Small mammals living today grow up quickly and have a short lifespan. Their primitive relatives, however, who lived at the time of the dinosaurs, lived much longer and were not fully grown even at one or two years of age. This is proven by two extraordinary fossil finds from Scotland, which include an adult animal and a young animal of the same species. The specimens were about the size of a mouse and are dated to be around 166 million years old.
The rule of thumb for mammals living today is that smaller animals grow faster and live shorter lives than larger ones. Mice, for example, are sexually mature at around two months of age and barely live past a year. Elephants, on the other hand, do not reach sexual maturity until they are ten years old and can reach lifespans of around 60 to 70 years. But when did this pattern develop in the course of evolution? “Since there is hardly any fossil material from young animals of early ancestors of today’s mammals, our understanding of the ontogeny of early mammals is limited,” explains a team led by Elsa Panciroli from the National Museums Scotland in Edinburgh.
Young animal of a prehistoric mammal
Now Panciroli and her team have come a significant step closer to answering this question. They were helped to do this by two 166-million-year-old skeletons of prehistoric mammals about the size of a mouse that were discovered on the Isle of Skye in Scotland. These are an almost completely preserved skeleton of an adult specimen of the species Krusatodon kirtlingtonensis and parts of another skeleton that, according to analyses, comes from a young animal of the same species. Krusatodon kirtlingtonensis is a representative of the so-called Docodontes, an extinct sister group of today’s mammals.
“These fossils are among the most complete mammalian skeletons from this period in the world,” says Panciroli. “They give us an unprecedented insight into the life of the first mammals at the time of the dinosaurs.” With the help of X-ray images, the researchers were able to digitally reconstruct the fossils and examine them non-destructively. Based on the size and proportions of the bones and teeth, Panciroli and her colleagues estimated that the adult animal weighed between 54 and 1568 grams, roughly in the size range between a shrew and a degu. The weight of the young animal was estimated to be between 32 and 80 grams.
Age estimation based on teeth
To determine the age of the two individuals at the time of their death, the team focused on the growth rings in the dental cement. “According to this, the adult animal was about seven years old at the time of its death and the juvenile between seven months and two years old,” report Panciroli and her colleagues. According to these results, Krusatodon lived significantly longer than similarly sized mammals today. “Although Krusatodon looked like a shrew or mouse on the outside, it was completely different on the inside,” write the researchers. “It grew more slowly and lived much longer than today’s small mammals. As a result, it probably had a very different physiology and life history.”
The analyses also showed that the young animal was still in the process of changing its teeth, meaning it still had some milk teeth, while others had already been replaced by adult teeth. “The relative sequence of tooth change in Krusatodon is similar to that observed in today’s mammals,” explains the research team. However, unlike in comparable species today, this development took place much later in life.
“The discovery provides an unprecedented insight into the developmental plan of an early relative of mammals,” the researchers summarize. “Our results suggest that the fundamental shift to faster growth and shorter lifespan in mammals may not have occurred until the Middle Jurassic.” The drivers of this change, however, are still unclear. More fossil young animals of other species of ancient mammals are needed, the team says.
Source: Elsa Panciroli (National Museums Scotland, Edinburgh) et al., Nature, doi: 10.1038/s41586-024-07733-1