Incredulous Teeth, I: With Twin Crests

Here begins a new series.

I will be spending some time showcasing the skulls of unusually-dentitioned animals (or showing the peculiar jaws of these animals, specially if they lack teeth). The first of these is Dilophosaurus wetherilli.

The recovery of the specimens that would later form the basis of Sam Welles’ new taxon Megalosaurus wetherilli (Welles, 1954) from the Early Jurassic Kayenta Formation, would show one of the largest Lower Jurassic theropods. The first skull described by Welles, part of the holotype UCMP 37302, is incomplete, but gave Welles the information needed to show that his large reptile was distinct enough from Megalosaurus bucklandii from the Middle Jurassic of England to warrant a new species name. However, Welles had with him additional specimens, including a skeleton (UCMP 37303) described along with the type specimen, which elaborated on what exactly was going on. Missing portions of the cranium, the holotype was mounted first with a megalosaur-like skull, including a continuous premaxilla-maxilla margin, with missing material of the ribs, pelvis, vertebrae and skull modeled after the other specimens.

Plate 1 from Welles, 1954, showing the skeleton and restored elements of UCMP 37302 as originally mounted.

The holotype skull was missing enough material that Welles had to draw upon a second skull (UCMP 77270) to fill in the gaps. The second skull, preserving a pair of large rounded crests arising from the lateral margins of the nasals and lachrymals, showed Welles enough information that he considered this second skull to belong to his species, wetherilli. The skull affirmed an unusual jaw morphology, with a kinked snout, elongated and slender teeth, and bore a pair of crests rising from each side of the nasals and lachrymals. This provided such an extreme variation from that of “megalosaurs,” Welles granted his species “generic” status alongside Megalosaurus as Dilophosaurus wetherilli (Welles, 1970), and later one would describe the skulls in detail, using both specimens as the basis for the species; Welles (1984) also noted that the holotype cranium bore the broken bases of the cranial crests, determining that two large theropods with virtually identical cranial features closely associated in space and time within the Kayenta was unlikely, and treated these two specimens as though they belonged to wetherilli.

Skulls of Dilophosaurus wetherilli Welles, 1970 (Welles, 1954), based on UCMP 37302 and UCMP 77270.

Dilophosaurus wetherilli‘s skull is here restored as reconstructed from UCMP 37302 and UCMP 77270. It is shown at left with its jaws open and sporting these fantastic dental “sabres” running along the maxilla and the hooked premaxilla with rearward-pointed teeth; these oppose in the lower jaw a slight radial array of enlarged mesial dentary teeth. The teeth are here shown as preserved with their roots slightly exposed, and thus when the jaw is closed (as determined by the extent to which the mesial dentary teeth can abut the ventral premaxilla and vomer (visible in lateral view beneath the premaxilla) they can extend below the mandibular gular margin. Such a feature has been cause to question the ‘tightness” of the teeth in their sockets. However, it might be more likely that the teeth are generally this long, and that “the exposure of the root” is actually exposure of the non-enameled base of the crowns, and in modern crocodilians and lizards, this is natural and marks the gumline. If so, then the maxillary teeth might preserve a deep gumline, and they would extend below the bony margin of the jaw. This does not necessarily say that the jaw lacked lips of any sort, as might be considered, as it does not describe the nature of the soft-tissue which might be inferred from the bone.

Dilophosaurus wetherilli possesses a relatively novel snout, with enlarged external narial fenestra which overhangs the rostral maxillary margin and nearly become level with the antorbital fossa when the skull is placed in “neutral” position (the lower right of the two skulls — determined here solely through a line drawn along the posterior ventral margin of the maxilla[n1]). The maxillary teeth have shallow roots, as largely determined by the depth of the maxilla ventral to the antorbital fossa and the inference from the medial side the position of the medial fossa and pneumatic structures of the antrum. However, the premaxilla appears as does the dentary to possess relatively deep roots. As determined by Sakamoto (2010), the bite mechanics of Dilophosaurus wetherilli show a very strong mechanical advantage in the posterior dentition than from the rostral teeth and by an extreme that differs from virtually all theropod dinosaurs. What, then is the value of a posteriorly-oriented or radial rostral dental array? This is a question that the twin-crests and a few other taxa may be able to answer.

Up next: Instead of extremely long teeth, how about extremely short teeth? The Thunderbolt of Wisdom.

Updated: I have now fixed the Dilophosaurus skull. The teeth were reconstructed as preserved and are too long, with slippage occurring exposing too much of the root. Additionally, an error above shows the dorsal process of the premaxilla lying to the outside of the nasal above the narial fenestra. This should be the opposite, and has been fixed below.

In addition, while doing this I prepared the “jaw closed” side to look at the effects the elongated teeth might have on the closure of the jaw. Above, the jaw is closed as far as the lower teeth will allow. But there is no particular reason this should be the case. If the line of foramina on the lower jaw present the “lower limit” for the extent of the upper teeth when the jaw is closed, this results in a jaw that very much open. But the two suggest extremes and the truth is likely somewhere in the middle. The latter problem may be solved by greater curvature of the jaw, and by under-exaggerating the curve produced by the teeth. As they may be in the act of shedding, using them to account for the shape of the jaw is irregular, though likely close. (A better method for account for relative tooth size would be alveolar diameter, but few studies have attempted to survey alveolar diameter to tooth length and “natural length” in some theropod dinosaurs; certainly none have done this across theropods.)

[n1] I note but did not align the skull to the mandibular long axis as proposed for Caenagnathus collinsi — when this alignment is applied, as I will show the next time you should see this skull, the skull is oriented around 10 degrees rostroventrally than above.

Sakamoto, M. 2010. Jaw biomechanics and the evolution of biting performance in theropod dinosaurs. Proceedings of the Royal Society of London, B: Biological Sciences 277-3327-3333.
Welles, S. P. 1954. New Jurassic dinosaur from the Kayenta formation of Arizona. Bulletin of the Geological Society of America 65:591–598.
Welles, S. P. 1970. Dilophosaurus (Reptilia: Saurischia), a new name for a dinosaur. Journal of Paleontology 44:989.
Welles, S. P. 1984. Dilophosaurus wetherilli (Dinosauria, Theropoda): Osteology and comparisons. Palaeontographica Abteilung A 185:85-180.