Mammals grew more slowly in the Jurassic than they do today

An international team of researchers led by National Museums Scotland discovered two Krusatodon kirtlingtonensis fossils, one adult and one juvenile, both in Skye. These mouse-sized mammals lived around 166 million years ago. The specimens represent the only juvenile Jurassic mammal skeleton known to science, while the adult is one of the most intact mammal skeletons from this time period in the world.

The fact that both specimens, the juvenile and the adult, belong to the same species of early mammals is unique and has allowed groundbreaking comparative analysis into their growth and life history. This was possible thanks to X-ray computed tomography carried out in several laboratories, including cutting-edge facilities, notably the ESRF’s ID19 beamline.

The ages of the specimens at death were determined using X-ray imaging to count the growth rings in their teeth. The adult was found to be around 7 years old and the juvenile between 1 - 2 years, and still in the process of replacing its baby teeth.

Shift in mammal growing patterns

Today, small mammals have significantly shorter lifespans, some living as little as 12 months, and maturing quickly, losing their baby teeth and weaning within months of birth. The Krusatodon fossils reveal for the first time that the earliest mammals didn’t finish replacing their teeth until well into their second year of life, possibly later. This tells us that a fundamental shift in the growth patterns and life expectancy of mammals must have taken place during or after the Middle Jurassic.

The specimens were discovered decades apart, with the adult being one of the earliest Jurassic finds on Skye in the 1970s, while the juvenile was discovered in 2016.

National Museums Scotland's Elsa Panciroli examines two krusatodon kirtlingtonesis fossils. Credits: Duncan Mc Glynn.

Elsa Panciroli, lead author and Associate Researcher of Palaeobiology at National Museums Scotland, explains the importance of the research: “These fossils are among the most complete mammals from this time period in the world. They give us unprecedented insights into the lives of the first mammals in the time of dinosaurs. Although on the outside Krusatodon looked like a shrew or mouse, on the inside it was quite different; it grew more slowly and lived much longer than small mammals do today. As a result, it probably had quite a different physiology and life history as well. Skye’s fossils are really putting Scotland firmly on the map when it comes to understanding mammal evolution. This is just the tip of the iceberg in terms of what they can tell us.”

This research is a long-standing collaboration between the team and the ESRF. Many fossils have been discovered in the Isle of Skye and a selected few have been imaged at the ESRF’s ID19 beamline. “Over the course of a single experiment at the ESRF, we acquired many data on very rare Jurassic animals, including mammals and lizards. This productive beamtime has resulted in several papers, with this Nature one being the fourth article published”, says Vincent Fernandez, paleontologist at the ESRF an co-author of the publication.

In addition to National Museums Scotland, the study also involved researchers from the American Museum of Natural History, University of Chicago, European Synchrotron Radiation Facility, and Queen Mary University of London.

Reference:

Panciroli, E. et al, Nature, 24 July 2024. DOI: 10.1038/s41586-024-07733-1 

Top image: Krusatodon kirtlingtonesis artist impression. Credits: Maija Karala.