As I mentioned a few days ago, Dr. Zhe-Xi Luo of the Carnegie Museum of Natural History gave two talks here yesterday, regarding paleontology in China. My apologies: first, I gave the wrong room # for his first talk, hopefully this didn’t cause too much confusion. Second, when urging people to attend these lectures on mammalian origins, I neglected to mention monotremes and marsupials–hopefully I didn’t offend any primitive-mammal-lovers.
Now to the lectures. The noon lecture was very interesting, and a bit more in depth than the evening talk. Luo discussed how fossil discoveries in China over the past few decades, but especially in the past 5-10 years, have addressed the issue of early mammalian diversity. He also discussed the discrepancy between molecular and fossil data, and ways to reconcile them (though I missed this important part because I had to go teach). His point was that there has been an incorrect notion that early mammals were not terribly diverse, and it wasn’t until the extinction of the dinosaurs at the end of the Mesozoic that mammals were able to diversify ecologically, and therefore morphologically. Moreover, it was assumed that early, ‘non-diversified’ mammaliaformes were just generalized insectivores.
Recent discoveries in China, however, have revealed great diversity in Mesozoic mammals. Late Jurassic-early Triassic Docodonts display vertebral morphology similar to modern beavers and otters, and their appendicular skeleton apparently appears to have been adapted to swimming and digging, ultimately suggesting that around 200 Ma Docodont mammals occupied an aquatic niche. Late Cretaceous Fruitafossor had a ‘primitive’ mandible lined with peg-like, open-rooted teeth which apparently grew throughout life. Fruitafossor, at some 145 Ma, shows Mesozoic mammals occupied a niche similar to extant Xenarthrans (i.e. aardvarks). Repenomamus was a dog-sized carnivore/scavenger; there were even gliders comparable to modern flying squirrels. The point: Mesozoic mammals filled a variety of niches, alongside dinosaurs, and only in the past 5-6 years have we seen fossils showing the diversity that molecular studies have suggested. One interesting thing is that these Mesozoic mammals, like their dinosaur contemporaries, similarly suffered from the mass extinction at the end of the Cretaceous. So these fossils, at least what was presented, do not tell us about the common ancestor of modern mammals.
So, one important lesson coming from these talks is that convergent evolution is not uncommon in the organismal world, even in mammals. Of course, this isn’t anything new: simply look at the convergence of forms of Australian marsupials with other placental mammals. The fact that convergence occurs, even though it’s arguably not “parsimonious” has been recognized in paleoanthropology for a long time now, but it never hurts to repeat the lesson, especially when so many out there are still so reliant on parsimony, at the neglect of other data, for phylogenetic reconstruction. Also, Luo posited that molecules suggested earlier divergence than did the fossil record. This is symptomatic of two things: the sparseness of the fossil record, and the possibility (or probability?) that molecules are overestimating divergence times, at least with regard to morphological divergence. This interesting in light of recent genetic studies suggesting a hominin-chimpanzee divergence time of 4-5 Ma. I’m too out of it to get too much into it now, plus I have to go to class, but suffice it to say that perhaps chimpanzees aren’t a good model for the last common ancestor of humans and chimps. Now I’m getting scatterbrain and am late for class. More organized and coherent thoughts on this later…