A new study, published in Nature, sheds light on the life history of the relatives of early mammals. An international team of researchers have discovered that these ancient creatures, known as mammaliaforms, lived longer and took more time to mature than their modern-day counterparts of similar size.

The exciting work, led by Dr Elsa Panciroli of National Museums Scotland, utilised exceptionally well-preserved fossils of Krusatodon, a rat-sized mammaliaform from the Middle Jurassic period, found on the Isle of Skye, Scotland.

A key contribution to the study came from Dr Elis Newham, a Postdoctoral Research Associate at the School of Engineering and Materials Science at Queen Mary University of London. Dr Newham developed a novel technique using synchrotron X-ray micro-computed tomography to analyse growth increments in the teeth of the fossils (X-ray cementochronology). This innovative technique allowed the researchers to estimate the age of the specimens, revealing that one adult Krusatodon lived for approximately seven years, while a juvenile was between 7 and 24 months old while it was still shedding its baby teeth.

The analysis yielded surprising results. "Living mammals typically grow rapidly when young and then stop growing entirely as adults," explains Dr Newham. "This study challenges our current understanding of how this life history pattern evolved. Our team found that Krusatodon exhibited a similar sequence of tooth development as modern mammals, but this process was significantly slower and occurred over a much longer time frame."

The research suggests that the shift towards faster growth and shorter lifespans observed in modern mammals may not have occurred until after the Middle Jurassic period. "This discovery highlights the importance of studying juvenile fossils to understand the evolution of mammalian life histories," says Dr Newham. "Our technique of X-ray cementochronology offers a powerful tool for unlocking the secrets hidden within these ancient specimens."