Early Triassic therapsid footprints from the Sydney Basin, Australia

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Abstract
A large rock slab collected in 1913 from the roof of the Bellambi Colliery in the southern Sydney Basin bears fossil tracks that are now known from recent radiometric and chemostratigraphic dating to be earliest Triassic, rather than latest Permian, in age. The tracks show two distinctive features of reptiles: scale impressions and claw marks. Both manus and pes are pentadactyl, ectaxonic, semidigitigrade and have an outer interdigital angle (digits IV–V) greater than inner interdigital angles. Digit proportions are consistent with a phalangeal formula of 23333. The fossil tracks are referred to the ichnospecies Dicynodontipus bellambiensis sp. nov. They are similar to the kinds of tracks thought to be produced by Lystrosaurus species. Given the abundance of these species in Early Triassic faunas of low diversity and the occurrence of members of the Lystrosaurus fauna in Queensland and Antarctica, chances are good that this is indeed a trackway of Lystrosaurus. If considered to be made by an animal of that type, the trackway was produced using the primitive alternate gait, rather than the mammalian amble, by an animal about 84 cm long and some 22 cm high. Preservation of bones of these creatures would not be expected given the non-calcareous nature of associated fossil soils in the Sydney Basin Triassic. Herbaceous lycopods, locally common in these and other Early Triassic strata worldwide, are among the most likely foods of these tusked, low-browsing herbivores.