Placental mammals consist of three main groups that diverged rapidly, evolving in wildly different directions: Afrotheria (for example, elephants and tenrecs), Xenarthra (such as armadillos and sloths) and Boreoeutheria (all other placental mammals). The relationships between them have been a subject of fierce controversy with multiple studies coming to incompatible conclusions over the last decade leading some researchers to suggest that these relationships might be impossible to resolve.
There are thus many outstanding questions such as which is the oldest sibling of the three? Did the mammals go their separate ways due to South America and Africa breaking apart? And if not, when did placentals split up?
"This has been one of the areas of greatest debate in evolutionary biology, with many researchers considering it impossible to resolve," said lead author Dr Tarver of Bristol's School of Earth Sciences. "Now we've proven these problems can be solved - you just need to analyse genome-scale datasets using models that accurately reflect genomic evolution."
A Simple Method for Estimating Informative Node Age Priors for the Fossil Calibration of Molecular Divergence Time Analyses
Authors:
1. Michael D. Nowak (a)
2. Andrew B. Smith (b)
3. Carl Simpson (c)
4. Derrick J. Zwickl
Affiliations:
a. Institute of Systematic Botany, University of Zürich, Zürich, Switzerland
b. Department of Palaeontology, The Natural History Museum, London, United Kingdom
c. Museum für Naturkunde der Humboldt-Universität zu Berlin, Berlin, Germany
d. Department of Ecology and Evolution, University of Kansas, Lawrence, Kansas, United States of America
Abstract:
Molecular divergence time analyses often rely on the age of fossil lineages to calibrate node age estimates. Most divergence time analyses are now performed in a Bayesian framework, where fossil calibrations are incorporated as parametric prior probabilities on node ages. It is widely accepted that an ideal parameterization of such node age prior probabilities should be based on a comprehensive analysis of the fossil record of the clade of interest, but there is currently no generally applicable approach for calculating such informative priors. We provide here a simple and easily implemented method that employs fossil data to estimate the likely amount of missing history prior to the oldest fossil occurrence of a clade, which can be used to fit an informative parametric prior probability distribution on a node age. Specifically, our method uses the extant diversity and the stratigraphic distribution of fossil lineages confidently assigned to a clade to fit a branching model of lineage diversification. Conditioning this on a simple model of fossil preservation, we estimate the likely amount of missing history prior to the oldest fossil occurrence of a clade. The likelihood surface of missing history can then be translated into a parametric prior probability distribution on the age of the clade of interest. We show that the method performs well with simulated fossil distribution data, but that the likelihood surface of missing history can at times be too complex for the distribution-fitting algorithm employed by our software tool. An empirical example of the application of our method is performed to estimate echinoid node ages. A simulation-based sensitivity analysis using the echinoid data set shows that node age prior distributions estimated under poor preservation rates are significantly less informative than those estimated under high preservation rates.
