Molecular dates and the mammalian radiation.

In a perspectives article, Bromham et al.1xBromham, L., Phillips, M.J., and Penny, D. Trends Ecol. Evol. 1999; 14: 113–118Abstract | Full Text | Full Text PDF | PubMed | Scopus (93)See all References1 raise some important concerns about the use of both molecular and paleontological data in assessing the timing of diversification of extant mammalian orders. In Box 1, they describe well the need to differentiate crown (or more generally, node-based) taxa from stem-based taxa. Unfortunately, their Fig. 2 is not clearly explained and thus confuses these kinds of taxa. As they note, the paleontologically based orders of extant placental mammals (except, perhaps, Insectivora) are recognized as appearing soon after the Cretaceous–Tertiary (K–T) boundary (thick lines in their Fig. 2). These dates are based on apomorphy- or node-based intraordinal diversifications for the order in question. In contrast, the extensions into the Late Cretaceous of clades shown in their Fig. 2 (the thin lines), estimated by molecular data, are interordinal separations. Thus, in at least this comparison, the molecular data indicate nothing about ordinal origination and diversification, but rather argue only that stem-based clades extend into the Late Cretaceous. Furthermore, the possible Cretaceous record of primates they mention in their text is a single tooth originally assigned to the primatomorph Purgatorius, which was discovered at a site now regarded as Paleocene in age2xLofgren, D.L. Univ. Calif. Publ. Geol. Sci. 1995; 140: 1–200See all References2.The authors’ biogeographical assessment of fossil taxa requires updating. The possible placental for the early Cretaceous of Australia is now regarded by most as symmetrodont3xKielan-Jaworowska, Z., Cifelli, R.L., and Luo, Z. Lethaia. 1998; 31: 267–268CrossRefSee all References3 or early therian. Thus, although marsupials are known for the early Eocene of Australia4xGodthelp, H. et al. Nature. 1992; 356: 514–516CrossRef | Scopus (53)See all References4, non-chiropteran placentals do not appear until the Pliocene5xClassification of Mammals. McKenna, M.C. and Bell, S.K. See all References5. In South America, all definite pre-Tertiary mammals are non-therians, with both marsupials and placentals appearing only after the K–T boundary6xFlynn, J.J. and Wyss, A.R. Trends Ecol. Evol. 1998; 13: 449–454Abstract | Full Text | Full Text PDF | PubMed | Scopus (128)See all References6. As the authors note, the Late Cretaceous of North America and Asia have a good record of mammals, but except for, perhaps, Insectivora, no modern orders of placentals are known. In fact, the latest Cretaceous record7xClemens, W.A. et al. : 7–58See all References7 is better known than the earliest Paleocene8xArchibald, J.D. et al. : 24–76See all References8. Europe is not well known but echoes what is known in Asia for placentals9xGheerbrant, E. and Astibia, H. Comptes Rendus Ser. II. 1994; 318: 1125–1131See all References9. As the authors also note, Africa is a cipher. Unless, however, one wishes to make the unsubstantiated argument that all 18 orders of extant placentals arose in Africa, the claim is not valid that the biogeography of placentals is too poor to help in deciphering ordinal appearances.