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.

23 Comments

  1. gbjames
    Posted February 11, 2013 at 3:03 pm | Permalink

    sub

  2. Matthew Cobb
    Posted February 11, 2013 at 3:05 pm | Permalink

    Another thoughtful and informative post, Greg! Thanks!

  3. Diane G.
    Posted February 11, 2013 at 3:20 pm | Permalink

    Nice summation of all the issues raised in the previous post; thanks. And I so agree with everything you said about publishing venues!

  4. Frank
    Posted February 11, 2013 at 3:43 pm | Permalink

    “And shouldn’t the richer information available in molecular sequence data that is time-calibrated by securely known fossil dates be used?”

    Yes, and many examples have shown how clock-like data can supersede fossil information. So I anticipate a lively exchange in the Science correspondence section from those who have constructed extensive molecular phylogenies of mammals with specified branch lengths.

    We should always remember when Vincent Sarich used the earliest, crude, clock-like molecular data (proteins) in the late 60s to date the human-chimp lineage split to only 5 million years ago. This was anathema to paleoanthropologists, who thought it was at least 15 million years ago! Sarich’s estimate turned out to be too recent, but he was closer to the mark, and, as he famously said,“I know my molecules have ancestors, you must prove your fossils had descendants.”

    Thanks for the NICE summary of the issue for those of us who don’t know anything about mammals.

  5. Posted February 11, 2013 at 3:46 pm | Permalink

    “Whose right?”? What are you, Greg, some kind of creationist? :-D

    Very insightful analysis. Many thanks.

    /@

  6. cherrybombsim
    Posted February 11, 2013 at 4:54 pm | Permalink

    Whose right? Maybe both, if molecular divergence does not parallel morphological divergence. Maybe it’s just me, but I am getting the feeling that this is starting to be a familiar pattern. Genetic divergence rooted long before a major break like the K-Pg boundary suddenly exploding into morphological diversity after it.

    • pjie2
      Posted February 12, 2013 at 8:19 am | Permalink

      Do cryptic species persist for long enough for this to be a plausible explanation? Savannah and forest elephants are an example among living species, but they diverged only a couple of million years ago. For this suggestion to be right, the species would have had to remain cryptic for tens of millions of years.

  7. Jim Thomerson
    Posted February 11, 2013 at 6:25 pm | Permalink

    http://www.sherwincarlquist.com/intercontinental-dispersal.html

    This is an interesting read. When you get about halfway down, you get some of the late Donn Rosen’s arguments against proposing dispersal as explaining the distributions of related organisms. I recall at one time he held that the only acceptable evidence of dispersal was the sympatric occurrence of sister species. I don’t recall how this all settled down.

    Donn Rosen was one of the most vociferous promoters of cladistics, to the extent that some began to refer to the “Cladistic Religion”. Anyway it looks like things have cooled down to the point that people, having done a major study, could feel comfortable proposing long distance dispersal as explaining distributions.

  8. Thanny
    Posted February 11, 2013 at 6:45 pm | Permalink

    Ahem.

    Whose right? As in, to whom does the right belong?

    I think you meant to ask, who’s right?

    • whyevolutionistrue
      Posted February 11, 2013 at 7:05 pm | Permalink

      Sorry. Fixed now.

      GCM

  9. John Harshman
    Posted February 11, 2013 at 9:28 pm | Permalink

    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.

    This isn’t necessarily commendable. There is considerable research showing that including even poorly known fossils can improve phylogenetic inference. John Wiens in particular has published a simulation studies on this.

  10. Hempenstein
    Posted February 11, 2013 at 9:55 pm | Permalink

    As a practical matter, if that’s an acceptable way to preface this, is the most immediate significance of this study that it gives support to the notion that the available fossil record does cover the early divergence of mammals?

    • whyevolutionistrue
      Posted February 11, 2013 at 10:29 pm | Permalink

      “… the early divergence of placental mammals”; mammals as a whole came much earlier. It isn’t so much that it supports this notion, as that it assumes this notion.

      GCM

      • Diane G.
        Posted February 11, 2013 at 10:42 pm | Permalink

        This would be a lot less confusing to people if, instead of “placental mammals” the group were known by something completely different sounding, like the monotremes and marsupials are…

        (Or even if they were called “monotreme mammals” and “marsupial mammals.” :D )

      • Hempenstein
        Posted February 12, 2013 at 7:09 am | Permalink

        Thx!

  11. madscientist
    Posted February 12, 2013 at 2:02 am | Permalink

    Don’t expect plate tectonics to have any influence over a relatively short period (a few million years). The damned plates are slow. The East African Rift has been separating for many millions of years. Parts of California won’t be islands just off the coast for a very long time yet. Of course ultimately the plates do play a part when islands are separated enough that animals can’t easily migrate. Many of the islands/continents would have separated long before the existence of the terrestrial tetrapods.

    • John Scanlon, FCD
      Posted February 12, 2013 at 7:17 am | Permalink

      The African rift is practically stalled, like the Rhine-Rhône scar across Europe. But look at India in the Cretaceous, that’s fast. :)

  12. Dominic
    Posted February 12, 2013 at 2:40 am | Permalink

    “Many fossil mammals are preserved only as teeth and thus hard to identify conclusively” – is it not true though that most palaeontologists work with teeth rather than bones? Most vertebrate fossils surely anyway?

    • John Scanlon, FCD
      Posted February 12, 2013 at 7:26 am | Permalink

      Many mammals can be identified and interpreted pretty well from jaws or even isolated teeth, but most mammal teeth (molars, anyway) are far more complicated (informative) than those of typical vertebrates. If you work on fossil reptiles, fish, amphibians, birds, or toothless mammals, you have to learn some actual anatomy :). It also helps with mammals (O’Leary et al. only scored 1451 of their 4541 characters from the dentition).

  13. gerdien
    Posted February 12, 2013 at 3:39 am | Permalink

    The geographical distribution of the fossil record from 50 Myr before to 10 Myr after the K-Pg boundary might be interfering with the dating of the phylogeny. All Cretaceous mammals are from China. The Paleogene mammals are from elsewhere. No early fossil Afrotheria from Africa – why? Do the appropriate fossil layers not exist? has nobody looked for them?
    Why by the way is Eomaia not an Eutherian mammal in this article?
    Generally, I’m betting on the molecules as more accurate for timing. Partly is this because the earliest Paleocene animals as Mimotona or the Carnivore Ravenictis (about 63 Myr ago)are not far from the K-Pg boundary. And that radiation really has to be fast if it is to be all in the Paleocene … Any comparitive evidence about radiation speed?

  14. John Scanlon, FCD
    Posted February 12, 2013 at 7:08 am | Permalink

    Regarding the accessibility of the paper, a colleague has pointed out that, seeing as the paper in Science itself is just an ‘extended abstract’ and the real meat is all in the 131-page Supplementary Material downloadable here, it’s not exactly hidden.

    Nor is the paper itself behind an actual paywall.

    • John Scanlon, FCD
      Posted February 13, 2013 at 8:18 pm | Permalink

      No response, everybody must be off reading the supplement…

  15. marksolock
    Posted February 12, 2013 at 7:34 pm | Permalink

    Reblogged this on Mark Solock Blog.


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