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Re: Platecarpus and Placodus papers in new JVP



From: Ben Creisler
bcreisler@gmail.com


My apologies for a cut-and-paste glitch--the authors of the
Platecarpus paper were not included in the last post.
Here's the correction:


Takuya Konishi, Johan Lindgren, Michael W. Caldwell & Luis Chiappe (2012)
Platecarpus tympaniticus (Squamata, Mosasauridae): osteology of an
exceptionally preserved specimen and its insights into the acquisition
of a streamlined body shape in mosasaurs.
Journal of Vertebrate Paleontology 32(6): 1313-1327
DOI:10.1080/02724634.2012.699811
http://www.tandfonline.com/doi/full/10.1080/02724634.2012.699811



LACM 128319, which was collected in western Kansas, U.S.A., and is
assignable to Platecarpus tympaniticus (Mosasauridae,
Plioplatecarpinae), represents arguably one of the most exquisite
mosasaur specimens known to date. Measuring 5.67 m from the tip of the
snout to the end of the tail, it comprises an exceptionally
well-articulated skeleton, accompanied by soft-tissue remains, such as
skin impressions and tracheal cartilage. P. tympaniticus is one of the
most numerously collected mosasaur taxa in North America, but as most
specimens are fragmentary or reconstructed to various degrees, LACM
128319 provides a unique opportunity to document the taxon's osteology
from a single skeleton. In this study, we first present a detailed
osteological description of LACM 128319. Following this, we present an
analysis of the evolution of a streamlined body shape in P.
tympaniticus, specifically by comparing the length distribution of the
dorsal ribs in relevant anguimorphan taxa. We conclude that both an
anterior migration of the rib cage and an increasing regionalization
within the dorsal vertebral series are key features contributing to
formation of a streamlined body profile in P. tympaniticus, and
probably in many other hydropedal members of mosasaurs.

===


James M. Neenan & Torsten M. Scheyer (2012)
The braincase and inner ear of Placodus gigas (Sauropterygia,
Placodontia)—a new reconstruction based on micro-computed tomographic
data.
Journal of Vertebrate Paleontology 32(6): 1350-1357
DOI:10.1080/02724634.2012.695241
http://www.tandfonline.com/doi/full/10.1080/02724634.2012.695241


Placodus gigas is an unarmored placodont marine reptile (Diapsida,
Sauropterygia) known from the Middle Triassic of Europe, most commonly
found in the shallow marine carbonate facies of the German Muschelkalk
(lower Anisian to middle Ladinian, approximately 243–235 Ma).
Generally, the morphology of the skull is well understood, with the
exception of the braincase, which is partially obscured by
dermatocranial bone. Two skulls that display well-preserved and intact
chondrocranial elements were scanned using industrial micro-computed
tomography (μCT), thus revealing parts of the braincase that were
previously obscured and allowing a new three-dimensional
reconstruction of the region to be constructed. This includes a
complete osteological description, the first reconstruction of a
sauropterygian vestibular apparatus, and a new virtual cranial
endocast. The morphology of the braincase and sphenoid region has been
revised, revealing the position of the hypophyseal pit. The enigmatic
‘alisphenoid bridge’ has been reinterpreted as being a dorsally
extended dorsum sellae. The vestibular apparatus is shown to have
strongly dorsoventrally compressed vertical semicircular canals, a
commonly observed morphology of other marine reptiles, and an ‘alert’
head position of about 20°, indicating a highly aquatic lifestyle.
Because placodonts are the sister group to all other sauropterygians,
these new data are of great comparative and phylogenetic significance,
providing insight into some of the morphological and functional
changes that occurred on the stem leading to the more derived
sauropterygians.