Convergence of several trackways toward a single point suggests repeated visits to drink or cross the river. One of the two large Moyenisauropus-type trackways has a narrow gauge that suggests an upright, parasagittal gait, whereas the other shows changes in gauge width, stance, and posture as it proceeded up the pointbar slope. At least three resting traces with manus, metatarsal, and tail impressions attributable to the Moyenisauropus-type ornithischian are also preserved. Discovery of two manus-pes pairs of chirotheroid-type footprints in the Moyeni section highlights a mismatch between the body-fossil and trace-fossil records. Chirotheroid tracks are generally thought to be restricted to the Triassic, and their discovery at the Moyeni tracksite compounds the problem of where to place the Triassic-Jurassic boundary in this succession. Three possible scenarios could explain the occurrence of chirotheroid-type tracks at Moyeni: (1) the tracksite is Late Triassic in age; (2) the chirotheroid tracks were made by archosaurs other than basal crurotarsans; (3) the tracks are correctly identified and the age of the Moyeni section is correctly assigned, but the inferred range of chirotheroid-type tracks is incorrect. We suggest that the latter two are the most likely explanations.