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Boyer diss 2009 1046..

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The zygapophyses increase in size between L1 and L3, but appear of roughly equivalent<br />

size on L3 to L7. The prezygapophyses face dorsomedially and are concave. The<br />

postzygapophyses face the opposite direction, and are slightly convex. They are distinctly<br />

squared in outline.<br />

Comparison.—Functional trends of the thoracic region are continued into the<br />

lumbus. Orientation of spinous processes, zygapophyses and large anapophyses suggest<br />

mobility in dorsiflexion and ventriflexion near the end of the thoracic region, as exhibited<br />

by Tupaia, for example (Jenkins, 1974). Prominent mammillary processes and<br />

prezygapophyses, and large, ventrally projecting transverse processes correlate with the<br />

use of a bounding gait (<strong>Boyer</strong> and Bloch, 2008), presumably because such morphology<br />

provides room for epaxial musculature, creating the capacity for powerful extension (and<br />

possibly also flexion) of the spine. Proportionally speaking, P. cookei has relatively small<br />

prezygapophyses and mammillary processes. Likewise, craniocaudally wide spinous<br />

processes typically characterize taxa with more rigid backs that do not engage in agile<br />

bounding and scampering (Sargis, 2001; <strong>Boyer</strong> and Bloch, 2008). Furthermore, taxa that<br />

rely on a bounding gait increase the length of the lumbus relative to the thorax (Sargis,<br />

2001; Shapiro and Simons, 2002). The lumbar region of P. cookei is estimated to have<br />

been 81% the length of the thoracic region. This contrasts with agile scansorial<br />

treeshrews in which the lumbar region is 94% the length of the thoracic region (Table<br />

4.24).<br />

Compared to smaller plesiadapids, P. cookei is similar in its lumbar vertebral<br />

morphology. The major differences are proportionally larger vertebral foramina and<br />

narrower spinous processes in the smaller taxa. The lumbar spinous process proportions<br />

369

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