The remains of a small mouse-like creature that lived 166 million years ago may help answer one of biology’s biggest questions about why mammals became so successful, fossil experts say.
Crusatodon kirtlingtonensis belong to the immediate predecessors of mammals and lived together with the dinosaurs during the middle Jurassic period. But while originally known only from individual teeth, researchers have now reported two partial skeletons.
Researchers say this shows not only that the animal lived much longer than mammals of similar size today, but evolved at a slower pace.
Dr. Elsa Panciroli, first author of the research of National Museums Scotlandsaid the research could help shed light on why mammals have become so successful.
“[Mammals] has the largest range of body sizes. They live in most habitats. They have the greatest variety of ecologies. And that’s why everyone asks, why?” she said. “Of course, to understand this, we need 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 says the fossils not only provide new insights into the animal’s anatomy, but offer an unprecedented chance to explore the rate at which it grew and reached maturity.
Writing in the journal Nature, Panciroli and colleagues described how they used an X-ray technique to scan the samples and then analyzed growth rings in the teeth, finding that the youth died between seven months and two years old, while the adult about seven years old.
Panciroli said the results support previous research suggesting that the immediate predecessors of mammals lived longer than similar-sized mammals today. Pet mice, for example, usually live less than three years.
“The earliest mammals looked like mice, but they lived very long,” Panciroli said.
The team also estimated the body masses of the adult and juvenile and analyzed their teeth. The results suggest the young child was getting its adult teeth, which could mean it was weaned or close to weaning when it died.
The results, Panciroli said, were a surprise given mammals of a similar size today wean at a much younger age. The team says this suggests the animal grew more slowly, weaning at a similar age to larger mammals such as the brush-tailed possum or Barbary macaque.
Panciroli said it’s unclear when, or why, the developmental schedule of early mammals sped up and life spans shortened, but it’s important to unpick the shift.
“We know that all mammals today have this rapid juvenile growth, defining growth [where growth stops at maturity], and that’s probably part of why they’re so successful, but that’s why we need to figure out how that happened,” she said. “And then, of course, once we know how, we might start to address why.”
Dr. Greg Funston of the Royal Ontario Museum in Toronto, who was not involved in the work, said the study showed that some of the characteristic habits of mammals were only established later 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 adulthood, and then a much longer life span than the live fast, die young approach of today small mammals,” he said. “However, other features, such as the order in which the teeth erupt, were already in place, and so this discovery helps us understand the piece-by-piece composition of the lifestyle we share today. “
