Skeletal accumulations of the parareptile Procolophon trigoniceps reflect fossorial response to Early Triassic climatic instability across southern Gondwana

The Lower Triassic parareptile Procolophon trigoniceps is known from hundreds of specimens, mostly from upper Katberg Formation of the South African Karoo, where it co-occurs with the dicynodont Lystrosaurus declivis and the diminutive temnospondyl Micropholis. The same taxon also occurs in the lower-mid Triassic Fremouw Formation of Transantarctic Basin and the coeval Sanga do Cabral Formation of Paraná Basin in Brazil. The Gondwana-wide distribution of P. trigoniceps raises questions as to how they so successfully colonised southern Gondwana and why their fossils occur in hyper-abundant skeletal concentrations.

We investigated the sedimentology and taphonomy of P. trigoniceps accumulations in South Africa, Brazil and Antarctica to determine the cause of such exceptional preservation. South African localities are mainly red mudrock with horizons of micrite nodules, some of which contain up to five fully- and partially articulated skeletons, commonly of adult and juveniles in bone-on-bone contact lying side-by-side, criss-crossing, or in curled-up poses. In situ cylindrical scratch-marked decline burrow casts occur in the same outcrops. Neutron tomography of a scratch-marked burrow cast revealed a curled-up adult and disarticulated juvenile. Rare, but notable, tooth puncture marks occur on some articulated specimens. The Antarctic specimens occur in only slightly rubified mudrock and have similar adult-juvenile associations although not within nodules. Cylindrical Reniformichnus burrow casts are also present. The Brazilian specimens are mostly disarticulated elements within lenses of intraformational conglomerate, however, some articulated specimens have recently been found in calcareous nodules.

The general palaeoenvironment of all the P. trigoniceps bearing intervals is of warm drought-and-deluge prone floodplains between low sinuosity anastomosing channels close to ephemeral ponds and further evidence for continued Gondwana-wide climatic instability following the end-Permian mass extinction. The taphonomic evidence supports previous suggestions that P. trigoniceps was a group-living, possibly communal, fossorial reptile analogous in its life habits to Gopherus agassizii, an extant North American desert tortoise.