More on placental mammals

by Greg Mayer

There have been a number of interesting comments by readers on my post on the recent paper on the radiation of placental mammals by Maureen O’Leary and colleagues. I want to respond briefly to a few of them here.

Biogeography. Does this paper imply that the origin and geographic distribution of the  major lineages of placental mammals are not well correlated with the breakup of the Mesozoic super-continents? Yes, it does. The authors explicitly say so, and therefore would invoke more dispersal events to account for mammalian distribution. Now their cladogeny and timing may be wrong, and lack of congruence with plate movement might be a reason for preferring an alternative phylogeny, but the authors do correctly recognize the biogeographic implications of their phylogeny.

G.G. Simpson, one of the founders of the Modern Synthesis, was also one of the most influential mammalogists of the 20th century, who often dealt with large scale issues of mammalian history. He delayed accepting plate tectonics until much later than most other zoogeographers, in part because many major continental movements took place well before the diversification of modern mammals, and thus plate tectonics seemed ‘unnecessary’ as part of an explanatory schema. He might well have been pleased by this aspect of O’Leary and her colleagues’ work.

Publication venue. I criticized the publication as a short paper in Science of a work that clearly deserves and requires monographic treatment. There is an obviously correct place to publish this work: in the Bulletin of the American Museum of Natural History. Many of the authors hold positions at the American Museum, and the Bulletin is explicitly designed for the publication of monographic works. Indeed, Simpson published one of his most important monographs on mammalian classification in the Bulletin. As the preceding link shows, the AMNH, quite admirably, makes all its publications available as free pdf’s, so there would be no question of access. In fact, access would be much greater, since all the material would be in a single freely available work, and not dependent on accessing a variety of websites of unknown permanence and varying cost.

Some readers have noted the tyranny of popularity and attention that journals like Science and Nature exert, and I can certainly sympathize with the authors’ desire to have their work widely read. But ultimately, scientific work must be judged by its data, methods, and conclusions, and publication in Science hampers the paper’s evaluation as a work of science. Science has published summary papers that present the main conclusions of monographic works; Jared Diamond’s 1973 paper in Science summarizing his 1972 monograph on New Guinean birds is an example. As Diamond wrote, “A recent book discusses in detail many of the examples summarized here”, but the monograph to explicate O’Leary’s work may never appear. Perhaps Science no longer does this, but a near simultaneous publication of short summary and Bulletin would have been far preferable.

Are the conclusions correct? This is the $64,000 question. I think the initial critiques come in two parts. First, don’t we already have fossil representatives in the Cretaceous of several of the modern orders of placental mammals? Well, a number of fossils have been so identified, but O’Leary et al. (and others) would dispute these identifications. Their paper does not include a careful analysis of these cases, and their fossil sample is not exhaustive, but does include most of the very well known Cretaceous mammals. Many fossil mammals are preserved only as teeth and thus hard to identify conclusively; O’Leary et al. commendably included only the most completely known forms so as to be able to observe as many as possible of the large number of characters they used. They do agree that there are basal placentals in the Cretaceous. (Or, to use the term they would probably prefer, “non-placental eutherians”. Eutherian and placental are treated as synonyms by some, but they formally distinguish the Placentalia as only members of the least inclusive clade that includes all living placental mammals; these taxon name questions are not important for their main points.) But these Cretaceous forms are, by their estimation, not in general ancestral to the Cenozoic forms– they believe only a single placental lineage survived into the Cenozoic.

Second, critics ask, isn’t using the literal fossil record a pretty crude way of determining ages of taxon splits, since such ages are always minimum ages? And shouldn’t the richer information available in molecular sequence data that is time-calibrated by securely known fossil dates be used? Well, the critics will answer “yes” to both questions, and will also point out that the fossil record is imperfect, so to say we don’t have any fossils dated to the Cretaceous is different from saying no such animals existed then. O’Leary et al. might reply that all molecular dating requires geological calibration, so that the fossil data is primary, not the molecular extrapolation; and that we have lots of Cretaceous mammal fossils, and none of them are obviously the varied precursors of the Cenozoic placental radiation.

(There are also questions about the exact sequence of splits in their phylogeny, and how molecular and morphological data agree or disagree. These discussion will be of most intense interest to specialists in the various groups, although there is considerable general interest in them as well.)

Who’s right? I don’t know. But that’s what the upcoming arguments will be about.