Wednesday, September 28, 2022

Mammals’ bodies popped out of their brains just after dinosaurs died

Modern mammals are known for their large brains. But new analysis of mammal skulls from creatures that lived shortly after the dinosaurs’ mass extinction shows that those brains weren’t always a foregone conclusion. For at least 10 million years after the dinosaurs disappeared, mammals became much more brainy, but not brainy, researchers report April 1. Science,

This is conventional wisdom to put it mildly. “I thought, it’s not possible, there must be something I did wrong,” says Ornella Bertrand, a mammal paleontologist at the University of Edinburgh. “It really threw me off. How would I explain they weren’t smart?”

Modern mammals have the largest brains in the animal kingdom relative to body size. How and when the brain evolved is a mystery. One view has been that the disappearance of all non-bird dinosaurs following an asteroid impact at the end of the Mesozoic Era 66 million years ago left a void for mammals (SN: 1/25/17The recent discovery of fossils belonging to the Paleocene—the epoch immediately after the extinction spanning 66 million to 56 million years ago—reveals a flourishing of strange and wonderful mammal species, much larger than their Mesozoic predecessors (SN: 10/24/19) This was the beginning of the era of mammals.

Illustration of Arctocyon and Hiracchius
The extinction of the dinosaurs soon opened a door to a peculiar threat of new mammal species, much larger than their earlier ancestors. among these were arctokyone (right in this artist’s illustration), a carnivore closely related to modern pigs and sheep. Although the brains of these Paleocene mammals were relatively small, brain size increased during the later era, the Eocene. Mammals emerged in that era such as hierachius modestus (at left), an ancestor of rhinos and tapirs.Sarah Shelley

Before those fossils were found, the prevailing wisdom was that after the mass dino extinction, mammals’ brains likely grew exponentially with their bodies, with everything growing together like an expanding balloon, Bertrand said. it is said. But Paleocene fossil discoveries in Colorado and New Mexico, as well as re-examinations of previously found fossils in France, are now uncovering that story by giving scientists a chance to measure the size of mammals’ brains over time.

Bertrand and his colleagues used CT scanning to create 3-D images of the skulls of a variety of ancient mammals before and after the extinction event. Those samples included mammals from 17 groups dating from the Paleocene and 17 from the Eocene, the epoch that spanned from 56 million to 34 million years ago.

What the team found was a shock: relative to their body size, Paleocene mammal brains were relatively small compared to Mesozoic mammals. It wasn’t until the Eocene that mammal brains began to grow, especially in certain sensory regions, the team reports.

To assess how the shape and size of those sensory areas changed over time, Bertrand looked for the edges of different parts of the brain within a 3-D skull model, tracing them like a sculptor working with clay . The size of mammals’ olfactory bulbs, responsible for their sense of smell, didn’t change over time, the researchers found—and that makes sense, since Mesozoic mammals were also good sniffers, she says.

The really big brain changes were to come in the neocortex, which is responsible for visual processing, memory, and motor control, among other skills. But such changes are metabolically expensive, Bertrand says. “To have a big mind, you need to sleep and eat, and if you don’t you get cranky, and your brain doesn’t work.”

Cranial diagram showing the neocortex in purple where Arctocyon primavus has a larger neocortex than Hierachius modestus
To track changes in mammal brain size over time, the researchers detected brain cases inside the mammal skull using CT scanning. On the left is the Paleocene mammal’s cranium arctokyone, with sensory regions including the olfactory bulb and neocortex highlighted in purple. On the right is the skull of an Eocene mammal hierachius modestus,Ornella Bertrand and Sarah Shelley

Therefore, the team proposes, as the world shook the dust of mass extinctions, the dispute was a priority for mammals, allowing them to spread rapidly across newly available ecological niches. But after 10 million years, metabolic calculations had changed, and competition within those niches was intensifying. As a result, mammals began to develop new sets of skills that could help them snatch hard-to-reach fruit from a branch, evade a predator, or capture prey.

Other factors – such as social behavior or parental care – have been important to the overall development of mammals’ large brains. But these new discoveries suggest that, at least as mammals age, ecology — and competition between species — gave a major push to brain evolution, wrote biologist Felisa Smith of the University of New Mexico in Albuquerque in a commentary. same issue of Science,

“An exciting aspect of these findings is that they raise a new question: Why did large brains evolve independently and concurrently in many mammal groups?” David Grosnickel, an evolutionary biologist at the University of Washington in Seattle, says.

Most modern mammals have relatively large brains, so studies examining only modern species can conclude that large brains once evolved in mammal ancestors, Grosnickel says. But what this study uncovered is a “much more interesting and nuanced story,” that these brains evolved differently in many different groups, he says. And it shows just how important fossils can be for stitching together a precise tapestry of evolutionary history.

Nation World News Desk
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