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Ichthyostega limb joint mobility

From: Ben Creisler

A new advance paper in Nature:

Stephanie E. Pierce, Jennifer A. Clack & John R. Hutchinson (2012)
Three-dimensional limb joint mobility in the early tetrapod Ichthyostega.
Nature (advance online publication)

The origin of tetrapods and the transition from swimming to walking
was a pivotal step in the evolution and diversification of terrestrial
vertebrates. During this time, modifications of the limbs—particularly
the specialization of joints and the structures that guide their
motions—fundamentally changed the ways in which early tetrapods could
move. Nonetheless, little is known about the functional consequences
of limb anatomy in early tetrapods and how that anatomy influenced
locomotion capabilities at this very critical stage in vertebrate
evolution. Here we present a three-dimensional reconstruction of the
iconic Devonian tetrapod Ichthyostega and a quantitative and
comparative analysis of limb mobility in this early tetrapod. We show
that Ichthyostega could not have employed typical tetrapod locomotory
behaviours, such as lateral sequence walking. In particular, it lacked
the necessary rotary motions in its limbs to push the body off the
ground and move the limbs in an alternating sequence. Given that
long-axis rotation was present in the fins of tetrapodomorph fishes,
it seems that either early tetrapods evolved through an initial stage
of restricted shoulder and hip joint mobility or that Ichthyostega was
unique in this respect. We conclude that early tetrapods with the
skeletal morphology and limb mobility of Ichthyostega were unlikely to
have made some of the recently described Middle Devonian trackways.