Data from: 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
completeness overall than other tetrapod clades, including the
contemporaneous anomodont synapsids. This discrepancy could be due to
differences in the diagnosability of their fossils, especially for
small-bodied species. Establishing the timing and assembly of derived
(‘mammalian’) anatomical features in Cynodontia is obscured by sampling.
Two of the major nodes at which acquisition of mammalian features is
concentrated (Cynodontia and Mammaliamorpha) suffer from lengthy intervals
of poor sampling prior to becoming abundant parts of tetrapod faunas. Low
completeness in these intervals limits our ability to determine when
certain ‘key’ mammalian characteristics evolved, or to identify the
selective pressures that might have driven their origins.