2023-09-22 17:15:07
66 million years ago, the asteroid that hit our planet caused the disappearance of the iconic dinosaurs and many groups of mammals. However, some mammalian lineages survived and gave rise to the modern placental mammals we know today, including humans, cats, bats and whales, called crown clades. Other placental mammals known as archaic groups survived until about 30 million years ago and became extinct.
An international team, with the participation of research personnel from the Miquel Crusafont Catalan Institute of Paleontology (ICP), the University of Edinburgh in Scotland and different institutions in the United States (Pittsburgh, Albuquerque and New York), has sought an explanation for this extinction of 30 million years ago.
“To shed light on the mystery, we decided to explore the senses and behaviors of this archaic group of mammals,” explains Ornella Bertrand, from the ICP and co-author of the new study. Since it is impossible to study the behavior of extinct species in the same way as in current mammals, paleoneurologists use the imprint left by the brain in the endocranial cavity to study its morphology and the difference in proportions between the parts of the brain. “This gives us an idea of the animal’s behavior. For example, large olfactory bulbs translate into a better sense of smell,” Bertrand clarifies.
The advancement of CT (computed tomography) scanning technologies in recent decades has allowed paleontologists to use this technique to see what is inside a fossil. It is a procedure very similar to having an MRI in a hospital.
The fossil skull they scanned with computed tomography belongs to the Tillodontia, an enigmatic group of mammals that lived from the Paleocene to the Eocene, during the Cenozoic. These mammals originated in Asia and migrated to North America and Europe. They belong to a group called Laurasiatheria (“Laurasia beasts”) and are remotely related to modern mammals. “Trogosus had a peculiar appearance, it was very different from any current species. Its size was approximately that of a modern-day wild boar,” says Marina Jiménez Lao, who worked on this project for her master’s degree at the School of Geosciences at the University of Edinburgh. It is possible that it supported its weight on its hind limbs and used its front limbs to dig up roots and tubers with its large, curved claws. “Perhaps the strangest thing was their incisors, similar to those of rodents and constantly growing, like squirrels and rats,” explains the co-author of the study.
Line drawing of Trogosus hillsii skull (USNM 17157) from the middle Eocene of the Huérfano Basin, Colorado. (Image: line drawing by Sarah Shelley (modified from the original in Bertrand et al. 2022))
All the specimens described in this study come from North America and the best preserved of Trogosus hillsii was unearthed in the Huérfano basin, in Colorado (USA), in sediments that correspond to the middle Eocene and have an estimated age between 52 and 48 million years old. The skull is relatively well preserved, including the complete cranial cavity, although it lacks the anterior part of the snout.
An interesting aspect of using CT data was discovering that in the back of the skull there were a large number of vessels draining the brain of Trogosus. In the 1950s, Charles Gazin, an American paleontologist, made a mold of the brain using another specimen and found that it had an unusually large cerebellum, but did not show any clear structure. Thanks to computed tomography, Bertrand and his team showed that there is a complex set of canals surrounding the cerebellum and that they most likely became dislodged after the animal’s death, which would have made the endocranial cavity appear much larger than it thought. it really was.
One particular aspect that Bertrand and his colleagues studied was the ratio of different brain regions. The neocortex is a part of the brain present in all mammals and that specifically integrates sensory and motor information. Species with a large neocortex show more elaborate behaviors, such as complex social behavior or a better sense of vision. “We have seen that the neocortex was relatively smaller in Trogosus and other archaic herbivores than in the archaic species of carnivores and the crown clades of herbivores that coexisted with it,” says 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 wild boars) in competing for the same resources, and perhaps were less able to escape predators than the species of the crown clades, which had more elaborate behaviors.
Climatic fluctuations could also have influenced the disappearance of this group. The Tillodontia were relatively specialized in the late Eocene, which could have led to their extinction due to a lack of flexibility to adapt to change. Within this group, Trogosus survived for millions of years before becoming extinct. It is believed that this species probably depended more on its sense of smell than on others such as sight.
The study is entitled “The virtual brain endocast of Trogosus (Mammalia, Tillodontia) and its relevance in understanding the extinction of archaic placental mammals”. And it has been published in the academic journal Journal of Anatomy. (Source: ICP)
#poorly #advanced #brain #driven #mammal #extinction #million #years