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Evolutionary halfway-house illuminates land assault

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Tiktaalik roseaeA new fossil species has cast light on one of the most important transitions in evolution - fish's move onto land. The missing link, dubbed Tiktaalik roseae was found in river sediments up in the Arctic circle.

A team of paleontologists found several specimens* of the plucky part-fish, part-croc during a grueling month long expedition in 2004 to the remote site on Canada's Ellesmere Island. The incredibly well preserved details, described in the journal Nature, reveal a set of primitive features Tiktaalik had evolved to help it to conquer land.

Ted Daeschler, one of the team who made the discovery said: “This find is a dream come true.” The group named the fish after a local native name for a large fish that lives in the shallows.

Tiktaalik's fins were on the way to becoming fully-fledged limbs. The bones are adapted to flex so that it could prop itself up on land. Natural History Museum paleontologist Andrew Miller explained the significance: “It is a stepping-stone in the water-land transition showing us a permutation of features not seen before, notably the combination of lobe-fins with the beginnings of a neck.”

The predator's gills were still fully operational. The researchers interpret this to mean it still spent most of its time sub-aqua.

There are still gaps in the fossil record showing the drastic changes to come before a full time landlubber's lifestyle would be an option.

Crucially, Tiktaalik's move onto solid ground paved the way for that host of other evolutionary innovations. Once out of the sea and on all fours, tetrapods immediately needed air-breathing lungs, modified ears and a sturdy pelvis to deal with the gravity they'd been unencumbered by underwater. Birds, reptiles, amphibians and mammals are all tetrapods. Tiktaalik fills in the early part of the transition.

The layer of rock Tiktaalik was found in dates from the late Devonian period, which ended 380m years ago. During its 50m years plants also colonised land properly for the first time.

Fish in general boomed during the Devonian, rapidly diversifying to fill newly-available niches. Paleontologists theorise that the land invasion by both tetrapods and insects was facilitated by the new vegetarian bounty on offer.

Tiktaalik joins an exclusive school of missing links, whose other alumni include the half-bird, half-dinosaur evolutionary poster child Archaeopteryx and monkey-headed football hardnut Wayne Rooney. ®

*Casts of Tiktaalik fossils are on display at London's Science Museum until May.

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