Something About Overbites

Some animals have overbites. it’s fairly common enough that animals (and humans) are born where the upper and lower dentition do not precisely match. Sometimes this alignment can be severe and affects diet. Other times, it is hardly noticeable. But a good number of animals have natural overbites, such that the upper jaw is always longer than the lower, and in some animals this disparity can be extreme. In these animals, the rostrum is much elongated relative to the mandible, and in many cases bears teeth that do not enmesh or engage at all with those of the mandible. Some dinosaurs have overbites, and it is a question worth asking about whether some of these overbites are natural or not. It helps to look at other animals with overbites to consider the variation. The biggest cases include marlin and other billfish, other fish including the prow-nosed Aspidorhynchus, the “eurhinosaurine” ichthyosaurs (Eurhinosaurus, Excalibosaurus), the proterosuchid and riojasuchid archosauriforms (such as Archosaurus rossicus). Many birds have a natural overbite merely because the upper bill is hooked and thus relatively elongated compared to the mandible, such that the jaw cannot extend further rostrally, and this is especially true in the “raptors” (eagles, owls, vultures, etc.)  but also in parrots, in which group it reaches its most severe length discrepancy.

Many mammals have an overbite as well, and the reasons for this have nothing to do with deformity, but because of certain mechanical advantages having a shorter jaw confers. It is more important to note that these advantages are not always related to the specific condition of having a shorter jaw, but due to the orientation of the teeth. Rodents are a good case for extreme overbites, more so than in lagomorphans (rabbits, pika, etc.), and in some rodents this is very, very severe. Rodent overbites, as noted, confer great mechanical advantage in having the ability to apply vertical shear forces without either excessive vertical movements of the head or vertical movements of mandible, aided in part by the presence of ever-growing incisors in both jaws, and by propalinal translation of the mandible, where the jaw moves forward and back on a mobile jaw joint rather than merely up and down (orthal jaw movement).

Skull of the chinchilla, Chinchilla lanigera (Bennett, 1829); jaw shown in partial depression. 1, Mandible in the protracted position; 2, mandible in the median, adducted position; 3, mandible in the retracted position. Positions do not reflect the extreme of positions. Arrows indicate direction of motion in propalinal bite stroke.

Many rodents are also fossorial or, at the least, inhabit small, cramped spaces and feed in these places. The utility of a propalinal bite with limited vertical clearance required, horizontally oriented, rostrally-projecting mandibular teeth means the effective length of the jaw is much longer than the actual bony length of the jaw. Not all overbite-bearing animals are so lucky.

Take Proterosuchidae. The mandible in proterosuchids extends only about a single tooth position longer than the maxilla, while the premaxilla extends further and thus overshoots the lower jaw. The largest teeth of the jaw are in the premaxilla, which is oriented subvertically rather than lengthwise.

Skull of Proterosuchus fergusi Broom, 1903. Jaw in the open and closed positions, respectively.

Bizarrely, the premaxillary dentition point caudally, and like spinosaurid theropods there is a small diastema between premaxillary and maxillary dental series. These teeth do not engage with the mandible: the articular cotylar surface of the mandible is restricted, preventing extensive movement to allow the dentition to engage on both sides. So what happens with that snout? If, as the bone surface suggests, it would have been surrounded by thick tissues on both upper and upper jaw, was there an upper lip, a gap, and the lower jaw had a band of tissue that, as in lizards, conceals the upper teeth from view when the jaw is closed? Yet, the premaxillary teeth would still be exposed. Or … perhaps … the soft tissue of the lower jaw was extensive enough to wrap around the entire upper dental series. This is easier to show with Tyrannosaurus rex:

Outline of soft tissue around a generalize Tyrannosaurus-ish tyrant, with bone in white and shaded, internal cavities in dark grey, and soft tissue in ochre. The jaw is illustrated in different lengths, to reflect the variable length of the mandible as preserved in known Tyrannosaurus rex specimens. The left side shows a silhouette of the head with a fully developed oral integument, and the right side shows the oral integument in sagittal section, and the length of the lower “lip.” A, jaw long enough that the first dentary tooth (d1) is positioned medial to the last premaxillary tooth (pmx3); B, jaw is shorter,, with d1 medial to mx2; C, jaw very short, with d1 medial to mx4.

It’s clear, at least, that amongst tyrannosaurid specimens, the length of the mandible is variable; not all specimens have mandibles that extend most of the length of the upper dental series, the anteriormost teeth often produce marginal or apical wear facets, being features that show the usually keel-like or serrated margins of the teeth are worn down, smooth, and similar to the surface of the rest of the tooth, and a small rounded or distal region where the enamel has been eroded away. It is not clear that this wear is produce by consumption of abrasive foods, or through thegotic behavior (tooth-tooth wear), but the latter is responsible for rodent incisor and hadrosaur/ceratopsian battery wear. In more posterior maxillary/dentary teeth, wear facets often form long facets or extreme fracturing (Schubert & Ungar, 2005), and these are the teeth which, in a macrophage like Tyrannosaurus rex, would be more likely responsible for heavy-duty feeding — bone processing, especially. This is also true in the marine macrophage Dakosaurus (Young et al., 2012), and even macrophagous whales such as Orcinus orca (Thewissen et al., 2011), in which the teeth are very large throughout the dental series and interlock finely rather than arranged into an outer and medial row.

Recall this seminal post on “lips” in theropods. I will give readers a chance to refresh yourself on it, or read it for the first time.

Okay, back?

Tyrannosaurus rex, generalized from diverse specimens, with soft-tissue outline modified relative to position of the mandible. A, rostral position; B, median position; C, posterior postition. 1 in all images shows ventral view of the rostrum and ventral view of the mandible in ventral view (above) and dorsal (oral) view (below). Red arrow indicates position of first dentary tooth.

Tyrannosaurids, like dromaeosaurids, have little room for an extra thick “lip” around the mandible that would fit between the teeth in some way. And given that thegotic wear surfaces are present, it is unlikely that the teeth wouldn’t exhibit some thegotic processes while biting. So there’s space between the teeth, but that space is very, very slim and isn’t filled with fake, illogical tissue structures that don’t appear in any other group of toothy sauropsidan. It’s either there (lizards, snakes, tuataras, monitors, amphisbaenians), or not there (extant crocodilians). Those extant sauropsidans without teeth (turtles, birds) have unique tissues (rhamphothecae) that render the issue moot about what they might have “in there.” Well, paleoart is rife with a sort of “halfway” point of art, where there’s a little bit of “lip,” but it only covers the base of the upper teeth, and there seems to be some teeth on the lower jaw covered by a similar tissue … but between the teeth of both upper and lower series. Recall, there’s not a lot of space there.

Greg Paul, whose art I cannot show here, is the greatest of these perveyors of the “half-lip” style, but it is not unique to his work: Charles Knight did this for Dimetrodon, as you can see here. But one of the most interesting things about Charles Knight’s work is that when theropods were drawn or modeled with their jaws closed, no teeth are visible. Extensive oral tissue is available. Even more striking, as the hypothesis supporting the idea hasn’t been formulated yet, Chas Knight also illustrated ornithischians with a divided “cheek,” effectively fleshy “lips,” and lighting details make the impression of lose, extraneous oral tissues unambiguous.

“Lip” extent in tyrannosaurs. Top, fully encompassing oral tissues, with an upper and lower band meeting lateral to the teeth. Middle, “malf-lip,” with bands of oral tissue around part of the upper jaw, but exposing the teeth. Bottom, the “croc” model, without bands of oral tissue. The top one is favored on this site based on present (circumstantial) data.

What, exactly, does this have to do with overbites? When the jaw margin has extensive tissue on the lower jaw that encloses a gap between the oral epithelium that covers the teeth and lines the sides of the mandible, as in lizards and snakes, this tissue isn’t merely on the sides: it wraps around the front with a band of thicker, ligamentous tissue, and thus teeth that are positioned rostral to the mandible are enclosed by this tissue, just as the rostral “beak” in tuataras is hidden within soft tissue and scales of the oral margin — and is likely the case in the more extremely “beaky” Paleopleurosaurus poseidoniae (Carroll, 1985), in which the fused dentition-premaxillae extend into a blade to at least the level of the ventral margin of the jaw when the jaw is closed. If, like other sphenodontians (tuataras and kin) these animals have their jaws swaddled in soft-tissue, you’d never see this.

I made that case for Daemonosaurus, Epidexipteryx, and Incisivosaurus, but it’s true for animals with overbites because, as it turns out, they’re just another version of overbite. Large rostral upper teeth projecting beyond the lower jaw or the upper jaw being a bit longer than the lower and the teeth being very large still require similar tissues, and without strong evidence for the absence of these tissues, it is likelier that the null hypothesis (our asusmption that all extinct sauropsidans should be first assumed to have tissues like most or phylogenetically bracketing encompassing extant taxa).

In fish with overbites, the upper jaw typically doesn’t have teeth that don’t fit with the other jaw: in billfish, the jaws are toothless; in Aspidorhynchus, the pointed rostrum isn’t toothed, and only the region of the jaw that opposes the mandible directly shows a set of teeth to match. In sawfish (Pristidae, Sclerorhynchidae, both types of ray) and sawsharks (Pristiophoridae, squaloid ray-like sharks) the marginal rostral “teeth” are modified dermal denticles and do not resemble or function the same as the oral dentition, which forms a complete aupper array within the mouth, and thus cannot be considered to have an “overbite.” Ichthyosaurs have upper and lower jaws of matching length, but in most when there are teeth they match in extent as well; but in Eurhinosauridae (Eurhinosaurus, Excalibosaurus) both jaws are fully dentate to their tips and involve teeth that certainly must have interlocked to form a “typical” piscivorous array, except as the rosum extends further from the mandible the teeth just keep going with it. It remains a mystery why, and no, I don’t think there was some saggy flesh that would have met the upper jaw when the jaw closes because … well, that’d be stretching things.

Some fish have a pronounced underbite, being veritable Hapsburg fishes, especially the Saurodontidae (a subgroup of Ichthyodectiformes, which includes the fabulously awesome Xiphactinus) whose teeth have been commonly associated — and confused — with spinosaurid teeth (and pterosaurs, in three cases: Aidachar paludalis and Sultanvaisia antiqua Nessov, 1981, then redescribed by the author, as discussed in Mkhitaryan & Averianov, 2011; and Gwawinapterus beardi, described as an istiodactylid by Arbor & Currie, 2011, but redescribed as an ichthyodectiform maxilla by Vullo et al., 2012), but the element that extends forward, often called a “predentary,” is sawed and denticulated, isn’t toothed, and isn’t even a part of the normal tooth-bearing bone as its name suggests.

But these aren’t the only “overbitten” animals. Some sauropsidans have overbites, but like in hook-beaked birds, I don’t argue they had “lips.” Those would be ornithischians. So I close this post with a repost of this illustration of a hadrosaur, showing off its marvellous overbite.

Almost as cool as Freddy Mercury.

Arbour, V. M. & Currie, P. J. 2011. An istiodactylid pterosaur from the Upper Cretaceous Nanaimo Group, Hornby Island, British Columbia, Canada. Canadian Journal of Earth Sciences – revue canadienne des sciences de la Terre 48 (1): 63-69.
Carroll, R. L. 1985. A pleurosaur from the Lower Jurassic and the taxonomic position of the Sphenodontia. Palaeontographica, Abteilungen A 189 (1-3): 28.
Mkhitaryan, T. G. & Averianov, A. O. 2011. New material and phylogenetic position of Aidachar paludalis Nesov, 1981 (Actionopterygii, Ichthyodectiformes) from the Late Cretaceous of Uzbekistan. Proceedings of the Zoological Institute of the Russian Academy of Sciences 315 (2): 181-192.
Nessov, L. A. 1981. [Flying reptiles of the Late Cretaceous of the Kyzylkum [Desert].] Палеонтологический Зурнал – Paleontological Journal 4: 98-104. [In Russian]
Schubert, B. W. & Ungar, P. S. 2005. Wear facets and enamel spalling in tyrannosaurid dinosaurs. Acta Palaeontologica Polonica 50 (1): 93-99. [PDF]
Thewissen, J. G. M., Sensor, J. D., Clementz, M. T. & Bajpai, S. 2011. Evolution of dental wear and diet during the origin of whales. Paleobiology 37 (4): 655-669.
Vullo, R., Buffetaut, E. & Everhart, M. J. 2012. Reappraisal of Gwawinapterus beardi from the Late Cretaceous of Canada: A saurodontid fish, not a pterosaur. Journal of Vertebrate Paleontology 32 (5): 1198-1201.
Young, M. T., Brusatte, S. L., Beatty, B. L., de Andrade M. B. & Desojo, J. B. 2012. Tooth-on-tooth interlocking occlusion suggests macrophagy in the Mesozoic crocodylomorph Dakosaurus. The Anatomical Record 265 (7): 1147-1158.

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