Reconstructive skeletal drawing- perspective foreshortening

[Mahnmut]

Hello dear fellow forum members,

I need some advice.

Until now, I have built my models mostly trying to follow the reconstructions others have made, often in form of profile-view skeletal drawings.

At the moment I am trying to build a model of Helveticosaurus, which is known from a rather complete, but heavily crushed skeleton.

The one Skeletal reconstruction I can find online comes from a source that I do not trust (the one calling itself heretic), so I try to start from the fossil.

In that online drawing, the lumbar ribs are showing a strong curvature of about 90 degrees, which is definitely there in the fossil. But I do not believe that all of this curvature would have been visible in the lateral view.

Below is my take starting from Kuhn-Schnyders drawing of the fossil itself:

http://www.thefossilforum.com/uploads/monthly_2021_03/54696752_HelveticosaurusafterKuhn-Schnyder1974.jpg.ce995add17ae5d89b765a7ca17a337a9.jpg

 

Concerning the reconstruction of Spinosaurus it has been discussed intensely if it is legitimate to show the leg bones as perspecive- foreshortened, or if they should be shown in their full length as if lying flat on the paper. Concerning ribs my impression was that skeletal drawings usually show what the anatomically connected ribcage would have looked like from the side.

Any advice would be much appreciated.

Best regards,

J

 

 

 

 

 

[pachy-pleuro-whatnot-odon]

4 hours ago, Mahnmut said:

Concerning the reconstruction of Spinosaurus it has been discussed intensely if it is legitimate to show the leg bones as perspecive- foreshortened, or if they should be shown in their full length as if lying flat on the paper.

 

I'm having difficulty even understanding this statement, if I'm honest. Would you have some references to pages discussing this particular topic with respect to Spinosaurus? If not too much of a read, it might give me an idea of the problem you're dealing with here...

[Mahnmut]

Hi,

thanks for pointing out where I am not making myself clear.

I am referring to the discussion following the quadrupedal reconstruction of Spinosaurus by Ibrahim et al 2014. In his answer to Scott Hartmanns critique

(https://www.skeletaldrawing.com/home/aquatic-spinosaurus-the-authors-responsd9182014)

Nizar Ibrahim mentioned that ...

"Also, in the 3D model, the femur is not hanging straight down - it is angled outward/laterally slightly and so foreshortened slightly in lateral view, making it look shorter."

This among other points has been criticized (in vain) by the Long-legged-Spino camp over the internet.

Now I wonder if it is common practice to draw skeletals as they would appear looking in an (constructed) undistorted way on an anatomically mounted skeleton, or if the bones are drawn to their measured proportions of length, even if the full length of a legbone for example would not be visible due to its oblique angle in the anatomical position.(imagine a crocodiles femur pointing directly at the lateral viewer, thus having very little apparent length)

I will try and include an image when I am somewhat further with my own reconstruction.

Best Regards,

Jan

 

Edited April 1, 2021 by Mahnmut
syntax

[pachy-pleuro-whatnot-odon]

Hi Jan,

 

I read through the blogpost you referenced. And while I think the discussion in that article mainly concerns a defence on how to determine normalized long-bone dimensions based on a standardized anatomical formulae, it does touch upon the issue of perspective as pertains to the representation of anatomical reconstructions, and the consequences of choices herein. I wouldn't, therefore, necessarily say that the blogpost was about whether or not it's legitimate to apply perspective foreshortening to skeletal reconstructions, but it put my mind on the right track, at least (I think).

 

That is - to me, as someone not immediately involved - the fact that there's discussion on this matter seems to indicate two things:

1. There doesn't seem to be a standard for the presentation of skeletal reconstructions yet, as this has probably never been an issue before. As such, this seems similar to the presentation of reconstructed pottery/vessel shapes in archaeological treatises, where an unspoken standard appears to exist but, as in actuality this is not the case, authors are free to deviate and create their own methods of presentation.
2. With the increasing uptake in 3D-modelling, it seems only natural for skeletal reconstructions to also start being made using these techniques, posing the reconstructed animals in more naturalistic stances.

Point one from the above would already seem to answer your question, but lets continue this mind-exercise a little further. For, I can see both one advantage and one disadvantage in using 3D-modelling when creating reconstructions - the obvious disadvantage being that for a 3D-model to be figured in a two-dimensional medium it needs to be rendered - that is, projected from a certain viewport - which will always create distortions. The advantage, of course, is that the reconstruction can be more true to the nature of the animal's anatomy - or at least the authors' interpretation thereof. Thus, while - especially in line with the ideals of the Dinosaur Renaissance - there are some obvious benefits to such dynamic and life-like reconstructions vis-à-vis ones that see the skeleton as if it were simply laid out on a floor or table like a big puzzle ready to be assembled, there are some pitfalls when it comes to their illustration and subsequent (re)interpretation by readers.

 

 

 

When it comes to your current project with the Helveticosaurus zollingeri, however, I think there are a few things to keep in mind:

1. The drawing you're using is not a reconstruction, but an interpretation. As such, and leaning on my knowledge of archaeological interpretive drawing, there should be as little perspective distortion as could be achieved. Although this is rarely the way things are done in reality (in the field of archaeology), you can still pretty much imagine the process through which interpretive drawings are made as first taking a photograph of the object to be drawn, so that you have a fixed perspective; then tracing the shapes on the photograph onto paper. In the process, hashing can be used to indicate slopes, with the slope being the steeper the more intense the hashing gets.
2. The moment in history when the drawing was made, as this may influence the specific style-rules that were applied. Although, in the case, I doubt there's much deviation from the procedure I described above.

If I look either at the interpretive drawing by Kuhn-Schnyder (1974) or actual fossil specimen, reproduced below from Mark Witton's blog and Wikipedia respectively, I see no reason why the ribs around the pelvic area should be reconstructed with a 90 degree angle, as in the reconstruction by ReptileEvolution, also below.

 

 

 

 

 

Rather, what I see is that there seems to be some displacement going on along the spinal column, may be some plastic deformation - but above all that the area surrounding the pelvic girdle is a mess: it appears to me to contain many broken and displaced rib fragments, which may lead one to interpret sharp concerns. However - whether you interpret the ribs as widening out around the pelvis or as flat and pointing straight at the pubis (i.e., interpret the drawing as a 3D or 2D image) - I don't believe such a condition would be known from any living animal, making it hard to believe a prehistoric one would have a couple - and only a couple - of such abhorrent ribs. Much rather, I think this is an interpretation error on the part of the person having created the reconstruction. For the direction the ribs flow, the lines they follow, in both Kuhn-Schnyder's drawing and the fossil seem to be much more mutually coherent - in particular when looking at the fossil. To me it seems the reconstruction was mainly drawn up from the interpretive drawing, and therefore did not take sufficient account of the fragmentation and displacement of the ribs.

[Mahnmut]

Hi Alexander,

thanks for your thoughts.

Thats about what I think.

The 90-degree curvature is really there in the fossil, and I think it is not mainly due to plastic deformation. Its just the curvature you would see looking from the head or tail into the ribcage. Hopefully my graphic below makes a little clearer what I mean.

Best Regards,

J

[LordTrilobite]

On 4/13/2021 at 7:39 PM, pachy-pleuro-whatnot-odon said:

as in the reconstruction by ReptileEvolution, also below.

The creator of that website is a crank and his skeletals cannot be trusted. He has wild ideas about phylogeny that don't match the data and he bases features of his skeletals on low detail photos without seeing the actual fossils. He often wholesale invents anatomy that simply is not there on the real fossils. Both the websites Reptile Evolution and Pterosaur Heresies should be ignored completely.

[pachy-pleuro-whatnot-odon]

1 hour ago, Mahnmut said:

Hi Alexander,

thanks for your thoughts.

Thats about what I think.

The 90-degree curvature is really there in the fossil, and I think it is not mainly due to plastic deformation. Its just the curvature you would see looking from the head or tail into the ribcage. Hopefully my graphic below makes a little clearer what I mean.

 

Hi Jan,

 

When talking about the 90-degree angle, I presumed you were talking about the area circled in red in the image below, which to me seems an unnatural bend and therefore misinterpretation:

 

 

 

However, as this doesn't seem to be what you meant, but rather - I presume - the two parts indicated by the red arrows, then: yes, it does look like the 90-degree curvature is actually present. That is, as you know, ribs' main function is to give the body shape and protect the organs in the thoracic region of the body. To this end, ribs - including gastralia, when present - work together to create a cylinder of bone. The parts of this cylinder don't have to connect, however, and, in fact, most often don't (e.g., the ribs of ichthyosaurs or the lower ones in our own species, or the ribs and gastralia in plesiosaurs). The way this works is that if you draw an imaginary line extending the curvature of the ribs, these lines will eventually cross, giving you the size of the rib cage/thorax. If gastralia are present, they would lie along this same imaginary line, however, again, not necessarily connected to the ribs. As in plesiosaurs gastralia often form a basket where individual gastralia align much more closely with one another rather than with the ribs. In other words, in your drawing above, I would move the grastralium down more towards the ventral side, meaning it would also automatically shift more to the right.

 

In itself, though, a curvature of 90 degrees for ribs is nothing special. In fact, plesiosaur ribs also exhibit the same degree of curvature:

 

Cast of Cryptoclidus oxoniensis from the Middle Jurassic of Peterborough in the Naturmuseum Senckenberg

[pachy-pleuro-whatnot-odon]

1 minute ago, LordTrilobite said:

The creator of that website is a crank and his skeletals cannot be trusted. David Peters has wild ideas about phylogeny that don't match the data and he bases features of his skeletals on low detail photos without seeing the actual fossils. He often wholesale invents anatomy that simply is not there on the real fossils. Both the websites Reptile Evolution and Pterosaur Heresies should be ignored completely.

 

I fully agree Olof. It's just that Jan mentioned this drawing it in his original post as the only reconstruction he could find, and I thought part of his question referred to it. Neither Jan nor me see either this reconstruction or the website(s) as useful sources of information and are well aware to keep away from them. But it's probably good to mention this again for those as are less familiar with them

[Mahnmut]

Thanks again, Alexander and LordTrilobite.

When I first found that drawing, I did not realize its dubious source. When I did, I decided to try and do my own reconstruction.

The 90 degrees I was talking about where indeed the ones you circled above, Alexander. To my astonishment, those ribs appear to be present in the fossil, circled by me below (Sorry, no access to photoshop at the moment.) The (blue) rib I picked for my cross section was the one that appeared the least damaged, though its curvature is less than 90 degrees. I still " work" under the assumption that there has been only a small amount of plastic deformation, the ribs do at least look as if they settled on the ground before being broken. So I take the curvature I see in the fossil as a base and add a little guessed curvature in the third dimension. That results in a relatively "conservative" ribcage with a round cross-section, as I put in the upper middle of the image in a first draft.

Best Regards,

Jan

 

[Mahnmut]

I wonder what it does to my reconstruction to put a greater distnace between the ribs and Gastralia. The curve seems to fit quite well with them nearly touching. Time for some more tinkering.

Good night,

J

[pachy-pleuro-whatnot-odon]

Okay, then my original answer did indeed target your question. And I do actually think there's quite a bit of plastic deformation and shifting going on in exactly that area of the fossil. For even from the drawing one can see that the vertebrae are missing from the right-hand side of the area you marked with the red circle, whereas the ones left on the left exhibit an unnatural drop that doesn't connect with the continuation of the spinal cord to the right of the circle. Also, the bones in that area are so densely clustered that it's hard to make out which bones are which. As such, I think that in this case it pays to take a closer look at the original fossil and draw your own conclusions. For, the way I interpret the ribs (see image below) doesn't show the extreme curvature the reconstruction by the heretic exhibits - although it is in line with the interpretive drawing by Kuhn-Schnyders.

 

 

Keep in mind, though, that plastic deformation is a post-depositional process that occurs due to the geological forces at play during fossilisation, whereas displacement may be peri-depositional or post-depositional. Both, however, are part of the fossil's taphonomic history.

 

I therefore think that a 90-degree curvature is not really an issue in this reconstruction, but the sharp carinated angle in the heretic's reconstruction is. Also, the ribs not making a full 90-degree curve may also not be a problem, just as long as the extrapolation of that curve somewhere connects to the other side of the rib cage... As to whether the gastralia should connect or not, those details will initially need to be gleaned from the fossil itself. in plesiosauria the gastralia show obvious compression on one or both ends of the bone, which indicates their overlap. Based on this, you can reconstruct the "gastral basket", which then, as a whole, needs to connect with the cylinder formed by the rib cage. For, as far as I'm aware, there's no one-to-one mapping between a pair of ribs and a matching pair of gastralia: that is, rather then ribs and gastralia being one-to-one, their relationship is one-to-many.

 

Hope this helps.

 

P.s.: For most simple drawings I do these days I also don't use Photoshop (though I have it installed on my desktop PC). Instead, I just use Microsoft Paint, which, though somewhat cumbersome and limited, does do the job. And if I really need to draw vectors, I use Inkscape, which is free to use.

[pachy-pleuro-whatnot-odon]

Hi Jan,

 

Just bumped into this image by Mark Witton that pretty much explains what I tried to express in words concerning ribs and gastralia above:

 

 

 

 

 

 

 

 

 

 

The blogpost itself also seems very informative in this context (though I haven't actually read it yet)