Spatiotemporal variation in completeness of the early cynodont fossil record and its implications for mammalian evolutionary history

Mammals are the only surviving group of Cynodontia, a synapsid clade that first appears in the fossil record in the late Permian, ~260 million years ago. Here, using three metrics that capture skeletal completeness, we quantify the quality of the early cynodont fossil record in time and space to evaluate the impact of sampling and preservational biases on our understanding of the group’s evolutionary history. There is no consistent global sampling signal for early cynodonts. Completeness of the cynodont fossil record increases across the Permian/Triassic boundary, peaking in the Early–early Late Triassic. This peak is dominated by specimens from southern Africa and South America, where a highly seasonal climate likely favoured preservation. Completeness is generally lower thereafter, correlated with a shift from a Gondwanan to a predominantly Laurasian fossil record. Phylogenetic and stratigraphic congruence in early cynodonts is high, although their fossil record exhibits less skeletal...