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Boyer diss 2009 1046..

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dorsoventrally than craniocaudally. The thoracic vertebrae have zygapophyses with<br />

rounded margins, although T12 appears to have postzygapophyses that are slightly more<br />

angular, or square. The postzygapophyses of T13 are too broken for meaningful<br />

comment. The postdiaphragmatic spinous process of T12 is broken, but was probably<br />

vertically oriented, revealing it as the anticlinal vertebra. The T13 process is badly<br />

broken, but its caudal and dorsocaudal margins are intact, revealing that it was cranially<br />

oriented and most likely shorter dorsoventrally than craniocaudally long. The two<br />

postdiaphragmatic vertebrae preserve the roots of large accessory processes<br />

(anapophyses).<br />

Comparison.—To have 13 thoracic vertebrae is slightly above average among<br />

small generalized, terrestrial to arboreal mammals (Table 4.24). It is the same number<br />

seen in tupaiid treeshrews (13), but one less than in Ptilocercus and Cynocephalus (14)<br />

(Sargis, 2001). It is generally thought that a longer thorax represents an emphasis on<br />

stabilization rather than mobility (Sargis, 2001). The presence of an anticlinal vertebra<br />

within the thoracic region, and large anapophyses, indicates that this region had<br />

pronounced mobility in dorsiflexion and extension, but was restricted in axial rotation or<br />

medial flexion. Animals that sometimes use a bounding, asymmetrical gait, and which<br />

therefore do not require dorsostability, are charactized by such features (e.g., Sciurus,<br />

treeshrews, many arboreal quadrupedal primates). Apes, humans, lorises, sloths,<br />

Cynocephalus and some opossums lack a thoracic anticlinal vertebra, and do not engage<br />

in quadrupedal bounding gaits (<strong>Boyer</strong> and Bloch, 2008). The thoracic region of P. cookei<br />

therefore indicates that it was not restricted to slow, cautious locomotion.<br />

367

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