Abstract
AbstractThere 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, while 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 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, which quantifies the completeness of fossil taxa as the percentage of phylogenetic characters available to be scored for any given taxon. Our data set comprised 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/Paleogene (K/Pg) 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 K/Pg boundary and diversified soon after.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 tectonic activity might play a role in controlling the completeness of the eutherian fossil record.
Highlights
One of the most significant problems in reconstructing the tree of life is reconciling conflicting signals from independent data sources, for example, between fossil and molecular data
The character completeness metric (CCM) increases toward the present; the rise in the CCM was mostly a gradual one, and no two consecutive geological-stage bins showed a significant difference from each other (Supplementary Table 3)
Variation in the CCM was greater in the Cretaceous than for the Paleogene, as shown, for example, by a doubling in the CCM in the space of just over 9 Myr from the Cenomanian to the Coniacian, and by larger error bars in many Cretaceous stages compared with those in the Cenozoic (Fig. 3)
Summary
One of the most significant problems in reconstructing the tree of life is reconciling conflicting signals from independent data sources, for example, between fossil and molecular data. The discrepancy between estimated dates and the fossil record has been considered to be unreasonably large; modern birds, for example, are predicted by various molecular studies to have originated and diversified early in the Cretaceous (Paton et al 2002; Brown et al 2008), but only disputed fragmentary fossils are known earlier than the latest Cretaceous (Brocklehurst et al 2012). Most unambiguous metazoan phyla first appear in the fossil record in strata that are early Cambrian in age, about 530 Ma (Wang et al 1999), but even recent molecular studies give divergence dates into the Cryogenian, well over 700 Ma (dos Reis et al 2015).
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