The head of Styxosaurus

[Sim]

This paper https://peerj.com/articles/1777/ comments on the presence of a preorbital boss/bulk in two skulls of Styxosaurus each classified as a different species, and says it's not clear if this boss/bulk is taphonomic or not.  I've looked at photos of those two skulls and to me it really looks like those areas have been crushed out of shape and the preorbital boss/bulk didn't actually exist on the living animals.  I also can't imagine what function that boss/bulk would have if it was real, unless it was actually part of the sagittal crest, but I don't know about jaw muscles extending that far to the front of the head...?

I've read that there's a third Styxosaurus skull, which makes two known skulls for the species Styxosaurus snowii.  This is the specimen known as SDSMT or SDSM 451.  I've seen an illustration of what this specimen's skull is like before being restored, and as far as I can see it doesn't have a preorbital boss/bulk.  Both S. snowii skulls are around the same size.  To me this appears to be additional evidence for the boss/bulk not being a natural feature of Styxosaurus skulls.

Oceans of Kansas has lots of photos of a mounted version of SDSM/SDSMT 451, here: http://oceansofkansas.com/sdsmt2.html
The skull of this specimen seen in these photos doesn't have a preorbital boss/bulk.  It seems to have a very low and small ridge along the top of the snout though.  I've seen a similar ridge on the snout in Eromangasaurus.  The paper that names Eromangasaurus (https://www.researchgate.net/publication/228657659_A_new_elasmosaurid_plesiosaur_from_the_Lower_Cretaceous_of_Queensland_Australia) says this about its ridge:

This bulges slightly in the area of the exter-
nal naris, although it is unclear as to whether this is an artifact of
lateral crushing or a potential homologue of the premaxillary
bump reported in Styxosaurus,Thalassomedon, and Terminona-
tator from the Late Cretaceous of North America (Sato, 2003).

If this ridge of bone was actually present in the animals when they were alive, I wonder what it was for, perhaps it wasn't externally visible and helped support the soft tissue around the nostrils and that part of the head?

At this point, there are two things I'm wondering about:

1. How likely is it that the preorbital boss/bulk of those two Styxosaurus skulls is/isn't a real feature of the animals when they were alive?
2. Whether the low ridge on the snout of that third Styxosaurus and the Eromangasaurus was really present on the living animals, and if it was, what function it could have?

I'd be particularly interested in hearing @dinotoyforum's thoughts on this?

[DinoToyForum]

Interesting topic, especially for me!

As the authors of those papers admit, it isn't certain whether the preorbital bosses are biological characters or just due to compression. The jury is out for now, but if they are genuine, then perhaps they are related to adaptations for musculature around the external nares (nostrils) or orbits (eyelids). Some relationship with salt glands is also a possibility. This is just speculation. You are right that the jaw muscles do not extend that far.

I wouldn't read much into the skull of SDSMT 451. Welles and Bump (1949) say of it: "The skull is crushed flat and has suffered much from gypsum, but a satisfactory restoration was made" (p. 524). So, the skull on display is basically a model.

However, a dorsomedian ridge is certainly present in many plesiosaurs including elasmosaurs (see Sato 2003 p 91). One other key taxon relevant to this discussion, although it is not an elasmosaurid, is Umoonasaurus. The paper describing Umoonasaurus even refers to it as a "crested" plesiosaur, and the paleoart in the press releases from the time depict it as such:



The paper is available here:
https://web.archive.org/web/20101204130422/http://plesiosaur.com/database/pdf/Kear%20et%20al.%202006-BL%20Umoonasaurus.pdf

The relevant part is on page 2: "There is a high, blade-like crest extending along the skull midline (premaxilla, parietal) from snout to past the pineal foramen; this is highest around the level of the external nares. Another pair of strongly arched crests is also present on the frontals (and possibly postfrontals), above the orbits."

and

"Function of the cranial crests in Umoonasaurus remains speculative; however, they appear too fragile for use in skull reinforcement, defence or male combat (see Molnar 2005). Their heavily sculpted surfaces suggest a horny covering in life; this would have substantially increased the crest height and perhaps (together with colour) made them effective display structures for species recognition and/or mating behaviours."

So, it is possible that the ridge or low crest in some elasmosaurids was also for display. Again, speculation abounds, but that's elasmosaurids for you!

[Sim]

Thanks for your reply!   It led me to do much further consideration of different possibilities.

I haven't been able to find somewhere I can read Sato 2003.  Do you know of somewhere?  I'm interested in seeing what it shows about the dorsomedian ridge in plesiosaurs, as well as the "premaxillary bump" as described in the quote in my previous post.

[Sim]

@dinotoyforum, I just want to make sure you don't miss my previous post.

[DinoToyForum]

I can email you a PDF of Sato 2003, next week