Lystrosaurus Subadults Were Trying to Keep Warm When They Died Creating Bonebeds in Induan (?) Triassic South Africa

Origin and palaeoenvironmental significance of Lystrosaurus bonebeds in the earliest Triassic Karoo Basin, South Africa

Authors:

Pia A. Viglietti, Roger M.H. Smith, and John S. Compton

Affiliations:

Earth experienced its most devastating extinction event at the end of the Permian period 251 million years ago (Ma). Despite an estimated 75 to 90% loss of species globally in both marine and terrestrial realms across the Permian–Triassic Boundary (PTB), around 20% of the terrestrial tetrapod genera in southwestern Gondwana survived and were immediately joined by a number of immigrant taxa to occupy most of the vacant niches of the earliest Triassic. Preserved in the Karoo Basin of South Africa is an almost continuous stratigraphic record of terrestrial sedimentation through the PTB that hosts a fossil record of ecosystem collapse, survivorship and recovery. The adaptation of the mammal like reptiles (therapsids) of the Lystrosaurus Assemblage Zone to a highly seasonal, potentially drought prone semi-arid earliest Triassic Karoo Basin is associated with changes in modes of fossilisation. Isolated dicynodont skulls and postcranial elements are commonly found in the latest Permian. However, in the earliest Triassic the dicynodonts occur as articulated, curled-up skeletons and multi-individual monotaxic bonebeds. Lack of epiphyses and relatively small skull length confirm that the bonebeds comprise several subadult Lystrosaurus declivis (L. declivis) carcasses. No significant evidence for hydraulic bone concentration along with clusters of ribs in life position points to complete carcasses being present at the site of death, and suggests that animals behaviourally congregated before perishing together. The bonebeds are hosted by an 8 m thick horizon of floodplain mudrocks in the lower Katberg Formation named the Lystrosaurus abundant zone. The bonebed horizon is overlain by sand-filled mud cracks capped by coarse sediments indicative of rapid deposition during waning floods. Stable isotope analyses of pedogenic and early diagenetic calcite nodules in association with the bonebeds yield average δ13C values ranging from − 9.5 to − 5‰ and δ18O values of 13.5 to 16‰, respectively. These isotope values support a seasonally cold, semi-arid climate at high latitude (~ 55°S). The presence of vertebrate burrow casts on bonebed horizons and evidence of shelter sharing suggests that tetrapods were attempting to escape extreme climatic conditions. Aggregation behaviour of subadult Lystrosaurus during unusual cold snaps may best explain the origin of bonebed assemblages.