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From ancient fish, insight into origin of limbs

Analysis of fossil evidence suggests Tiktaalik roseae, a 375-million-year-old crocodile-like fish, was a transitional species between fish and the first legged animals that walked on land.
Tikaalik roseae
Discovered in northern Canada in 2004, Tikaalik Rosea, is a lobbed-fin fish with a broad flat head and sharp teeth. It had gills, scales and fins. Growing up to nine feet long, it hunted in shallow freshwater environments.
Fossils recovered
A reconstruction of Tikaalik Rosea from fossil bones recovered at the Canadian excavation site revealed an almost complete pectoral fin and a partial pelvic fin.
Analysis of the bones
Pectoral bone The front fins of the fish featured a shoulder, elbows, and a partial wrist, which would have allowed it to support its weight.
Pelvic bone Comparable in size to the pectoral bone, with prominent ball-and-socket joint and a large area for muscles to attach.
Femur From fossil record, the missing rear femur is believed to be as strong as the front one.
Place in evolutionary scale
Acanthostega
(365 million years ago)
This early tetrapod had a weight-bearing pelvic girdle, allowing it to shift propulsion from the front to hind limbs.
Tiktaalik roseae
(375 million years ago)
Pelvic bone Evidence of "all-wheel drive" in a fish. The pelvis is comparable in scale to those of early tetrapods.
Eusthenopteron
(385 million years ago)
Femur The lobe-finned fish, an early relative of tetrapods, had a pelvic girdle much smaller than the pectoral one, suggesting it had "front-wheel drive" propulsion.
James Abundis/Globe Staff

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