Researchers discovered the remains of the oldest-known fur seal, and the finding provides a missing link in the evolutionary chain.
The discovery resolves a more than five million-year gap in the known evolutionary history of fur seals and sea lions, the University of Otago reported. The researchers named the new species Eotaria crypta, which means "dawn of the sea lion."
The fossil, which consists of a partial jaw bone with well-preserved teeth, was recovered from a 15 to 17 million-year-old rock formation in Southern California back in the early 1980s. Until now the sample had been misidentified as belonging to a walrus species, but researcher Robert Boessenecker immediately noticed it was from a tiny early fur seal.
"This was very exciting as fur seals and sea lions--the family Otariidae--have a limited fossil record that, up until now, extended back to about 10 [to]12 million years ago. Yet we know that their fossil record must go back to around 16-17 million years ago or so, because walruses--the closest modern relative of the otariids--have a record reaching back that far," he said.
This type of evolutionary gap is known of as a "ghost lineage," and the new research has now eliminated it in terms of history of the fur seal.
"The mystery remains of why there has only been one of these fur seals ever found given that there have been extensive fossil excavations of similarly aged rocks in California," Boessenecker said.
In the past, Japanese palaeontologist Naoki Kohno proposed that the earliest fur seal most likely lived in the open ocean and were rarely seen around continental shelf areas, where fossils would be visible to scientists. This theory could help explain why fossils of the ancient animal have remained so elusive.
"This hypothesis is supported by this fossil having been collected from rock formed by sediments deposited in what was then continental shelf, rather than extensively studied inland fossil sites, such as Sharktooth Hill, that formed in bays," Boessenecker concluded.
The findings were published in a recent edition of the journal Biology Letters.
