Fossil remains of a 16 million-year-old fur seal, named Eotaria crypta, have shed light on the evolution of fur seals and sea lions.
The two seals on the right are a life restoration of Eotaria crypta, whose adult size was only slightly larger than a sea otter; the pinniped depicted above them is the larger-bodied seal Allodesmus, which also existed at the time. Image credit: Robert Boessenecker.
Eotaria crypta was tiny, with adults being only slightly larger than a sea otter and around the size of a juvenile New Zealand fur seal.
“Its fossilized partial jaw, with several well-preserved teeth, was recovered from a 15-17 million-year-old rock formation in Southern California in the early 1980s, but until now had been misidentified as belonging to a walrus species,” explained Robert Boessenecker from the University of Otago, the first author of a paper published in the journal Biology Letters.
The fossil was deposited in what is now the John D. Cooper Archaeological and Paleontological Center, where Mr Boessenecker found it while searching through its collections. He instantly realized that it was not the small walrus Neotherium but a tiny, early fur seal.
“This was very exciting as fur seals and sea lions have a limited fossil record that, up until now, extended back to about 10-12 million years ago.”
“Yet we know that their fossil record must go back to around 16-17 million years ago or so, because walruses have a record reaching back that far.”
In paleontology, a gap like this is known as a ‘ghost lineage,’ and Eotaria crypta has now eliminated it.
“Until now we had no fossil evidence for the first five million years of fur seal and sea lion evolution. It’s extremely satisfying to have remedied that,” Mr Boessenecker said.
“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.”
Japanese paleontologist Dr Naoki Kohno of the Natural History Museum and Institute in Chiba has previously proposed that the earliest fur seals lived in the open ocean and only rarely strayed into continental shelf areas where they would be more likely to be preserved as fossils.
“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,” Mr Boessenecker said.
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Robert W. Boessenecker & Morgan Churchill. The oldest known fur seal. Biology Letters, published online February 11, 2015; doi: 10.1098/rsbl.2014.0835
