Pinnipeds are typically sleek bodied and rather large. The smallest pinniped, the Galapagos Fur Seal weighs about 30 kilos when full-grown and is 1.2 metres long; the largest, the male Southern Elephant Seal, is over 4 metres long and weighs up to 2.2 tonnes. All are carnivorous and live on fish, shellfish, squid, and other marine creatures.
It has long been believed that the pinnipeds are descended from a land-based carnivore, something approximately like a dog. During the 20th Century there was considerable debate about the relationship between them; some taxonomists maintaining the traditional view that they share a common ancestor, others suggesting that the eared seals (sea lions and fur seals) are distinct from the true seals, and that the similarities between the two groups are the result of convergent evolution. If this were so, Pinnipedia would be a paraphyletic grouping with no taxonomic meaning. Recent studies of mitochondrial DNA, however, have strongly supported the monophyletic hypothesis: that is, the evidence is currently on the side of a single-ancestor theory.