A too small neocortex may have led to extinction a 50-million-year-old mammal

CERDANYOLA DEL VALLES, Spain - Sept. 26, 2023 - PRLog -- 66 million years ago, the asteroid that hit our planet led to the demise of the iconic dinosaurs and many mammalian groups. However, some mammalian lineages survived and partly gave rise to modern placental mammals that we know today including us, cats, bats and whales called crown clades. Other placental mammals known as archaic groups, only survived until about 30 million years ago and went extinct. This time no asteroid could be blamed, and it is still unclear why these groups disappeared.

"To bring light to this mystery, we decided to explore the senses and behaviours of these archaic group of mammals", explains Ornella Bertrand, main author of the recent article published in Journal of Anatomy and researcher at the ICP. Since it is impossible to study the behaviour of extinct species the same way as modern-day mammals, paleoneurologists use the imprint of the brain against the endocranial cavity to study its morphology and the difference in proportions existing between brain regions.

The fossil skull that they CT scanned belong to Tillodontia, an enigmatic group of mammals that lived from the Paleocene to the Eocene, during the Cenozoic era. These mammals originated in Asia and migrated to North America and Europe. They belong to a group called Laurasiatheria and are distantly related to modern mammals. "Trogosus had a peculiar appearance, nothing that we would see today. This animal was close to the size of a medium size wild boar", claims Marina Jimenez Lao, who worked on this project for her Master thesis at the School of GeoSciences from the University of Edimburgh. It may have put more weight on its hindlimbs and used its forelimbs to unearth roots and tubers thanks to large and recurved claws.

One particular aspect that Bertrand and her colleagues studied was the proportion of different brain regions. The neocortex is a portion of the brain present in all mammals and that specifically integrates sensory and motor information together. Species with large neocortices, display more elaborated behaviours such as complex social behaviour or enhanced vision. "We have seen that the neocortex was relatively smaller in Trogosus and in other archaic herbivores compared to contemporaneous archaic carnivoran species and herbivorous crown clades", states Bertrand.

The researchers hypothesized that Trogosus and other archaic placental herbivorous mammals may have been outcompeted by crown herbivores (such as the ancestors of ruminants or boars) for the same resources and may have been less well-equipped to escape predators than crown clades that had more complex behaviours.

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