NEW YORK, NY.- Small mammals often live fast and die young. Rodents and shrews mature quickly, mate within months and usually go belly up in a year or two. Some giant rats kick the bucket in just six months.

But miniature mammals have not always burned out so quickly. Researchers recently analyzed a pair of fossilized skeletons belonging to a mouse-size mammal relative that lived among dinosaurs during the Jurassic period. Their findings, published Wednesday in the journal Nature, reveal that these critters lived much longer and grew more slowly than their similarly sized descendants.

The two specimens were discovered decades apart on the Isle of Skye in Scotland. This craggy island was home to swampy lagoons fringed by dense forests 166 million years ago. Sauropod dinosaurs stomped across the mud as pterosaurs flew overhead. Scurrying underfoot was a menagerie of Mesozoic mammal relatives.

Krusatodon kirtlingtonensis was among these ancestral mammals. The species was previously known only from fossilized teeth. The two newly described specimens provide a more complete picture of Krusatodon, which resembled a pint-size possum and weighed less than a hockey puck.

The larger of the Krusatodon specimens was discovered in the 1970s. The smaller Krusatodon was discovered in 2016 by Elsa Panciroli, a paleontologist at National Museums Scotland and the lead author of the new study, and her team. It remains the only relatively complete skeleton of a juvenile Jurassic mammal known to science.

Panciroli said she was delighted “to realize that the two of them were an adult and a juvenile of the same species.”

According to Stephanie Pierce, a paleontologist at Harvard University who was not involved in the study, the fossils are also remarkable for how they were preserved, “in three-dimensions, which is rare for that part of the mammal evolutionary tree.” Most fossilized mammals from this time period are stamped flat onto slabs of stone, obscuring crucial details about how they moved and lived.

The team took high-resolution CT scans to compare the two fossil skeletons. To gauge how old each Krusatodon was when it died, the researchers analyzed rings of mineralized dental tissue called cementum. This tissue is deposited in increments as the animal ages, leaving light and dark bands around the roots of the tooth. Each pair of light and dark bands usually represents one year, making estimating the age of an ancient mammal akin to counting tree rings.

The cementum bands revealed that the adult Krusatodon was around 7 years old when it died — a ripe old age compared with living mammals of similar size. The juvenile, which was roughly half as large as the adult, was somewhere between 7 months and 2 years old. Surprisingly, this Krusatodon was still in the process of replacing its baby teeth when it died.

“We didn’t expect it to be such an old juvenile,” Panciroli said. Based on its size, “you would expect it to replace its teeth within weeks or months, not two years.”

Additional research showed that most small mammals lost their baby teeth within months. This made Krusatodon more similar to mammals like hyraxes, a creature resembling a marmot more than 20 times larger. It takes years for adult teeth to grow in hyraxes, which can live for a decade.

The extended time frame for tooth replacement, along with the older Krusatodon’s age, led the team to conclude that these ancient mammals enjoyed surprisingly long life spans.

This most likely extended their growth period. Most modern mammals experience rapid growth early in life before plateauing as they approach adult size. Krusatodon and other early Mesozoic mammals may have slowly grown throughout their long lives.

However, Krusatodon still exhibited some aspects of mammalian development. The juvenile Krusatodon possessed roughly half of its adult teeth, which meant it had only recently weaned off its mother’s milk. Other factors, like tooth growth, appear to also be similar to living mammals. “Although they’re growing more slowly, the way in which they’re growing appears to be very similar to mammals now,” Panciroli said.

Determining when the mammalian growth process sped up is difficult. Panciroli thinks development was most likely turbocharged as mammals’ metabolism increased, and as they became warm-blooded. These traits would emerge later in the Mesozoic Era and help early mammals adopt more energetically demanding lifestyles that included swimming and gliding.

Pierce added that more Mesozoic mammal fossils are needed to further understand how early mammals became primed for a global takeover. Such specimens “will be central to unraveling the factors that drove the ecological success and diversity of mammals living today.”

This article originally appeared in The New York Times.