Atopodentaus skull rekonstruction

[Mahnmut]

Hello together,

its been some time since I posted a model, and there are quite a few unfinished ones in the making.

realizing how small Atopodentatus'  iconic head was in relation to its body, I decided to rather try and print a lifesize skull than a complete downscaled skeleton.

Morphing recent species' skulls has the advantage that you get anatomically looking detail, although on the other hand it is wrong detail. So I would much appreciate feedback when you spot something particularly wrong. @pachy-pleuro-whatnot-odon?  And Atopodentatus took some morphing.

Here is how far I got:

Best Regards,

J

[pachy-pleuro-whatnot-odon]

Hi Jan,

 

Glad to see you're finding time again to work on your excellent models! Making a separate skull this time definitely makes for an interesting variation, I think! I also believe that, depending on your expectations on the anatomical accuracy of the desired outcome, it may be a bit more difficult to achieve than for a complete skeleton, as singling out an individual piece of anatomy means that all attention to detail that would normally be divided across a skeleton - parts of which will undoubtedly be forgiving, allowing you to make up for less certainty in other parts - is now focussed just on the skull.

 

That having been said, I'd say you did an excellent job and were smart to modify an existing model for your project. Saves a lot of work and potential frustration, I can imagine, and, by picking a quality base model, increases the quality of your own work. Unfortunately, it'll be difficult for me to advise you on this project, I think, seeing as I've got little experience with the anatomy of Atopodentatus unicus and its skull compares so poorly to other known animal species. That said, however, there are a couple of things that don't quite feel right to me...

 

1. Sutures

In your reconstruction the wide snout part of the skull appears to consist of a single bone at the top and a single one at the bottom. This isn't anatomically correct, as can be seen in the below diagram (figure 2 from Chun, Rieppel, Long and Fraser, 2016). Firstly, in most animal species, the dentary, which forms the bottom half of this snout, is comprised of two separate bones which, even when conjoined, normally leave a distinctly recognizable suture-line - one that seems to be interdigitating in A. unicus. Now having this suture in your model might simply be too much detail. But even so, the shape of the dentary differs from how you've reconstructed it, as well as the shape of it influencing the direction in which the bone fibres would flow (which currently can be seen on your model, I believe). From what I can tell from the diagram, the dentary bones have a bit of a "hammer shape", running straight forward from the back of the skull, never in a line to meet each other in the middle as in your model. Instead, where the vertical part of the dentary transitions into the horizontal part, the latter fans out both medially and laterally to form a wide spoon-shape that connects when it hits the midline of the skull, at which point the two dentaries form a rough suture.

 

 

Similarly, and in contrast to most other animal species in which the premaxilla is strongly fused (to the point of taking the appearance of a single bone - even if bilateralism, of course, informs us that this would originally have comprised a pair of mirrored bones), the premaxilla consists of two separate bones with a very loose connection, as can be seen in the gap that exists between them in the actual fossil on which the above diagram was based (figure 1, ibid.). It's hard to say, however, how such a loose suture line would've taken effect within the broader context of the skull's morphology.

 

 

 

2. Nares

Interestingly, the above diagram shows two sets of paired holes in the skull roof, which seems a bit confusing as to which of these represent the nares (especially since these haven't been explicitly indicated in the diagram). As such, reconstructions placing the nose higher up on the skull exist, such as the one below taken from Wikipedia.

 

 

The same Wikipedia page also features another reconstruction that places the nasal openings lower on the skull, towards the beak - which, indeed, seems to be Chun, Rieppel, Long and Fraser seem to favour as well, and would appear indicated by the greater size of the openings between the maxillae and premaxillae (figure 3).

 

 

 

Your nares should therefore move lower onto the skull, towards the tip of it, sitting between maxilla and premaxilla.

 

But what then about this second set of openings? I suspect these are salt glands, an anatomical feature present in various marine species, including sharks and marine-based archosaurs, and used to get rid of the excess salt consumed through their aquatic lifestyle. While these glands are typically located in the immediate vicinity of eyes, nose or mouth in archosaurs, it's not inconceivable, especially with respect to Atopodentatus' other unusual anatomical features, that the salt glands would've taken a different position along the skull. Keep in mind that Atopondentatus unicus was a Triassic species, living during a time when many archosaurs first became secondarily aquatic, and thus a period that saw a lot of experimentation in adjustments to a marine lifestyle. Your reconstruction will thus need a second pair or smaller openings in the skull at about the position that your nares are in right now.

 

3. Pineal foramen

Looking at the diagram and fossil provided above, yours is too big and located too far towards the back of the skull.

 

4. Orbits

This one is, in fact, the most difficult for me to put a finger on, but: I think your orbits have the wrong shape, being much too tear-shaped. Most marine reptiles I know of have more or less circular orbits, whereas yours is almost triangular. And while the aforementioned diagram and its fossil seem to suggest this is the correct shape for the orbits, I believe this is rather an artefact of the plastic deformation, the crushing, the fossil went through. For if we look at the original skull that was found, one that presents a much better lateral view of the Atopodentatus skull morphology, you'll notice the orbit is decidedly rounded. Of course you'll need to make compensation for the fact that this skull, rather than vertically compressed, has its rostrum pressed into the back of its cranium. But even stretching that back out makes for a much rounder orbit, I believe, than what you currently have. As the length of the orbit does seem correct for the size of the skull, however, I'd suggest the skull remains somewhat taller for a bit longer than it does in your current model, therefore having a steeper dip between orbits and premaxillae. I also wonder, in that case, whether a straight slope as you have now would be an accurate representation of the skull shape, or whether the gradient in that spot would be more sinuoidal - that is, taking a quick dive after the orbits, and then flattening out towards the tip of the snout.

 

(source)

 

 

As said, a lot of this is just argued opinion. But I hope it helps. In any case, your work looks great so far!

[Mahnmut]

Thanks Alex for your good observations, I will try to adjust my model accordingly.

The snout is of course the part of the original scan (Amblyrhynchus cristatus) that I had to distort most heavily, loosing much of the fine structure like sutures that I will have to sculpt in again.

Interesting thought about the salt glands.

I also wonder about the teeth arrangement (see below) On a purely intuitive level the second version feels more plausible, but I cannot tell what the fossil really shows.

Best Regards,

J

[pachy-pleuro-whatnot-odon]

On 8/10/2022 at 5:43 PM, Mahnmut said:

Thanks Alex for your good observations, I will try to adjust my model accordingly.

 

No worries! As you know, always glad to help

 

On 8/10/2022 at 5:43 PM, Mahnmut said:

The snout is of course the part of the original scan (Amblyrhynchus cristatus) that I had to distort most heavily, loosing much of the fine structure like sutures that I will have to sculpt in again.

 

Having done 3D-modelling myself I know what you mean and how nasty my comments were in that regard. However, I thought I'd still better give you my ideas on the anatomical correctness of the skull, leaving you to decide how much effort you want to invest in it

 

On 8/10/2022 at 5:43 PM, Mahnmut said:

I also wonder about the teeth arrangement (see below) On a purely intuitive level the second version feels more plausible, but I cannot tell what the fossil really shows.

 

 

Interesting issue you're raising here... I haven't really considered this, nor, as you say, do the actual fossils provide indications for one or the other solution. Although - if I look at the second skull (the one in lateral view), at the point where the snout/rostrum has been compressed, it looks like there's a gap in the dentition. And, indeed, intuitively, it'd make sense for the teeth to not run all around the widened rostrum: anatomically it'd be way to costly to grow teeth in areas where they're not used. As such, I agree that the arrangement ont he right seems the more likely.

[Mahnmut]

Ok,

I adressed Alex' points of constructive critique concerning the nares and sutures.

Cheers,

J

[pachy-pleuro-whatnot-odon]

Hard to say without seeing the rest of the skull. But this view, at least, is looking a lot better now