Friday, May 4, 2012

Narwhals in temperate latitudes is ancient history

            Speculation that a dead narwhal had washed ashore last month in Ireland, a thousand miles from its normal range, got me thinking about the whale’s evolution and ancient range.  Although it turns out that the Ireland whale was misidentified and was actually a long-finned pilot whale, Ireland had been part of the narwhal’s range many thousands of years ago.
The line of evolutionary development that led to narwhals and the rest of today’s whales – and to humans as well – can be traced back to the Cretaceous and the carnivorous land mammals from which they descended. The branch that became whales follows that of the artiodactyls, hoofed mammals like antelopes and camels, as well as the hippopotamus, the whales’ nearest living relative on land.  The discovery in 1978 of a 52-million year old skull of what became known as Pakicetus was found to have features that showed a transition between terrestrial mammals and aquatic animals, including modifications that allowed for directional hearing under water, one of the first hints that mammals were returning to the sea.  But there were numerous additional steps before the first true whales emerged.
Narwhal fluke, by Paul Nicklen
            An amphibious animal called Ambulocetus, with hind feet clearly adapted for swimming, followed Pakicetus, and later still came Rhodocetus, which had additional adaptations for a marine lifestyle, including legs disengaged from its pelvis.  By the Eocene, about 40 million years ago, Basilosaurus emerged as a fully adapted marine mammal, with a streamlined body, paddle-like flippers, a strong whale-like tail, and the remnants of hind limbs that soon would disappear.  Other physiological changes took place along the way as well, including the relocation of the nostrils from the snout to the top of the head, the addition of an insulating layer of blubber, and changes in the circulatory system for the management of oxygen and for withstanding the pressures encountered at great depths.
            The two groups of modern cetaceans, the toothed whales and baleen whales, descended from a third group, the Archaeocetes, which disappeared about 30 million years ago.  It was just 500,000 years ago that narwhals evolved as a species, sometime in the late Pleistocene, about the same time that polar bears diverged from brown bears and the period when many large mammals and birds evolved and went extinct.  It was also a time when great changes in climate occurred.  While always believed to be an Arctic species, the range of the narwhal expanded and contracted over the millennia as variations in climate dictated.  During the last glaciations about 50,000 years ago, when ice extended as far south as England, narwhals were forced southward.  Fossils of the whales have been found there, along the coast of Norfolk.  When the glaciers began their retreat, narwhals followed them north again, with some traveling to the east of Greenland and others to the west.  That was the last time that narwhals from West Greenland and Canada came into contact with narwhals from East Greenland.  The 10,000 year separation of the two populations has led to genetic differences between them, a signal that evolutionary changes are still taking place, changes that one day far in the future could result in two distinct species of narwhals.  If they last that long.

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