Rise of the Mammals Redux

21 06 2007

Several months ago the blogosphere was all a-twitter about a new paper suggesting that modern mammals diversified much earlier than originally thought, DNA analysis of living mammals showing that modern mammal lineages may have developed well before the extinction of dinosaurs at the end of the Cretaceous (see my posts Oodles of Confusion, and Oodles of Confusion Part II). The study did note that mammals in general did benefit from the extinction of dinosaurs, many groups experiencing greater diversity after the Cretaceous, but these groups (like mesonychids and multituberculates) are now extinct and have left no descendants that we know of. Some scientists weren’t so sure about the new mammalian supertree however, and now a new Nature paper entitled “Cretaceous eutherians and Laurasian origin for
placental mammals near the K/T boundary” seems to support the idea that modern mammals did not experience large-scale diversity until they were “out from under the feet” of dinosaurs.

[If you have a Nature subscription, you can see the Letter here, as well as two commentaries on the DNA vs. Fossils debate here and here]

One of the major problems with reconstructing the origin and diversification of living mammals is that mammals do not preserve well. To the best of our understanding, there were not Cretaceous rhinos, giraffes, or elephants, nor would we expect to find them. Mammals were typically small and fragile (at least compared to their dinosaurian contemporaries), and so every mammal fossil is a treasure for those who want to reconstruct them and their history. Teeth are much more durable and long-lasting than bone, however, and many small extinct mammals are known primarily (if not exclusively) from their teeth. Because mammals have different shaped teeth throughout their jaws, teeth and jaws can quite diagnostic, and for the purposes of our discussion the typical eutherian dental formula is (incisors, canines, premolars, molars [upper jaw/lower jaw]) (Ji et al., 2002), as seen in the most basal known eutherian Eomaia.

The initial focus of the paper is a new mammal from Mongolia named Maelestes gobiensis, a eutherian (but not a true placental) mammal that lived between 75 and 71 million years ago, which means that Maelestes was more closely related to placental mammals than to marsupials or monotremes. Of course, its teeth take the prominent spot in the discussion of its skull in the letter, and Maelestes has a dental formula of ?.1.5.3/ (the ? mark for the upper incisors is because that part of the skull was missing, and while we can infer that Maelestes had incisors, how many is not known).

In order to find the proper place for Maelestes, the researchers compared 408 characters across 69 extinct and extant taxa (see the Supplementary Information, you’ll be glad you did). What immediately jumps out at the reader when looking at the results of the character states (i.e. is a canine tooth in the upper jaw present or absent?) is how fragmentary our evidence is. While the 408 questions were based on what we do have from Maelestes there are many, many “?” marks in the comparisons, suggesting that we can’t compare the two because we don’t have the fossil evidence for this. Teeth and skull structure are indeed important, but the more we can find from these small mammals the better as it will greatly clarify results when comparing different characters across taxa.

Nevertheless, based upon the 408 characters that have been gleaned from the remains of Maelestes, the relationship among mammals appears to support the “explosive” model of mammals greatly diversifying after the K/T Boundary. As the study points out, all known fossil placental mammals are younger than their closest sister group (identified by the genera Protungulatum, Oxyprimus and Purgatorius), and although their lineages could reach back into the Cretaceous, the lineages don’t appear to extend back to the early- or mid-Cretaceous like some molecular studies have proposed. This all brings to mind one of my favorite paintings by one of my favorite paleo-artists, Mark Hallett, depicting a multituberculate and opossum facing each other on top of a bleached Triceratops skull, the sun just peeking out from behind the clouds in the background signaling the “new dawn” of mammals (I can’t find the picture online to share it, but it is in Mitchell’s The Last Dinosaur Book if you own it).

The new study by Wible & co. is impressive because of the number of taxa and number of characters examined, making it perhaps the most rigorous comparison yet made. Indeed, they take some biogeopgraphy into account as well, the new tree suggesting that extant placental lineages arose in Asia, not South America as some have proposed (the basal status of Xenarthrans like anteaters seemed to support a South American origin, but the new paper removes the group from its basal status). I feel this is very important because we are often concerned when lineages arose, but in order to construct an accurate evolutionary tree we need to understand who was living where and when, too.

The case is far from closed, however, and I’m sure there will be further molecular studies and further fossil discoveries that may change the way the tree looks. As the supporting articles in Nature and others have noted before, there will hopefully be greater collaboration between those engaging in DNA studies and paleontology, but at the moment I would expect to see scientists as being fairly divided over the emergence of mammals. I would highly recommend reading Carl Zimmer’s article “The Genome: An Outsider’s View” if you’re interested in what he describes as the “blind fistfight” between paleontologists and computational/molecular biologists. For the sake of honesty, I have to admit that I often find myself on the side of paleontologists as I am more inclined toward the “traditional” naturalist role than lab work, but that is no excuse not to be familiar with other fields. Personally, I find supertrees, molecular clocks, and cell biology are nearly dizzying, but how foolish would I be to dismiss the results of such studies just because I had a hard time understanding the findings of another field? The issues that are often central in the conflict between “old” and “new” natural sciences are far more complex than anyone probably thought, and I don’t see us arriving at the right answers by relying solely upon the small percentage of animals preserved as fossils or DNA. When varying disciplines come together, arguments will abound, but as far as I can tell our understanding of nature does not advance when everyone agrees with each other.

Ji, Q., et al. The earliest known eutherian mammal. Nature 416, 816-822 (25 April 2002)

Wible, J.R., et al. Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary. Nature 447, 1003-1006 (21 June 2007)

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6 responses

21 06 2007
Zach Miller

I’ll have to find my way to this paper too. The thing, to me, about evolutionary clocks is that they depend on a constant mutation rate. Supposedly, a more feasable evolutionary “clock” that would take the fossil record into account would be to compare mitocondrial DNA of various mammalian lineages. You could see where the points of divergence are between the families (thus cementing a crown-group Mammalia tree) and then look at the fossil record to see how far back those lineages seem to go.

You probably wouldn’t get as far back as the K/T boundary, but it’s a start.

21 06 2007

Thanks Zach; the calibration of mutation rates has always bothered me, especially since it’s essentially a reasonable hypothesis, but we don’t really know. The dating and analysis of fossils is so much more direct to me, based on fewer assumptions, and I guess that’s why I prefer it over the still-developing genetic sciences.

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