In the wake of the mass extinction 66 million years ago that wiped out all of the dinosaurs apart from the birds, mammals underwent an evolutionary explosion. The small species that survived the consequences of the asteroid-triggered 鈥渆nd-Cretaceous鈥� event diversified and began to evolve into new niches, filling forests that sprung up from the ashes of the Cretaceous world. Now, palaeontologists have learned that during this formative time, beasts evolved bigger bodies millions of years before their brain size caught up.
鈥淭here are so many mysteries surrounding the mammals who survived the end-Cretaceous extinction,鈥� says at the University of Edinburgh, UK, including how mammals began to evolve proportionally larger and more complex brains.
To find out more, Bertrand and colleagues looked at CT scans of mammal skulls from before and after the end-Cretaceous extinction. Some of these are new discoveries from places like the Colorado Basin in the US, which embody mammal evolution in the million years after the likes of Triceratops went extinct.
Advertisement
It was already known that the body size of mammals alive immediately following the event increased. Mammals at the time of the extinction were no bigger than a badger 鈥� which tend to be under a metre in length 鈥� while some of those 10 million years later were the size of a black bear. But the researchers discovered that the brain size of the mammals stayed about the same even as their bodies grew larger.
鈥淢ammals needed the opportunity to increase their body size first,鈥� says Bertrand.
It wasn鈥檛 until about 56 million years ago, during a time called the Eocene, that the palaeontologists detected sweeping changes in mammal brains. 鈥淒uring this time,鈥� says Bertrand, 鈥渢he part of the brain that increases the most is the neocortex where the integration of complex senses such as vision, hearing, motor control and memory occurs.鈥�
This brain increase may be explained in part by the lush forests that spread across the world during the early Eocene. Mammals underwent a second evolutionary burst as new species began to adapt to living in habitats thick with vegetation 鈥� which made navigating, finding food and avoiding predators more complex.
The new study helps make sense of a pattern palaeontologists knew from other fossil studies. During the Eocene, the early ancestors of today鈥檚 mammals 鈥� the dogs and cats and bats and rats 鈥� out-competed earlier groups of mammals that had survived alongside the dinosaurs. In fact, the greatest increases in brain size and complexity were among the ancestors of mammal groups that are still alive today as compared to those that had persisted through the dinosaur-dominated world prior to the extinction.
at High Point University in North Carolina says the new study 鈥渨ill allow palaeontologists to really begin to dig into neurosensory evolution of mammals鈥�, uncovering the deeper stories of how they came to dominate the modern world.
Science
Sign up to Wild Wild Life, a free monthly newsletter celebrating the diversity and science of animals, plants and Earth鈥檚 other weird and wonderful inhabitants
