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New Dimetrodon, Sphenacodon sail study

From: Ben Creisler

Although these critters are not dinosaurs, they still may 
be of interest to dinophiles:


Adam K. Huttenlocker, Elizabeth Rega, Stuart S. Sumida 
Comparative anatomy and osteohistology of hyperelongate 
neural spines in the sphenacodontids Sphenacodon and 
Dimetrodon (Amniota: Synapsida). Journal of Morphology 
(advance online publication)
Article first published online: 30 SEP 2010

Osteohistological investigations of hyperelongate 
vertebral spinous processes (neural spines) are presented 
to elucidate previously unknown aspects of dorsal sail 
form and function in two, closely related genera of ?sail-
backed? synapsids: Sphenacodon and Dimetrodon. Although 
recent and classic surveys of bone histology in extinct 
vertebrates have sampled the genus Dimetrodon, new 
sectioning of Sphenacodon material allows a comparative 
analysis of these structures among Sphenacodontidae for 
the first time. Variability within the histological 
organization of the neural spine is assessed by examining 
multiple regions along its length, and implications for 
soft tissue correlates, growth and mechanics are 
considered here. Both genera exhibit extensive parallel-
fibered and fibrolamellar bone, in addition to lamellar 
bone. Several features vary along the length of the spine 
in each species. Muscle scars and extensive Sharpey's 
fibers are present at the base of the spine; no scars and 
fewer fibers are manifested ∼55?60 mm above the 
zygapophyses in mature individuals. The distal cortex of 
the spine does not exhibit greater vascularity than the 
proximal region in either genus. However, both genera 
manifest distinct vascular grooves of variable size along 
the distal periosteal surface, some of which become 
incorporated into the distal cortex. The observed 
histovariability appears to record the transition from 
the proximal (epaxial muscle embedded) to the distally 
protruding portion of the spine. These observations and 
independent pathological evidence support the existence 
of a short dorsal crest in Sphenacodon and possibly other 
basal sphenacodontids. Although the thermoregulatory 
capacity of such a crest remains uncertain, developmental 
and mechanical features are readily interpretable and are 
discussed with respect to the origins and early evolution 
of the dorsal sail in sphenacodontid synapsids.