Jurassic fossils show modern mammals grow faster than ancient ones

In the middle of the Jurassic Period, small mammals had much longer lives than their modern counterparts, and spent years instead of weeks being cared for by their parents. This suggests that at some point, small mammals underwent a significant shift in how quickly they grow – but it is unclear precisely what caused it.

The finding is based on two fossilised skeletons of an extinct mouse-sized creature called Krusatodon kirtlingtonensis, which lived on the Isle of Skye in Scotland around 166 million years ago. The fossils were unearthed decades apart: the first in the 1970s, and the second in 2016.

Together, they are a rare find – two fossils from the same species, one an adult, one a juvenile. The research team could compare the specimens to see how the animals grew and developed. “It meant that we could start asking questions we couldn’t have dreamt of with just one specimen,” says Elsa Panciroli at the National Museum of Scotland.

First, the scientists used X-ray imaging to count the annual growth rings in the specimens’ teeth, which, similar to rings in a tree trunk, can be used to estimate age. They found that the adult specimen was around 7 years old and the juvenile was between 7 months and 2 years old.

Because the youngster still had its baby teeth, Panciroli says she expected the juvenile fossil to be much younger. “That was quite surprising because this animal is about the size of a squirrel or shrew,” says Panciroli. “We would expect it to be replacing its teeth much earlier – within weeks or months. So we knew immediately that it must have grown very differently [than modern species].”

The finding suggests K. kirtlingtonensis took up to two years to wean from its mother, a notable leap from the handful of weeks it takes most modern small mammals. Analysis of the fossils’ bone lengths and size revealed that the animals “were also growing throughout their whole lives”, says Panciroli. Today, small mammals like mice grow rapidly when young, but stall once they reach adulthood.

Exactly when and why small mammals underwent this developmental shift isn’t clear. Panciroli says it could have been linked to environmental changes, or be the result of mammals developing warmer blood and faster metabolisms.

Panciroli’s team returns to the Isle of Skye each year, and she is optimistic that they will be able to more fully understand this change. “Hopefully, in years to come, we’ll find more fossils or new methods of asking those questions,” she says.