The two otter species seen here (through an individual sea otter and a family of river otters), share common ancestors. Today, they belong to different groups in the family Mustelidae. Want to know more? Read this Episode 4 of Evolution Events!
Mustelidae encompasses familiar creatures such as badgers, martens, mink, weasels and otters (of 67 species); the family first appeared in evolutionary history possibly between 35-16.1 million years ago with its the first members believed badgers and martens, with other groups appearing later (read on).
Mustelids share some common characteristics, however for simplicity’s sake we will focus on the differences between sea otters and river otters. Sea otters and river otters differ in a few ways; the following will aid you in your I.D. of these interesting creatures. Maximum weights for sea otters in comparison to river otters are 100 pounds to 30 pounds. Sea otters have broader faces and thick, “teddy bear like” fur; whereas river otters have a more petite face and a streamlined, smooth coat. Sea otters rarely walk and travel on land; and when they do, they appear stocky and less agile than the river otter. Sea otters are often found floating on their backs in the ocean, and are solely marine, rather than being found in both salt and freshwater like river otters.
Despite their differences, all otters are grouped into subfamily Lutrinae. This group appeared about 8 million years ago (during the Miocene).
The ancestors of sea otters today first began to split from other otters about 5 million years ago (in the Pliocene). In terms of evolution, this is considered fairly quick in relation to that of some other lineages! The sea otter pictured above is known as species Enhydra lutris, which is further grouped in three subspecies (details we will not go into here). The origin of the North American river otter species, like the family pictured above (Lotra canadensis) follows a similar time frame as the sea otter. Early relatives of this creature also appeared in the Pliocene.
Article By: Alexa D., B.Sc. Five Star Marine Naturalist