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Ichthyosaur limb development evolution

From: Ben Creisler

A new online paper:

Erin E. Maxwell, Torsten M. Scheyer, and Donald A. Fowler (2013)
An evolutionary and developmental perspective on the loss of
regionalization in the limbs of derived ichthyosaurs.
Geological Magazine (advance online publication)
DOI: http://dx.doi.org/10.1017/S0016756812001070

Ichthyosaurs, a lineage of extinct Mesozoic marine reptiles, have
garnered attention in both the palaeontological and developmental
literature for the unique limb morphology seen in derived genera.
These morphologies include an increase in the number of phalanges per
digit (hyperphalangy) and in the number of digits (hyperdactyly), but
most interestingly also a shift in element identity. Elements distal
to the stylopodium acquire characteristics of mesopodial elements,
such as a rounded, nodular shape and a loss of perichondral bone on
the anterior and posterior surfaces. Here, we examine numerous aspects
of the loss of proximodistal identity in ichthyosaur limbs including
phylogenetic progression of the loss of perichondral bone, histology
and microstructure of the elements retaining perichondral bone in
derived taxa, and correlates of intraspecific variation in degree of
perichondral bone reduction in a derived ichthyosaur, Stenopterygius
quadriscissus. Results show that loss of limb element identity
occurred progressively over ichthyosaurian evolution, and the notches
seen on the anterior surface of limb elements in derived ichthyosaurs
are homologous to the long bone shafts in terrestrial tetrapods.
Variation in the number of notches in S. quadriscissus can best be
explained through delayed ossification of the anterior perichondrium,
indicating a heterochronic component to the loss of identity. We
propose a developmental mechanism – gradual expansion of the
polyalanine region of HoxD13 over ichthyosaurian evolution – to
explain the progressive loss of limb regionalization as well as the
heterochronic delay in perichondral ossification.