Cranial-Caudal vs Anterior-Posterior

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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13 Responses to “Cranial-Caudal vs Anterior-Posterior”


  1. 1 Tor Bertin 23/11/2009 at 1:35 pm

    Caudalmost portion of the caudalmost caudal.

  2. 3 Thomas R. Holtz, Jr. 23/11/2009 at 10:04 pm

    You are not in as small a minority as you think…

  3. 4 Jerry D. Harris 23/11/2009 at 10:33 pm

    and posterior caudals are caudal caudals. How can this possibly be considered clear?

    No, in the tail, as in any appendage, the terms “proximal” and “distal” apply, so there are not “cranial caudals” and “caudal caudals,” but “proximal caudals” and “distal caudals.” I have no idea where the idea that anyone thought that “caudal caudals” was correct came from…

    but I’ve yet to hear a convincing argument as to why ‘cranial-caudal’ is better than ‘anterior-posterior’

    Actually, I’ve never argued that it’s better — only easier in terms of who to convert: a couple thousand paleontologists to the terminology everyone else (except human anatomists) use, or tens of thousands of neontologists to the terminology paleontologists use (the mammal specialists also use “cranial” and “caudal” — except some paleontologists, that is). It’s also workable that each profession has its own language, but given the push to include an understanding of evolution via phylogenetics and homology in the study of any extant organism, the easier it is for the two groups to communicate, the better, IMHO, and when it is easy to recognize homologies (which can only happen if homologous structures have the same names across taxa), then understanding (and explaining) evolution is facilitated. I’d be perfectly happy if all the neontologists got rid of “cranial” and “caudal” and went back to “anterior” and “posterior” — I’d make the switch, too, if they would — as I said, I’ve never advocated that the A/P system is inherently bad in any way — just that it’s the minority view among those that study tetrapods and therefore is (hypothetically) the easier group to change.

    • 5 Mike Taylor 24/11/2009 at 5:23 am

      Don’t forget all the extant herpetologists.

    • 6 David Hone 24/11/2009 at 9:08 am

      It may not be correct, but I have seen ‘caudal caudals’ I’m sure. As I note that, I still find “caudal dorsals” or what ever cause me to stop mid sentence and mentally calculate that the sentence refers to dorsals and not cuadals *every single time* and it drives me nuts.

      “Actually, I’ve never argued that it’s better ”
      I wasn’t targeting you specifically Jerry! If you asked me for a list of who used which, I would not be able to give you one. Apologies if you feel got-at.

      To return to your main point, we might be in the minority, but as I argue above I think our system is better. Surely it matters not if one group is a small minority, if their way of doing things is better the other side should take note, we should not switch to a less intuitive system surely? (This sounds like two militant factions at war, but you get my point).

  4. 7 Mike Taylor 24/11/2009 at 5:24 am

    This matter seems terribly simple to me: cranial and caudal already have perfectly good meanings, so it’s dumb to overload them with second meanings when we already have terms that mean what we want.

    • 8 David Hone 24/11/2009 at 9:09 am

      That’s exactly it Mike. In this context caudal means two different things and that is just not helpful. Clarity is demanded and this obfuscates things, not clarifies them.

  5. 9 Jerry D. Harris 24/11/2009 at 11:00 pm

    This matter seems terribly simple to me: cranial and caudal already have perfectly good meanings

    By this same logic, though, “anterior” and “posterior” also have perfectly good meanings: forward and backward. But these terms were invented to apply to humans in which “anterior” is 90° to “anterior” in…well, every other, sane tetrapod. Same with “posterior” (for which I’ll note “backward” means “toward the back,” which is “up” in anything other than a human!) So zoologists working on organisms other than humans decided to leave those terms to humans and have other terms for their own groups (and, by the way, terms that don’t have any inherent external directional specifiers, such as “forward” and “backward” — not that “anterior” and “posterior” have inherent specifiers etymologically — but they have come to be intimately associated with those directions because of anthropocentrism through the ages). And, as it turns out, “cranial” and “caudal” work just fine in humans, too — it doesn’t matter where the head is, for example — a direction can still be “toward the head” without implying “forward,” and “caudal” can still mean “toward the tail” without implying “backward” (or, in humans, “down”). This just makes sense to me.

    , so it’s dumb to overload them with second meanings when we already have terms that mean what we want.

    This is only because you’ve got the bias of pre-education with one set of terms with which you are comfortable. Had you been educated initially with “cranial” and “caudal,” you’d be just as comfortable with those terms. And, of course, one can educate oneself to become comfortable with the other system — I’m living proof of that! It’s just a matter of willingness to actually do it. (NOTE: there is no accusation or judgment intended anywhere in that statement!)

    t may not be correct, but I have seen ‘caudal caudals’ I’m sure.

    Yes, you have — as have I! But it isn’t correct.

    I wasn’t targeting you specifically Jerry!

    No worries; I didn’t take any offense. But I’m the one that started this whole debate openly, so I feel compelled to defend my point of view until I’m convinced otherwise!

    Surely it matters not if one group is a small minority, if their way of doing things is better the other side should take note, we should not switch to a less intuitive system surely?

    Well, to take these two issues separately:

    (1) Of course it doesn’t matter that one side is in the minority — after all, people that thought the world was round were once in the minority, but that didn’t make them wrong! I mentioned that point of view simply from a logistics standpoint. However, I also feel compelled to point out that the avian and mammalian terminologies were not concocted willy-nilly — they were debated by anatomists far better than I, and ultimately they decided to jettison the human-based terminology in favor of a more universally applicable one. I’m not saying they were necessarily correct — I have no idea what their rationales were — but surely that must count for something…?

    (2) “Less intuitive” is, I guess, a matter of perception. For me, a system in which my own directions (as a human) are different than every other organism, including the ones I’m studying, is bizarre and “unintuitive.” Having a system in which all directional terms apply universally is intuitive. And using identical terms to describe identical features (homologies) across taxa is also intuitive to me. Maintaining two separate nomenclatural systems when identifying homologies as the products of evolution and phylogeny is a primary concern is most definitely not intuitive (and I think we all agree on that point!). Given that pretty much all paleontologists run around declaring that phylogenies must be based on homologies, it seems particularly unintuitive — not to mention openly hypocritical — that they then reject the most logical (= quickest) way to apply homology to their nomenclatures: by using what the majority uses and what has been standardized for the specific purpose of increasing communicability and comprehension globally. (That’s the logistical part.)

    As I said, I have nothing against the “anterior”-”posterior” system (except that I think it makes less sense in a “humans vs. the world” kind of way), and if the avian and mammalian nomenclatures would switch to using it, so would I. I guess my question, then, is if you (anyone; not you, Dave, in particular!) think it’s a more intuitive system, how do you intend to convince the neontological world? Continuing to use it in papers that pretty much only paleontologists will see isn’t going to cut that mustard! (Not that I have any brilliant ideas, either, mind you!)

    • 10 David Hone 25/11/2009 at 9:00 am

      Well I do have a couple of points to make here Jerry and I hope i don’t stick my foot in it since I know I know much less than you on this subject so apologies if I make a howling error and feel free to correct me and ignore any point based on it. That said…

      Speaking personally, I didn’t have any pre-education when it came to anatomy. I started reading various dinosaur and bird and reptile papers and encountered both terms pretty much at the same time and in equal amounts and I never found cranial-caudal intuitive at all. Obviously I read lots of appears that use it have have been doing so for years now and “caudal dorsals” *still* gets me. Every. Single. Time. One data point and a personal anecdote, but valid just the same! ;-)

      I know that it was debated about the use of terms but was this not done quite some considerable time ago? And was everyone involved? My point essentially being that some (great) anatomists might have had a good say, but is this relevant now? The science has moved on, and the simple fact that not everyone adopted the scheme surely means that there was and remains disagreement. (Again, I don’t know, my bad if this is wrong).

      I guess my point is that this split has been around for a while and there has been no obvious civil war among the anatomists! I do think that ‘anterior-posterior’ is better, and I’d be delighted if everyone switched to it, but the two systems have survived side by side for decades if not centuries and show no great signs of almagamating or of huge uncorrectable confusion between the two. In other words, I think we can live and let live and not worry too much about it, though I don’t think I’ll ever get ‘caudal dorsals’ right in my head.

      As a final separate point, while I do agree with you on principle that humans should not differ from other animals when it comes to terminology, I would note that if you are looking for an exception, they are it. Humans after all get whole branches of science (anthropology, archaeology, medicine) that others do not and much of that does not often overlap with more mainstream biology / palaeo so if there was to be a separation, I can see that humans are the most intuitive place to do it.

  6. 11 Mike Taylor 25/11/2009 at 8:08 am

    As it turns out, “cranial” and “caudal” work just fine in humans, too — it doesn’t matter where the head is, for example — a direction can still be “toward the head” without implying “forward,” and “caudal” can still mean “toward the tail” without implying “backward” (or, in humans, “down”). This just makes sense to me.

    So you’re happy that the knee is caudal to the ankle?

  7. 12 Jerry D. Harris 25/11/2009 at 11:13 pm

    So you’re happy that the knee is caudal to the ankle?

    Well, I suppose this is correct from a global perspective…but since they’re both in the leg (an appendage), appendicular terminology applies and the knee is proximal to the ankle (or the ankle is distal to the knee).


  1. 1 To me – to you: directions and descriptions « Dave Hone’s Archosaur Musings Trackback on 24/11/2009 at 9:01 am

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