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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."