What do we mean by the directions “cranial” and “caudal” on a vertebra?

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1 Department of Earth Sciences, University of Bristol, Bristol, United Kingdom
2 College of Osteopathic Medicine of the Pacific, Western University of Health Sciences, Pomona, California, United States
DOI
10.7287/peerj.preprints.27437v2
Published
Accepted
Subject Areas
Evolutionary Studies, Paleontology, Zoology, Anatomy and Physiology
Keywords
vertebra, orientation, horizontal, cranial, caudal, anterior, posterior, sauropod, giraffe
Copyright
© 2019 Taylor et al.
Licence
This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, reproduction and adaptation in any medium and for any purpose provided that it is properly attributed. For attribution, the original author(s), title, publication source (PeerJ Preprints) and either DOI or URL of the article must be cited.
Cite this article
Taylor MP, Wedel MJ. 2019. What do we mean by the directions “cranial” and “caudal” on a vertebra? PeerJ Preprints 7:e27437v2

Abstract

In illustrating vertebrae, it is important to consistently depict their orientation, so we can objectively assess and compare the slope of the neural arch, neural canal, or articular surfaces. However, differing vertebral shapes across taxa and across regions of the spinal column make it difficult to maintain consistency, or even define what we mean by the directions “cranial” and “caudal”. Consequently, characters such as “Neural arch slopes cranially 30° relative to the vertical” are disputable rather than objective measurements. Cranial and caudal are defined as directed along the horizontal axis, but several different notions of “horizontal” are possible:

1. Long axis of centrum is horizontal. This is appealing for elongate vertebrae such as sauropod cervicals, but is not always well defined, and is difficult to determine for craniocaudally short vertebrae such as most caudals.

2. Articular surfaces of centrum are vertical. Difficult to determine when dealing with facets that are concave or (worse) convex; and ambiguous for “keystoned” vertebrae in which the facets are not parallel.

3. Neural canal is horizontal. Anatomically informative, but difficult to determine in vertebrae that have not been fully prepared or CT-scanned, and impossible to see in lateral view. Ambiguous for vertebrae where the dorsal and ventral margins of the canal are not straight or not parallel.

4. Similarity in articulation (“horizontal” is defined as a line joining the same point on two similarly oriented copies of the same vertebra when optimally articulated). This is less intuitive than definitions 1–3, but takes the entire vertebra into account.

We advocate explicitly stating a definition and using it consistently. In most cases, definition 3 (“Neural canal is horizontal”) best reflects anatomical and developmental realities, and it is therefore preferred. Low-tech techniques can be used to determine neural canal orientation with adequate precision for most purposes.

Author Comment

This is the full manuscript version for submission to peer-review.