A fossil bone, mislabelled and forgotten for decades in a museum drawer, has been identified as a fragment from a giant prehistoric reptile, rewriting the timeline of Antarctic life. The find, announced today by a team from the University of Cambridge and the British Antarctic Survey, reveals that a diverse ecosystem thrived on the continent as recently as 66 million years ago, just before the asteroid impact that ended the Cretaceous period.
The specimen, a partial femur from a mosasaur, was originally collected in 1986 from Seymour Island, a key fossil site off the Antarctic Peninsula. It was stored at the San Diego Museum of Natural History and erroneously catalogued as a rock. Only during a routine audit was its true nature noticed by Dr. Amelia Thompson, a palaeontologist now leading the reanalysis.
“The bone had been sitting there for nearly 40 years, mistaken for a piece of sandstone,” Dr. Thompson said. “Our team was flabbergasted to realise it belonged to a mosasaur, a marine predator that could reach 10 metres in length. This pushes back the earliest known occurrence of these apex reptiles in Antarctica by 15 million years.”
Mosasaurs were not strictly dinosaurs; they were large marine lizards that dominated the oceans at the end of the age of dinosaurs. Previous Antarctic mosasaur fossils were dated to around 70 million years ago, but this femur is firmly placed in the Maastrichtian stage, the final geological interval of the Cretaceous, ending at 66 million years ago. This suggests that mosasaurs, and by extension the entire Antarctic marine ecosystem, were healthy right up to the Chicxulub impact.
“This changes how we understand the southern polar environment at the end of the Cretaceous,” explained Dr. Vance in a concurrent analysis. “Antarctica was then forested, with temperatures 10 to 15 degrees Celsius higher than today. The presence of a large predator like a mosasaur implies a food web supporting large prey. It contradicts the idea that polar life was somehow in decline before the asteroid struck.”
The fossil also hints at migration pathways. The bone’s microstructure shows growth rings indicating seasonal changes, typical of animals living in high latitudes that experience prolonged periods of darkness. Similar features are seen in other Antarctic mosasaurs, but this femur is the largest yet found. Its size and shape match those of Mosasaurus hoffmanni, a species known from North America and Europe, suggesting that mosasaurs swam between hemispheres.
“This bone is a window into the last moments of a thriving world,” Dr. Vance added. “It underscores the importance of museum collections. Over 90% of fossils remain unstudied, often misidentified. A proper audit can yield discoveries that reframe our understanding of Earth’s history.”
The team used CT scanning and chemical analysis to confirm the bone’s identity. They compared it to mosasaur collections worldwide, matching its unique features. The specimen will now be rehoused at the British Antarctic Survey facility in Cambridge, where it will form part of a new exhibit on Antarctic prehistory.
The timing of the announcement is poignant. As the modern Antarctic faces unprecedented warming and ice loss, this fossil serves as a reminder of a time when the continent was ice free and teeming with life. “The past teaches us that our planet is dynamic,” Dr. Vance reflected. “The carbon we are releasing now is driving conditions not seen since these animals ruled the seas. It is a sobering parallel.”
The discovery has already sparked renewed interest in other museum specimens from the 1980s expeditions. Researchers are now scouring drawers in Buenos Aires, Washington D.C. and London for similarly overlooked treasures. For now, this single bone, hidden in plain sight, forces a rewrite of the textbooks.
As Dr. Thompson concluded: “We think we know the fossil record, but often we have only scratched the surface. This femur is proof that the greatest discoveries can still come from the quietest corners of a collection.”
