by Greg Mayer (Updates below.)
A new study just published in Science by Maureen O’Leary and colleagues examines the phylogeny of 40 fossil and 46 extant mammals based on a very large data set of morphological and molecular characters (the latter only from the living taxa). The study has gotten a fair amount of attention in the press, where it seems to have been misinterpreted; more on that later. First, let’s see what they were trying to do and what they found.
There are three major groups of mammals alive today: the egg-laying monotremes (the platypus and the echidnas), the marsupials (opossums, kangaroos, bandicoots, etc.: a few hundred species) and the placentals (cats, dogs, cattle, deer, and all the rest, including primates: several thousand species in about 18 orders in all). The placentals are the overwhelming majority of extant mammals, and dominate the mammalian fauna of all parts of the world except Australia, which has mostly marsupials. (There are a fair number of marsupials in South and Central America, but they are still outnumbered by placentals.)
In the fossil record, although basal placentals are known from the Cretaceous (some of these fossils are disputed, including by O’Leary et al., but all agree there were some), the great radiation of placental mammals did not occur until the early Cenozoic, after the extinction of the dinosaurs (at least those that had not evolved into birds) at the end of the Cretaceous, about 65 mya. Although the first two-thirds of mammalian history had occurred during the Mesozoic (the “Age of Reptiles”), they really broke out, biodiversity-wise, in the Cenozoic (the “Age of Mammals”).
There’s been considerable debate over whether the placental lineages that radiated in the Cenozoic arose just before the radiation (the “explosive model”), had existed since earlier in the Cretaceous but did not radiate until the Cenozoic (the “long-fuse model“), or had undergone considerable diversification in the Cretaceous (the “short-fuse model“). The latter model would require that the fossil record be seriously incomplete, but has been supported by various molecular phylogenetic studies that estimate various splits among the extant placentals to have occurred well before 65 mya.
The chief question O’Leary and colleagues addressed was which of these models is correct. To do so, they scored over 4000 morphological characters (including soft-tissue characters generally not scorable in fossils) and utilized 27 nuclear gene sequences to estimate the branching sequence. They then added in the known stratigraphic range of the fossils to get a phylogenetic tree (their Fig. 1) that looks very much like the explosive model above, except that the common ancestor of extant placentals (the “P” in the figure) occurred jut over the line, in the earliest Cenozoic rather than the Cretaceous, making it even a little bit more explosive-y. “Explosive it is, sir!”, as Apu on the Simpsons might have put it.
They make two further interesting inferences from their tree. First, they use their morphological data set to estimate what this earliest Cenozoic common ancestor of all placental mammals looked like. It looks like this:
With such a large data set this is interesting, but it does look pretty much like what people have long thought early placentals would look like. Remember, this is a hypothetical common ancestor, not a newly found fossil.
Second, because much of the breakup of the Mesozoic super-continents had occurred by the early Cenozoic, they infer that a lot of dispersal occurred in the placental radiation, and not just passive floating around on the drifting continental plates. The exact arrangement of lineages in the tree is also of interest, and will be discussed and debated by mammalogists. It’s not clear to me that a huge data set is necessarily an advantage in inferring this large scale phylogeny, because we don’t understand the dynamics of conservatism and lability of morphological characters in the way we do for genetic sequence data. Our understanding of the latter allows us to select genes and use methods of analysis appropriate for a particular question. Using thousands of morphological characters seems a bit too reminiscent of the old pheneticists’ hope that if they could score enough characters, “parametric overall similarity” could be known (phenetics didn’t pan out as hoped). I hasten to add that morphological characters are more difficult because they are more complex and more crucial to the organism, and consequently more interesting– indeed, what most biologists are really interested in– not because there is something wrong with studying morphology.
In the media, there has been considerable confusion about this study, in part because the distinction between mammals and placental mammals has not always been kept clear. The New York Times initially led with the headline:
“Common Ancestor of Mammals Plucked from Obscurity”;
but, of course, the study is not about the common ancestor of mammals, but only placentals. And furthermore, there is no particular known fossil which is being identified as or compared to this placental common ancestor; the ancestor in the picture, as stressed here, is hypothetical. Yet, the Times article identifies Protungulatum as the ancestral placental, O’Leary et al. most definitely do not do do: they identify Protungulatum as a member of the lineage that gave rise to (most) hoofed mammals (i.e. quite far from the common ancestor of all placentals). Protungulatum is the oldest known member of the clade that includes all extant placentals, but that does not make it the common ancestor.
This misunderstanding has infected the news media, and spread widely. Gizmodo labels a picture of the hypothetical form (a version of the figure above) as Protungulatum, and states
This rat with way too many sharp teeth is your great x 4 x 10^6-grandmother. That’s what scientists have discovered after six years of research—the Protungulatum donnae is the common ancestor to all mammals, from humans to horses to lions.
This is pretty much completely wrong. And UPI labels the same figure as
An artist’s rendering of Protungulatum donnae,
which it isn’t at all. The Times, at least, subsequently changed its headline to
“Rat-Size Ancestor Said to Link Man and Beast”,
which is still pretty obscure, but not actually wrong. But the article retains its misstatements about Protungulatum (at least last I checked). The Times did correct another error. They had initially stated that only a single mammalian lineage had survived the end-Cretaceous extinction, but it is known that there were at least four surviving lineages (one monotreme, one marsupial, one placental, plus one multituberculate– a now extinct mammal group which survived the end-Cretaceous extinction, but died out in the Oligocene). The article has now been corrected to say that the study concludes only one placental mammal survived (which is indeed what its major conclusion is).
The article in the BBC was better, getting the headline right:
“Earliest placental mammal ancestor pinpointed”
and not mentioning Protungulatum at all. The BBC front page headline, however, was off:
“Earliest mammal ancestor pinpointed”.
UPDATE. I had left the following comment out, because I thought I was saying it too often, but this paper really shouldn’t have been published in Science. There is much too much data, methods, analysis, and discussion left out of the paper because of Science‘s severe length limits. There are two online “supplements”, one at Science and another at morphobank.org. The one at Science is 132 pages long. The morphobank supplements are not organized as a file, so it’s hard to tell how much is there, but it’s a lot. Now some of this material (e.g. lists of which authors examined which specimens) need not be published, but it’s simply impossible to fully understand or critique the paper with out referring to a great deal of this material, which is not readily available to someone in possession of a copy of the paper. The authors have shortchanged themselves and their readers by publishing in such a venue. I was moved to add this update after an alert reader noted an error in my statement of the number of species included, and I had to pore though the supplements to verify the correct numbers because of ambiguous wording in the paper. I was able, while doing so, to confirm that the same extant 46 species were used for the genetic and morphological analyses. (And since I’m kvetching, I’ll note that the authors substitute the grotesque, poorly defined, and unnecessary word “phenomic” for “morphological” (or a similar word) throughout their paper.)
UPDATE 2. The errors in the media coverage do not stem from SUNY Stony Brook’s press release, which correctly summarizes the claims of the paper.
O’Leary, M.A., et al. 2013. The placental mammal ancestor and the post-K-Pg radiation of placentals. Science 339:662-667. (abstract)
Rose, K.D. 2006. The Beginning of the Age of Mammals. Johns Hopkins University Press, Baltimore. (Google Books)
Yoder, A.D. 2013. Fossils versus clocks. Science 339:656-658. (abstract)