Boyer diss 2009 1046..

vertebrae. It is also similar to that of the most proximal of the preserved caudal vertebrae
(see below).
Comparison.—The orientation and large size of the spinous process of the first
sacral vertebra is similar to that of the caudal lumbar vertebrae. This suggests that the
sacrum was integrated into the relatively rigid segment formed superiorly (cranially) by
the caudal lumbar vertebrae. In highly agile taxa such as treeshrews, Sciurus and
richochetal rodents (Gambaryan, 1974), the spinous process of the first sacral vertebra is
often reduced. This morphology is associated with a relatively large range of flexibility
at the lumbosacral joint, important for agile locomotor behaviors. The presence of a large
first sacral spinous process thus indicates less flexibility and less agile locomotion.
However, there is some evidence that the reduction of this process only correlates with
agility in taxa that mainly use pronograde postures: a brief survey of taxa that frequently
use orthograde postures reveals them to have a large, cranially oriented first sacral
spinous process, even including those that also exhibit agile (e.g., Callithrix) or acrobatic
(e.g., Galago) locomotor behaviors. Given independent evidence suggesting a somewhat
agile locomotor repertoire in P. cookei (e.g., position of the anticlinal vertebra), the sacral
spinous process size and orientation may be an indicator of substantial reliance on
orthograde postures.
The sacra of N. intermedius and N. gidleyi are virtually identical to that of P.
cookei in the morphological features mentioned above. The paromomyid plesiadapiform
Ignacius clarkforkensis differs in having a reduced first spinous process (Boyer and
Bloch, 2008). All treeshrews exhibit the reduced condition (Sargis, 2001). Cynocephalus
exhibits the plesiadapid condition.
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