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Ichthyosaur flipper evolution

From: Ben Creisler

A recent online paper not yet mentioned on the DML:

Erin E. Maxwell (2012)
Unraveling the Influences of Soft-Tissue Flipper Development on
Skeletal Variation Using an Extinct Taxon.
Journal of Experimental Zoology Part B: Molecular and Developmental
Evolution (advance online publication)
DOI: 10.1002/jez.b.22459

Adaptation to an aquatic habitat results in dramatic changes in
tetrapod limb morphology as limbs take on the roles of propulsion and
steering and lose their weight-bearing function. Changes include
enclosure of the limb in a soft-tissue flipper and proportional
lengthening of the distal limb, often accomplished through the
addition of skeletal elements (hyperphalangy). The flipper structure
itself and changes to the developmental architecture permitting
hyperphalangy are hypothesized to increase observed limb variation,
based on a cetacean model. These hypotheses are examined in the
ichthyosaurs Stenopterygius and Mixosaurus. Hyperphalangy combined
with high levels of variation in phalangeal counts were observed in
both genera. The amount of variation was not proportional to the
number of phalanges in a digit, but was related to functional digit
length. In addition, qualitative variants were catalogued in both
genera. Polyphalangy, phalangeal fusion, and additional ossifications
in the zeugopodial row were not observed in Mixosaurus, but were
common in Stenopterygius, even though both genera exhibited a similar
degree of hyperphalangy. These results suggest that while the flipper
structure and processes resulting in hyperphalangy may increase
observed variation in phalangeal counts, these factors are unlikely to
be causing high levels of qualitative variation in ichthyosaurs.
Patterns of variation in ichthyosaur limbs, and thus variability, are
unique to species but can change over evolutionary time.