Friday, 29 July 2022

Attempted adventures in dinosaur facial restoration, starring Giganotosaurus

Giganotosaurus carolinii carries the remains of a juvenile rebacchisaurid while sporting a bunch of crazy tissues on its face. Big scales, horns, and is that some kind of thick pad over its snout? Is all this artistic speculation or something inferred from fossils? Read on...

Up until now, my palaeoart career has not crossed paths with carcharodontosaurids, the gigantic, charismatic and famous allosauroids best known for Acrocanthosaurus, Giganotosaurus, Carcharodontosaurus and, most recently, Meraxes. This month, however, I finally had cause to restore Giganotosaurus carolinii, the largest of the group and, potentially, the largest of all theropods.

Carcharodontosaurids are, at first glance, not too challenging to restore: take an Allosaurus, turn everything up to 11 and job done, right? Well, maybe not. Not only are the proportions of carcharodontosaurids (and, to be fair, carcharodontosaurians in general) subtly different from their allosauroid ancestors, but their jaws and eye regions are characterised by a suite of complex sculpting and rugosities. It’s thought that these are epidermal correlates (Sereno and Brusatte 2008): distinctive bone surfaces and histological patterns that record different skin types interacting with the underlying bone (Hieronymus et al. 2009). I’ve written quite a lot about epidermal correlates at this blog because they provide heaps of important external soft-tissue information without direct soft-tissue fossilisation and learning to spot them, in my view, is an essential skill for any budding palaeoartist.

The presence of epidermal correlates on carcharodontosaurid skulls means that we can’t take an “anything goes” approach to restoring Giganotosaurus or its close relatives; instead, there probably is a “right”, or at least "more defensible", way to approach depictions of their faces. Alas, to my knowledge, no specific investigation has been conducted into what carcharodontosaurid skull textures represent despite our interest in other dinosaur epidermal correlates (e.g. Hieronymus et al. 2009; Carr et al. 2017; Delcourt 2018). This means there’s not yet a “go-to” study to provide artists with answers for restoring these animals and anyone wanting to illustrate Giganotosaurus credibly has to make their own interpretations from descriptions and illustrations in scientific literature. Having just been through this process myself, and realising that Giganotosaurus is a fan-favourite, I thought it might be of interest to share my thoughts here. I want to be upfront by declaring that the following deductions are little more than best guesses; without having direct experience of Giganotosaurus fossils I can’t write anything definitive about what Giganotosaurus looked like. Think of the following more as a discussion piece than a rigorous guide, and I welcome input and insight from others if I’ve made errors.

As I understand it, most of what's been illustrated of Giganotosaurus is shown above in this compilation of figures from Coria and Salgado (1995; greyscale graphics) and Novas et al. (2013; colour). Not as much as you might expect, right? There's a lot more known of this species that hasn't been published yet, some of which contain crucial information for palaeoartists (and I guess for scientists too).

And it’s in this spirit that, right off the bat, we need to mention that researching Giganotosaurus is pretty challenging. Its fossils are thinly documented despite Giganotosaurus being one of the more completely known carcharodontosaurids and, even today, almost 30 years since it was announced to the world, we only have a fraction more information available to us than when it was first named in 1995. Just a handful of its bones have been figured so good photos or illustrations of several fossils relevant to this conversation have not been published (Coria and Salgado 1995; Coria & Currie 2002; Novas et al. 2013). Thus, anyone trying to restore this animal from scientific papers alone will struggle for information and a lot of secondary sources — online photographs of fossils and casts, skeletal reconstructions and museum mounts etc. — are essential to obtaining basic information about its proportions and size, even if they risk introducing reconstruction errors. Closely related taxa and comparative descriptions (i.e. “Mapusaurus has a more rugose snout than Giganotosaurus”) are critical too, providing crucial details not mentioned in dedicated Giganotosaurus papers. I mention this because it means that, from the get-go, we’re not in an ideal research scenario for a palaeoartwork, and this makes the possibility of errors in interpretation all the greater.

With appropriate caveats established, let’s dive into this discussion. As with most theropods that have rugose, textured faces, our attention here is going to be on the bones of the snout and orbital region, as these are the principal areas to bear features that might signify epidermal tissues. The carcharodontosaurid fossil record contains a large number of maxilla bones (the main tooth-bearing bone of the upper jaw) and this is good news for artists, as the lateral surfaces of these potentially tell us a lot about the skin on the side of the upper jaw. The typical maxillary rugosity for carcharodontosaurids is well documented across several species, especially Carcharodontosaurus saharicus, Eocarcharia, Mapusaurus and Meraxes. It comprises a series of sub-vertical grooves and pits (Stromer 1936; Sereno et al. 1996; Coria and Currie 2006; Brusatte and Sereno 2007; Sereno and Brusatte 2008; Canale et al. 2022) and some species (e.g. C. saharicus, Eocarcharia, Mapusaurus) supplement these with prominent ridges extending along the base of the antorbital region. These bars separate the rugose maxillary body from the smoother bone of the antorbital fossa: that slightly impressed region of bone surrounding the antorbital fenestra.

The maxilla of Eocarcharia dinops, as illustrated by Brusatte and Sereno (2008), shows the texturing typical of carcharodontosaurid snouts. Note the ridge dividing the textured region from the smoother antorbital fossa: this feature isn't seen in all carcharodontosaurids but might tell us something about skin types all the same. The fossa region is particularly big in this species.

The texturing characterising carcharodontosaurid maxillae may be somewhat less pronounced in Giganotosaurus and thus, perhaps like Acrocanthosaurus, its maxillae may have been on the smoother end of the rugosity scale (Coria and Currie 2006; Eddy and Clarke 2011; Novas et al. 2013). All else being equal, this might imply differences in facial anatomy within Carcharodontosauridae: whatever those grooves and pits signify may not have been as exaggerated in some species as others. A caveat here is that, as is often the case with skin-altered bones, larger carcharodontosaurid individuals tend to have more exaggerated rugosity profiles than smaller ones (Coria and Currie 2006; Canale et al. 2014), suggesting a link with body size or age as well as differences between species. We probably want a number of maxillae from a range of differently aged individuals to establish whether a species has consistently smoother jawbones than its relatives.

Comparing these maxillary features with existing interpretations of dinosaur epidermal correlates provides potential insights into their significance. The textures in question are often likened to those adorning the lateral surfaces of abelisaur skulls and, if so, we might follow Delcourt (2018) in inferring that they represent scale correlates. This seems sensible to me, certainly more than other possible jaw coverings. Carcharodontosaurid maxillae lack the projecting rugosities consistent with armoured dermis or the branching neurovascular channels and oblique foramina found under beak or horn tissues (Hieronymus et al. 2009). Furthermore, the ridges bordering the antorbital regions in some carcharodontosaurids are inconsistent with beaks: cornified sheath tissues tend to terminate with obvious steps downwards into smoother neighbouring bone, not upwards to ridges of rugose bone (Hieronymus et al. 2009).

The skull of a common snapping turtle Chelydra serpentina. The more rugose parts of this skull correspond to regions covered in large scales, while the slightly finer rugosity around the jaws demark the distribution of the beak (also note the stepped topography at the beak/scale transition). Prominent ridges occur around the eyes and nose where large scales meet softer tissues: perhaps this is analogous to what we're seeing in those ridged carcharodontosaurid maxillae?

Raised bony ridges are seen, however, around the skull openings of reptiles with tough, tightly-adhering facial skin like crocodylians and certain turtles, marking some boundaries between thick, relatively immobile skin and softer, more flexible regions. We might expect the antorbital skin of theropods to flex slightly during breathing, as it does in birds, and I wonder if we're picking up some evidence of that in carcharodontosaurids? The notion that carcharodontosaurid maxillary skin might be tough and immobile is not without precedent, as early members of the broader Carcharodontosauria clade are thought to have had maxillary skin of this nature (Barker et al. 2019). If there really was a distinction in skin flexibility in the snouts of these animals it may have been obvious in life, as it is in crocodylians and turtles (I realise this sounds like advocating some form of shrinkwrapping — lightning flashes in the distance, thunder rumbles — but we can't overlook the fact that osteological features do, sometimes, correlate with skin types in living animals). I took these reptiles as inspiration in my reconstruction, giving Giganotosaurus a series of large, thick scales over the side of its upper jaw that terminate sharply around the antorbital region. I retained a full set of lips for reasons that have been thrashed out too many times to bear repeating here, except to mention that — like those of tyrannosaurs — carcharodontosaurid maxillae seem to constrain their rugosity to regions above the toothrow, suggesting whatever skin anchored above the labial foramina (the row of perforations along the jaw) was not so tightly anchored next to the teeth.

Immediately above the maxillae are another set of sculpted bones: the nasals and lacrimals. Collectively, these bones form the various fins and crests that line the top of the snouts in many allosauroids (see Chure and Loewen 2020 for a great visual of these), but the carcharodontosaurid condition is not typical of this wider clade. Nasal material is known for Giganotosaurus but it was not featured in its original description, nor (to my knowledge) has it been illustrated elsewhere. What’s hinted at in various reconstructions and papers is that Giganotosaurus joins Mapusaurus, Meraxes and Carcharodontosaurus in having especially sculpted nasal bones over the maxillary region, specifically bearing deep, generally parallel-sided grooves crossing transversely over the dorsal surface and vertically on the lateral face (Coria and Currie 2006).

The right nasal of Mapusaurus rosae, one of the better-illustrated examples of the crazy rugosities developed on these bones by some carcharodontosaurids. A shows the lateral view, B is dorsal, from Coria and Currie (2006).

These bones cannot be described as forming narrow crests as they can for Allosaurus and kin because their texturing meets in the middle of the skull and they are not pinched into long, narrow fins (Sereno et al. 1996; Coria and Currie 2006). Accordingly, some common artistic interpretations of these structures as supporting crests or a series of hornets over carcharodontosaurid faces (which I first assumed when embarking on this painting project, I think incorrectly: see below) may be erroneous: whatever skin made these features extended over the entire dorsal surface of the nasals as well as across the upper lateral region of the snout. Exactly what’s happening here is unusual among theropods, but the rugosity depth almost certainly implies some extensive cornificiation. I'll go further to say that, to the best of my knowledge, deep, subparallel grooves are uniquely associated with cornified pads growing at shallow angles to the underlying bone (Hieronymus et al. 2009). If correct, might we infer that heavy, thick bars of densely keratinised tissue adorned the top of carcharodontosaurid skulls? Cornified pads are predicted in this region elsewhere within Theropoda (e.g. within abelisaurids: Delcourt 2018) so such a suggestion isn’t entirely without precedent, but I'm not sure we've viewed carcharodontosaurids with such heavy ornament before. It would be great to see some actual research on this to investigate what’s really going on with these bones. Giganotosaurus striding around with fat cornified pads atop its face would be all sorts of awesome, especially given that we already think other regions of carcharodontosaurid faces might be adapted for headbutting (e.g. Sereno and Brusatte 2008; Cau et al. 2013).

An earlier version of my Giganotosaurus reconstruction with individual hornlets above the snout: thinking again on this topic, I probably got this wrong as the nasal texturing isn't consistent with the bones that underly hornlets in living species. I've already sent myself to bed without dinner as punishment.

The corrugated nasals are bordered posteriorly by further rugosities around the orbit. This is actually one of the better-known parts of the Giganotosaurus skull and it has been illustrated (Coria and Salgado 1995) so we can be pretty confident about what this region generally looked like, even if a lack of a comprehensive description means it’s difficult to know exactly what sort of rugosities it bears. In terms of basic structure, a rounded, horn-shaped process sits atop the lacrimal (the bone in front of the orbit) and a prominent boss projects above and somewhat laterally from the postorbital (the bone behind the eye). As seems typical for carcharodontosaurids and, indeed, for carcharodontosaurians in general, the latter slopes back and downward somewhat such that Giganotosaurus and kin probably looked perpetually worried, their postorbital bosses creating the appearance of a furrowed brow. 

From what I can gather, the ultra-rugosity of the nasal bones doesn’t extend fully over the eyes in Giganotosaurus or its relatives. I suspect, based on what we see in better-illustrated carcharodontosaurids, that this reflects adornment of the lacrimal process with a cornified sheath rather than a pad. This creates the potential for a sharper horn than implied by the underlying bone shape, although it just as easily could be an exaggeration of the relatively blunt underlying bone structure. As is widely known in palaeoart circles, it can be difficult to predict the exact shapes cornified sheaths will take, even in modern species (Angst et al. 2020).

The postorbital boss variation of carcharodontosaurids, as illustrated by Sereno and Brusatte (2008): A shows the simpler morphology of Eocarcharia dinops; B shows Carcharodontosaurus saharicus.

Similar textures seem to have extended continuously onto the postorbital boss in derived carcharodontosaurids, such that we might imagine a continuation of the sheathed skin of the lacrimal onto this region (Coria and Currie 2006; Sereno and Brusatte 2008; Canale et al. 2022). There is, however, some variation of boss morphology within the clade in that some species have relatively smooth, rounded bosses (e.g. Sereno and Brusatte 2008; Cau et al. 2012): this is another area where more information specifically on Giganotosaurus would be welcome. For those species lacking pronounced texturing, I wonder if we’re dealing with big scale correlates rather than a surface covered with thick, densely keratinised tissue? These skin types may not be mutually exclusive however, as there is precedent for scale correlates showing signs of cornification in some dinosaurs (Hieronymus et al. 2009). A scaly postorbital boss in a young animal could well develop into a more cornified, horny structure in an adult. Again, more specimens of different growth stages might be needed here to be certain of true differences between species.

The result of all this noodling: Giganotosaurus looking a little more knobbly than usual, and also a bit world-weary thanks to that postorbital boss. Maybe the pressure of the "who's the biggest theropod" competition is pretty intense for these guys.

Putting all this together resulted in the image of Giganotosaurus that accompanies this post. Thanks especially to the big cornified pad bridging the middle of the skull, this is a face that looks more heavy-duty than we’re used to and maybe less generically “allosaurian”. But unfamiliar as it is, I’m happy with this outcome because following evidence to unexpected results is one of the great joys of palaeoart, and I always enjoy rationalising an unusual reconstruction from a foundation in science rather than mere speculation. But, again, I want to stress that this is just my interpretation of information gleaned from a less-than-ideal representation of Giganotosaurus in technical literature. This means I may have made errors obvious to those more experienced with these fossils and, moreover, when the structures discussed here are finally studied for their soft-tissue significance, the outcomes may be very different.

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