Theropod Facial structure theory
I have been trying to write this paper out so many times and the lack of funds to hire an editor/publish as well as getting this organized in a fashion that's considered proper for a scientific paper has caused me to hit so many walls, not including the learning disabilities that have made this worse to get this out. So i'm doing this to get what information i've been gathering since 2018.
looking at young corvid images of their open mouth, they had a lip area in their mouth that the upper beak would overlap and lay in perfectly, this is what started me down the road along with noticing the formina on the sides of theropod faces matched that of goose skulls I saw online. As I had no access to actual specimens in person.
feeling as though waterfowl specimens were the easiest to obtain(they were actually more difficult than thought) as well as being closer to dinosaurs on the evolutionary tree next to ratites, and even living along dinosaurs during the late cretaceous ie presbyornis, they seemed to be the best to study for this.
After obtaining a pair of peking duck and an african goose, from some local farmers that donated them, they were dissected and preserved using techniques learned from taxidermy for study. The specimens had one side of their bills removed while leaving the skin intact on the otherside. One of the peking ducks kept the tissue on both sides, while only one side had the top layer of the epidermis removed, exposing the harder dermis underneath. A noticeable attribute was that before preservation the bills were plump and didn't show any texturing, except for the goose around the back nearest to the corner of the mouth. Once preserved however, the skin on the bills shrank, revealing grooves and texturing where nerves and blood vessels would run.
The sensory organs and nerves that ran in the bills were observed using a candlelight method/ie holding them up to the light.
Tyrannosaurus Juvenile Jane specimen for comparison
The deep grooves and formina on the bill, as well as the formina and grooves that are seen in T.Rex have many similarities, which was the main focus of this study as they're are numerous studies pertaining to their sensory organs. Also something to compare is that there is a distinct row of formina that's more prominent on older crocodiles and alligators along the dentiary similar to that of T.Rex, on both the top and bottom row of the mandibles.
saltwater crocodile head from crocodiledarwin.com and alligator from encyclopediaofarkansas.net
komodo dragon profile view(comparing formina)
Though waterfowl having lamillae cannot be used to compare enamel ware. While crocodiles and alligators have teeth, but are known for not having lips, have ware and tare on their teeth than animals like monitor lizards that have lips as shown in this study using a relative of Tyrannosaurus rex to prove they had some form of lip to protect their tooth enamel.
Another observation is that in alligators the bottom row of dentary is tucked under the upper mandible and covered more than in crocodilians. It would be interesting to see the tooth study done with other teeth on both animals, as some of the posterior dentary on the upper mandible can sometimes be seen tucking away in a pocket of skin as protection in some alligators.
Also looking at neonatal alligators shows a depression in the lower jaw area that the upper dentary could sit in, almost as though their reminants of lips
neonatal alligator from natgeo on alligator facial mechanoreceptors
and while alligators and crocodilians seem to have scaling on their face this is just thickened sensitive skin with cracked texturing imitating scales. Bills on waterfowl are also thickened as this allows them to forage in merky waters and plants without damaging their bills. While both animals have thickened skin on their manidbles, it is still a very sensitive part of their body that they use to navigate and explore/manipulate their environment as well as interact with other of their species. While reptiles have less sensitivity and do not share these organs along their faces(the skin sense organ of some iguanian lizards). This could be also a lack of information on the anatomy of reptile skulls and facial structures that could be expanded more for comparison.
However there was a study done on the facial sensitivity of Tyrannosaurus along with alligators
See Complex Neurovascular system in the dentary of Tyrannosaurus
See Despite their thick skins, alligators and crocodiles are surprisingly touchy
and while it took awhile to find and make available, there had been a paper written on a very similar organ within the bills of waterfowl, namely mallard ducks.
See The Morphology and Distribution of Cutaneous Mechanoreceptors(Herbst and Grandry Corpuscles in Bill and Tongue of the Mallard)
And while Monitor lizards and other reptiles share a similar formina pattern, they lack the textured grooves and sensitive organs archosaurs and certain avians share, as well as other anatomical features of moniotor lizard skulls that they share more closely with other reptiles and snakes than they do with the archosaurians and avians.
komodo dragon skull ventral view
Alligator(left) Crocodile(right) ventral views of skulls
Peking Duck skull with bill skin(ventral view)
another observation in many reptile skulls is how the upper and lower dentary interact. While some had teeth that overlapped the lower mandible, there seemed to be somewhere that the teeth met, either at the anterior end of the skull or continually all around, in monitors there was some sort of gap at the anterior portion of the dentary to make way for the tongue to be able to slide in and out without being obstructed too much while allowing the mouth to stay closed. Where as archosaurs and avians the two overlapped one another allowing for a scissoring affect with the teeth, the most noticeable being the anterior end of the mandibles. And while waterfowl lack the dentary that archosaurs and reptiles have, the lamillae still slide past one another similar to what's observed in many theropods, even in non avian dinosaurs. The thought of bone to tooth contact is considered as well, and there can be a sizeable closure without the need for the amount of soft tissue observed in reptiles.
proposed crocodilian like contact of tooth on mandible(top), proposed soft tissue similar to that of monitor lizards(bottom left), new proposed soft tissue without scaled lips(bottom right)
Having soft tissued lips that is similar to that of a waterfowls bill allows for the mechanosensory corpusicules to interact as well as still allows for protection of the tooth enamel.
The way their upper dentary line up with their lower mandibular formina seems to follow the same pattern, compared to reptiles that didn't curve upwards towards the anterior portion of the lower mandible. There's also a possibility of having a nail, like waterfowl, or some large scale, like many other reptiles with dentary, at the anterior ends of the upper and lower manible. This is due to the amount of formina clustering in a pattern like structure similarly seen in both waterfowl and reptiles.
These observations and studies allowed for an idea of how to reconstruct Tyrannosaurus Rex
Juvenile Jane Tyannosaurus Rex skull, with open and closed mouth(top) and reconstructions
Even constructing a physical reconstruction to better observe how the lips and teeth would interact.
Paper mache head reconsturction done in a similar manner to the duck and goose specimens used in this study
Also when reconstructing, and also accounting for tooth slippage, there was an issue with a sizable gap at the anterior end with the front dentary when closing the jaws when attempting to imitate the mouth closing similar to that of previous reconstructions using reptile lips, where as with the more waterfowl skin reconstruction there was less of an issue and also less of an overhanging bottom lip.
Pages from personal study notes
There was also the issue of either exposing the jaw muscles similar to how crocodilians, alligators, and other reptiles are observed to have, or to cover it in skin like birds. One of the arguments is that the mouth wouldn't be able to open to the extent allosaurians and tyrannosaurus are able to open their mouths. When there are several avian species with skin over the area that can very well open their mouths just as wide, ie Bell Birds, which have a widened jugal bone like that of tyrannosaurus.
Young tyrannosaurus specimen(chomper)
Also to note, in most reconstructions where there is a bony knob on the jugal, circled in the above example, is more than likely a muscle attachment point, and the divet in the space before the attachment point could allow for the corner of the lips to sit.
In conclusion, Tyrannosaurus Rex and possible other theropod dinosaurs with similar skull anatomy of the upper and lower mandibles more than likely had thickened yet sensitive skin with a lip like structure similar to that of waterfowl and not a scaled skin.
Additional research papers on duck bill anatomy/mechanreceptors
Histomorphogenesis of Upper Beak in Muscovy Ducks (Cairina moschata)
3D architecture and a bicellular mechanism of touch detection in mechanosensory corpuscle
Molecular basis of tactile specialization in the duck bill
Personal Notes on research
