Size plays a role
X-ray tomography was used to examine a newly described fossil of an extinct ancestor of modern mammals found with 38 petrified offspring. The study provides new evidence that the first mammals traded the brood size for the size of the brain.
Mammals have the largest brains compared to body size and produce some of the smallest offspring. It seems that at some point in evolution, the brains overtook the brood. Now, Eva Hoffman of the University of Texas in Austin, USA, and her colleagues have looked at an extinct mammalian relative and her 38 offspring to help them understand how this transition took place. This is a rare finding as it clearly shows offspring with the mother of a mammalian precursor, a Kayentatherium. The fact that there are more than twice as many babies with the mother as with living mammals suggests that this ancestor still lays a large number of eggs, as do any modern reptile and the most primitive living mammals, the monotremes, such as the platypus. The team believes that the offspring were either still in their eggs, which were lost over time or had just hatched, perhaps as a result of a flood or a mudslide. In the journal Nature, the team explains that these species are a missing link between the numbers of reptiles and the emergence of greater brain forces accompanied by smaller mammals of modern mammals.
"These babies are from a really important point in the evolutionary tree," explains Hoffman, who was a graduate student at the Jackson School of Geosciences at the University of Texas when the work was done. "They had many features similar to those of modern mammals, properties that are relevant to understanding the evolution of mammals," she adds. Hoffman co-authored the new study with her advisor, Timothy Rowe.
Kayentatherium wellesi was a dog-sized, hairy herbivore that lived next to the dinosaurs 1
Researchers would have to wait for advances in Austin's high-resolution X-ray computed tomography (UTCT) device and extensive digital processing before complete skulls and sub-skeletons within the fossil could be uncovered. Hoffman used 3D visualizations to do a thorough analysis and verify that the tiny bones belonged to the babies and were the same species as the adult.
"There are more deep stories about the evolution of evolution and the evolution of mammalian intelligence and behavior and physiology that can be extracted from a remarkable fossil like this, now that we have the technology to study it," says Rowe.
Nature 2018, online: "Law-Tailed Mammal Perinates and the Origin of Mammal Reproduction and Growth"
Article by David Bradley
The views expressed in this article are exclusively those of the author and not necessarily those of John Wiley and Sons, Ltd.
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