Brain over brooding: Fossil explained with X-ray tomography – Ezine

Size matters

X-ray tomography has been used to study a newly described fossil of an extinct ancestor of modern mammals found with 38 petrified offspring. The study provides new evidence that when the first mammals developed, they exchanged brood size for brain size.

Mammals have the largest brain in relation to body height and produce some of the smallest nests of offspring. It seems that at some point in the evolution the brain caught up with brood. Now, Eva Hoffman of the University of Texas in Austin, USA, and her colleagues have looked for an extinct mammalian relative and her 38 offspring to help them understand how this transition took place. This is a rare find because it clearly shows offspring with mother of a mammalian precursor, a Kayentatherium. The fact that there is more than twice the average number of babies in the mother compared to a living mammal suggests that this ancestor still lay a large number of eggs, such as every modern reptile and the most primitive living mammals, the monotremes, such as the platypus. The team believes the offspring were still in their eggs, lost by time or just hatched when they died, perhaps as a result of a river flood or a mudslide.

Tooth and claw

In the journal Nature, the team describes how this species could be a missing link between the numbers of offspring of reptiles and the emergence of greater brain capacity that was associated with the smaller brood of modern mammals.

"These babies are of a very important point in the evolutionary tree," explains Hoffman, who graduated from the University of Texas Jackson School of Geosciences when the work was performed. "They had many functions similar to modern mammals, functions that are relevant to understanding the evolution of mammals," she adds. Hoffman was co-author of the new study with her advisor, Timothy Rowe.

Kayentatherium Wellesi was a dog-like, hairy herbivore that lived next to the dinosaurs 185 million years ago. Rowe originally collected the fossil more than 18 years ago from a rock formation in Arizona and then assumed that he had found a single specimen. Former graduate student Sebastian Egberts revealed for the first time that the fossil could contain more than a single specimen when a grainy tooth enamel of tooth enamel caught his attention in 2009 when he unpacked the fossil.

Remarkable

The researchers would have to wait for progress in Austin's High-Resolution X-ray Computed Tomography Facility (UTCT) and extensive digital processing before complete skulls and partial skeletons within the fossil could be revealed. Hoffman used 3D visualizations to perform an in-depth analysis and to verify that the small bones were from babies and were the same species as the adult.

"There are extra deep stories about the evolution of development and the evolution of mammalian intelligence and behavior and physiology that can be squeezed out of a remarkable fossil like this now that we have the technology to study it," says Rowe.

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