Gross spinal anatomy and limb use in living and fossil reptiles

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.”

Does postcranial palaeoneurology provide insight into pterosaur behaviour and lifestyle? New data from the azhdarchoid Vectidraco and the ornithocheirids Coloborhynchus and Anhanguera

“…Giffin () introduced hypotheses formulated on these data to the field of palaeobiology; she described how correlations between the relative size of the spinal cord and increased innervation were linked to locomotory function and lifestyle in living animals, an idea then applied to a selection of fossil reptiles and mammals following examination of data on inferred nerve pattern and size. She found a close relationship between the size of the neural canal and size of the spinal cord (Giffin , ), though the relative size of the spinal cord compared to the neural canal does vary (Giffin ). She found that in Alligator mississippiensis , the cross‐sectional area of the spinal cord is relatively constant throughout the vertebral column, occupying a maximum of 47% (in the brachial and lumbosacral maxima), and a minimum interlimb segment fill of 38%.…”

“…While the pigeon ( Columba livia ) generally has a relatively larger spinal cord (73% maximum vs 53% minimum fill of interlimb segments) the overall pattern is the same. The lumbosacral regions of both the pigeon and the ostrich have spinal cord fills that are lower than the brachial and interlimb segments as well as in Alligator (29% and 32%, respectively; Giffin ). This is due to the presence of the glycogen body of birds, a structure not found in any other living vertebrates, or in the brachial region of birds.…”

“…This is due to the presence of the glycogen body of birds, a structure not found in any other living vertebrates, or in the brachial region of birds. Although the fill percentage is lower in the lumbosacral region of birds, the overall cross‐sectional area of both the spinal cord and the neural canal increases along with the spinal cord area in this region (Giffin ). This suggests that the size of the neural canal can be used as a proxy for the size of the spinal cord in a given section.…”

“…The relative cord/canal size was studied first in extant lepidosaurs, birds and crocodyliforms (Giffin , , ). For example, the sacral enlargement, which innervates the sacral plexus (lateral to the vertebral column on both sides) and therefore the hindlimbs, was much larger relatively in an ostrich than in a pigeon or lizard, while the ostrich brachial enlargement (for innervation of the brachial plexus and therefore the forelimbs, also laterally to the vertebral column) was comparatively small (Giffin ). She then applied this technique to non‐avian dinosaur, crocodyliform and plesiosaur fossils with semi‐complete vertebral columns, with varying success.…”

Synsacra of the Eocene Antarctic penguins: new data on spinal maturation and an insight into their role in the control of walking

“…An interesting feature of the lumbosacral intumescence of the spinal cord in birds is its dorsal split ("cleft" in Jelgersma 1951, "groove" in Necker 2005 leading to the formation of the so−called dorsal rhomboid (or lumbosacral) spinal fossa (or sinus). The fossa houses an enigmatic ovoid glial structure called the glyco− gen body (Jelgersma 1951;Breazile and Kuenzel 1993;Butler and Hodos 2005;Kai− ser 2007; see also Giffin 1990). The position of the lumbosacral intumescence and glycogen body can be easily located, for the corresponding part of the synsacral canal is enlarged (Necker 2006).…”