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Mosasaur swimming evolution

From: Ben Creisler

In the new Paleobiology:

Johan Lindgren, Michael J. Polcyb, and Bruce A. Young (2011)
Landlubbers to leviathans: evolution of swimming in mosasaurine mosasaurs.
Paleobiology 37(3):445-469. 2011 
doi: 10.1666/09023.1 

Incremental stages of major evolutionary transitions within a single animal
lineage are rarely observed in the fossil record. However, the
extraordinarily complete sequence of well preserved material spanning the
27-Myr existence of the marine squamate subfamily Mosasaurinae provides a
unique exception. By comparison with extant and extinct analogs, the tail
morphology of four mosasaurine genera is examined, revealing a pattern of
evolution that begins with the generalized varanoid anatomy and culminates
in a high-aspect-ratio fin, similar to that of sharks. However, unlike the
epicercal caudal fluke of selachians in which the tail bends dorsocaudally,
derived mosasaurs develop a hypocercal tail with a ventrocaudal bend.
Progressive caudal regionalization, reduced intervertebral mobility,
increased tail depth due to a marked downturn of the posterior caudal
segment, and the development of finlike paired appendages reveal a pattern
of adaptation toward an optimized marine existence. This change in
morphology reflects a transition from anguilliform or sub-carangiform
locomotion to carangiform locomotion, and indicates a progressive shift
from nearshore dwellers to pelagic cruisers?a change in foraging habitat
independently corroborated by paleobiogeographic, stable isotope,
osteohistological, and paleopathological data. Evolutionary patterns
similar to those observed in mosasaurine mosasaurs are seen in other
secondarily aquatically adapted amniotes, notably metriorhynchid
crocodyliforms, cetaceans, and ichthyosaurs, and may be explained by
developmental modularity governing the observed phenotypic expression.

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