The roots of mammals stretch deep into Earth’s history. Long before modern species appeared, small creatures experimented with traits that shaped the lineage we come from.
Fossils are our only link to these ancestors, and sometimes a single discovery can change the whole picture.
This is exactly what happened in Greenland, where scientists uncovered the oldest definitive docodontan mammal ever found.
Oldest fossil from Greenland
Dr. Sofia Patrocínio and her team at the Universidade de Évora have described the fossil in the journal Papers in Palaeontology. It came from the Rhætelv Formation, a site that holds rare traces of early life.
The research closes a gap in mammal history, narrowing a 40-million-year mystery to 33 million years.
The docodontan fossil, named Nujalikodon cassiopeiae, includes part of a jawbone, a preserved molar, and the roots of another tooth. From these pieces, researchers pieced together a story about how early mammals ate, lived, and adapted.
Adaptable teeth shaped survival
“Docodontans are one of the earliest groups of mammaliaforms,” said Dr. Elsa Panciroli. “They have more complex teeth than most other mammaliaforms at this time, with a lot of cusps and ridges, rather than a simple arrangement of only a few cusps in a row.”
That complexity gave docodontans a real advantage. With teeth that could handle many foods, they spread into new environments.
“This probably made them able to eat a wider range of foods, making them more ecologically diverse,” Dr. Panciroli said. By the Middle to Late Jurassic, they were thriving in ways few other early mammals managed.
Rise and fall of docodontans
But success did not guarantee survival. Docodontans dissapeared, and no direct descendants remain today. Why? No one knows for certain.
“It is not possible to say why they became extinct. Extinction is a natural phenomenon that happens, often when conditions change (new habitats, changing climates, or altered ecosystems),” said Dr. Panciroli.
Shifts in climate and habitat may have played a role, but the fossil record leaves questions where answers should be.
Docodontan fossils fill history gaps
The placement of docodontans in the evolutionary tree is relatively clear, but relationships among species remain debated. Fossils help untangle these links, though they are scarce.
“There are very few places in the world where the fossils of terrestrial animals are preserved from the Early to Middle Jurassic,” said Dr. Panciroli.
“This is why there is such a big gap in our knowledge. It’s a problem not just for the study of mammals, but for all land-living animal groups.”
Fossil reveals docodontan link
Detailed analysis showed Nujalikodon cassiopeiae was either an early docodontan or its closest relative, making it a transitional species.
It also represents the oldest definitive docodontan yet identified, reducing the evolutionary gap by seven million years.
Its teeth reveal how simple cusp patterns evolved into the complex structures seen later. “This gives us a clue about how their complex teeth may have evolved from the simpler cusp patterns of their ancestors,” said Dr. Panciroli.
The fossil’s location matters too. With similar finds in Great Britain and France, researchers believe docodontans first appeared in Europe.
From there, they spread widely, reaching regions that include Portugal, Germany, Russia, China, and the U.S. Their ability to adapt to new diets likely made this expansion possible.
The history of mammals
The Greenland fossil does not answer every question, but it takes scientists much closer to understanding the bigger picture.
It offers a glimpse into how docodontans lived, the regions they spread into, and the way their teeth developed into more complex forms that allowed them to survive in different environments.
Still, one of the biggest puzzles remains unsolved – why did they eventually disappear, leaving no direct descendants behind?
Every new discovery adds a missing piece to the vast puzzle of mammal evolution. With Nujalikodon cassiopeiae, researchers now hold one of the most important clues uncovered so far.
The study highlights both the progress that has been made and the mysteries that remain, reminding us how much more there is to learn about the deep history of mammals.
Image credit: Two Nujalikodon cassiopeiae in the Rhætelv Formation, Greenland. Credit: Pedro Andrade in Patrocínio et al. 2025
The study is published in the journal Papers in Palaeontology.
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