A research paper published this week in the journal “Nature” has revealed that early mammals may have lived much longer than their modern-day counterparts.  The study describes two Krusatodon kirtlingtonensis fossil specimens.  These mouse-sized mammals lived approximately 166 million years ago (Bathonian faunal stage of the Middle Jurassic).  The fossils come from the remarkable Jurassic strata found on the Isle of Skye.

The research led by National Museums Scotland describes the fossilised remains of one adult and one juvenile.  The adult K. kirtlingtonensis is thought to have been around seven years old when it perished.  The juvenile is thought to have been between seven to twenty-four months old when it died.  It was still in the process of replacing its deciduous dentition (milk teeth) with its final, adult set of teeth.

National Museums Scotland’s Dr Elsa Panciroli with enlarged, 3D printed models of Krusatodon kirtlingtonensis skeletons. Picture credit: Duncan Mc Glynn.

Picture credit: Duncan Mc Glynn

The scientists conclude that early diverging mammaliaforms did not experience the same life histories as extant small-bodied mammals. Furthermore, the fundamental shift to faster growth over a shorter lifespan may not have taken place in mammaliaforms until during or after the Middle Jurassic.

Two Krusatodon kirtlingtonensis Fossil Specimens

These two Krusatodon kirtlingtonensis fossils are extremely significant.  The adult specimen represents one of the earliest fossil discoveries from the Isle of Skye.  It was found in the 1970s.  This specimen is one of the most complete Jurassic mammaliaform skeletons known to science.  The juvenile fossil was found more recently (2016).  It is the only known skeleton of an immature Jurassic mammaliaform. Indeed, there is no other record of an adult and juvenile from the same Jurassic mammaliaform taxon.

The Isle of Skye is one of the most important locations in the world for Middle Jurassic fossils.  Vertebrate body and trace fossils confirm the presence of a rich biota of dinosaurs, pterosaurs and early mammals. This new study focusing on K. kirtlingtonensis underscores the significance of the Scottish island to palaeontology.

An article about the discovery of a large, new species of pterosaur (Dearc sgiathanach): Fantastic Pterosaur from the Isle of Skye.

Dr Elsa Panciroli examines the barnacle encrusted fossil which contained Krusatodon kirtlingtonensis material. Picture credit: Duncan Mc Glynn.

Picture credit: Duncan Mc Glynn

X-ray Computed Tomography Employed

The discovery of a juvenile and adult of the same species of Jurassic mammaliaform is unique.  As a result, a ground-breaking comparative analysis into their growth and life history (ontogeny) was undertaken. The ages of the specimens at death were determined using X-ray imaging to count the growth rings in their teeth.  Therefore, the age of each animal at the time they died could be established.  The X-ray computed tomography was carried out in several laboratories, including the European Synchrotron (ESRF) located in Grenoble, France.

Extant small mammals have much shorter lifespans.  Many live for less than a year.  They mature quickly, losing their milk teeth and weaning within months of birth.  In addition, most small mammals living today rapidly reach sexual maturity and breeding age. The Krusatodon fossils reveal for the first time that the earliest mammals didn’t finish replacing their milk teeth until well into their second year of life, possibly later.  The researchers conclude that mammals grew more slowly in the Jurassic than they do today.  This suggests a fundamental shift in the growth patterns and life expectancy of mammals must have taken place during or after the Middle Jurassic.

An adult and young Krusatodon kirtlingtonensis life reconstruction. A stegosaur can be seen in the background. Fossilised tracks found on the Isle of Skye indicate the presence of stegosaurs in the ecosystem.  Picture credit Maija Karala.

Picture credit: Maija Karala

Dr Elsa Panciroli, lead author and Associate Researcher of Palaeobiology at National Museums Scotland, commented:

“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.”

National Museums Scotland’s Dr Elsa Panciroli pointing out details on a high-resolution scan of the Krusatodon kirtlingtonensis fossils. Picture credit: Duncan Mc Glynn.

Picture credit: Duncan Mc Glynn

Analysis of Teeth Growth Patterns Yields Surprising Results

Co-author Dr Elis Newham, a Postdoctoral Research Associate at Queen Mary University of London explained:

“Our study benefited greatly from a new technique we developed using synchrotron X-ray technology. This allowed us to analyse growth patterns in the teeth of these fossils, much like counting tree rings. It was surprising to discover that Krusatodon, while showing a similar development pattern to modern mammals, grew much slower and lived longer. This research emphasises the value of studying juvenile fossils, and the X-ray cementochronology technique offers a powerful tool for unlocking these secrets from the past.”

Dr Stig Walsh, Senior Curator of Vertebrate Palaeobiology at National Museums Scotland and fellow co-author on the research, said:

“Even in the context of the amazing palaeontological finds on Skye in recent years, these fossils are remarkable. Mammal fossils of this age are exceptionally rare worldwide, and most are just single teeth found by sieving sediment. To find two such rare fossil skeletons of the same species at different growth stages has rewritten our understanding of the lives of the very earliest mammals. We’re thrilled they are both now part of Scotland’s National Collection, an important part of the global fossil record, and will be preserved for generations of researchers to come.”

National Museums Scotland’s Dr Elsa Panciroli examines two Krusatodon kirtlingtonensis fossils. Picture credit: Duncan Mc Glynn.

Picture credit: Duncan Mc Glynn

International Collaboration

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

Co-author Professor Roger Benson (American Museum of Natural History), described the juvenile fossil material:

“When we found the tiny juvenile skull, I didn’t realise what we’d found right away. The part of the fossil that was sticking out of the rock was blasted by erosion, surrounded by barnacles, and looked just like a piece of ash. I used micro-CT scanning, a form of 3D X-ray imaging, and was very surprised to see a whole skull in the rock. For science, it’s really remarkable to have this fossil and it told us a lot about growth and life history in some of the earliest mammals.”

To read a blog post from 2019 about greater legal protection for Scottish fossil sites: Legal Protection for Isle of Skye Fossils.

Everything Dinosaur acknowledges the assistance of a media release from National Museums Scotland in the compilation of this article.

The scientific paper: “Jurassic fossil juvenile reveals prolonged life history in early mammals” by Elsa Panciroli, Roger B. J. Benson, Vincent Fernandez, Nicholas C. Fraser, Matt Humpage, Zhe-Xi Luo, Elis Newham and Stig Walsh published in the journal Nature.

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