Pinipeds are fin-footed, semiaquatic mammals widely distributed around the world. Three families are recognized: Odobenidae (walrus), Otariidae (sea lions), and Phocidae (seals). Otarids and phocids, commonly grouped as "seals" in the eyes of most non-specialists, are actually quite different animals. Sea lions (otariids) have external ears, are generally larger than phocids, and are overall more adapted to terrestrial life - these are the ones barking and galloping across the stage for our entertainment at marine parks. True seals (phocids) have no external ears and more tapered snouts, are less vocal, and are overall more streamlined than sea lions, and are thus better adapted for life in water. Habitat specialization in these two groups of seals is largely the result of the configuration and function of the hind pair of flippers. Those of sea lions can turn forward, enabling the sea lion to walk on land on all fours. Those of true seals, in contrast, form essentially a tail at the end of the body, which acts as a rudder during swimming but is more-or-less useless on land.
Recent molecular analyses suggest that pinnipeds evolved from a bear-like ancestor approximately 30 million years ago (late Oligocene). However, evolutionary pathways within the Pinnipedia have been more difficult to unravel - a problem not uncommon to any highly specialized group of organisms. It has long been assumed that all "seals" originated from a single, at least partially aquatic, ancestor. The more-terrestrial sea lions and the more-aquatic true seals have been interpreted as representing two stages along a pathway toward greater acquaticism. However, Carolin Kuhn and Eberhard Frey at the Staatliches Museum fuer Naturkunde in Karlsruhe, Germany, would disagree They have found sufficient differences in the musculoskeletal anatomy and locomotor styles of the two groups of seals to suggest that they had separate origins. While true seals (in their words) "walk like caterpillars" on land, on their bellies undulating their entire bodies, sea lions more or less walk using all four limbs. Sea lions, on the other hand can "fly like bats" through the water using their fore flippers, whereas true seals use a full-body lateral undulation (propelled by the hind "tail") to maneuver. The most significant difference lies in what they do in the water - the two styles are not progressions along a continuum - they are completely different, using different limbs. Thus the two kinds of "seals," say Kuhn and Frey, must have acquired their seal-like characteristics in parallel, each originating from its own unique "pre-construction," thus suggesting separate aquatic ancestors (but of course not precluding their common descent from the terrestrial bear-like "pre-seal" long before). This interpretation disagrees with currently accepted genetic data and phylogenetic analyses, but it will be interesting to see what effect they will ultimately have on a combined reanalysis.
Find the original article in Palaeobiodiversity and Palaeoenvironments.
Watch sea lions "flying like bats" via National Geographic's Crittercam.
