Mammals began their takeover long before the death of the dinosaurs

[muriel_volestrangler]

It's a familiar story—the mighty dinosaurs dominated their prehistoric environment, while tiny mammals took a backseat, until the dinosaurs (besides birds) went extinct 66 million years ago, allowing mammals to shine. Just one problem—it's not true. A new article in the Proceedings of the Royal Society B reports that mammals actually began their massive diversification ten to twenty million years before the extinction that ended the age of the dinosaurs.

"The traditional view is that mammals were suppressed by the dinosaurs' success, and that they didn't really take off until after the dinosaurs went extinct. This study shows that therian mammals, the ancestors of most modern mammals, were already diversifying before the dinosaurs died out," says lead author David Grossnickle, a Field Museum Fellow and PhD candidate at the University of Chicago.

The old hypothesis hinged upon the fact that many of the early mammal fossils that had been found were from small, insect-eating animals—there didn't seem to be much in the way of diversity. But over the years, more and more early mammals have been found, including some hoofed animal predecessors the size of dogs. The animals' teeth were varied, too. Grossnickle, along with his co-author Elis Newham at the University of Southampton, analyzed the molars of hundreds of early mammal specimens in museum fossil collections. They found that the mammals that lived during the years leading up to the dinosaurs' demise had widely varied tooth shapes, meaning that they had widely varied diets. These different diets proved key to an unexpected finding regarding mammal species going extinct along with the dinosaurs.

Not only did mammals begin diversifying earlier than previously expected, but the mass extinction wasn't the perfect opportunity for mammal evolution that it's traditionally been painted as. Early mammals were hit by a selective extinction at the same time the dinosaurs died out—generalists that could live off of a wide variety of foods seemed more apt to survive, but many mammals with specialized diets went extinct.

Read more at: http://phys.org/news/2016-06-mammals-began-takeover-death-dinosaurs.html#jCp

Therian mammals experience an ecomorphological radiation during the Late Cretaceous and selective extinction at the K–Pg boundary

It is often postulated that mammalian diversity was suppressed during the Mesozoic Era and increased rapidly after the Cretaceous–Palaeogene (K–Pg) extinction event. We test this hypothesis by examining macroevolutionary patterns in early therian mammals, the group that gave rise to modern placentals and marsupials. We assess morphological disparity and dietary trends using morphometric analyses of lower molars, and we evaluate generic level taxonomic diversity patterns using techniques that account for sampling biases. In contrast with the suppression hypothesis, our results suggest that an ecomorphological diversification of therians began 10–20 Myr prior to the K–Pg extinction event, led by disparate metatherians and Eurasian faunas. This diversification is concurrent with ecomorphological radiations of multituberculate mammals and flowering plants, suggesting that mammals as a whole benefitted from the ecological rise of angiosperms. In further contrast with the suppression hypothesis, therian disparity decreased immediately after the K–Pg boundary, probably due to selective extinction against ecological specialists and metatherians. However, taxonomic diversity trends appear to have been decoupled from disparity patterns, remaining low in the Cretaceous and substantially increasing immediately after the K–Pg extinction event. The conflicting diversity and disparity patterns suggest that earliest Palaeocene extinction survivors, especially eutherian dietary generalists, underwent rapid taxonomic diversification without considerable morphological diversification.

http://rspb.royalsocietypublishing.org/content/283/1832/20160256