[dinosaur] Describing 'cranial' and 'caudal' directions on a vertebra (free pdf)

Ben Creisler
[email protected]

A new open access preprint paper:

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Michael P. Taylor & Matthew J. Wedel (2019)
What do we mean by the directions 'cranial' and 'caudal' on a vertebra?
PeerJ Preprints 7:e27437v2
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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.

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