Teeth and jaws and turds

As with many carnivorous animals, theropod heads tended to be proportionately large. In the case of the biggest, the heads could be upward of 1.75 m in length. In general, theropod skulls are rather primitive, reminiscent of those of many non-dinosaurian ornithodirans. Yet

Carnotaurus Skull Size

Figure 9.11. Left forelimb of

(a) Struthiomimus, (b) Tyrannosaurus, and

(c) Carnotaurus.

20 cm

20 cm

20 cm

Figure 9.11. Left forelimb of

(a) Struthiomimus, (b) Tyrannosaurus, and

(c) Carnotaurus.

20 cm

20 cm

20 cm there are differences: tyrannosauroids had robust, deep-jawed skulls, suggesting a powerful bite. Other theropods - even large ones like Carcharodontosaurus - had much more lightly built skulls (Figures 9.12 and 9.13).

5 cm

5 cm

5 cm

5 cm

Carnotaurus Skeleton Side View

Figure 9.12. Left lateral view of the skull of (a) Herrerasaurus, (b) Cerato-saurus, (c) Dilophosaurus, (d) Coelophysis, (e) Carnotaurus, and (f) Allosaurus.

Dilophasaurus Skull Front View
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All theropod teeth - in the case of those that had them - tended to be flattened from side to side, recurved (curved backward), pointed and serrated. With the jaw joint at the level of the tooth row, the effect was like that of a pair of scissors, slicing from back to front (Figure 9.14). The design clearly lacks the chewing specializations of genasaurs.

It was the sharply pointed, recurved and serrated teeth in the upper and lower jaws that handled the prey. The recurved shape kept prey from escaping from the mouth. The teeth of smaller theropods such as Troodon, with their prominently pointed serrations and narrow cross-sections, sliced like hacksaw blades. Tyrannosauroids, at the other end of the spectrum, with bulbous teeth and rounded serrations, had a weaker cutting ability, but greater strength, suggesting that they could withstand complex, strong, and violent forces, such as might occur with a powerful, actively struggling prey (Figure 9.15). They may have even been able to crush bone (see below).

With the differences in skull and teeth, theropods evidently bit in different ways. Recent studies have paired computed tomography (CT) scans and computer-modeled stress analyses to the architecture of theropod skulls (Figure 9.16). We now know, for example, that Allosaurus, with its relatively lightly built skull, used a "slash-and-tear" attack on its prey, in which powerful neck muscles drove the skull downward rather than delivering a crushing bite with the jaw muscles alone. When the head was retracted, the teeth sliced and tore flesh. Such a wound might not kill prey immediately - but blood loss and possible bacterial

Skull Herrerasaurus

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Figure9.13. Left lateral view of the skull of (a) Ornitholestes, (b) Oviraptor, (c) Albertosaurus, (d) Tyrannosaurus, (e) Saurornithoides, (f) Gallimimus, (g) Dromiceiommus, (h) Demonychus, and (i) Veloaraptor.

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Figure9.13. Left lateral view of the skull of (a) Ornitholestes, (b) Oviraptor, (c) Albertosaurus, (d) Tyrannosaurus, (e) Saurornithoides, (f) Gallimimus, (g) Dromiceiommus, (h) Demonychus, and (i) Veloaraptor.

Carnivorous Teeth Diagram
Figure 9.14. In its manner of slicing and in the placement of its hinge, the carnivorous theropod mouth was very similar to the design of a pair of scissors.
(b)

5 cm

Figure 9.15. Extremes of theropod teeth compared. (a) Blade-like meat-slicing tooth of Dromaeosaurus; (b) bulbous bone-crunching (?) tooth of Tyranno-saurus.

infection would work their relentless damage. Tracking and waiting may have been part of the killing technique of dinosaurs with lightly built skulls and thin, blade-like teeth.

This contrasts with tyrannosauroids (see Figures 9.13d and 9.17) or perhaps abelisaurids such as Carnotaurus (see Figure 9.12e), whose more bulbous teeth and larger, more heavily built skulls likely delivered a bone-crushing, heart-stopping bite. They may also have suffocated their victim by seizing their snout or neck between their jaws and clamping down. This kind of attack is consistent with the large gape, powerful jaws, and stout teeth of these predators. In all cases, however, the skull had considerable mobility on the neck because of a well-rounded occipital condyle and its articulation with the Figure9.16. The skull ofAHosaurus marked with its finite element analysis model, first part of the cervical (neck) vertebrae. indicating the regions of stress that pass through it as a function of biting.

Is all this who-ate-whom speculation? The answer comes from an unlikely source, a

44 cm long, 13 cm high, and 16 cm wide coprolite. Its age, geographical location, and, err, size point to Tyrannosaurus rex as the culprit. The specimen contains between 30% and 50% of bone fragments, thought to be the remains of limb bones or parts of a ceratopsian frill. In combination with other information about theropod diets (see "Eaters and eatees," below), this coprolite provides physical evidence that tyrannosauroids crushed, consumed, and incompletely digested large quantities of bone.

Allosaurus Gape Articulated Jaws
Compression Tension
Figure 9.17. The upper teeth of Tarbo-saurus as seen from the right side of the skull. Scale in centimeters.

Toothless. At least two times in their history,2 theropods drastically reduced or lost all of their teeth. With the exception of one primitive genus, all ornithomimosaurs, a group of small-skulled, long-legged theropods that look very much like ostriches with long tails (see Figure 9.8), lost all their teeth (see Figure 9.13f). Ornithomimosaurs had a beak, and in the case of Gallimimus, at least, the beak had a ridged feature that appeared almost sieve-like along its margin, provoking a controversial suggestion that it fed aquatically, much like a modern duck. Later interpretations of the beak edge suggest that it is less a sieve and more of a shearing feature, consistent with grinding fibrous plant matter.

Ornithomimosaurs are also known to have gastroliths, and these indicate the presence of a muscular gizzard for grinding plant matter. These, in combination with its powerfully clawed hands and superb running capability, suggest a terrestrial existence rather than a duck-like lifestyle. Consistent with the evidence from the beak, these animals were likely fast-running creatures that used their gastric mills to grind up fibrous plant matter as do modern plant-eating birds.

Oviraptorosaurs were also toothless (see Figures 9.13b and 9.18). The skull was very short with apparent pneumaticity (see Chapter 8), and the jaw musculature was very well developed. Located between their shortened upper jaws is a pair of peg-like projections dead center in the middle of the palate. One analysis of the mechanics of the oviraptorosaur skull suggested that the jaws were designed to feed on hard objects that required crushing, such as clams, oysters, and mussels. Oviraptorosaurs presumably cracked them open by the brute force of their jaw muscles acting on the thick horny bill covering the margins of the mouth and the palate, and especially the stout pegs in the center.

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  • david jones
    How big are dinosaur turds?
    8 years ago
  • Mustafa
    Which toothless dinosaurs had a horny bill with stout pegs in the middle?
    4 months ago

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