No abstract

The spinal quotient (S.Q.) is an osteologically defined estimate of the enlargement of the spinal cord at limb levels over that at interlimb levels. It is an efficient predictor of limb use in living reptiles and birds and may be used to predict limb function in fossil vertebrates. Among living reptiles, this ratio of limb to interlimb innervation is greatest in arboreal genera, followed by terrestrial sprawlers, aquatic forms, and undulatory forms. Birds show a wide range of brachial S.Q. values that are roughly commensurate with flight ability. S.Q. values for the manipulative forelimbs of some dinosaurs fall well above those of locomotory limbs. Dinosaur hind-limb values are either well within ranges predicted by living reptiles and birds (most taxa), or highly inflated (stegosaurs, sauropods). This inflation may be the result of presence of a glycogen body similar to that of birds. In no case does the lumbosacral S.Q. support the presence of a “sacral brain.”

A statistical and functional relationship between neural canal anatomy and locomotor mode is demonstrated in living marine mammals of the Order Carnivora. This relationship is interpreted to be the result of differential innervation and territory of musculature involved in generating the six locomotor patterns analyzed. This osteological reflection of a behavioral trait allows prediction of locomotor pattern in extinct genera of closely related taxa. The robust data allow such predictions even when a considerable number of presacral vertebrae are missing in the fossil specimens. In some cases, these predictions conflict with interpretations based solely on limb osteology.

With 15 figures in the text)The relationships between the size of the spinal cord and the size of the neural canal, and between gross spinal cord anatomy and locomotor style, were documented in a wide range of living diapsids. Observed relationships were used to make predictions about spinal cord anatomy and about limb use and position in related fossil taxa. In particular, the data suggest that the brachial plexus, and therefore the cervical/dorsal vertebral transition, of the theropod dinosaurs studied was located considerably posterior to its presently accepted location, and that the forelimbs of the giant carnosaurs Tyrannosaurus rex and Curnotauras sustrei were of biologically insignificant use. Neural canal measurements support previous interpretations of locomotor style in extinct crocodilians, and can be used to predict limb placement in plesiosaurs.