The Potamotherium valletoni, also known as a “river beast,” was an ancient relative to seals with an otter-like appearance. The early animal wandered the Earth about 23 million years ago, and it likely used its whiskers to find food and explore underwater.
Modern-day seals or pinnipeds, such as harbor, elephant and ringed seals, are highly adapted to their watery environments and use their whiskers to detect vibrations underwater. However, when and how they adapted to life underwater has stumped researchers. So, a team of experts turned to where they could find an answer: ancient seal brains.
A recent study published in Communications Biology sheds light on how ancient seals transitioned from life on land to living underwater. Before, it was also unclear when seals and their ancient relatives began to use their whiskers to forage.
A Seal's Evolution
Ancient seals appeared more otter-like than modern-day ones, dwelling in freshwater environments or walking on legs. Most early relatives also had forms less specialized for swimming.
For example, Puijila, a fossil that showcases the evolutionary path of pinnipeds, also known as the walking seal, had webbed feet and a long tail. During the transition into a more stream-lined body for water-based life, whiskers to forage for resources may have aided this shift for the early ancestors.
Analyzing Seal Whiskers
Modern-day seals use their whiskers to forage for food, utilizing them to sense vibrations in the water. Animals that use whisker-foraging behavior have thicker infraorbital nerves, or the nerves that pass from the whiskers into the skull.
But, looking at the infraorbital nerves alone does not inform on the sensitivity of whiskers. So, researchers also looked at casts of modern and ancient skulls to see how the coronal gyrus changed shape with whisker use.
The team studied the skulls of seven ancient, fossilized specimens and compared them to the skulls of 31 modern-day carnivorous mammals and made endocranial casts. These creatures included bears, mustelids and seal relatives. Casts were made because preserved remains of whiskers or soft tissues of early pinnipeds are unknown.
Ancient Seal Findings
The coronal gyrus, located inside the cranial cavities, receives signals from whiskers. Based on the shape of the skulls, researchers can see how the area received signals from whiskers and how it changed with use over time.
By comparing the skulls, the team found that the coronal gyrus is bigger in animals that use their whiskers significantly. Otters, seals, civets and sea lions had larger areas. In carnivorous mammals that only use their hands to forage, this area did not expand. In Potamotherium, the coronal gyrus was larger than ancient and living land-based mammals that use their arms to forage.
The coronal gyrus was similar in size to animals that used their whiskers to explore their environments. This find suggests that this ancient relative used its whiskers to find food and possibly their hands, too. Overall, whisker-based foraging may have already been present in seal relatives before their transition to a fully aquatic lifestyle.
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