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Stratesaurus, Avalonnectes, Eoplesiosaurus, new plesiosaurs from UK Lias (earliest Jurassic)



From: Ben Creisler
bcreisler@gmail.com


New in PLoS ONE:

Benson, R.B.J., Evans, M., & Druckenmiller, P.S. (2012)
High Diversity, Low Disparity and Small Body Size in Plesiosaurs
(Reptilia, Sauropterygia) from the Triassic–Jurassic Boundary.
PLoS ONE 7(3): e31838.
doi:10.1371/journal.pone.0031838
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0031838

Invasion of the open ocean by tetrapods represents a major
evolutionary transition that occurred independently in cetaceans,
mosasauroids, chelonioids (sea turtles), ichthyosaurs and plesiosaurs.
Plesiosaurian reptiles invaded pelagic ocean environments immediately
following the Late Triassic extinctions. This diversification is
recorded by three intensively-sampled European fossil faunas, spanning
20 million years (Ma). These provide an unparalleled opportunity to
document changes in key macroevolutionary parameters associated with
secondary adaptation to pelagic life in tetrapods. A comprehensive
assessment focuses on the oldest fauna, from the Blue Lias Formation
of Street, and nearby localities, in Somerset, UK (Earliest Jurassic:
200 Ma), identifying three new species representing two small-bodied
rhomaleosaurids (Stratesaurus taylori gen et sp. nov.; Avalonnectes
arturi gen. et sp. nov) and the most basal plesiosauroid,
Eoplesiosaurus antiquior gen. et sp. nov. The initial radiation of
plesiosaurs was characterised by high, but short-lived, diversity of
an archaic clade, Rhomaleosauridae. Representatives of this initial
radiation were replaced by derived, neoplesiosaurian plesiosaurs at
small-medium body sizes during a more gradual accumulation of
morphological disparity. This gradualistic modality suggests that
adaptive radiations within tetrapod subclades are not always
characterised by the initially high levels of disparity observed in
the Paleozoic origins of major metazoan body plans, or in the origin
of tetrapods. High rhomaleosaurid diversity immediately following the
Triassic-Jurassic boundary supports the gradual model of Late Triassic
extinctions, mostly predating the boundary itself. Increase in both
maximum and minimum body length early in plesiosaurian history
suggests a driven evolutionary trend. However, Maximum-likelihood
models suggest only passive expansion into higher body size
categories.