Tuesday, October 26, 2010

Did Dinosaurs Eat Your Ancestors?

The "Cretaceous Terrestrial Revolution" (abbreviated "KTR") refers to the appearance of many modern groups of plants in the Cretaceous period.  Marsupials show up then, as do the first eutherian mammals (though there is no fossil evidence yet that true placental mammals existed during the Cretaceous -- though some estimates for the diversification of mammalian orders suggests that they must have existed at least as far back as ninety million years ago).  Flowering plants appeared and diversified rapidly.  Grass seems to have been present; at least, phytoliths (mineral inclusions in plants) similar to those in the grass family have shown up in coprolites (fossilized dung) from the late Cretaceous.  Birds of course were diversifying and taking on fairly modern appearances, as were many modern orders of insects such as wasps, ants, butterflies, etc.  But a study performed at the University of Bristol suggests that dinosaurs did not take part in this explosive diversification, and questions whether they exploited the new biological resources that were becoming available at this time (said resources including our tiny, shrew-like ancestors).

Their research involved trying to construct a cladogram of approximately 400 species of dinosaurs, as well as determining when the various species had branched off from earlier ancestors.  This was of course complicated by the notorious imperfection of the fossil record cited in previous posts: not only is fossilization rare, and not only are most fossils still undiscovered and undescribed, but some periods are better represented in accessible rock formations than others.  To compensate for this, researchers discarded data from the better-represented periods until they had samples from a similar volume of sediments for each era (they note that this method can be criticized, but it seemed more objective than inflating the number of species from poorly-sampled ages).  When this was done, dinosaur taxa seemed to radiate early on in their history and by the Cretaceous the production of new taxa had leveled off: the dinosaurs (birds presumably excepted) were no longer generating new species as fast as other groups like mammals, lizards, and insects.  The implication drawn was that they were holding their own but not innovating drastically.

Now, obviously, if those phytoliths mentioned above were really from grasses, and the coprolites containing them were in fact from dinosaurs (from their size and type, they were thought to be from titanosaur sauropods), then dinosaurs must have been at least occasionally exploiting some of the new resources.  And typically, carnivores are less choosy about their diet than herbivores; one must suspect that many of the maniraptoran theropods would not have turned down a chance to dine on early primates, if any were around in the late Cretaceous.    All we can be sure of was that if dinosaurs did eat any of our ancestors, it was after they had already reproduced, so thank a Troodon for its self-restraint.

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