Thewissen et al. reply

Abstract

Replying to: J. H. Geisler & J. M. Theodor Nature 458, 10.1038/nature07776 (2009)

The analysis of Geisler and Theodor¹ confirms our main phylogenetic result², that raoellids are, or include, the sister group to cetaceans. Their study expands on our findings by inferring that hippopotamids are the sister group to the combined raoellid–cetacean clade, whereas our paper had explicitly stated that our data could not address the position of the extant artiodactyl families.

Main

Geisler and Theodor¹ place confidence in their results with regard to hippopotamids by stating that their analysis is consistent with “previous phylogenetic studies”. However, one of the two articles that they cite³ was published after the publication of our paper, and they do not cite a recent paper⁴ that disagrees with their (and our own) results.

Geisler and Theodor¹ execute two analyses, one with equally weighted characters and one with down-weighted characters (Appendix Fig. 3 of their paper). They state that in both analyses Hippopotamidae is the closest extant taxon to Cetacea, but their Appendix is inconsistent with this statement. In the Appendix, a large polychotomy lies at the base of the raoellid–cetacean clade in the equally weighted analysis. This polychotomy dissolves when Andrewsarchus is excluded from the study, which results in a strict consensus tree where hippopotamids are indeed the sister group to the raoellid–cetacean clade. Instead of discussing this consensus tree, Geisler and Theodor¹ mention that the equally weighted analysis yielded two most parsimonious trees: one showing hippopotamids as the sister group to the raoellid–cetacean clade and the other showing (apparently undeleted) Andrewsarchus as more closely related to the raoellid–cetacean clade than to hippopotamids. Notably, it took the deletion of Andrewsarchus before hippopotamids could be identified as the raoellid–cetacean sister group in the unweighted analysis of the Appendix of Geisler and Theodor¹; the text seems to identify Andrewsarchus as a possible close relative of hippopotamids and this is consistent with another recently published cladogram⁴. Andrewsarchus is poorly known, and this may result in its unstable phylogenetic position, although it may still be part of the hippopotamid–raoellid–cetacean clade.

Geisler and Theodor¹ agree with us that the phylogenetic link between cetaceans and raoellids is closer than that between cetaceans and hippopotamids, and we consider the former link to be more robust than the latter. We also believe that improving fossil collections of poorly known taxa is important in advancing understanding of cetacean relationships. Hippopotamids are certainly closely related to cetaceans, yet they appear about 35 million years after the first cetaceans, and their origins are in Africa, not the birthplace of cetaceans.

Geisler and Theodor¹ further elaborate on our results by discussing the origin of osteosclerosis of limbs and bulla, calling this process pachyostosis (for definitions, see refs 5, 6). Osteosclerosis, but not pachyostosis, of the limbs occurs in hippopotamids, raoellids and pakicetids, whereas pachyosteosclerosis of the bulla occurs in raoellids and pakicetids, but not in hippopotamids. Experimental studies with lipoprotein-receptor-related proteins in transgenic mice suggest that pachyostosis and osteosclerosis are underlain by different genetic mechanisms⁷. The mechanisms that lead to such hyperostoses deserve further study, and such a study is necessary to test the speculation of Geisler and Theodor¹ that hyperostoses of the limbs and bulla are related. Such a study should be rooted in a sound understanding of the developmental controls of bone formation, a field that is growing rapidly⁸ and that promises to elucidate cetacean evolution considerably.