This is something I have been avoiding for a while as I know for a fact that this is divisive in the dinosaur / archosaur community and incidentally I’m probably on the minority side. However, regardless of the divide, it is important to know that the division in there and why.
Although I have not really covered the issue of directions in detail before, the central idea that scientific writing should be clear and concise is something I have expounded on before. In short, when describing bones (or teeth, positions, rotations about joints etc.) anatomists use a series of words to accurately describe directions to make things clear and concise. Obviously terms like up, down, left, back, rear and so on can vary depending on your point of view or if something can rotate (you can hold you palm facing up or down for example) so having a set of directions based around the skeleton itself eliminates much of the confusion.
However since palaeontology and anatomy as subjects evolved well before phylogenetics, initially different groups of animals ended up with different shorthands to reflect the different people working on them and the different issues at hand. Thus while reptile (and dinosaur) researchers largely used anterior and posterior to denote a direction towards the front or rear, most bird workers used cranial and caudal (literally towards the head and towards the tail) to mean essentially the same thing. Of course once it became clear that birds were dinosaurs, many dinosaur researchers started to use cranial and caudal as opposed to anterior and posterior (the situation is of course more complex than this, but that’s my take on it in as few words as possible).
The consensus seems to be moving towards this as the right thing to do but I have to say I disagree. I think the clarity issue is at stake with these terms for two slightly different but related reasons:
1. Frankly I think the terms can sound silly when in use the beak or rostrum of a skull is anterior (i.e. in front of) the orbit (eye socket). Under the ‘avian’ system you would say that the rostrum is cranial to the orbit, but both of them are in the head itself, so essentially something is in the head direction of the head relative to something else in the head. Sure you know what it means, but I don’t think it is as clear as it can be and it is almost oxymoronic.*
2. Similarly but even worse, we have the problem with vertebrae. Vertebrae in reptiles are typically divided into cervical (neck), dorsal (back), sacral (in the pelvis) and caudal (tail) vertebrae – you may therefore have already spotted the issue. If you want to talk about posterior dorsal vertebrae these are caudal dorsals, and posterior caudals are caudal caudals. How can this possibly be considered clear? You actively have to stick the word caudal into positional details of vertebrae that abut the caudals or relate to the caudals but are not caudals? You can end up saying that the caudal dorsal vertebrae resemble the cranial caudal vertebrae but not the caudal caudal vertebrae or the caudal cervical vertebrae. Oh good.
That’s it really. In short cranial = to the front and caudal = to the rear. These terms are common and indeed are increasingly so, but I often find them awkward and as such I don’t think they serve the community as well as they could. Perhaps I’m just slow, but I really often do have to re-read phrases about ‘caudal cervical vertebrae’ to remember that we are dealing with the neck and not the tail and it seems so unnecessarily complex. I suspect at least a couple of readers will try to correct me in my thinking and I’m willing to be persuaded, but I’ve yet to hear a convincing argument as to why ‘cranial-caudal’ is better than ‘anterior-posterior’.
*Note: here ‘rostral’ meaning ‘towards the snout’ can also come into ply in place of ‘cranial’ though of course this still does not help when you want to say that the rostrum is rotrally positioned (OK, that’s obvious, but the point is valid), and of course a horn say that extends in front of the rostrum becomes very hard to describe!
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