After the dinosaurs: Mammals had bodies before brains

Mammals today are distinguished by the fact that they have the largest brain compared to their body among vertebrates. But how long did this relationship, or encephalization, become so prominent? “For a long time it was thought that brain volume increased in parallel with the size of the initial mammalian irradiation,” recalls Emmanuel Gerbrant, a CNRS and MNHN paleontologist who specializes in ancient irradiation of African mammals. The rapid growth of this surviving lineage during the Cretaceous-Tertiary Crisis was driven by the disappearance of archaic lineages such as the dinosaur lineage and the liberation of many ecological niches. However, a new study from the University of Edinburgh, led by paleontologist Ornella Bertrand, tends to shift the significant rate of mammalian encephalization several million years after the crisis, at a time when competition raged between different evolutionary lineages…

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First Conquest: Size Up

Almost 66 million years ago there was a mass extinction of animals that have dominated until now. Among them were the dinosaurs, whose diversity extended to every branch of the vast web of ecological niches. With their sudden end, the Paleocene begins, when the surviving species will try to conquer the environment as quickly as possible … “We are seeing rapid and convergent diversity among various families of birds and mammals. For example, over several million years, mammals have increased in size just phenomenally! ‘, depicts Emmanuel Geerbrant. Thus, weight gain, as well as diversification of food adaptations and locomotion, are necessary to develop now empty niches, in particular those previously occupied by the giant herbivores of the Cretaceous period.

This dramatic change in the size and morphology of the skeletons at the very beginning of the Tertiary period has been known since the 1960s thanks to the numerous fragments of jaws and limb bones found, in particular, in North America – the first indicators of anatomy as an adaptation of their owners. . However, the fossil record is not rich enough to determine an increase in the rate of encephalization in the mammalian lineage. Thus, the scenario that seems most logical remains valid: while the animals were getting bigger, their brains should have been increasing in size. It was necessary to wait for the last twenty years and the advent of new technologies in laboratories to enable in-depth analysis of the ever-growing collection of fossils. Among these technological revolutions, the tomograph has made the work of paleontologists especially much easier …

Computed tomography at the Center for Microscopy and Imaging at the American Museum of Natural History in New York. Credits: Ornella Bertrand.

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Second conquest: increase brain size

So at Ornella University in Edinburgh, Bertrand and her team scanned fossils long archived as well as more recently discovered specimens, such as these skulls found in the San Juan Basin in New Mexico, Denver in Colorado and east. France. “This method allowed them to observe the internal characteristics of fossilized bones and, in particular, their intracranial cavity. An area previously inaccessible to researchers,” explains the paleontologist. Ornella Bertrand noticed the marked evolution of the mammalian brain of the Paleocene (-66 to -55 million years) and Eocene (-56 to -34 million years). Thus, the Edinburgh team highlighted an increase in the volume of brain regions, such as the neocortex, associated with sensory and cognitive functions, which should help the animal adapt to the environment and to increasingly complex …

“In the Eocene, the ecological niches occupied after the extinction of the dinosaurs are saturated. Faced with competition between species, some lineages will develop sensory tools and superior cognition, resulting in better vision or better dexterity,” says Emmanuel Geerbrant. With regard to height, the evolution of encephalization appears independently in different groups in response to the same problem: superiority in competition in the exploitation of habitat resources! This evolution occurs during the Eocene in modern placental groups such as carnivores, primates, and ungulates, in contrast to the archaic mammals of the Paleocene, whose brains remain small. The second period of ecological conquest is underway… Thus, the study conducted by Ornella Bertrand shows us the anatomical and adaptive evolution of mammals in several phases, something like… an evolutionary ladder!

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