Data from: Completeness of the eutherian mammal fossil record and implications for reconstructing mammal evolution through the Cretaceous/Paleogene mass extinction

There is a well-established discrepancy between paleontological and molecular data regarding the timing of the origin and diversification of placental mammals. Molecular estimates place interordinal diversification dates in the Cretaceous, whilst no unambiguous crown placental fossils have been found prior to the end-Cretaceous mass extinction. Here, the completeness of the eutherian fossil record through geological time is evaluated in order to assess the suggestion that a poor fossil record is largely responsible for the difference in estimates of placental origins. The completeness of fossil specimens was measured using the Character Completeness Metric, a metric that quantifies the completeness of fossil taxa as the percentage of phylogenetic characters that are available to be scored for any given taxon. Our dataset was comprised of 33 published cladistic matrices representing 445 genera, of which 333 were coded at the species-level. There was no significant difference in eutherian completeness across the Cretaceous/Palaeogene boundary. This suggests that the lack of placental mammal fossils in the Cretaceous is not due to a poor fossil record, but more likely represents a genuine absence of placental mammals in the Cretaceous. This result supports the ‘explosive model’ of early placental evolution, whereby placental mammals originated around the time of the Cretaceous/Palaeogene boundary and diversified soon after the event. No correlation was found between the completeness pattern observed in this study and those of previous completeness studies on birds and sauropodomorph dinosaurs, suggesting that different factors affect the preservation of these groups. No correlations were found with various isotope proxy measures, but Akaike Information Criterion analysis found that eutherian Character Completeness Metric scores were best explained by models involving the marine carbonate strontium isotope ratios (87Sr/86Sr), suggesting that the tectonic activity might play a role in controlling the completeness of the eutherian fossil record