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.