The Mammals Who Lived
When the asteroid slammed into prehistoric Mexico and drew the curtain on the Cretaceous, dinosaurs did not fare very well. All but the toothless birds soon died. The aftermath of the impact wasn’t kind to the rest of life on Earth, either. Entire plant communities withered and died, their rotting leaves and trunks feeding pioneering fungi that eventually gave way to ferns and then forests as sunlight started to peek through the dusty skies.
Mammals are often presented as being more resilient. Life beneath the claws of the dinosaurs had kept them small and wily enough to escape underground, shuddering in their burrows as the worst of the extinction fallout slowly passed. But this is myth.
Mammals, as a taxonomic group, survived the K/Pg extinction – just as dinosaurs did – but they suffered major losses that irrevocably changed the evolutionary pathways open to those beasts that survived. This fact has traditionally been taken right from mammalian mouths. Fragments of jaw and shiny black teeth tell who lived and who perished. But paleontologists can draw from more than dental details. In a new Cretaceous Research paper, University of Washington paleontologists Lauren DeBey and Gregory Wilson looked to leg bones to gauge the ebb and flow of mammal evolution across the fifth great extinction.
DeBey and Wilson chose to focus on the femur. That’s the long thigh bone that articulates with the hip and the lower leg. The choice presents both difficulties and benefits.
Exactly which species each of the 64 partial femora used in the study belonged to isn’t certain. The bones are usually found in isolation, just like the tiny teeth and jaws, so matching mouth to leg is difficult. Still, the leg bones present enough anatomical detail to narrow down the lineages of mammal they represent, the size of the critters, and how the animals moved. Taken together, the bones DeBey and Wilson selected add to the picture of how mammals evolved as the dinosaur-dominated Cretaceous gave way to the Paleocene.
All of the leg bones from the end of the Cretaceous came from one lineage of mammal – multituberculates. These extinct beasts would have looked superficially rodentish, but they had extravagantly-complex teeth. But even though other types of mammals that definitely lived at the same time – such as metatherians – were missing from the sample, DeBey and Wilson nevertheless counted eight distinct multituberculate femur shapes (or morphotypes) in the collection.
Even without the mammals that the dental record indicates were present, this was a greater diversity of form than in the million years that followed the end-Cretaceous disaster. Mammals had actually flourished during the time of the dinosaurs. Then the sky came crashing down.
Some of the multituberculates didn’t survive into the Paleocene. Depending on the exact age of one tricky deposit, DeBey and Wilson write that two to four different “multi” morphotypes disappear across the K/Pg boundary. Dental fossils have also shown that the proto-marsupials – the metatherians – were also severely cut back.
In the rock right after the end-Cretaceous event, DeBey and Wilson count five different femur morphotypes. Three belong to multituberculates, and the other two represent archaic hoofed mammals. The anatomy of the early ungulate fossils shows that they belonged to groups that did not originate in the area, but rather immigrated from elsewhere. These mammals were bigger than those that had evolved in the region, DeBey and Wilson point out, meaning that the apparent increase in size over time was caused by larger species moving in rather than local mammals becoming bigger.
Not that the ungulates stuck around long. In the next Paleocene layers, dated between 65.99 and 65.15 million years ago, both of those morphotypes are absent, along with a missing multituberculate. At the same time, a new pair of eutherians and a duo of multituberculates appear for the first time. And it’s during this narrow window that the mammals start to branch out again. This slice’s sample includes a multituberculate that burrowed, unlike the earlier ones that just ran along the ground, and a eutherian that may have lived in the trees.
The first million years of the Age of Mammals wasn’t a time of beastly rejoicing or straightforward progress towards bigger and weirder forms. Mammals were devastated by the end-Cretaceous catastrophe, just as other forms of life were, and the survivors continued to struggle according to the fluctuations of evolution and extinction. The great dinosaurs were gone, yes, but the world’s ecosystems were still knitting themselves back together. This is how the third act of our own evolutionary story began.
DeBey, L., Wilson, G. 2014. Mammalian fauna across the Cretaceous-Paleogene boundary in eastern Montana. Cretaceous Research. 51: 361-385. doi: 10.1016/j.cretres.2014.06.001