Figure 6.4. Left lateral views of the skeletons of (a) Huayangosaurus, (b) Dacentrurus, and (c) Lexovisaurus.
Obviously even these running speeds were not of much importance to a hungry stegosaur, for these animals were herbivorous. Their general body form (large abdominal region for a capacious gut), small head, toothless snout, and simple blunt teeth send a clear message that these animals ate plants. Echoing these features is the thought that stegosaurs had muscular cheeks and strong chewing muscles, both of which help enormously in the way feeding on foliage is conducted, whether you're a dinosaur or a mammal.
For a stegosaur - as for all vertebrates with jaws - the business end of feeding begins at the front of the jaws. Therefore, it might be possible to learn a great deal about how these animals procured their food (Figure 6.5). Here a horn-covered beak or rhamphotheca (rhampho - beak; theca -cup or sheath) covered the fronts of both the upper jaw (the premaxilla) and the predentary bone of the lower jaw, similarly to rhamphothecae seen in modern turtles and birds. The rhamphothecae of the upper and lower jaws were probably sharp edged, but not hooked like those of a bird-of-prey or a snapping turtle. By bringing these sharp edges together,
Figure 6.6. Inner views of an upper tooth of (a) Stegosaurus and (b) Paranthodon.
Figure 6.6. Inner views of an upper tooth of (a) Stegosaurus and (b) Paranthodon.
Figure 6.5. Left lateral views of the skull of (a) Stegosaurus, (b) Huayangosaurus,
(c) Tuojiangosaurus, and (d) Chunkingosaurus. Dorsal views of the skull of (e) Stegosaurus,
(f) Huayangosaurus, and (g) Tuojiangosaurus.
the rhamphotheca became quite an effective apparatus for cropping or stripping foliage from plants. Only in Huayangosaurus - where the upper rhamphotheca was relatively small - was this cropping ability aided by the premaxillary teeth (remember, in those other stegosaurs where we have some evidence, these teeth are absent).
Once enough food was in the mouth, chewing began. But how this was accomplished is hard to tell. The chewing (i.e., cheek) teeth of stegosaurs were relatively small, simple, triangular (Figure 6.6), and not tightly appressed. These teeth apparently did very little grinding, because they lack regularly placed, well-developed wear surfaces on their crowns. Furthermore, the jaw musculature doesn't seem to have been particularly powerful: the coronoid process on the lower jaw was low, lending little mechanical advantage to the jaw musculature.
In view of the small teeth and weak oral musculature, one might look elsewhere for the means by which stegosaurs broke up their food. Where to look would be in the stomach, for it is known that some animals (birds and crocodilians, as well as sauropodomorphs and psittaco-saurs; see Chapters 9 and 11) use stones (gastroliths) to fragment food within the muscular part of the stomach. While it might seem logical that stegosaurs, with their seemingly inadequate chewing apparatus, might have had such gastroliths, skeletons of these animals have never been found with associated gastroliths.
Still, to judge from the inset position of the teeth, stegosaurs probably possessed cheeks. Here is an animal which has relatively weak adaptations for handling food at the mouth but is equipped with cheeks, an adaptation normally associated with sophisticated oral food-processing. Did these animals process food extensively in their mouths, or did they not?This apparent contradiction in stegosaurs has yet to be fully explained. The coexistence in stegosaurs of simple, irregularly worn teeth, a large gut capacity, cropping rhamphothecae, and cheeks all conspire to make stegosaurs unique, yet still poorly understood herbivores.
Even if we cannot understand all of the "hows and whys" of stegosaur food processing, perhaps we can get a glimmer of some more obvious aspects of feeding styles in these plant-eaters. First, recall that the snout is quite narrow in all stegosaurs, suggesting a fair degree of selectivity in the food these animals were seeking. Secondly, remember that all but Huayangosaurus have a great disparity in size between the forelimb and hindlimb, yielding the characteristic downturn of the vertebral column in the shoulder and neck region. From these observations, it is obvious that the head must have been brought very close to the ground, most likely near the 1 m level. And if the head was naturally positioned low to the ground, it is highly likely that these stegosaurs were principally low-browsing animals, consuming in great quantities the leaves and perhaps even succulent fruits and seeds of such ground-level plants as ferns, cycads, and other herbaceous gymnosperms.
The Mesozoic world of the low-browsers was not filled just with stegosaurs. It is very likely that these plant-eaters competed with a variety of other forms, principally any number of the smaller, agile, bipedal euornithopods. Although also confined to feeding on low-level vegetation, the latter may have fed on more fibrous, chewable leaves and other portions of the plants.
But were stegosaurs confined to such low-browsing, highly competitive levels? Not necessarily so, suggest some paleontologists. For example, R. T. Bakker has argued that some forms, particularly Stegosaurus, were able to rear up on their hindlimbs in order to forage at higher levels, perhaps even into the crowns of tree. This land of posture is similar to that generally assumed by kangaroos and occasionally by elephants. But stegosaurs may have gone one better on the elephant. First, with the center of gravity near the hips, the hindlimbs would already be supporting nearly 80% of the weight of the body. In addition, the strong, flexible tail would have been able to act as a third "leg" to form a tripod under the animal as it attempted to rear up. Should this kind of behavior have been utilized by stegosaurs, then these animal may have been able to feed on leaves quite high up in the trees (perhaps as much as 3 m for Kentrosaurus, and 6 m for Stegosaurus). Perhaps, like elephants, which assume a rearing, bipedal stance to feed on very high leaves for only very short periods of time, stegosaurs also foraged in the crowns of contemporary trees, but they may have done so for longer periods of time thanks to the aid of their strong, supporting tails.
Brains Whatever the aspirations of the dinosaurs to deep thought, stegosaurs cannot have been ranked among their crowning intellects. With very small brains (an estimated 0.001% body weight), it is certainly well justified to consider stegosaurs to be near the bottom of the gray-matter scale. Only sauropods and perhaps ankylosaurs had proportionately smaller brains (see Chapters 7 and 11).
The means by which these measures are obtained and their meanings interpreted are exceedingly important and worth describing. How, after all, do we know the size and shape of a dinosaur brain? The brain is soft tissue, and commonly decomposes long before the process of fossilization can begin. As many workers have shown, however, casts can be obtained of the interior of the braincases of fossil vertebrates. Latex is painted onto the inside of a well-preserved, three-dimensional braincase (one that was not crushed during fossilization). When the latex has dried (and is flexible), it can be peeled off the inside of the braincase, and pulled through the foramen magnum ("big hole"), the opening through which the spinal cord enters the skull. The result is a three-dimensional cast of the region occupied by the brain (Figures 6.7 and 15.6). Such casts give some inkling about the shapes and sizes of brains. Dinosaur brains can be no larger than the volume of these casts, but they were probably significantly smaller. Observations made of the brains of living lizards, snakes, and crocodilians show that these brains take up less room within the braincase than do those of mammals or birds. Researchers have long thought that the brains of non-avian dinosaurs should similarly be smaller than the entire volume of the braincase and have provided correction factors, which are reflected in calculations of dinosaurian gray matter.
Starting with general studies of fossil "brains" by Harry Jerison of the University of California at Los Angeles and, further, more detailed research by J. A. Hopson of the University of Chicago (see Box 15.4), it is now clear that brain size in vertebrates scales negatively allometrically with body size (technically, to the 0.67 power). What this means is that, as animals become bigger, either ontogenetically or phylogenetically, their brains also become large, but at a rate not equal to their size. Even for large-brained mammals such as ourselves, as we reach maturity and stop growing, our brains have grown less than have our bodies. The same pattern applies to shrews and elephants, as well as to lizards and croco-dilians. And presumably to extinct dinosaurs as well, for we know that the same pattern is found in the living variety: birds.
Hopson used this relationship between estimated dinosaurian brain size (calculated from the expected brain size of lizards, snakes, and croc-odilians scaled up to dinosaur size) and dinosaurian body size to make comparisons with measured brain size in actual dinosaurs. Among stegosaurs, casts of the braincase are available only for Kentrosaurus and Stegosaurus, both of which are reasonably similar in shape and size. The brains of these two stegosaurs are relatively long, slightly flexed, and above all else small. The only aspect of the stegosaur brain that appears to have been somewhat large is its extraordinary olfactory bulbs, those portions of the brain that provide the animal with its sense of smell. Why this region should have been so large, we can only guess: perhaps sense of smell was raised to a high art in stegosaurs. Otherwise, the mental and sensory faculties of stegosaurs - as we have already mentioned - were not particularly well developed. Clearly for stegosaurs, animals that assuredly had a relatively unhurried life style and possibly a relatively uncomplicated range of behaviors, these brains were adequate.
Yet the small-brained stegosaur defied even those nay-sayers who spoke about dinosaurs as dullards, because stegosaurs apparently looked elsewhere in order to embellish their anomalous nervous systems. It was 0. C. Marsh who, over a century ago, observed the very much enlarged canal - upwards of 20-times the volume of the brain - that accommodated the spinal cord in the hip region. Here began the legend of the dinosaur with two brains: a diminutive one in the head, which monitored and controlled not quite enough; and another in the sacral region, which handled the balance of neural functions. The subject of poetry (Box 6.1), this sacral enlargement has caused a great deal of controversy. Some suggested, as did Marsh, that the enlarged spinal cord "covered" for what the standard brain could not manage, while others hypothesized that it was the logical enlargement for the hindlimb and tail region in an animal that emphasized both, for reasons of locomotion (hindlimb domination) and protection (the tail). While this latter explanation is more in keeping with comparative neuroanatomy, neither has garnered much acceptance in the paleontological literature. Thus a new idea about the stegosaur sacral enlargement has created much excitement.
In 1990, E. B. Buchholz (formerly Giffin) of Wellesley College reexamined the issue of enlargements of the neural canals in sacral vertebrae in dinosaurs. She did so with an eye for how large the neural canals should be, given the size of the spinal cord, in much the same way that Hopson and earlier Jerison did with brain size. She found out that the sacral enlargements in stegosaurs (also in sauropods, as it turns out; see Chapter 11) were more than large enough to accommodate the
The poetry of dinosaurs
Dinosaurs have been subjects of numerous poems, limericks, and other bits of doggerel, virtually since the time of their earliest discovery. Most have centered around their enormity and putative lack of brain power and social graces, with few recent ; efforts to balance such dismal views.
The most famous dinosaurian poem celebrates the mental claims of Stegosaurus, in particular the cerebral gymnastics supplied by its double brains, the standard issue and the one in its rear end.The piece, by Bert L.Taylor; a columnist in the 1930s and 40s for the Chicago Tribune goes like this:
Behold the mighty dinosaur; Famous in prehistoric lore, Not only for his power and strength But for his intellectual length. You will observe by these remains The creature had two sets of brains -One in his head (the usual place), The other at his spinal base. Thus he could reason a priori As well as a posteriori No problem bothered him a bit He made both head and tail of it. So wise was he, so wise and solemn, Each thought filled just a spinal column. If one brain found the pressure strong It passed a few ideas along. If something slipped his forward mind 'Twas rescued by the one behind. And if in error he was caught He had a saving afterthought. As he thought twice before he spoke He had no judgement to revoke. Thus he could think without congestion Upon both sides of every question. Oh, gaze upon this model beast, Defunct ten million years at least.
As a poetic counterpoint to the range of Mesozoic intelligensia,we also provide some thoughts by John Maynard Smith, evolutionary biologist extraordinaire at the University of Sussex in Brighton, England, entitled The Danger of Being too Clever.
The Dinosaurs, or so we're told Were far too imbecile to hold Their own against mammalian brains; Today not one of them remains. There is another school of thought, Which says they suffered from a sort
Of constipation from the loss Of adequate supplies of moss.
But Science now can put before us The reason true why Brontosaurus Became extinct in the Cretaceous. A beast incredibly sagacious Lived & loved & ate his fill; Long were his legs, & sharp his bill, Cunning its hands, to steal the eggs Of beasts as clumsy in the legs As Proto- & Triceratops And run, like gangster from the cops,
To some safe vantage-point from which It could enjoy its plunder rich. Cleverer far than any fox Or STANLEY in the witness box It was aVERY GREAT SUCCESS. No egg was safe from it unless Retained within its mother's womb And so the reptiles met their doom.
The Dinosaurs were most put out And bitterly complained about The way their eggs, of giant size, Were eaten up before their eyes, Before they had a chance to hatch, By a beast they couldn't catch.
This awful carnage could not last; The age of Archosaurs was past. They went as broody as a hen When all their eggs were pinched by men. Older they grew, and sadder yet, But still no offspring could they get. Until at last the fearful time, as Yet unguessed by Struthiomimus Arrived, when no more eggs were laid, And then at last he was afraid. He could not learn to climb with ease To reach the birds' nests in the trees, And though he followed round and round Some funny furry things he found, They never laid an egg - not once. It made him feel an awful dunce. So,thin beyond all recognition, He died at last of inanition.
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