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Biomechanics of Ornithopod Feet

1: J Morphol. 2006 Dec 4; [Epub ahead of print]  
Morphological changes in pedal phalanges through
ornithopod dinosaur evolution: A biomechanical
approach.Moreno K, Carrano MT, Snyder R. 
Department of Earth Sciences, University of Bristol,
Bristol BS8 1RJ, UK.

The evolution of ornithopod dinosaurs provides a
well-documented example of the transition from
digitigrady to subunguligrady. During this transition,
the ornithopod pes was drastically altered from the
plesiomorphic dinosaurian morphology (four digits,
claw-shaped unguals, strongly concavo-convex joints,
phalanges longer than wide, excavated collateral
ligament fossae, presence of sagittal ridge, and
prominent processes for the attachment of tendons) to
a more derived condition (tridactyly, modification of
the unguals into hooves, phalanges wider and thinner
than long, lack of collateral ligament fossae, loss of
sagittal ridge and tendon attachment processes,
relatively flattened articular surfaces). These
changes are particularly noteworthy given the overall
conservatism in pedal morphology seen across
Dinosauria. But what are the functional consequences
of these specific morphological transitions? To study
them, we examine a wide range of pedal morphologies in
four non-avian dinosaurs and two birds. Our analyses
of the external morphology, two-dimensional models
(using Finite Element Analysis), and internal bone
structure demonstrate that this evolutionary shift was
accompanied by a loss of digit mobility and
flexibility. In addition, pedal posture was modified
to better align the pes with the main direction of the
ground reaction force, thus becoming well suited to
support high loads. These conclusions can be applied
to other, parallel evolutionary changes (in both
dinosaurs and mammals) that involved similar
transitions to a subunguligrade posture.