EAVP 2021: You heard it here first: results support a Cretaceous origin for placental mammals

Carlisle, Sivestro and Donoghue 2021 report,
“Recent molecular clock analyses have suggested that placental mammals originated in the mid- to late Cretaceous, before the Cretaceous-Paleogene (K-Pg) mass extinction. However, there are no unequivocal fossils of placental mammals from the Cretaceous to support this.”

Incorrect. The large reptile tree (LRT, 1890+ taxa; subset Fig. 1) recovers several Cretaceous and Late Jurassic mammals as placentals (Fig. 1). Run your own tests. University textbooks must be behind the times if this is what students and workers believe coming out of Bristol.

Figure 1. Select basal cynodonts and mammals set chronologically. The divergence times for placentals (Eutheria), marsupials (Metatheria) and monotremes (Mammalia) are estimated here.
Figure 1. Select basal cynodonts and mammals set chronologically. The divergence times for placentals (Eutheria), marsupials (Metatheria) and monotremes (Mammalia) are estimated here.

Carlisle, Sivestro and Donoghue 2021 continue,
“Definitive fossils of placental mammals only appear after the K-Pg boundary, at which point they rapidly radiate leading into the ‘Age of Mammals’.”

This should be the last time this myth is repeated or printed.

“Here we use the Bayesian Brownian Bridge model to estimate the age of origin of placental mammals based on the fossil record. The model uses fossil diversity through time to inform a random walk from the clade’s present-day diversity back to the estimated origin of the clade within a Bayesian framework. This model works well with clades that have poor fossil records, such as the early placental mammals, and does not require a phylogeny, thereby mitigating the lingering uncertainty over the branching pattern at the root of the placental tree of life.”

No reason to estimate. Just create a cladogram. It’s a powerful tool that readily answers many questions like this. Not sure why more students and professionals don’t do this. Leaving issues like this for retired amateurs is no way to run a profession. And don’t borrow someone else’s cladogram. Create your own. You’ll have a powerful tool you can use the rest of your career into retirement.

“Our results support a Cretaceous origin for placental mammals, in agreement with the molecular data, and demonstrate that the group was already present before the K-Pg mass extinction and experienced a radiation during the Paleogene. The Bayesian Brownian Bridge model can therefore help to reconcile paleontological data with molecular data when estimating the origin of clades.”

Who knows what their molecular data recovers? We do know that molecule clades, like Afrotheria and Laurasiatheria, too often deliver false positives. Please use traits, not deep time DNA. The title of this abstract is a wee bit misleading. The authors did not use fossils from Cretaceous and Jurassic strata. They used estimates.

References
Carlisle E, Silvestro D and Donoghue P 2021. The origin of placental mammals according to the fossil record. EAVP abstracts 2021.

Bayesian Brownian Bridge model

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