Ossification patterns and the phylogeny of mammals

Afrotherian embryos. Reproduced from Hautier et al. (here)

© 2013 The Authors

High resolution X-ray microtomography is among new methods available to comparative anatomists. It enables non-invasive imaging of museum specimens and subsequent 3D visualisation of the tissues. One field where it has been applied is to study the sequence in which bones become ossified during embryonic and fetal development.

An obvious requirement is that developmental series are available as for the African elephant (here) and tail-less tenrec (here). A new study from the Department of Zoology at Cambridge (here) analyzes data from a wider range of afrotherians. It then compares ossification patterns in "southern" mammals (Afrotheria and Xenarthra) and "northern" mammals (Laurasiatheria  and Euarchontoglires).

The "northern" mammals share a common ancestor (together constituting Boreoeutheria). There is little variability in developmental sequences between species available for study. In contrast both groups of "southern" mammals exhibit shifts in ossification sequences. At first sight this might seem to argue for a common ancestor for Afrotheria and Xenarthra. However, the shifts are different in the two clades and this could argue against a common ancestry.

The data are intriguing as we still lack certainty about the root of the tree. This is largely because the four major clades diverged so rapidly early in the history of placental mammals. Even phylogenomics has yielded conflicting results (discussed here).