Aquatic adaptation in the skull of Spinosaurids

[MLMjp]

This is both a question and post for a new study that I found.

So, by a chance I came across a reddit post about Spinosaurus (https://www.reddit.com/r/Dinosaurs/comments/9etjbr/fluff_question_about_spinosaurus/) and one of the responses to the topic cough my attention:

So let's just address this first: The 15m estimates assigned to Spinosaurus aegyptiacus (Stromer, 1915) actually belong to Sigilmassasaurus brevicollis (Russel, 1996). Based on a 2018 study conducted by Arden et al., which describes a left and partial right frontal (FSAC-KK-3209), measuring ~114mm, and paired frontals and parietals (FSAC-KK-3210), measuring ~113mm. These remains belong to Sp.aegyptiacus. The elevated orbits suggest aquatic lifestyle, and are fused, indicating near or full maturity. The most accurate measurements (based on Franoys) for Sp.aegyptiacus are:
FSAC-KK-18888 (Ibrahim, 2014) (Neotype) Partial skull, relatively small, partial sail, near complete hip girdle and legs, various tail vertebrae. Some other elements. Fused elements, indicating near or full maturity.
Skull length: Premaxilla to squamosal; 1.20 m. Premaxilla to quadrate condyle; 1.12 m.
Body length (along centra): ~11.38 m.
Snout-sacrum length: ~5.78 m.
Femur length: ~0.61 m.
Tibia length: ~0.67 m.
Leg length: ~1.63 m.
Hip height: ~1.78 m.
Weight: ~5 tonnes.
However, Arden et al. also described larger remains belonging to Si.brevicollis. It's referred material (FSAC-KK-7715) are left and right frontals, measuring ~160mm, considerably larger than Sp.aegyptiacus. It also boasted a width-length index of .93. It was noted that this specimen was not fully grown due to it's high vascularity. It's snout was considerably broader than Carcharodontosaurus, in which it shared it's habitat. This indicates a large animal, despite not being fully grown. It's robust build indicate it wasn't as heavily specialised in aquatic hunting, like Sp.aegyptiacus. Because of it's massive size, the largest Sp.aegyptiacus specimen (MSNM-V-4047) must now be assigned to Si.brevicollis. The most accurate measurements (based on Franoys) for Si.brevicollis are:
MSNM-V-4047 Large rostrum (premaxilla and maxilla).
Skull length: Premaxilla to squamosal; 1.58 m. Premaxilla to quadrate condyle; 1.48 m.
Body length (along centra): ~15 m.
Snout-sacrum length: ~7.61 m.
Femur length: ~0.803 m.
Tibia length: ~0.883.
Leg length: ~2.15 m.
Hip height: ~2.35 m.
Weight: ~6-7 tonnes.
So from this, we can infer that Si.brevicollis was considerably larger than Sp.aegyptiacus, so now, we have a new largest predator. Arden et al. also noted that Ibrahim's reconstruction of a spinosaurinin is still quite accurate, meaning that Sp. aegyptiacus and Si.brevicollis had short hind limbs and quite robust arms, the latter's legs somewhat longer, so it does give validity to the short leg hypothesis.

So I looked for that supposed study by Arden et al., and I found this: https://www.sciencedirect.com/science/article/pii/S0195667117303427

I can't access to the full paper, so if someone can a summary will be very helpful. specially in the part that say that the rostrum MSNM-V-4047 now needs to be assigned to Sigilmassasaurus, if there is a true reason for that.

[Sim]

The first link you provided appears to have become a bit incorrect because you put "and" connected to the link.  Clicking on it then shows the page without the original post.  I think this is the correct link: https://www.reddit.com/r/Dinosaurs/comments/9etjbr/fluff_question_about_spinosaurus/

I haven't been able to access the full paper either, but in the link to the paper that you provided the abstract can be read and it makes it clear the referral of the specimens is only tentative.

The paper is used as the reference for the following:
On the Spinosaurus Wikipedia page, referring to MSNM V4047:

Arden et al. (2018) tentatively assinged this specimen to Sigilmassasaurus brevicollis given its size. In the absence of associated material, however, it is difficult to be certain which material belongs to which taxon.

On the Sigilmassasaurus Wikipedia page:

The largest specimen of Spinosaurus cf. aegyptiacus, MSNM V4047, was tentatively assinged to S. brevicollis and on the basis of vetebrae Sigilmassasaurus may have grown larger than Spinosaurus. Although in the absence of associated material, it is difficult to be certain which material belongs to which genus.

[MLMjp]

Alright, so it is not very clear for now...Thank you @Sim!
I wish I could read the rest of the study though...Seems interesting.

[Halichoeres]

I have a pdf of the Arden et al. study. If anybody wants it, just pm me your email address and I'll send it.

From the discussion section:
"There are therefore two spinosaurines in the fauna on the basis of vertebrae, rostra, frontals, and quadrates. In the absence of associated material, it is difficult to be certain which material belongs to which taxon."

So yes, as Sim says, very tentative. For both specimens, they use a question mark in the "Systematic Paleontology" section of the description.

[Neosodon]

It seems they're implying that having a robust or wide skull implies that Si.brevicollis was less adapted to an aquatic environment than Sp.aegyptiacus. Yet alligators usually have proportionally wider more robust skulls than crocodiles but I don't think their any less adapted to water. Just wondering what the rational behind this is.

[Cretaceous Crab]

It seems they're implying that having a robust or wide skull implies that Si.brevicollis was less adapted to an aquatic environment than Sp.aegyptiacus. Yet alligators usually have proportionally wider more robust skulls than crocodiles but I don't think their any less adapted to water. Just wondering what the rational behind this is.

The wider robust skulls of alligators and caimans are more adapted to a diet of turtles and large game, while the slender jaws of crocs and gharials are better for grabbing fish.

[HD-man]

Something I've been meaning to ask: Is it just me, or does the top Spino have different proportions from the bottom Spinos (especially the legs, arms, & torso)?