Tip dating supports novel resolutions of controversial relationships among early mammals

King, B; Beck, RMD

Abstract

The estimation of the timing of major divergences in early mammal evolution is challenging due to conflicting interpretations of key fossil taxa. One contentious group is Haramiyida, the earliest members of which are from the Late Triassic. Many phylogenetic analyses have placed haramiyidans in a clade with multituberculates within crown Mammalia, thus extending the minimum divergence date for the crown group deep into the Triassic. A second taxon of interest is the eutherian Juramaia from the Middle-Late Jurassic Yanliao Biota, which is morphologically very similar to eutherians from the Early Cretaceous Jehol Biota and implies a very early origin for therian mammals. Here we apply Bayesian tip-dating phylogenetic methods to investigate these issues. Tip dating firmly rejects a monophyletic Allotheria (multituberculates and haramiyidans), which are split into three separate clades, a result not found in any previous analysis. Most notably, the Late Triassic Haramiyavia and Thomasia are separate from the Middle Jurassic euharamiyidans. We also test whether the Middle–Late Jurassic age of Juramaia is ‘expected’ given its known morphology by assigning an age prior without hard bounds. Strikingly, this analysis supports an Early Cretaceous age for Juramaia, but similar analyses on twelve other mammaliaforms from the Yanliao biota return the correct, Jurassic age. Our results show that analyses incorporating stratigraphic data can produce results very different from other methods. Early mammal evolution may have involved multiple instances of convergent morphological evolution (e.g., in the dentition), and tip dating may be a method uniquely suitable to recognising this due to the incorporation of stratigraphic data. Our results also confirm that Juramaia is anomalous in exhibiting a much more derived morphology than expected given its age, which in turn implies very high rates of evolution at the base of therian mammals.

Citation

King, B., & Beck, R. (2020). Tip dating supports novel resolutions of controversial relationships among early mammals. Proceedings of the Royal Society B: Biological Sciences, 287(1928), https://doi.org/10.1098/rspb.2020.0943

• Journal Article Type: Article
• Acceptance Date: May 13, 2020
• Online Publication Date: Jun 10, 2020
• Publication Date: Jun 2, 2020
• Deposit Date: May 13, 2020
• Publicly Available Date: May 13, 2020
• Journal: Proceedings of the Royal Society B: Biological Sciences
• Print ISSN: 0962-8452
• Electronic ISSN: 1471-2954
• Publisher: The Royal Society
• Volume: 287
• Issue: 1928
• DOI: https://doi.org/10.1098/rspb.2020.0943
• Publisher URL: https://doi.org/10.1098/rspb.2020.0943
• Related Public URLs: http://rspb.royalsocietypublishing.org/