Pinniped ancestor: fossil of a walking seal discovered [merged]

[DMB]

http://www.timesonline.co.uk/tol/news/uk/science/article6149818.ece

I expect someone less lazy than me will link to the Nature paper.
This evolutionary process has been difficult to study precisely because the earliest known pinniped, a creature called Enaliarctos, already had flippers, and scientists did not have access to transitional forms in the fossil record.

That has changed with the discovery of Puijila, a nearly complete fossil skeleton that is one of these missing links. The creature, about 110cm (3ft) long, shares some features with modern pinnipeds and has been identified as the earliest known member of the group, but it also has anatomical characteristics that are found in modern bears, skunks, badgers, weasels and otters.

“The remarkably preserved skeleton of Puijila had heavy limbs, indicative of well-developed muscles, and flattened phalanges [finger and toe bones] which suggests that the feet were webbed, but not flippers,” said Mary Dawson, of the Carnegie Museum of Natural History in Pittsburgh, a member of the discovery team.

“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.”

Now how will the creationists tackle this one (apart from the extra gaps argument, of course)? How about, "It didn't have flippers, so it wasn't a pinniped"?

[lpetrich]

Link: http://nature.ca/puijila/

From the Canadian Museum of Nature,
Puijila
A Prehistoric Walking Seal
Le Phoque que Marchait (The Seal that Walked)

It shows some pictures of the fossil, where it was discovered, and reconstructions of Puijila darwini and its habitat.

It lived in the early Miocene, about 20 million years ago, and back then, the Canadian Arctic was warm enough to allow forests to grow.

It looked much like an otter, but its skull had seal-like features, its body was long and slender, and its feet were partially turned into flippers.

So this walking pinniped joins walking cetaceans like Ambulocetus and walking sirenians like Pezosiren.

[Oolon Colluphid]

Nature 458, 1021-1024 (23 April 2009)

A semi-aquatic Arctic mammalian carnivore from the Miocene epoch and origin of Pinnipedia

Rybczynski, Dawson & Tedford

Modern pinnipeds (seals, sea lions and the walrus) are semi-aquatic, generally marine carnivores the limbs of which have been modified into flippers. Recent phylogenetic studies using morphological and molecular evidence support pinniped monophyly, and suggest a sister relationship with ursoids (for example bears) or musteloids (the clade that includes skunks, badgers, weasels and otters).

Although the position of pinnipeds within modern carnivores appears moderately well resolved, fossil evidence of the morphological steps leading from a terrestrial ancestor to the modern marine forms has been weak or contentious. The earliest well-represented fossil pinniped is Enaliarctos, a marine form with flippers, which had appeared on the northwestern shores of North America by the early Miocene epoch.

Here we report the discovery of a nearly complete skeleton of a new semi-aquatic carnivore from an early Miocene lake deposit in Nunavut, Canada, that represents a morphological link in early pinniped evolution.

The new taxon retains a long tail and the proportions of its fore- and hindlimbs are more similar to those of modern terrestrial carnivores than to modern pinnipeds. Morphological traits indicative of semi-aquatic adaptation include a forelimb with a prominent deltopectoral ridge on the humerus [upper arm bone], a posterodorsally expanded scapula [shoulder blade], a pelvis with relatively short ilium (http://en.wikipedia.org/wiki/Ilium_(bone)), a shortened femur and flattened phalanges [toe bones], suggestive of webbing.

The new fossil shows evidence of pinniped affinities and similarities to the early Oligocene Amphicticeps from Asia and the late Oligocene and Miocene Potamotherium from Europe.

The discovery suggests that the evolution of pinnipeds included a freshwater transitional phase, and may support the hypothesis that the Arctic was an early centre of pinniped evolution.

Puijila darwini gen. et sp. nov.

Etymology. Puijila (Inuktitut): young sea mammal, often referring to a seal; darwini: for Charles Darwin, who wrote with his usual prescience, "A strictly terrestrial animal, by occasionally hunting for food in shallow water, then in streams or lakes, might at last be converted into an animal so thoroughly aquatic as to brave the open ocean".

FIGURE 2. Puijila darwini skeleton (NUFV 405, holotype).

http://www.nature.com/nature/journal/v458/n7241/images/nature07985-f2.2.jpg

Reconstruction of skeleton showing preserved bones in dark grey.

The presence of enlarged, probably webbed feet, robust forelimbs and an unspecialized tail suggests that Puijila swam quadrupedally using its webbed fore and hind feet for propulsion. It was almost certainly not specialized for swimming under water using simultaneous pelvic paddling, as seen in Lontra [American otters]. Mammals that swim using simultaneous pelvic paddling do so without the aid of their front legs, relying instead on simultaneous propulsive thrusts of the hindlimbs in combination with dorsoventral tail (and sometimes body) undulations. In contrast, most living pinnipeds swim using one of two disparate modes: true seals (Phocidae) use their hind feet in a side-to-side pelvic oscillation, whereas fur seals (Otariidae) oscillate their fore flippers, in a movement akin to flying. As a possible quadrupedal swimmer, Puijila represents a form that could have given rise to both of the major swimming modes observed in pinnipeds today. Enaliarctos has been variously interpreted as using fore- and hindlimbs and the axial skeleton in swimming9 or as being a hindlimb-dominated swimmer.

The discovery of Puijila and the results of the phylogenetic analysis presented here support the hypothesis that pinnipeds diverged from an arctoid ancestral population by the early Oligocene. The non-marine pinniped Potamotherium was present in mid-latitudes of Europe and North America, and is known from the Oligocene/Miocene boundary through to the end of the Miocene. Puijila itself appears to be a relict stem pinniped. It is the least aquatically specialized of all known pinnipeds (except possibly Amphicticeps, for which postcrania [non-skull bones] are unknown), yet it appears in the fossil record in the early Miocene, approximately contemporaneously with the more highly derived pinniped Enaliarctos, and not long before a significant radiation of other early marine pinnipeds.