Ancient dates or accelerated rates? Morphological clocks and the antiquity of placental mammals

Beck, RMD; Lee, MSY

Abstract

Analyses of a comprehensive morphological character matrix of mammals
using ‘relaxed’ clock models (which simultaneously estimate topology, divergence
dates and evolutionary rates), either alone or in combination with an
8.5 kb nuclear sequence dataset, retrieve implausibly ancient, Late Jurassic–
Early Cretaceous estimates for the initial diversification of Placentalia
(crown-group Eutheria). These dates are much older than all recent molecular
and palaeontological estimates. They are recovered using two very different
clock models, and regardless of whether the tree topology is freely estimated
or constrained using scaffolds to match the current consensus placental phylogeny.
This raises the possibility that divergence dates have been overestimated
in previous analyses that have applied such clock models to morphological
and total evidence datasets. Enforcing additional age constraints on selected
internal divergences results in only a slight reduction of the age of Placentalia.
Constraining Placentalia to less than 93.8 Ma, congruent with recent molecular
estimates, does not require major changes in morphological or molecular evolutionary
rates. Even constraining Placentalia to less than 66 Ma to match the
‘explosive’ palaeontological model results in only a 10- to 20-fold increase in
maximum evolutionary rate for morphology, and fivefold for molecules.
The large discrepancies between clock- and fossil-based estimates for divergence
dates might therefore be attributable to relatively small changes in
evolutionary rates through time, although other explanations (such as overly
simplistic models of morphological evolution) need to be investigated.
Conversely, dates inferred using relaxed clock models (especially with discrete
morphological data and MRBAYES) should be treated cautiously, as relatively
minor deviations in rate patterns can generate large effects on estimated
divergence dates.

Citation

Beck, R., & Lee, M. (2014). Ancient dates or accelerated rates? Morphological clocks and the antiquity of placental mammals. Proceedings of the Royal Society B: Biological Sciences, 281(1793), 20141278. https://doi.org/10.1098/rspb.2014.1278

• Journal Article Type: Article
• Acceptance Date: Jul 29, 2014
• Publication Date: Oct 22, 2014
• Deposit Date: Jan 20, 2015
• Journal: Proceedings of the Royal Society B
• Print ISSN: 0962-8452
• Electronic ISSN: 1471-2954
• Publisher: The Royal Society
• Peer Reviewed: Peer Reviewed
• Volume: 281
• Issue: 1793
• Pages: 20141278
• DOI: https://doi.org/10.1098/rspb.2014.1278
• Publisher URL: http://dx.doi.org/10.1098/rspb.2014.1278
• Related Public URLs: http://rspb.royalsocietypublishing.org/
• Additional Information: Projects : Using ancient fossils and new methods to unravel Australian mammal evolution in deep time