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Missing evolutionary link between land and sea found

Modern seals, walruses, and sea lions are all descendants of animals that once lived on land but eventually swapped their terrestrial lifestyle for a life in the ocean. Until now, the morphological evidence for this transition from land to water has been weak, but researchers from Canada and the United States have now found a remarkably well preserved skeleton of a newly discovered carnivorous animal: Puijila darwini.

Puijila darwini Missing evolutionary link between land and sea found
Skeletal illustration of Puijila darwini.
Credit: Mark A. Klingler/Carnegie Museum of Natural History

Seals, walruses and sea lions all have flippers; a type of limb perfectly adapted for swimming and moving around in water. But how could a land living animal develop flippers? The adaptation evolved gradually over a long period of time, as some land living animals adapted semi-aquatic habits. New research now suggests that the genus Puijila is the “missing” evolutionary link between our modern seals, walruses and sea lions and their terrestrial ancestors.

Puijila darwini is described as having fore-limbs comparatively proportionate to modern carnivorous land animals rather than to pinnepeds*, a long tail, and webbed feet.

The remarkably preserved skeleton of Puijila had heavy limbs, indicative of well developed muscles, and flattened phalanges which suggests that the feet were webbed, but not flippers. This animal was likely adept at both swimming and walking on land. For swimming it paddled with both front and hind limbs. Puijila is the evolutionary evidence we have been lacking for so long,” says Mary Dawson, curator emeritus of Carnegie Museum of Natural History.

The Puijila darwini skeleton was found in Nunavut, Canada in the remains of what was once a crater lake on coastal Devon Island. The first pieces of the skeleton were found in 2007, but the important basicranium wasn’t found until researchers paid a new visit to the site in 2008. Without a basicranium it is much more difficult to determine taxonomic relationships.

Based on Paleobotanic fossils, Devon Island had a cool, coastal temperate climate during the Miocene when Puijila darwini roamed the seashore. The conditions were quite similar to modern-day New Jersey and the lakes would freeze during the winter, something which probably prompted Puijila darwini to move over land from the lake to the sea in search of food.

The find suggests that pinnipeds went through a freshwater phase in their evolution. It also provides us with a glimpse of what pinnipeds looked like before they had flippers,” says Natalia Rybczynski, leader of the field expedition.

The idea that semi-aquatic mammals may have undergone a transition from freshwater to saltwater is not new. In the On the Origin of Species by the Means of Natural Selection, Charles Darwin writes “A strictly terrestrial animal, by occasionally hunting for food in shallow water, then in streams or lakes, might at last be converted in an animal so thoroughly aquatic as to brace the open ocean.”

The oldest well-preserved pinniped animal belongs to the genus Enaliarctos and was a sea living creature with flippers. This species has been found on North Americas northern Pacific shores which have lead researchers to believe that the evolution of pinniped animals may have taken place mainly around the Arctic. This new finding of Puijila darwini strengthens that notion.

You can find more information about Puijila darwini and the origin of pinnipeds in the April 23 issue of the journal Nature.

http://www.nature.com

http://www.nature.com/nature/index.html

* The pinnipeds are a widely distributed and diverse group of semi-aquatic marine mammals. It contains the families Odobenidae (walruses), Otariidae (eared seals, including sea lions and fur seals), and Phocidae (earless seals). The name is derived from the Latin words pinna, which means wing or fin, and ped, which means foot. The pinnipeds are therefore also known as fin-footed mammals.

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