Saturday, March 24, 2012

What the teeth tell us

There is a group of pervasive extinct mammals you've likely never heard of (I hadn't): multituberculates. They made their appearance in the weird news (artist's rendition 3rd image, also here) and caught my eye because of their diet of flowers. As I found out, this order of animals has the longest fossil history of any mammal lineage - 120 million years - and existed in as many varieties as the angiospores (2nd image) they favored. There were tree-dwelling squirrel-like varieties, terrestrial types as small as a mouse and as large as a beaver, and burrowing gopher-like "multis," as they are called. They are named after their teeth ("tubercles" are cusps) and these are the most prevalent remnants in the fossil record. It was by studying the changes in the teeth and developing a new way to quantify their bumpiness that University of Washington paleontologist Greg Wilson (1st image, holding a cast of a multi's jaw) and his colleagues concluded that multis flourished during and after the age of the dinosaurs. As they explain in their scientific paper, "Here we show that in arguably the most evolutionarily successful clade of Mesozoic mammals, the Multituberculata, an adaptive radiation began at least 20 million years before the extinction of non-avian dinosaurs and continued across the Cretaceous–Paleogene boundary." But to me and other laypeople, Wilson's quote to the popular press is more understandable: "If you pull out a textbook, it will basically say that with the extinction of the dinosaurs, the opportunity became available so that mammals could evolve into diverse forms. What we're seeing is that at least in this important group of mammals, extinction of the dinosaurs wasn't the driver." So they survived the mass extinction of the dinosaurs 65 million years ago and lived throughout the world for another 30 million years, but then lost out to primates, ungulates, and rodents. Today multituberculates - represented by more than 200 species - are the only major branch of mammals to have become completely extinct, and have no living descendants.

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