A life reconstruction of Krusatodon. Not only are the specimens remarkably complete, but one belongs to a juvenile — making it the oldest known juvenile mammal fossil.Credit…Maija Karala

The Trilobites column at the New York Times has a post about some unexpected discovery in some Jurassic mammals.  Small mammals usually live fast and die young.  However, recent research published in the journal Nature found that a pair of mouse-sized mammal fossils from the island of Skye grew more slowly and lived much longer than their modern day counterparts.  Krusatodon kirtlingtonensis lived near swampy and thick forests about 166 million years ago during the Jurassic Period.

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.

Dr. 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.