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Platecarpus had a tail fluke
A well-preserved _Platecarpus_ specimen shows evidence (albeit indirect) of a
terminal tail fluke, hitherto known only in the advanced _Plotosaurus_ among
mosasaurs. So it would have swam more like an ichthyosaur: the term is
'carangiform', which I'd never seen before. (Funnily enough, mosasaurs first
make an appearance in the mid Cretaceous, around the same time that
ichthyosaurs went extinct.)
Lindgren, J., Caldwell, M.W., Konishi, T., and Chiappe, L.M. (2010).
Convergent Evolution in Aquatic Tetrapods: Insights from an Exceptional Fossil
Mosasaur. PLoS ONE August 2010 | Volume 5 | Issue 8 | e11998
Abstract: "Mosasaurs (family Mosasauridae) are a diverse group of secondarily
aquatic lizards that radiated into marine environments during the Late
Cretaceous (98â65 million years ago). For the most part, they have been
considered to be simple anguilliform swimmers â i.e., their propulsive force
was generated by means of lateral undulations incorporating the greater part of
the body â with unremarkable, dorsoventrally narrow tails and long, lizard-like
bodies. Convergence with the specialized fusiform body shape and inferred
carangiform locomotory style (in which only a portion of the posterior body
participates in the thrust-producing flexure) of ichthyosaurs and
metriorhynchid crocodyliform reptiles, along with cetaceans, has so far only
been recognized in _Plotosaurus_, the most highly derived member of the
Mosasauridae. Here we report on an exceptionally complete specimen (LACM
128319) of the moderately derived genus _Platecarpus_ that
preserves soft tissues and anatomical details (e.g., large portions of
integument, a partial body outline, putative skin color markings, a downturned
tail, branching bronchial tubes, and probable visceral traces) to an extent
that has never been seen previously in any mosasaur. Our study demonstrates
that a streamlined body plan and crescent-shaped caudal fin were already well
established in _Platecarpus_, a taxon that preceded _Plotosaurus_ by 20 million
years. These new data expand our understanding of convergent evolution among
marine reptiles, and provide insights into their evolutionâs tempo and mode."