The remains of a diminutive mouse-like creature that lived 166m years ago could help answer one of biology’s biggest questions of why mammals have become so successful, fossil experts say. Krusatodon kirtlingtonensis belongs to the immediate predecessors of mammals and lived alongside the dinosaurs during the middle Jurassic age. But while it was originally known only from individual teeth, researchers have now reported two partial skeletons. Researchers say these show not only that the creature lived far longer than similar sized mammals today, but developed at a slower pace. Dr Elsa Panciroli, first author of the research from National Museums Scotland, said the research could help shed light on why mammals have become so successful. “[Mammals] have the biggest range of body sizes. They’re living in the most habitats. They have the widest range of ecologies. And so everybody’s asking, Why?” she said. “Of course, to understand that, we have to understand where their unique biology comes from. So this is a piece of that puzzle”. Found on the Isle of Skye, one set of remains, discovered in 2016, is a juvenile, while the other, discovered in the 1970s but not studied, is an adult. The team say the fossils not only offer new insights into the creature’s anatomy, but provide an unprecedented chance to explore the pace at which it grew and reached maturity. Writing in the journal Nature, Panciroli and colleagues describe how they used an X-ray technique to scan the specimens and then analysed growth rings in the teeth, finding that the juvenile died between seven months and two years old, while the adult died at about seven years old. Panciroli said the results supported previous research suggesting the immediate predecessors of mammals lived longer lives than similar sized mammals today. Pet mice, for example, typically live less than three years. “The earliest mammals looked like mice, but they were living a very long time,” Panciroli said. The team also estimated the body masses of the adult and juvenile and analysed their teeth. The results suggest the juvenile was in the process of gaining its adult teeth, which could mean it was weaned or close to being weaned when it died. The results, said Panciroli, were a surprise given mammals of a similar size today wean at a much younger age. The team say that indicates the creature grew more slowly, weaning at a similar age to larger mammals such as the brush-tailed possum or Barbary macaque. Panciroli said it is unclear when, or why, the developmental schedule of early mammals sped up and the lifespan shortened, but that unpicking the shift is important. “We know that all mammals today have this rapid juvenile growth, determinate growth [where growth stops at maturity], and that’s probably part of why they’re so successful, but so we need to find out how that happened,” she said. “And then, of course, once we know how, we might be able to start addressing why.” Dr Greg Funston of the Royal Ontario Museum in Toronto, who was not involved in the work, said the study showed some of the hallmark habits of mammals were only established later on in our lineage. “Specifically, these little shrew-like animals, despite appearances, probably lived and died more like reptiles, with a long growth trajectory through childhood to maturity, and then a much longer lifespan than the live fast, die young approach of today’s small mammals,” he said. “Other features though, like the order that the teeth erupt, were already in place, and so this discovery helps us to understand the piece-by-piece assembly of the lifestyle we all share today.”