Ancient fossils reveal slower growth rates in mammals compared to modern species from Jurassic period

During the Middle Jurassic, small mammals lived much longer than modern ones and received parental care for years rather than weeks, suggesting that at some point there was a major change in the growth rates of small mammals, although the exact cause is unknown.

The discovery is based on two fossil skeletons of extinct mouse-sized creatures. Cruxatodon kiltlingtonensis, It lived on the Isle of Skye in Scotland about 166 million years ago, and its fossils were unearthed decades apart, the first in the 1970s and the second in 2016.

The unusual discovery of two fossils of the same species, one adult and one juvenile, allowed the team to compare the specimens to study how the animals grew and developed. “That meant we could ask questions we never dreamed of with just one specimen,” he says. Elsa Panciroli At the National Museum of Scotland.

First, the scientists used X-ray images to count the growth rings on the specimens' teeth, which are similar to growth rings on tree trunks and can be used to estimate age. They found that the adult specimens were about 7 years old, and the juvenile specimens were between 7 months and 2 years old.

Panciroli said he expected the fossil to be much younger, since the pup still had its baby teeth. “This was quite surprising, as this animal is about the size of a squirrel or a shrew,” Panciroli said. “We would have expected its teeth to grow back within a few weeks or months, so we could see straight away that it must have been developing quite differently. [than modern species].”

This discovery K. Quiltrington Mice took up to two years to wean from their mothers, a big jump from the few weeks most small mammals require today. Analysis of the length and size of the fossil bones reveals that the animals “grew throughout their lives,” Panchiroli says. Today, small mammals like mice grow rapidly when they're young but then stop growing as adults.

It's unclear exactly when and why small mammals evolved this way, but Panchiroli said it could be linked to environmental changes or it could be the result of mammals having warmer blood and a faster metabolism.

Panciroli and her team return to Skye every year, and are optimistic that they will be able to better understand these changes: “Hopefully in the coming years we'll find more fossils and new ways to ask these questions,” she says.