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Polycotylus (Plesiosauria) redescribed from nearly complete skeleton



Ben Creisler
bcreisler@gmail.com

A new paper:

Bruce A. Schumacher & James E. Martin (2015)
Polycotylus latipinnis Cope (Plesiosauria, Polycotylidae), a nearly
complete skeleton from the Niobrara Formation (Early Campanian) of
southwestern South Dakota.
Journal of Vertebrate Paleontology (advance publication)
DOI:10.1080/02724634.2015.1031341
http://www.tandfonline.com/doi/full/10.1080/02724634.2015.1031341

A nearly complete skeleton of Polycotylus latipinnis (SDSM 23020) from
the upper Niobrara Formation (early Campanian) of South Dakota (U.S.A)
greatly improves information for this formerly poorly known taxon.
Specimens SDSM 23020 and YPM 1125 (paratype P. latipinnis) exhibit
numerous postcranial characters that distinguish Polycotylus, in
particular presacral vertebral count, nature of chevron facets, unique
ilia, and highly derived paddles including five epipodial
ossifications and increased hyperphalangy. Greater vertebral and
phalangeal counts of Polycotylus are accompanied by exceptional
foreshortening; thus, the relative body and limb proportions are
likely similar to less derived polycotylids, with fewer, more elongate
vertebrae and phalanges. The complete skull, derived among
polycotylids, has short temporal fenestrae, elongate frontals,
exceptionally slender parasphenoid with prominent cultriform process,
and exceptionally long extensions of the angulars and splenials within
the mandibular symphysis. Cladistic analysis indicates that
Polycotylus nests firmly within derived Polycotylidae as a potential
sister taxon to Dolichorhynchops osborni and Trinacromerum, despite a
‘primitive’ presacral vertebral count. Future improvements in
available data, in particular better information regarding basal
polycotylids, and refinement of character selection to nullify
homoplasy could significantly alter the tree structure to show
distinct subgroups within Polycotylidae. North American polycotylids
with elongate temporal fenestrae, elongate podials, and relatively
long vertebrae are confined to occurrences of late Cenomanian and
earliest Turonian age, whereas polycotylids distinctly foreshortened
in these respects are restricted to post-Turonian occurrences. Ilium
morphology is highly variable among polycotylids and, in addition to
taxonomic utility, may be in part attributable to sexual dimorphism.