Mammal Skulls and Evidence of Creation?
There's a fascinating new research paper from last week's Science looking at large-scale trends in mammal skull shapes, and I'm still still thinking about it. These comments are very preliminary.
Authors Goswami and colleagues examined a sample of 322 mammal skulls covering the majority of mammalian diversity. They analyzed changes in shape using 3D models of the skulls and landmarks to track how skulls differ from group to group and how they believe mammals have evolved over the last 100 or so million years on the conventional timeline. They reach a few very interesting conclusions based on their results. First, the highest rates of skull change in mammal "evolution" occur primarily at the base of the mammal tree, near the beginning of the Cenozoic. Second, the highest rates of skull change among the mammals occur in groups like the whales, elephants, and sea cows. Third, the lowest rates of skull change occur in the rodents and bats.
That sounds really interesting from a creationist perspective. I'd like to say that the high "rates of change" at the base of the mammalian fossil record actually represents discontinuity between different created kinds, and the later slow rates of change represent diversification within created kinds. I don't think that's quite right though. To begin with, to represent as much mammalian diversity as possible, the sample of 322 skulls includes roughly one skull per family, meaning that each data point in this sample could represent a single created kind. At the very least, we're not looking at a lot of diversity within created kinds. We also have to be cautious about the "rate," since that might not be directly equivalent to a large magnitude of difference. We could be looking at a moderate magnitude of difference over a short time.
Still, these results are very curious. From a creationist perspective, I would expect there to be a lot of disparity at the first appearance of mammals, and that's not exactly what we get. The disparity appears fairly quickly though, so that by the time you get to Eocene rocks, you have highly diverse skull forms. Again, since we're looking at multiple created kinds, this may be records of the first fossilizations of different created kinds.
Also curious is the "rapid" skull changes found in some of the slowest reproducers on the planet. I (perhaps naively) would expect diversification to be linked to intrinsic rate of increase, the speed at which organisms produce offspring and reach reproductive maturity. Here, that relationship seems to be inverted. Rapid reproducers like rodents or bats have very moderate skull changes, while slow reproducers (i.e., the largest mammals) are marked by rapid skull changes. Perhaps what we're seeing is rodents and bats exploring the full range of their skull forms (hence any individual difference is small), while the larger mammals only sample a larger morphospace making their individual differences of larger magnitude? But even in that case, how do large mammals with slow reproduction speciate at a rate that allows them to sample a morphology quite different from the parent form?
On the other hand, the PCA of the skull shape shows a nice yawning gap between whales and terrestrial mammals, so that's very nice. Fully expected.
I wonder what we could do with this at the level of a created kind? What would we find?
Goswami et al. 2022. Attenuated evolution of mammals through the Cenozoic. Science 378:377-383.
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