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Placodont long bone histology and microanatomy (free pdf)



Ben Creisler
bcreisler@gmail.com

A recent paper not yet mentioned (pdf is free):


Nicole Klein, Alexandra Houssaye, James M. Neenan & Torsten M. Scheyer  (2015)
Long bone histology and microanatomy of Placodontia (Diapsida: Sauropterygia).,
Journal Contributions to Zoology 84(1): 59-84
http://www.scriptageologica.nl/cgi/t/text/text-idx?c=ctz;sid=30b6f86f5f29ca0ac63fde1d2b73661c;view=text;idno=m8401a05;rgn=div1;node=m8401a05:1

Free pdf:

http://www.ctoz.nl/cgi/t/text/get-pdf?c=ctz;idno=8401a05

Placodontia, an enigmatic group of durophagous and in part heavily
armoured animals, were members of Sauropterygia, the most diverse and
successful group of Mesozoic marine reptiles. Microanatomy and
histology of long bones of several armoured and non-armoured
Placodontia were studied, covering most of their taxonomic breadth, to
elucidate the paleoecology, physiology, and lifestyle of its members.
Results reveal an unexpected and not phylogenetically or
stratigraphically related disparity of microanatomical and
histological features for the group. The non-armoured Paraplacodus and
the heavily armoured Psephoderma grew with lamellar-zonal bone tissue
type, which is typical for modern sauropsids. In the former, the
tissue is nearly avascular surrounding a compacted medullary region,
whereas in the latter, the lamellar-zonal bone tissue is vascularized
framing a large open medullary cavity and a perimedullary region.
Armoured Henodus and Placodontia indet. aff. Cyamodus as well as
non-armoured Placodus exhibit a reduced medullary cavity and grew with
highly vascularized plexiform to radiating fibro-lamellar bone.
Several long bones of Placodontia indet. show circumferential
fibro-lamellar bone and can be distinguished into two groups on the
basis of microanatomical features. In addition, all bones that grew
with fibro-lamellar bone show locally primary spongeous-like
architecture and had secondarily widened primary osteons throughout
the cortex, resulting in a secondarily spongeous tissue. The highly
vascularized fibro-lamellar bone of these Placodontia indicates growth
rates comparable to that of open marine ichthyosaurs. Differences in
microanatomy and bone histology as expressed by a principal component
analysis, thus clearly indicate different paleoecologies, including
differences in lifestyle and swimming modes and capabilities in
Placodontia. This would have reduced competition in the shallow marine
environments of the Tethys and might be a key to their success and
diversity. A certain developmental plasticity among the studied
placodonts is interpreted as response to different environmental
conditions as is obvious from inter- and intraspecific histological
variation. Most striking is the difference in life history strategy in
armoured Psephoderma and non-armoured Paraplacodus when compared to
armoured Henodus, Placodontia indet. aff. Cyamodus, non-armoured
Placodus, and Placodontia indet. Bone tissue of Psephoderma and
Paraplacodus indicates low growth rates and a low basal metabolic
rate, as many modern sauropsids have such as the marine iguana,
whereas the others grew with extremely fast growth rates, more typical
for birds and mammals, indicating an increased basal metabolic rate.