Lips of Tyrannosaurus rex

[Vladimír Socha]

Good day to all listmembers!

I was wondering what is the current prevailing opinion on the topic of whether T. rex and other large tyrannosaurid theropods had lips, fully covering their teeth. I guess the well known 2023 study was definitely not the last word on this issue, as Thomas Carr and some other paleontologists were not convinced by it (and many uncertainties clearly remain). It's also interesting that Robert Bakker drew a substantial fleshy lips on T. rex's head back in 1986 already. Thank you for your thoughts! VS.

References:

• Cullen, T. M.; et al.
 (2023).

Theropod dinosaur facial reconstruction and the importance of soft tissues in paleobiology.Science.379:1348–1352.

Carr, T. D.; et al. (2017). A new tyrannosaur with evidence for anagenesis and crocodile-like facial sensory system. Scientific Reports. 7: 44942.

Carr, T. D. (1999). Craniofacial ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria). Journal of Vertebrate Paleontology. 19: 497–520.

Molnar, R. E. (1991). The cranial morphology of Tyrannosaurus rex. Palaeontographica Abt. A Paläozool. Stratigr. 217: 137–176.

Paul, G. S. (2018). Non-ornithischian dinosaurs probably had lips. Here’s why. Prehistoric Times. 127: 44–49.

Ford, T. L. (1997). Did theropods have lizard lips? in Southwest Paleontological Symposium – Proceedings (Southwest Paleontological Society and Mesa Southwest Museum, 1997), pp. 65–78.

Bell, P. R.; et al. (2017). Tyrannosauroid integument reveals conflicting patterns of gigantism and feather evolution. Biology Letters. 13: 20170092.

Bouabdellah, F.; Lessner, E.; Benoit, J. (2022). The rostral neurovascular system of Tyrannosaurus rex. Palaeontologia Electronica. 25 (1): a3.

Meers, M. B. (2003). Maximum bite force and prey size of Tyrannosaurus rex and their relationships to the inference of feeding behavior. Historical Biology. 16 (1): 1–12.

Erickson, G. M.; Lappin, A. K.; Vliet, K. A. (2003). The ontogeny of bite-force performance in American alligator (Alligator mississippiensis). Journal of Zoology; The Zoological Society of London. 260 (6): 317–327.

Snively, E.; Russell, A. (2007). Craniocervical feeding dynamics of Tyrannosaurus rex. Paleobiology. 33 (4): 610-638.

Bates, K. T.; Falkingham, P. L. (2012). Estimating maximum bite performance in Tyrannosaurus rex using multi-body dynamics. Biological Letters. 8 (4): 660–664.

Bates, K. T.; Falkingham, P. L. (2018). Correction to ‘Estimating maximum bite performance in Tyrannosaurus rex using multi-body dynamics’. Biology Letters. 14 (4): 20180160.

Gignac, P. M.; Erickson, G. M. (2017). The Biomechanics Behind Extreme Osteophagy in Tyrannosaurus rex. Scientific Reports. 7. 2012 (2017).

Peterson, J. E.; Tseng, Z. J.; Brink, S. (2021). Bite force estimates in juvenile Tyrannosaurus rex based on simulated puncture marks. PeerJ. 9: e11450.

Rowe, A. J.; Snively, E. (2021). Biomechanics of juvenile tyrannosaurid mandibles and their implications for bite force The Anatomical Record. 305 (2): 373–392.

Johnson-Ransom, E.; et al. (2023). Comparative cranial biomechanics reveal that Late Cretaceous tyrannosaurids exerted relatively greater bite force than in early-diverging tyrannosauroids. The Anatomical Record. 307 (5): 1897–1917.

Wroe, S.; McHenry, C.; Thomason, J. (2005). Bite club: comparative bite force in big biting mammals and the prediction of predatory behaviour in fossil taxa. Proceedings of the Royal Society B. Biological Sciences. 272 (1563): 619–625.

Erickson, G. M.; et al. (2012). Insights into the Ecology and Evolutionary Success of Crocodilians Revealed through Bite-Force and Tooth-Pressure Experimentation. PLOS ONE. 7 (3): e31781.

[Jaime Headden]

As you say, those are the latest words.

The arguments come down to constraining to the most limited extant phylogenetic bracket (extant birds, extant crocs) while appealing to certain crocodilian or avian style facial features; versus a broader EPB with appeal to general constraints on facial tissue and the actual lack of data regarding how dental exposure relating to dessication (which might be a red herring) cares about all this.

There may be room in the middle of this to have one's cake and eat it, too. It's not like people necessarily want to deprive tyrannosaurs of lips. We have sufficient evidence for the fact that thrannosaurids, T. rex especially, to have more crocodilian facial anatomy, in comparison to, say, Allosaurus. In contrast, the evidence for a limited EPB constraining tyrannosaurs would have to apply to all dinosaurs, pterosaurs, etc. which would require a far more nuanced application of this argument. The same features of the skull that might suggest that croc-like faces in tyrannosaurs don't necessarily apply the same way to dromaeosaurs, troodontids, or even many basal ornithischians.

And that's the kicker: if there's no good supportive evidence for fleshy, mammalian cheeks (maxilla-dentary muscles not confined to the adductor complex) and they lacked lips, then we'd be supporting a croc-like face for them, too, with exposed teeth. Indeed, there was preliminary work suggesting this that never got out of the conference level. The short of it is, with ankylosaurs and ceratopsians having "horn-faces," much as might be present in tyrannosaurs, there was a possibility that the faces (but not the jaws in most) were so heavily cornified as to suggest a beak-like sheath across the entire skull. New data from Sharpe et al. now adds evidence for this for some hadrosaurs.

This is exciting stuff, and with care we might see a synthesis of various competing views converge towards agreement on some fundamental answers.

Cheers,

Jaime A. Headden

The Bite Stuff: http://qilong.wordpress.com/

"Innocent, unbiased observation is a myth" - P. B. Medawar (1969)

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[Lucas Matias Castro]

Recently this year Carr also described another error or misinterpretation from Cullen and Witton regard to the "foramina argument", which as been stated before and online but is the first time reported after the lips paper. 

" 10.3. Number of lateral maxillary foramina rows
Cullen et al. (2023) claimed that T. rex had a single row of neurovascular foramina extending along the ventral margin of the lateral surface of the maxilla. However, six rows that penetrate the maxilla of CM 9380 were counted here (Figure 4B), and multiple rows are typical of Tyrannosauridae in general. Also, the distribution of neurovascular foramina in tyrannosaurids extends across the dorsum of the snout, which is more extensive than that seen in living crocodylians (e.g. Alligator mississippiensis) where dorsal rows in the nasals are not seen. Therefore, the pattern in tyrannosaurids cannot be described as ‘low density, linear pattern’ (Cullen et al., 2023: 1349). In both clades, the largest foramina follow the ventral margin of the upper jaw, whereas those piercing the rest of the snout are smaller in diameter (Cullen et al., 2023: Figure 2C,E)."

Carr, T. D. (2025). Observations on the skull of the type specimen of Tyrannosaurus rex Osborn, 1905. All Earth, 37(1). https://doi.org/10.1080/27669645.2025.2539638

Also this year on a conference it was reported an Allosaurus hatchling which had the same corrosion on his teeth than digested anolis lizzards (and some fossil lacertids). But that alone doesn't say the type of oral tissue but to some degree the extension of the gums on that hatchling

Malafaia, E., Maggia, B., Rauhut, O. 2025. Taphonomic insights from a corroded maxilla of a hatchling Allosaurus from the Upper Jurassic of Guimarota (Portugal). Abstracts Book X Jornadas Internacionales sobre Paleontología de Dinosaurios y su Entorno. 4-6 septiembre. Salas de los IInfantes. Burgos, España. 143-146. https://www.researchgate.net/publication/395850822_Taphonomic_insights_from_a_corroded_maxilla_of_a_hatchling_Allosaurus_from_the_Upper_Jurassic_of_Guimarota_Portugal

Smith, K.T., Comay, O., Maul, L. et al. A model of digestive tooth corrosion in lizards: experimental tests and taphonomic implications. Sci Rep 11, 12877 (2021). https://doi.org/10.1038/s41598-021-92326-5

[Thomas Cullen]

I don't like commenting on social media and message boards, so I’ll keep this brief, but I do think it important to clarify that what you've noted above is not an 'error or misinterpretation' on our part.

Contrary to that claim, we actually never wrote that T. rex only had a single row of neurovascular foramina (doubtless an innocent mistake in recollection by Carr and/or others). What we did was compare the general orientation of the foramina along the jaws to various other taxa, and discussed whether the most prominent foramina were clustered linearly vs. being more evenly distributed (alongside the several other lines of evidence we presented). It would be rather foolish for us to say that tyrannosaurids had no other foramina. There are obviously other foramina there, though I’ll admit I’m not sure I follow the exact logic for deciding how to connect the dots on the other foramina present to make them into discrete rows, as he's interpreted in that figure. In any case though, the condition in most living crocodilians differs from what he’s illustrated there for that T. rex, so I can repeat what we already said in our 2023 paper (in the supplementary discussion) and disagree with it implying a crocodilian pattern of facial integument.

Tom

[Jaime Headden]

>  I’m not sure I follow the exact logic for deciding how to connect the dots on the other foramina present to make them into discrete rows

Without referencing the comment and paper in question, I'd like to note the only reason we discriminate "lip holes" in the maxilla, premaxilla, and dentary, is that they are continuous along the dentigerous margin. They're not always discriminated by texture of the surrounding bone, which can be "smooth" or "grooved" (but still in smoother bone), and not always associated with large, angular depressions or pits, or even neurovascular channels (which are some of the grooves mentioned earlier). In fact, the only reason we call it a neurovascular "row" is because they are close to the dentigerous margin. That's in.

In some tyrant skulls, there's a second row (approximately), it's more an arch, corresponding to the arch of the maxillary branch of the trigeminal around or adjacent to the medial (palatal) process of the maxilla. In the Premaxilla, there's less of a regular row, and even the dentigerous-adjacent foramina tend to break apart as a group, as in the rostral dentary. Does the "row" suddenly end at the anterior maxilla and concordant point on the mandible? Is only that portion of the jaw "lipped," but the rest covered in some croc-like cornified integument? Was it a BEAK?! (ahaha no, not serious).

There's also a complimentary ventral row on the dentary corresponding to the mandibular branch of the trigeminal, and at one time this secondary row, which follows Meckel's canal internally, was considered diagnostic in maniraptoriforms? Someone help me on this point. Nevertheless, "rows" are arbitrary, but the only ones I would argue are closer to constrained by other anatomy are those associated with the trigeminal and other osteological features, rather than "the neurovascular canal happens to coincide with the sudden disappearance of bone."

Cheers,

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[Lucas Matias Castro]

"In any case though, the condition in most living crocodilians differs from what he’s illustrated there for that T. rex, so I can repeat what we already said in our 2023 paper (in the supplementary discussion) and disagree with it implying a crocodilian pattern of facial integument."

While i'm being now just  a student, i think it doesn't match either the varanid pattern which anyways is derived even among squamatans and other lepidosaurs. It is just far more.

" Does the "row" suddenly end at the anterior maxilla and concordant point on the mandible? Is only that portion of the jaw "lipped," but the rest covered in some croc-like cornified integument? Was it a BEAK?! (ahaha no, not serious)."

I don't think the  lepidosaurian oral tissue "lips" could be a great explanation, but that is with EPB as you already said on the first message and teeth can be covered with another kind of oral tissue. While you are joking about "beaks" i think the fact true beaks (toothless) or beak like structures evolved many times across all Archosauria (aetosaurs, poposaurids, crocodilomorphs, pterosauromorphs many times, silesaurids, ornitischians, sauropodomorphs, theropods many times) independently and in different ways and portions says something, plus the fact birds and crocodilians don't have scales on their mouths. For example dromaeosaurids have a pattern more similar to keratinizated mouths from  ducks.

Whether exposed teeth or covered teeth, that could only be answered with a new study to dinosaurs mouths articulation with simulations, or on this particular rex's case about the theropod's ectopterygoid, because any can argue if they prefer Ford or Keillor hypothesis but if not tested meh.

Regard the dessication and damage problem, well as you said can be red herring, the lips paper was criticized since the start for the low sample but understandable because the curation problem about cutting things, but that gives some advantage because the contra can't be tested from actual skulls. But related about how dinosaurs closed their mouths, some dinosaurs don't show slippage from their teeth and indeed the teeth can't go more inside the socket because the teeth beneath. Plus the fact toothed pterosaurs didn't seem to have that problem, although some had rapid replace rate as shown this year with a ornithocherid; and anyways if i don't remember bad, work is being done about dinosaurs teeth replace rate being more faster than currently estimated. 

But yeah as you said it is exiting stuff regard archosaur evolution, i hope more people study this and on the next years before i graduate. Or a fossil but since many people do claims about skin dessication, if something appears with fossilizated oral tissue either with scales or keratinizated  the research about dinosaurs mouth articulation would be fundamental. 

[Thomas Yazbek]

I have a question that sort of zooms out from the details of the discussion here. What exactly are the adaptive advantages of a croc-like or lizard-like facial integument? What does a theropod need either of these morphologies for? Tyrannosaurs are not living a lifestyle anything like that of modern crocodilians. I'm not well-versed in the anatomy but I am wondering if one morphology or the other makes more sense for the assumed behavior of T. rex.

Thomas Yazbeck

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[Jaime Headden]

Thomas Yazbek wrote:

> I have a question that sort of zooms out from the details of the discussion here. What exactly are the adaptive advantages of a croc-like or lizard-like facial integument? What does a theropod need either of these morphologies for? Tyrannosaurs are not living a lifestyle anything like that of modern crocodilians. I'm not well-versed in the anatomy but I am wondering if one morphology or the other makes more sense for the assumed behavior of T. rex.

There's been some work on the topic, this time deriving from Lautenschlager's work on osteological correlates to beaks and how they appear. This was done with FEA analysis of strain relief and created a model by which the facial and mandibular skeleton developed closed sutures, or eventually fusion, as a means to alleviate strain, by passing along whole-body strain throughout the skull rather than borne on one singular element thereof. That unequivocal beaks only appear in edentulous portions of the skull is important in this, and it presents ideas we should explore regarding ornithischians with toothed premaxillae, but large predentaries.

I digress. The concise answer is that lizards retain scales even in the most-hard-biting and akinetic-skulled species, and scales are doing things that naked armored dermis would do otherwise.

Crocodilians have a close-knit, closed or fused cranial sutures, but also cranial anatomy that helps relieve strain along the skull and prevents torsional or compression distortion during biting, but given they're all aquatic, that may be relevant. Oreinorostral crocodilians (snouts taller than wide) or crocs with circular snout cross-sections (such as gharials) have different bite mechanics than platyrostral alligators or crocodiles. Still, crocs have a hardened, cornified dermis that may assist in distributing strain through the skin, and may reflect the closure of cranial sutures more than in most other reptiles. One question to ask is why mammals don't produce the same issues, and I think a good answer to that is that somewhere after the synapsid-therapsid split stem-mammals developed external facial muscles that more or less cover the head and prevent the skin from affecting the skull beneath---except in the frontoparietal region, which sees a great deal of adaptation towards display features. It also means less muscle expression in uintatheres, rhinos, etc.

Anyways.

Lizards mostly don't need that: their skulls are generally far more kinetic, and where they're not---especially with hard-biters like tegu, gilas and the like---we see the so-called hummocky texture of the skull roof and close appression of the dermis to the skull and the overlying squamation, which may tend to ossify more readily; the extreme of this is turtles, in which the cranial armor is formed of large fused scales, which includes the "beak," making it quite different from how it's been assumed bird beaks formed (through a hardening of the outermost epidermis, a caruncle). That needed flexibility does things to the skin, especially in snakes, that are less well-known (engulfing snakes have less keratin in their dermis to allow the skin to be more flexible and stretch, something Carl Gans noted and cited for egg-eating snakes in his study on Dasypeltis). A flexible skull correlates with flexible skin, and thus a looser connection between the two. Crocs lack this, lacking cranial scales entirely.

Tyrannosaurids have low-kinetic skulls. They're still somewhat kinetic, with open sutures between several major facial bones. The nasals are interfingered with all adjacent bones (premaxilla being the loosest, interfingering with the maxilla and lacrimal, while connecting to the frontals with an open scarf join). Despite this, the nasals are unusually fused together: they have a small possibly pneumatic chamber within them, and are basically a thick-walled, flattened tube. This assists in the arguments that the facial skeleton of tyrannosaurids, as numerous studies at this point on biting mechanics have supported, have a strong midline "brace" to the skull against which the remaining bones release strain over a close, but open suture into the maxilla, jugal, palate, and the rest of the suspensorium. This mechanism reduces the high strain at the tip of the snout by distributing it into the braincase and thus likely into the neck. The maxilla and jugal complement this with strongly interdigitating components with the lacrimal and postorbital, which has a broad contact with the jugal, and thus complements that midline brace into a pair of kinetic absorbers for mid-jaw and rearward biting. But there'd be more give. It would be more flexible.

Tyrant faces aren't fused, like most terrestrial (but extinct) crocodilian species they're still oreinorostral which confers expectations of their bite strategies, with flexibility in their skulls being of some advantage to prevent strain from buckling the bones themselves, passing through sutures, and of course, across teeth.

Cheers,

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[Gregory Paul]

A research analysis problem is that there has been very little research on the lips and gums of lizards. What is the advantage of those both being so well developed that when a monitor opens its mouth as a threat the upper and lower teeth cannot be seen or maybe just barely some tips? Same for iguanas, etc. (some lizards seem to expose the lower teeth). And what are the thick soft tissues on the palate about? Forming two long roof ridges at least in oras (see my recent field guides for illustrations, including based on an ora head scan, as well as fall 2018 Prehistoric Times). What function does all this serve? I have been assuming that the roof of a dinosaur mouth was basically the palatal elements with a thin covering of soft tissues, but maybe not. The saurischians had lips is pretty much certain. Did they also have the gums? Is there a way to figure that out? Having the lips and gums serves to protect the teeth. And monitors show they do not hinder using the teeth to kill prey and consume carcasses. 

GSPaul

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[Dalton Meyer]

I agree that there is a paucity of research on extant taxa (especially lepidosaurs) that would serve to ground truth a lot of what we might look for in the fossil record. I tend to default to a view of lips (of some sort) for toothed dinosaurs, simply because it seems to be the ancestral tetrapod condition (and is decently conserved).

Regarding the "gumminess" of lizards (and especially varanids), I also wonder about it's informativeness for dinosaurs. However I can't escape the suspicion that the abundance of gum-tissue is helping to support the un-socketed, pleurodont teeth, and potentially may be elaborated in varanids as part of the venom support system. I have a suspicion that thecodont dinosaurs would have been different, but again this requires actual research into the oral tissue of modern lizards.