By Andrea Hargraves 
Small mammals often live fast and die young. Rodents and shrews mature quickly, mate within months, and usually die within a year or two. Some Giant rats They reached Earth in just six months.
But small mammals didn’t always die so quickly. Researchers recently analyzed a pair of fossilized skeletons belonging to a mouse-sized mammal that lived among the dinosaurs during the Jurassic period. Discoveriespublished on 24 in the magazine natureIt reveals that these creatures lived much longer, and grew more slowly, than their similarly sized descendants.
The two specimens were discovered decades ago on Scotland’s Isle of Skye. The rocky island was home to swampy lakes surrounded by dense forests 166 million years ago. Sauropod dinosaurs stomped through the mud and pterosaurs soared above. A variety of Mesozoic mammal relatives scurried beneath their feet.
Crosatodon kirtlingtonensis Among these ancient mammals was the genus. Previously known only from fossilized teeth, the two newly described specimens provide a more complete picture of Crosatodonwhich resembles a miniature opossum and weighs less than a hockey puck.
The largest specimens of Crosatodon Discovered in the 1970s and younger, in 2016 by paleontologist Elsa Pancirolli, of National Museums Scotland and lead author of the new study, and her team, it is the only relatively complete skeleton of a juvenile Jurassic mammal known to science.
Pancirolli said she was happy “to realize that the two were an adult and a juvenile of the same species.”
The fossils are also notable for the way they are preserved, “in three dimensions, which is rare for this part of the mammalian evolutionary tree,” said paleontologist Stephanie Pierce of Harvard University, who was not involved in the study. Most fossilized mammals from this period have been flattened into slabs of stone, obscuring important details about how they moved and lived.
The team conducted high-resolution CT scans to compare the two fossil skeletons to determine their age. Crosatodon He died, and the researchers analyzed rings of mineralized tooth tissue called cementum. This tissue gradually deposits as an animal ages, leaving light and dark bands around the roots of the teeth. Each pair of light and dark bands typically represents a year, making estimating the age of ancient mammals similar to counting tree rings.
The cement gangs revealed that Crosatodon The adult was about seven years old when it died, which is advanced compared to living mammals of similar size. The juvenile, which was about half the size of an adult, was between seven months and two years old. Surprisingly, this Crosatodon He was still in the process of replacing his baby teeth when he died.
“We didn’t expect it to be a very old event,” Pancirolli said. Based on its size, “it’s likely that it will replace its teeth in weeks or months, not two years.”
Other research has shown that most young mammals lose their baby teeth within months. This makes Crosatodon More similar to mammals are hyracoids, a badger-like creature that is 20 times larger. It takes years for permanent teeth to grow into hyracoids, which can live for ten years.
Longer period of tooth replacement, with age CrosatodonThis led the team to conclude that these ancient mammals enjoyed long life spans.
This likely extended their growth period. Most modern mammals experience rapid growth early in life before reaching a plateau as they approach adult size. Crosatodon Other Mesozoic mammals may have grown slowly over their long lives.
However, Crosatodon Some aspects of mammalian evolution are still evident. The calf had about half of its adult teeth, suggesting it had recently been weaned from its mother’s milk. Other factors, such as tooth development, are clearly similar to living mammals. “Although it grows more slowly, the way it grows seems very similar to modern mammals,” Pancirolli said.
It’s hard to pinpoint when mammals’ growth accelerated. Pancirolli says he thinks evolution likely accelerated as mammals’ metabolisms increased and they became warm-blooded. These traits emerged later in the Mesozoic and helped early mammals adopt more active lifestyles, including swimming and gliding.
More Mesozoic mammal fossils are needed to better understand how early mammals prepared for global dominance, Pierce added. Such specimens “will be essential to uncovering the factors that led to the success and ecological diversity of mammals living today.”