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Most comprehensive seal family tree reveals the hidden history of walruses
Seals are successful marine predators found in oceans around the world.
But their current diversity is just a snapshot of the species that used to exist. A new study has now explored the evolutionary history of living and fossil seals, revealing in the process how walruses had extraordinary periods of speciation followed by extinction.
There is only one species of walrus alive today, but in the past there were many others.
The diversification of seals was once thought to have been fairly steady and relatively uninteresting. But a new study looking into the evolution and diversity of these marine mammals has found that walruses stand out as having been quite dynamic.
The history of walruses is characterised by dozens of bizarre species evolving and then being driven to extinction, resulting in our single remaining species. Dr Natalie Cooper is a researcher at the Natural History Museum who was involved in this latest study.
“When you take just the living species of seals and you look at their diversification, extinction, and speciation rates, it’s basically a flatline,” explains Natalie. “It’s so dull.”
“Whereas if you add in the fossil species, suddenly you start seeing these really interesting shifts. And the group that shows the most interesting stuff going on is the walruses. We’ve only got one walrus alive today, but there were 20 or 30 fossil ones.”
It seems that fluctuating sea levels in the past resulted in the isolation and speciation of many different types of walruses, before climatic shifts drove them to extinction just as rapidly. The results have been published in the journal Evolution.
The 'true' seals are defined by their lack of external ears and how they move on land ©Enessa Varnaeva/Shutterstock
The diversity of seals
From first appearing around 30 million years ago, seals have spread around the world to become one of the most successful marine predators alive today.
Seals are mostly found in the colder waters of the northern and southern hemispheres. There are 34 surviving species of seals, also known as pinnipeds, and these are split up into three main groups.
The first of these are the earless or ‘true’ seals. As the name suggests, these are seals that lack external ears. They also move on land by ‘galumphing’, which is the technical term for when seals bounce along the ground on their bellies. These are the most widespread species of seals and include the grey and common seals found across much of northern Europe, as well as the elephant seals that frequent the coastlines of North America and the Antarctic.
The second largest group are the eared seals. This group contains sea lions and fur seals, which are predominantly found in the southern hemisphere. They have obvious external ears and can effectively walk on their front flippers when on land.
The final group contains the walrus. These are distinctive not only for their large tusks, which are used to help maintain holes in the sea ice and by males to compete for females, but also because they are the last surviving species in a once much more diverse group.
Eared seals, which include sea lions and fur seals, are predominantly found in the southern hemisphere ©Smit/Shutterstock
This new paper shows just how much turnover there has been within the walrus lineage. Compared to the other groups of seals, walruses have gone through huge rates of speciation and extinction, leading to a massive array of fossil walruses, including short-tusked and giant species.
But today, all of that diversity is only survived by one species. This is probably down to the restricted range that this group has historically occupied.
Dr James Rule is a researcher who focuses on seals and their evolution, and was also involved in this latest study.
“Today the modern walrus has a circumpolar distribution meaning you can find it in the Arctic Ocean, and a little bit in the North Atlantic and the North Pacific Oceans,” explains James. “Most of the fossil species were also in the North Pacific.”
“The ocean changed a lot throughout their evolution, and so there’s a lot of climatic change going on in the background. Apart from the fact that the walruses had specialised diets, a likely reason why they all went extinct is because they never achieved a worldwide distribution.”
James suspects that this is also why a fourth group of fossil seals, known as the Desmatophocidae, also went extinct. In a similar fashion to the modern walruses, this entire group of extinct seals was confined to the North Pacific.
The restricted distribution of walruses likely contributed to their higher rates of extinction ©Hal Brindley/Shutterstock
The cradle of seal evolution
The origin of seals is surprisingly murky. There are two fossil candidates for what could be considered the first seal ancestors, called Potamotherium and Puijila. Both were likely living similar lifestyles as otter-like animals, but the issue is that they come from completely different parts of the world.
“One is known from Europe and one lived in the USA,” says James. “So it doesn’t really give us a clear answer. Either one could have been where seals evolved from.”
But the mirroring of the distribution between walruses and the extinct Desmatophocidae could potentially help resolve where modern seals first appeared. It seems that the northern Pacific Ocean was key for all four main groups.
“So even though we don’t know where exactly seals entered the water and made the transition, we know that once they did, the North Pacific Ocean is where most of the early evolution occurred,” explains James.
“But at some point, two of the groups broke free. The true seals broke free of the North Pacific quite early and spread all around the world. Then closer to the present – right before the ice ages – the fur seals and the sealions crossed the equator and dispersed in the southern hemisphere.”
This dispersal could be what saved the eared seals. As the planet cooled around 2.6 million years ago, the ice at the poles grew and the sea levels dropped. It is likely that the dramatic change in sea level radically altered the coastal ecosystems that the seals were dependent on. Any seals left in the North Pacific, namely the walruses, suffered the worst.
Whilst this latest study can’t yet give a definitive answer on where seals originated, it has created the first unified evolutionary tree for all extinct and living pinniped species.
