Teeth can fit inside a closed mouth
Could the teeth fit behind a closed T. rex mouth, and what suggests that tissue actually covered them? Proportions address the first question; tooth surfaces and facial bones help test the second. Together they support a lizard-like, scaly margin, although no preserved T. rex lip has settled the reconstruction.
Here, “lips” means tissue extending beyond the jaw bones and covering the teeth. It does not mean a human mouth or the muscular expressions of a snarling dog. The researchers’ explanation of the proposed reconstruction uses lizard-like, scaly coverings as the comparison.
Two questions should stay separate: could the teeth fit, and what evidence suggests that they were actually covered? Showing that coverage is possible removes an objection. It does not, by itself, reconstruct the face.
Why living relatives give different clues
Crocodiles and alligators make an understandable starting point. They are living relatives of dinosaurs and retain exposed teeth when their mouths close. Birds, the surviving dinosaur branch, have beaks rather than a comparable toothed mouth. Neither supplies a living example of the exact anatomy being reconstructed.
As the 2023 research team’s account explains, comparisons with other reptiles test alternatives to copying a crocodilian face. Relatedness guides which animals to examine; it does not establish that every feature remained unchanged along each branch.
The same distinction appears in the evidence that birds are living dinosaurs. A family tree describes relationships, while a reconstruction must account for the particular animal’s anatomy. T. rex was neither an enlarged crocodile nor a monitor lizard wearing a different skeleton.
Giant teeth need a proportional comparison
The 2023 Science paper by Cullen and colleagues compared tooth crown height with skull length in theropods and monitor lizards. Their scaling patterns were similar. A crocodile monitor in the sample had proportionally taller teeth than the sampled T. rex while retaining covered dentition.
That finding addresses the claim that tyrannosaur teeth were simply too large to hide. The relevant measurement is the crown relative to the head, not the total length of an isolated fossil tooth, including its root.
An everyday drawing mistake illustrates the problem: placing the whole fossil outside the jaw makes the mouth impossible before any soft tissue is considered. Part of the tooth belonged within the socket. Even the portion above the bone need not have been exposed all the way to its base.
A tooth records a particular history
The same Science study examined microscopic tooth structure. Its detailed comparison included a tooth from Daspletosaurus, a tyrannosaur relative. Enamel was similarly preserved on its inner and outer surfaces, contrasting with pronounced outer-surface wear in the alligator comparison. The authors interpreted this as support for protection inside surrounding tissues.
That is evidence about a specimen, not a preserved lip. Growth lines also require care: hundreds of daily lines record tooth formation, not necessarily the same number of days functioning above the gums.
A 2026 review of tyrannosauroid dentition describes widespread evidence of tooth wear, including facets, microscopic scratches and damaged surfaces. Tooth position, age and use can all matter when interpreting a surface. “Covered teeth” should therefore never become “teeth that experienced no wear.”
The useful test concerns the pattern and character of damage, with comparative samples large enough to distinguish competing explanations. A dramatic photograph of one tooth cannot supply every part of that comparison.
Replacement was a process, not an instant reset
The dentition review also describes cyclical replacement, with new teeth developing in the jaws. Estimates for adult T. rex include a replacement interval of roughly two years at an individual position, rather than a new tooth appearing immediately whenever needed.
This matters to the argument without deciding it. A replaceable structure still needs to function between replacements. Equally, replacement history can influence which tooth a researcher samples and how its wear is interpreted. The presence of replacement teeth cannot, by itself, tell us whether lips existed.
Small openings produce a real disagreement
Jaw bones contain openings, called foramina, that carried nerves and blood vessels toward facial tissues. Their distribution can provide clues to what was present above the bone. The 2023 team interpreted theropod patterns as compatible with a lizard-like mouth margin.
But a 2025 description of the T. rex type specimen by Thomas Carr challenges the simple version of that account. Carr counted six rows of foramina on the specimen’s upper-jaw bone and noted their wider distribution over tyrannosaur snouts. He rejected describing the pattern as a sparse, linear arrangement.
This objection concerns actual T. rex anatomy. It deserves a place beside the evidence favoring coverage, rather than being treated as a preference for a different-looking dinosaur.
Its scope is also specific. More rows of openings challenge one proposed anatomical indicator; they do not directly reveal a mouth edge or independently dispose of tooth-proportion and wear comparisons. A persuasive reconstruction has to explain the whole combination.
What a careful reconstruction can show
A lipped T. rex is a defensible interpretation of several kinds of evidence. Its exact mouth outline, thickness and flexibility remain less certain. Illustrations should communicate that difference between general structure and detailed contour.
When assessing a new claim, ask whether it supplies preserved tissue, another comparative specimen or a different interpretation of an existing indicator. Those discoveries can all be useful, but they answer different questions. The teeth were formidable either way; whether a closed mouth displayed them remains a question about a functioning animal around the bones.
