Two Rare Jurassic Skulls Could Help Unlock the Secrets of Mammals’ Evolutionary Success

Roughly 166 million years ago, a pint-sized mammal lived alongside the dinosaurs on Scotland’s Isle of Skye. On the outside, the 3.5-ounce creature, called Krusatodon kirtlingtonensis, looked very similar to a modern shrew or mouse. But after studying two fossils from the species, scientists have now discovered that on the inside, it was very different.

The study, published in the journal Nature last week, reveals that these early Jurassic animals matured slower and lived longer than similar mammals of today. The findings could shed light on how mammals diversified and evolved to be so successful across the globe.

“These fossils are among the most complete mammals from this time period,” study lead author Elsa Panciroli, a paleontologist at National Museums Scotland, says in a statement. “They give us unprecedented insights into the lives of the first mammals in the time of dinosaurs.”

Previously, the only information scientists knew about Krusatodon had come from fossilized teeth. But analyzing these two skulls discovered on the Isle of Skye—an adult in the 1970s and a juvenile in 2016—gave researchers a new look at the mysterious creature.

To determine at what age the animals had died, researchers used X-ray imaging to count the growth rings on their teeth, like how botanists count the rings of a tree. They found that both fossils were incredibly unique: One was an adult specimen of around 7 years old—quite long-lived compared to modern-day small mammals—and the other was a rare juvenile between 1 and 2 years old.

“When we found the tiny juvenile skull, I didn’t realize 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,” study co-author Roger Benson, curator of paleobiology at the American Museum of Natural History, says in the statement. “For science, it’s really remarkable to have this fossil.”

This younger specimen, which died while it was still replacing its baby teeth, is the only known juvenile mammal skeleton from the Jurassic Period. Combined with the skull of the longer-lived specimen, it allowed researchers to compare the life cycles of this mammal ancestor with the mammals of today.

The fact that the 2-year-old creature was still in the process of replacing its teeth “is unusual and tells us a lot about how mammals’ evolution took place,” Panciroli tells BBC News’ Helen Briggs. Namely, in the words of the New York Times’ Jack Tamisiea, it signals to researchers that these early mammals “lived slow and died old.”

Small mammals today lose their baby teeth within weeks, and they live as little as one year—but the study shows this hasn’t always been the case.

“[Krusatodon], despite appearances, probably lived and died more like reptiles, with a long growth trajectory through childhood to maturity, and then a much longer lifespan than the live fast, die young approach of today’s small mammals,” Greg Funston, a paleontologist at the Royal Ontario Museum in Canada who was not involved in the study, tells the Guardian’s Nicola Davis. “Other features though, like the order that the teeth erupt, were already in place, and so this discovery helps us to understand the piece-by-piece assembly of the lifestyle we all share today.”

The study suggests small mammals’ shift to shorter life cycles happened during or after the Middle Jurassic, between 161 million and 174 million years ago.

Perhaps this change gave today’s mammals an evolutionary leg up because of their ability to mature quickly, Panciroli tells the Guardian. Though researchers haven’t yet found a reason to explain this change, the answer might reveal why modern mammals have seen so much success.

Margherita Bassi | READ MORE

Margherita Bassi is a trilingual storyteller and freelance journalist with a particular interest in ancient history, astronomy and human interest stories.