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

Mike Taylor. Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK. dino@miketaylor.org.uk

Mathew J. Wedel. College of Osteopathic Medicine of the Pacific and College of Podiatric Medicine, Western University of Health Sciences, Pomona, California, USA. mathew.wedel@gmail.com

Contents

Introduction

In late 2017, one of us submitted a paper (Taylor 2018b) redescribing the sauropod dinosaur Xenoposeidon and assigning it to the group Rebbachisauridae, based on the holotype and only specimen NHMUK PV R2095. Among the five diagnostic characters given for Xenoposeidon was #2, “Neural arch slopes anteriorly 30°–35° relative to the vertical” (Taylor 2018b:5). In a helpful and detailed peer review, Phil Mannion (XXX Mannion 2018a) commented:

The strong anterior slant of the neural arch appears to be dependent on how you've chosen to orientate the vertebra, but there doesn't appear to be any need to orientate it in this way.

I (Taylor) carelessly failed to directly address this criticism in my response letter, although I did add a brief discussion of the orientation. Consequently Mannion raised the matter again in the second round of review (XXX Mannion 2018b):

I still don't agree with this, and I don't see any clear evidence for orientating it this way. I went into the NHM to re-look at this. No aspect of the posterior articular surface of the centrum leads me to orient the vertebra in the same way of shown in your figures. In addition, as currently orientated, the floor of the neural canal is strongly tilted - it seems more conservative to assume that this is horizontal. Similarly, by following that orientation, this would then make the long-axis of the lateral pneumatic opening closer to horizontal.

By orientating the vertebra this way, the anterior margin is sub-vertical, with a very gentle anterior deflection, and the M-lamina is much closer in orientation to that of Rebbachisaurus.

I responded (Taylor 2018a):

Phil remains convinced that the proper orientation of the vertebra gives is a lesser forward slope that as described in the manuscript. Having once more revisited my photos and 3D models, I remain convinced that the present orientation is essentially correct. It could be out by five degrees or so, so I have changed “35 degrees” to “30-35 degrees” throughout.

Mannion was gracious enough to accept this, and the paper proceeded to publication with the relevant section (Taylor 2018b:5) essentially unchanged. But the question he had raised continued to occupy me: what exactly is the “correct” orientation of the vertebra, relative to which we can measure the angle of the sloping neural arch? And what do we even mean by “correct”? Figure A shows the difference between the slope as published (part A), and as interpreted by Mannion (part B).

Figure A. rotated Xenoposeidon

Figure A. NHMUK PV R2095, the holotype dorsal vertebra of Xenoposiedon proneneukos in left lateral view. A. In the canonical orientation that has been used in illustrations in published papers (Taylor and Naish 2007, Taylor 2018b, in blog-posts and on posters (XXX cite SV-POW! post) and mugs (XXX cite the SV-POW! post). B. Rotated 15° “backwards” (i.e. clockwise, with the dorsal portion displaced caudally), yielding a sub-vertical anterior margin in accordance the recommendation of Mannion (2018b). In both parts, the blue line indicates the horizontal axis, the green line indicates the vertical axis, and the red line indicates the slope of the neural arch as in Taylor (2018b: figure 3B, part 2). In part A, the slope (i.e. the angle between the red and green lines) is 35°; in part B, it is 20°.

The neural arch slopes relative to the vertical. Vertical is defined as being orthogonal to the horizontal. That in turn is defined by the cranial-caudal (= anterior-posterior) axis. But what do those directions mean? How can we define them for a given vertebra?

In the present paper, we aim to answer that question. We will propose and discuss four candidate criteria, recommend the one we consider most practical and informative, and determine the slope of Xenoposeidon's neural arch more precisely.

Note that the present question is nothing to do with life posture, which is a much more difficult problem, subject to many more degrees of uncertainty. Animals do not hold their vertebral columns any anything close to true horizontal — not even though that we characterise as having horizontal posture — and we do not want to tie the meaning of our very nomenclature to something so variable and unpredictable. Otherwise we would have to define “horizontal” for the mid-cervical vertebrae of parrots as upside-down (Figure B).

Figure B. Parrot with 'S'-curved neck

Figure B. Parrot skeleton with hemisected integument (probably Amazona ochrocephala) in left lateral view, in the Natuurhistorisch (XXX is this spelled right?) Museum of Rotterdam, from a post on Love in the Time of Chasmosaurs, 4 August 2012 (XXX cite properly). Note the very strong 'S'-curve of the neck, such that the most caudal cervical vertebrae are inclined downwards, then more cranial vertebrae are, progressively, inclined upwards, near vertical, sloping backwards, then vertical again, and finally sloping upwards to the skull.

Anatomical nomenclature

As dinosaur palaeontologists, we generally use and prefer the Owenian system of anatomical directions, with anterior and posterior indicating the forward and backward directions accordingly (XXX Owen ref) — hence the use of these terms in the Xenoposeidon paper, its reviews, and the associated discussion. However, for the present paper, we seek directional definitions that are unambiguous for all vertebrates: not only those like dinosaurs, dogs and fish, which hold their vertebral columns essentially horizontal; but also those like humans, penguins and XXX third example, which hold their vertebral columns essentially vertical. FOr this reason, avoiding ambiguity in humans, where “anterior” means ventral (towards the belly) rather than cranial (towards the head), we will use terms cranial and caudal, and derived terms such as craniodorsal.

Institutional abbreviations

  • NHMUK PV — Natural History Museum, London, UK; vertebrate palaeontology collection.

Discussion

Relevance to other work

XXX

Open peer review

In publishing the Xenoposeidon revision (Taylor 2018b) in the journal _PeerJ-, I (Taylor) was pleased to take advantage of the journal's policy of allowing submitted drafts, peer-reviews, response letters and handling editors' comments to be published alongside the final paper. It is because these materials are published (### Young et al. 2018) that the sequence of discussion is preserved, and Mannion's helpful and gracious comments are available to be read — not only as the extracts in the present paper, but in their full context.

We endorse the publication of peer reviews, and both take this option whenever it is offered. Aside from their value as part of the scholarly record, published peer-reviews are visible evidence of the reviewers’ broader contribution to science, and can be taken into account in evaluating researchers for jobs, promotions, tenure and grants. Sets of reviews, accompanied by the corresponding versions of the manuscript, can be an important pedagogical tool for teaching students in practical terms how peer-review works (e.g. XXX pers. comm.). Crucially, reviews can play an important role in the origination of new research questions, and should be acknowledged: the present work on defining vertebral orientation arises directly from Phil Mannion's peer-review comments (Mannion 2018a, 2018b).

Open composition

This work first began to take shape as a series of blog-posts (Taylor 2018c, Taylor 2018d, Wedel 2018a, Wedel 2018b, Wedel 2018c) which were drawn together in a talk (XXX reference) presented by Taylor as part of the 1st Palaeontological Virtual Congress (http://palaeovc.uv.es/) and announced online (Wedel 2018d). This manuscript was developer in the open, in a public GitHub repository (XXX cite SV-POW! post, and hopefully some issue reports and/or pull-requests). We commend this approach as valuable for soliciting informal feedback early in the process, and in making the research itself available quickly.

Acknowledgements

First, we thank Phil Mannion (XXX Imperial?) both for his multiple rounds of review of the Xenoposeidon manuscript and for giving us permission to quote relevant excepts in the current paper. XXX seek this permission!

We are also grateful to the curators and collection managers for access to specimens used in this study, including

  • Daniela Schwarz (Museum für Naturukunde Berlin) for Giraffatitan.
  • Julia McHugh (Dinosaur Journey) for Haplocanthosaurus.
  • Bill Simpson (Field Museum of Natural History, Chicago, IL) for Brachiosaurus and the giraffe.
  • Neftali Camacho (LACM XXX expand this) for the Komodo dragon.
  • Sandra Chapman (Natural History Museum, London, UK) for Xenoposeidon.
  • Ken Noriega (XXX check Western University of Health Sciences, Veterinary Medicine, CA) for the horse head.

Finally, we thanks John Yasmer and Thierra Nalley (Western University of Health Sciences, CA) for their assistance in CT scanning and 3D modelling the Haplocanthosaurus caudal vertebra.

References