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Models Point to Ongoing Placental Mammal Diversification During Mass Extinction Event

NEW YORK (GenomeWeb) – Placental mammals may have continued to diversify during a mass extinction event in which large swaths of plants and animals fell by the wayside.

Researchers from China, the US, Singapore, and the UK brought together sequences spanning nearly 4,400 genes in dozens of existing mammalian taxa in an effort to understand placental mammal diversification in relation to a mass extinction that took place during the shift from the Cretaceous Period to Paleogene portion of the Tertiary Period, known as the KPg boundary — a time when large swaths of plant and animal species are believed to have been snuffed out.

"[W]e present a hypothesis of mammalian divergence timing that is more consistent with the fossil record than previous molecular clock reconstructions, suggesting that placental mammals underwent a continuous radiation across the KPg boundary," co-corresponding authors Shaoyuan Wu, a phylogenomics and comparative genomics researcher affiliated with Jiangsu Normal University and Tianjin Medical University, and his colleagues wrote in the Proceedings of the National Academy of Sciences online this week.

With the help of clues from the fossil record, the researchers used several analytical approaches to put together placental mammal phylogenies based on sequences for as many as 4,388 loci in 82 mammalian species, spanning 20 existing placental mammal orders. After folding in gene sequence data for eight non-mammal vertebrate animals, they focused on nearly 3,900 genes present in all 90 mammalian and non-mammalian taxa.

The team also established new transcriptomic profiles for flying lemur and pangolin representatives with Illumina HiSeq 2000 RNA sequencing on fibroblast cell lines from the Malayan flying lemur and from Sunda pangolin frozen muscle samples.

"To achieve reliable dating, we scrutinized molecular clock methodologies in detail, showing that gene partitioning schemes, clock models, and genic deviation from the assumptions of a molecular clock have a pronounced impact on dating placental divergence," the authors explained.

Results from the researchers' models refined placental mammal relationships, for example, while their molecular clock estimates pointed to a range of potential diversification dates for placental mammals. Even so, by applying multiple models to distinct data sets, they were able to conclude that placental mammal diversification rates did not change significantly over the course of the KPg boundary.

Instead, the team reported, this diversification seemed to follow a so-called trans-KPg model marked by a steady flow of diversification that was relatively unchanged before, after, and during the KPg transition.

"Our results suggest resolution of some major controversies in the mammalian tree and a hypothesis of placental diversification, termed the 'trans-KPg model,' which uniquely envisions the rise of placental orders as an uninterrupted radiation across the KPg boundary," Wu and co-authors wrote.