Neural Arch-Nemesis

The tibia originally used to distinguish what was then Saurophagus from Allosaurus was not distinguishable from those found in Allosaurus. We have mentioned that a few times so far this week. The distinguishing characteristics of the vertebrae used by Chure (1995) to separate Saurophaganax and Allosaurus, however, were obviously different enough to warrant the widely accepted separation of the two genera though they reside in the same family. The question then becomes what is different between thevertebrae of Allosaurus and Saurophaganax and what exactly is a neural arch? A neural arch, for those wondering, is the posterior (rear) part of a vertebra. It is the arch that spans the hole through which the spinal cord is strung down the entire series of the vertebral column from brain to tail, to put it simply. As we can see from the diagram at right, the neural arch is composed of, and has coming from it, a number of processes of bone, not all of which are labeled here. Some of these labels sometimes change depending on both the animal and the source. For example, the dorsal spine is referred to as the spinous process in some diagrams. The number of these bones differs based on the animal and the number within each region of the body also differs depending on the animal. Humans have 24 vertebrae arranged in three sections; cervical, thoracic, lumbar with an additional 9 fused vertebrae in the sacral region (the last 9 are typically left out of the count of the vertebral column in human anatomy meaning we have 33 but only 24 are considered vertebrae) whereas cattle have between 49 and 51 vertebrae arranged over five regions; cervical, thoracic, lumbar, sacral, and caudal. Typically dinosaur vertebrae are looked at as being either cervical, dorsal, sacral (typically a fused set of vertebrae), or caudal. The guys at SVPOW, specifically Matt Wedel for this post, do a good job of going over dinosaur vertebral anatomy themselves.

Anyway, the neural arches of Saurophaganax show the following characteristics, as restated by Mickey Mortimer that distinguish them from that of Allosaurus:

- atlas lacks prezygapophysis for proatlas, does not roof over neural canal
- dorsal vertebrae with horizontal lamina along base of each side of neural
spine arising from spine base cranially, free caudally
- chevrons craniocaudally expanded distally

The atlas is the first vertebra in the vertebral column, meaning it is right behind the skull of the animal (below in our case). The pro-atlas is a dorsal (back of the animal) aspect of the atlas and the prezygapophysis is an aspect of the vertebra which projects forward (upward in humans) to help fit with the preceding vertebra in the column snugly (in this case it would extend to the base of the cranium had it existed). The second aspect, the lamina arising cranially and being free caudally means that the caudal (tail-ward) end of the vertebrae were not covered by that aspect of the bone as the cranial (skull-ward) end was. Chevrons being extended craniocaudally distally means that the chevrons of the spine, typically found on tail elements to protect nerves and blood vessels ventrally (from the underside, meaning these aspects of the bone are underneath the vertebrae) were expanded as they moved away from the center of the body, toward the tail.