Eurypoda Stegosauria hot plates

Stegosaurs were medium-sized dinosaurs, 3-9 m in length and weighing 300-1,500 kg, characterized by osteoderms that developed into spines and plates, as well as by their quadrupedal stance (Figure 5.3). Their profiles sloped strongly forward and downward toward the ground as a result of the hindlimbs being substantially longer than the forelimbs (Figure 5.4). All toes had broad hooves. They seem to have been relatively uncommon dinosaurs, yet clearly had a global distribution (Figure 5.5).

Figure 5.3. Tuojiangosaurus, a stegosaur from the Late Jurassic of Sichuan Province, China.
Global Plate Distribution Jurassic

Figure 5.5. Global distribution of Stegosauria.

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Figure 5.5. Global distribution of Stegosauria.

Stegosaurian lives and lifestyles

Locomotion. The general body plan of stegosaurs does not suggest life in the fast lane (Figure 5.6). Indeed, with their long back legs and short front legs, stegosaurs must have had a locomotor conundrum: at the same cadence (the rate of feet hitting the ground), the hindlimbs would have covered much more distance than the forelimbs. At high speeds, therefore, the rear of the animal would have overtaken its head! This problem could be avoided in two ways: (1) by drawing the forelimbs up from the ground (that is, temporarily being bipedal while running) or (2) by limiting movement to a slow walking gait. Because of the mass distribution of stegosaurs, the first option is unlikely. Our best guess is that the pace of

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Top View Stegosauria

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Figure 5.6. Left lateral views of the skeletons of (a) Huayangosaurus, (b) Dacentrurus, and (c) Lexovisaurus.

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Figure 5.6. Left lateral views of the skeletons of (a) Huayangosaurus, (b) Dacentrurus, and (c) Lexovisaurus.

stegosaur life was leisurely, on the order of 6.5 to 7.0 km/h maximum speed (see Box 12.3). Chasing fleet prey was not too important to a hungry herbivore.

Dealin' with mealin'. The business end of feeding began at the rhamphotheca, similar to those seen in modern turtles and birds, which covered the fronts of both the upper and lower jaws (Figure 5.7). The rhamphothecae were probably sharp-edged, and were used to crop and strip foliage.

Like all genasaurs, stegosaurs had an inset tooth row, implying cheeks, which in turn suggest chewing; however, exactly how that must have worked is baffling. The cheek teeth of stegosaurs were relatively small, simple, and triangular (Figure 5.8), and not tightly pressed together in a block for efficient grinding. Moreover, the teeth lack regularly

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Figure 5.7. Left lateral views of the skull of (a) Stegosaurus, (b) Huayangosaurus, (c) Tuojiangosaurus, and (d) Chunkingo-saurus. Dorsal views of the skull of (e) Stegosaurus, (f) Huayangosaurus, and (g) Tuojiangosaurus.

Figure 5.7. Left lateral views of the skull of (a) Stegosaurus, (b) Huayangosaurus, (c) Tuojiangosaurus, and (d) Chunkingo-saurus. Dorsal views of the skull of (e) Stegosaurus, (f) Huayangosaurus, and (g) Tuojiangosaurus.

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Figure 5.8. Inner views of an upper tooth of (a) Stegosaurus and (b) Paranthodon.

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placed, well-developed worn surfaces, features present in herbivores that chew by grinding. Furthermore, the coronoid process was low, lending little mechanical advantage to the jaw musculature. Chewing? Perhaps, but not particularly efficient when compared with other chewing vertebrates.

So how else might stegosaurs have ground their food? Birds use gastroliths (stones within the muscular part of the stomach; see Chapter 8) to grind food. The problem is that gastroliths have never been found with stegosaur remains, as they have with other dinosaurs (prosauro-pods, sauropods, psittacosaurs, and ornithomi-mosaurs). Ultimately, however, the co-existence in stegosaurs of simple, irregularly worn teeth, large gut capacity, cropping rhamphothecae, weak jaw musculature, and cheeks all conspire to make the business of dealin' with mealin' poorly understood in these dinosaurs.

What might stegosaurs have eaten? In most, the head was held near the 1 m level. Thus

Stegosaurus Tooth

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Figure 5.8. Inner views of an upper tooth of (a) Stegosaurus and (b) Paranthodon.

stegosaurs were likely low-browsers, consuming ground-level plants such as ferns, cycads, and other herbaceous gymnosperms (see Chapter 13).

The Mesozoic world of the low-browsers was not filled only with stegosaurs. It is very likely that stegosaurs competed with a variety of other dinosaurs, many of whom appear to have been very efficient chewers. Could stegosaurs have used their narrow skulls to select only the most nutritious parts of the plant, while everybody else dined less discriminately?

On the other hand, maybe stegosaurs weren't confined to low-browsing. Some paleontologists have argued that stegosaurs could have reared up on their hindlimbs in order to forage. Then the strong, flexible tail might have acted as a third "leg" to form a tripod. If so, these animals could have reached as high as 6 m in the largest forms.

No brains, one brain, or two brains? It is as clear as most anything can be at a distance of 100 million years that stegosaurs were just not all that bright. Their brains were an estimated 0.001% of the adult stegosaur body weight, putting them near the bottom of the dinosaur -for that matter, vertebrate - gray-matter scale (Figure 5.9). Brainy-ness must not have been part of the stegosaur life strategy, as indeed they were so small-brained that early workers felt compelled to assign them an extra brain: based upon an enlargement of the canal in the centra of the vertebrate (see inset to Figure 4.5) in the hip region, in which the spinal chord rests. Here began the legend of the dinosaur with two brains: a small one in the head and another in the pelvis, presumably to pick up the slack left by the first. All of this inspired literary outpourings, two of which we offer in Box 5.1.

The enlargement of the stegosaur spinal canal in the pelvic region, the putative rear "brain," is yet another stegosaur mystery. Many vertebrates have enlargements in the sacrum for nerves going to the hind legs, but the neural canal at the front of the stegosaur pelvis is upward of 20 times the volume of the brain. Some living birds have a similar enlargement that houses an organ whose function is thought to supply glycogen (a complex sugar-based molecule which the body stores, but can break down to obtain energy) to the nervous system. Could the enlargement in the stegosaur sacrum have housed a glycogen body?

Speculation aside, the two stegosaurs in which the brain cavities are known -Kentrosaurus and Stegosaurus - suggest that stegosaur brains were relatively long, slightly flexed, and small (Figure 5.9; Box 5.2). Only the olfactory bulbs, the portions of the brain that provide the animal with its sense of smell, are somewhat enlarged. Clearly stegosaurs, animals that had an unhurried lifestyle and possibly a relatively uncomplicated

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  • betty vest
    What kind of hips did paranthodon have?
    9 years ago
  • DIANE
    What paranthodon BODY PARTS was found?
    9 years ago

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