The discovery of the oldest known fur seal, a tiny creature slightly larger than a sea otter, has helped close a five million year old gap in the evolutionary history of these semi-aquatic marine mammals, according to research published in the latest edition of Biology Letters.
Robert Boessenecker, a PhD student in geology at the University of Otago in New Zealand, and Morgan Churchill from the University of Wyoming identified the new genus and species of fur seal (Eotaria crypta) from a fossilized partial jaw and several teeth recovered from a 15 to 17 million year old rock formation in Southern California in the early 1980s.
While the remains were first discovered decades ago, they had previously been misidentified as belonging to a species of small walrus. Boessenecker found the specimen while looking through the collections at the John D. Cooper Archaeological and Paleontological Center in California and said he could tell almost instantly that they belonged to 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 10-12 million years ago,” Boessenecker said in a statement. “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.”
In paleontology terms, this type of gap is known as a “ghost lineage,” meaning that there is an inferred relationship between two different groups of organisms, but no fossil record to support the existence of said relationship. Previously, there was no evidence of the first five million years of fur seal and sea lion evolution, the authors said, but the new discovery fixes that problem.
To determine the phylogenetic placement of E. crypta, Bossenecker and Churchill analyzed 115 morphological characters (including three novel ones) coded for 23 different modern types of the creatures from across all families. They focused on characteristics most useful for resolving the phylogenetic relationships of stem otariids and early pinnipeds, and coding efforts focused on the male specimens in order to minimize the impact of sexual dimorphism, the authors said.
Boessenecker explained that E. crypta’s status as a critical transitional fossil is proven by its teeth, which essentially bridge the gap between the complex bear-like teeth of the earliest known pinnipeds and the most simplified teeth of modern-day sea lions.
However, the find also raises an important question: why, despite extensive excavations of similarly aged rocks in California, has only one specimen of this new fur seal species ever been discovered? Japanese paleontologist Dr. Naoki Kohno previously hypothesized that early fur seals lived in the open ocean and only rarely ventured into continental shelf areas.
If Dr. Kohno’s proposal is true, it means that the creatures would not have frequented regions where they would be more likely to be preserved as fossils. This hypothesis is supported by the fact that the newly discovered fossil was collected from sedimentary rocks that had been formed by deposits in what was once a continental shelf instead of at an inland fossil site.