Fossil skull turns seal evolution upside down
Seals can be found in many places around the world. But especially in the polar regions, these marine mammals inspire tourists and fascinate scientists with their adaptability. The group of true seals in particular has settled in both the Arctic and Antarctic. But how this diversification within the largest seal family came about was not entirely clear. A study of new skulls from New Zealand has now turned upside down the previous theory that a part of the family had developed in the northern hemisphere.
The Australian-New Zealand research team, led by PhD student James Rule of Monash University in Australia and Dr Felix Marx, paleontologist and curator at the Te Papa Museum in New Zealand, examined seven fossil skulls discovered by hobby fossil hunters in Taranaki, New Zealand. It turned out that it was a 3– 3.5 million-year-old skull of a previously unknown seal species. Because the skulls were in surprisingly good condition, the team was able to examine the skulls in detail. A more detailed analysis and comparisons then revealed the surprise: they held the ancestor of the monk seals in their hands.
According to their research, the seals were about 2.5 meters long and weighed between 200 and 250 kilos, similar to leopard seal or crabeaters, but larger than today’s monk seals. The researchers suspect that the species was more native to temperate to warm waters. But the real discovery lies in the fact that it is an ancestor of this genus of seals. Monk seals previously had been exclusively assigned to the northern hemisphere. Based on the research results of Rule and his colleagues, the entire family tree of the true seals had to be rearranged. Until now, we thought that all true seals originated in the northern hemisphere, and then crossed the equator just once or twice during their entire evolutionary history,” explains James Rule, the study’s lead author. “Instead, many of them appear to have evolved in the southern Pacific, and then criss-crossed the equator up to eight times. This discovery really turns seal evolution on its head.”
According to the results of the study, the north-south dichotomy in seal evolution also happened much earlier than previously thought, i.e. about 15 million years ago. The researchers write that the true seals first split in the North Atlantic and then the Monachinae migrated to the South Pacific, where they formed the ancestors of elephant seals, crabeater-like seals and monk seals, which then crossed the equator several times during their evolution histories. The results also suggest that the monk seals, which today live in the Mediterranean, among other things, migrated there only about 3 million years ago. The extinction of the southern monk seals was also the starting signal for the immigration of the ancestors of the southern sea lion and fur seal species and their spreading.
Elephant seal 
Crabeater seal 
Leopard seal 
Weddell seal 
Ross seal 
Monk seal
The research, published in the journal Proceedings of the Royal Society B, not only sheds new light on seal evolution. The fact that the skulls had been discovered by non-researchers also puts paleontology in a new light and is good news for Citizen Science projects. This is also the opinion of Dr. Felix Marx, co-author of the study and curator at Te Papa Musem, where the skulls are today. “This discovery was a triumph for Citizen Science. This new species has been discovered thanks to numerous, exceptionally well-preserved fossils, all of which were found by members of the public,” he explains. The researchers hope that there will be other treasures in the soil of Taranaki and elsewhere that support their groundbreaking results of seal evolution.
Dr Michael Wenger, PolarJournal
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