Something that has both fascinated me and frustrated me to no end as of late are the lacrimal crests of allosauroids and other such theropods. No living theropods, to my knowledge, possess such structures. The closest analog I could find were Geese. I'm being driven up a wall trying to figure out what they would have been covered with in life. How would they have been incorporated into the face of the theropod? What display structures could they support? Air sacs, keratin, or caruncles? How pneumatized were they? Were they even for display? Most paleoartists just reconstruct them as odd stones sitting on top of the animals head, or say they were just for shading the eyes, but that can't be it, right? It feels like something is missing.
As a Paleoartist, is there anything you can say on this? Because I don't know nearly enough.
Oh boi, this is a tricky one.
Cranial ornaments come in all kinds of shapes, sizes and surface textures. For Allosaurus it seems likely that they simply had keratin sheets on top of the lacrinal crests, at least that's what the rough surface and striations suggest. However it also seems like air sacs were reaching up from the antorbital fenestra
This is how I would reconstruct that part of Allosaurus in the moment.
There could have been more though, you can see in this photo of the skull of Big Al II that this animal had large opening behind the lacrinal crests, so there could have been other structures involved.
Maybe even better to look at is this 3d scan of Arkhane, from Brussels.
This is the skull of “Arkhane”, an adult allosaurus specimen (70% complete, 8.7m). Wyoming, USD, 157 to 152 million years old. &
This could have been related to theroregulation, more display structures or simply further weight reduction.
There is quite a bit of wiggle room when it comes to the interpretation of dinosaur facial features and there is way too much already published and at the same time too little to put it all into a single blog post.
Also not all dinosaurus follow this patters, the cranial crests of oviraptorosaurs show no good indication for keratin crests for example. So it's better to say good bye to cassowary interpretations of these animals and instead cover them in skin and/or inflatable sacs, because these hollow chambers appear to be extensions of the sinuses. (my version of Corythoraptor on the right, Citipati from Wikipedia on the left.)
And then again in other theropods like Abelisaurs we see very rugose skull tops but without the striations you see on the lacrinals of Allosaurus, potentially indicating large scales. Here my, slightly over the top, interpretation of Skorpiovenator. And even then: within abelisaurus you have stuff like Carnotaurus which shows clear signs of keratin sheets on its horns.
This all has been complicated in recent years with new methods in bone histology, because as it turns out the internal structure of a bone can tell us stuff about it's outside, that's how we for example found out that Amargosaurus neck spines were no horns and probably conected with soft tissue, or that the osteoderms of notosuchians were covered by skin like in leatherback turtles.
Going back to Allosaurus and other allosauroids. Keratin sheets, scales, air sacs, all these were probably present in these animals, but their exact distribution is still a question of debate.
Here a paper from a few years back that maps para-nasal sinuses in extent and extinct archosaurus which gives you a little idea how complex their internal anatomy could get
https://anatomypubs.onlinelibrary.wiley.com/doi/pdfdirect/10.1002/ar.20794
