Website written and created by Ben Kitchen & Claire Hamilton, students of University of Bristol

Evolution and Diversity of Pinnipeds

Pinnipeds are split into three different families; eared seals, true seals and walruses. They all have limbs that are modified for swimming. Their bones are short and have almost completely withdrawn into the body to leave only their flippers protruding. Their flippers have a large surface area as the digits are connected by a web of skin. Their body shape and lack of limbs mean that pinnipeds are fairly streamline, an advantage for their aquatic life. Pinnipeds are able to exploit the colder waters of the world as they have a layer of blubber which acts as an insulating layer as well as helping to give their streamlined appearance. Some, such as the fur seal, have a layer of hair which also helps with insulation.

The skeleton of a pinniped (walrus)
Walrus skeleton

Seals were once thought to be an 'unnatural' polyphyletic unit, containing two unrelated groups; eared seals and walruses. Walruses were said to have evolved from bear like carnivores. True seals were thought to be the most closely related to the mustelid family (weasel family). More recent research shows that all three groups all originate from a group of land dwelling carnivores.

The two proposed phylogenies of pinnipeds. Picture taken from: Marine Mammal Evolutionary Biology (Berta and Sumich 1999)
Phylogeny

True seals are easily identifiable as they have hair covered flippers and their hind flippers cannot be brought underneath their body meaning that they are not able to take any of their body weight and move on land by wriggling and sliding over the ground. They have retained many characteristics of their land dwelling ancestors including fur on their skin (also known as a pelt). True seals, along with walruses do not have obvious external pinna. This is unlike eared seals which, as their name suggests, have an inch long, pointed and furled pinna. Walruses and eared seals are able to turn their hind flippers forward to support their weight and walk on land.

Antlers dating back to the Palaeolithic times with drawings of seals have been found. Seals were hunted by Neolithic Scandinavians who carved their bones into weapons and used their teeth as decoration. Coastal natives used seal skin for clothing and over the years they developed many legends about it. They believed that if seal skin was drawn around the edge of a field it would protect the crop from hailstones and that seal skin clothing would protect the owner from lightening. A seals flipper was also believed to be able to cure insomnia by being placed under a person's pillow each night. Some Eskimos also believed that a seal's soul and urinary bladder remained together after the seal had been hunted and if the bladder was returned to the sea then the soul would be able to join fresh bodies that could be hunted.