Divergence Times of Eutherian Mammals

In the continuing debate about the timing of the origin of major extant placental clades, both proponents of a divergence after the Cretaceous-Tertiary (K-T) boundary and those advocating divergence deep within the Cretaceous too often miss what the Late Cretaceous record of placentals actually shows. For example, Foote et al. (1) are correct that many Late Cretaceous reports of extant ordinal and superordinal placental taxa are now questioned (2). They are incorrect, however in extending this statement to all such taxa, notably the “zhelestids.” Foote et al. essentially dismiss “zhelestids” as being “archaic eutherians allied with either Prokennalestes or zalambdalestids,” thus placing them outside the clade leading to extant placentals. These assertions are not based on the most recent, extensive phylogenetic studies, but rather come from English language publications that cite older, preliminary Russian descriptions of “zhelestids.” Recent fieldwork has yielded extensive marine invertebrate faunas overlying the “zhelestid” sites, thus demonstrating a minimum age of 85 million years ago (Ma) (3). Recent monographic studies and phylogenetic analyses strongly support the hypothesis that “zhelestids” form a series of stem taxa relative to early Tertiary archaic ungulates (so-called condylarths) that in turn are ancestral to a number of extinct and extant placental orders (4). Conversely, some molecular studies have used the “zhelestids” as evidence that some extant orders extend well back into the Late Cretaceous (5). Such conclusions, however, are unfounded. The most recent studies of “zhelestids” place them as a series of Late Cretaceous stem taxa most likely related to later ungulates, but the extant orders are not said to extend well into the Late Cretaceous. Thus, the fossil evidence (at least for now) supports the argument that there were some superordinal clades of extant placentals present by the Late Cretaceous, but such evidence cannot be used to extend extant ordinal appearances into the Late Cretaceous. In fact, another empirical study (6) that statistically examined the actual first appearances of extant orders in the fossil record does support the contention by Foote et al. that extant orders did not appear until shortly after the K-T boundary. Neither of these studies, however, contain data regarding the timing of appearance of superordinal clades of placentals. J. David Archibald Department of Biology, San Diego State University San Diego, CA 92182–4614, USA E-mail: [email protected] References 1. M. Foote, J. P. Hunter, C. M. Janis, J. J. Sepkoski Jr., Science 283, 1310 (1999). 2. M. J. Novacek, K. Gao, M. A. Norell, G. Rougier. JVP Abst. 18, suppl. to 3, 67A (1998). 3. J. D. Archibald et al., N. Mex. Mus. Nat. Hist. Sci. Bull. 14, 21 (1998). 4. L. A. Nessov, J. D. Archibald, Z. Kielan-Jaworowska. Bull. Carnegie Mus. Nat. Hist. 34, 40 (1998); J. D. Archibald, Science 272, 1150 (1996). 5. A. Cooper and R. Fortey, TREE 13, 151 (1998). 6. J. D. Archibald and D. H. Deutschman, paper presented at the International Symposium on the Origin of Mammalian Orders, Hayama Japan 21–25 October 1998.