The armored and plated dinosaurs were all low-browsing animals, picking plants from ground cover, bushes, and other material at the base of the flora. They ate below the line of plants normally consumed by the taller sauropods and iguanodontians. They probably plowed into the vegetation with their sharp beaks, snipping off branches and twigs with a snap of the jaws and a sideways pull of the head.
The herbivorous stegosaurs and ankylosaurs were equipped with simple but effective jaws and teeth. The teeth were inset from the edges of the jaws, forming significant ridges of bone on the upper and lower jaws that protruded quite a bit from the location of the teeth; this suggests that these creatures had fleshy cheeks in which to hold food that was being chewed. The animals' spatula- or leaf-shaped teeth were set in rows and were well suited for tearing vegetation rather than grinding it. As in the sauropods, the breakdown of food was probably done in the stomach and gut. Stegosaurs and especially the ankylosaurs had huge guts in which the natural digestive fermentation of food took place slowly but effectively.
Plant life on Earth changed dramatically during the rise and evolution of these dinosaurs. Three major kinds of land plants were common during the Jurassic and Cretaceous Periods in which the stegosaurs and ankylosaurs lived. Their diet may have consisted in part of the following menu.
Pteridophytes. These included the ferns. Ferns grew in forms that hugged the ground but also grew as trees, with a single unadorned, stemlike trunk and a single growing point at the top. These plants generally required a moist environment. They were fast growing and so could be grazed without being killed; this made them an excellent renewable source of food. Pteridophytes were abundant throughout the age of dinosaurs. They were an especially important source of food for lower-browsing armored and plated dinosaurs.
Equisetopsids. These include horsetails and scouring rushes. Their leaves are greatly reduced, and the stems are jointed and pho-tosynthetic. Some early species were trees measuring more than 100 feet (30 m) tall.
Gymnosperms. There were, and are, many different kinds of seed plants, but during the Mesozoic, two groups, the conifers and the cycads, were among the most abundant. They reproduced by means of a "naked seed," in contrast to the seeds of angiosperms, which are enclosed within a fruit. Other branches of the conifer group include the cypress and bald cypress. In contrast to the conifers, the cycads were mostly short plants with bulbous or palmlike trunks, although they also had some treelike forms. They were capped with fronds reminiscent of palm trees. Gymnosperms were neither moist nor soft forms of vegetation; they were not an easy food to digest. They were gradually displaced as a source of food by the rise of the flowering plants, or angiosperms, in the Cretaceous Period.
Angiosperms. These are flowering plants, the last of the major plant groups to evolve. They are distinguished by having a seed borne within a fruit, unlike the gymnosperms, which bear naked seeds. Angiosperms first appeared in the Early Cretaceous but did not begin to become common until the Late Cretaceous. This means that only the later Cretaceous thyreophorans really had to deal with angiosperms; Middle and Late Jurassic thyreophorans contented themselves with gymnosperms, pteridophytes, and other plants. Angiosperms diversified and spread rapidly in the form of flowering shrubs to become the dominant plant group by the end of the
Cretaceous. Angiosperms reproduced and grew more quickly than gymnosperms this made them abundantly available as dinosaur food. The foliage of most angiosperms was generally more digestible, moist, and nutritionally sound than that of either pteridophytes or gymnosperms. The ability of these plants to spread and grow quickly made them ideally suited for food for herbivores.
This chapter described the rise and diversification of the stegosaurs and ankylosaurs.
1. Ornithischian dinosaurs made up one of the two major clades of dinosaurs and included a variety of bipedal and quadrupedal herbivores. Ornithischians were highly specialized plant eaters, many of which grew to large size and lived in herds.
2. The first widely successful groups of ornithischians were the Stegosauria, or plated dinosaurs, and the Ankylosauria, the armored dinosaurs. Stegosaurs and ankylosaurs belong to a larger clade called Thyreophora, or "shield bearers." All members of the group were quadrupedal, had leaf-shaped cheek teeth for tearing vegetation, had some form of body armor or plating, and were widely distributed geographically.
3. The earliest stegosaur fossils have been found in China and date from the Middle Jurassic. All stegosaurs had small, narrow heads and heavy limbs and bore some form of vertical, bony plates or spikes running in a double row along the back. Some grew up to 30 feet (9 m) long.
4. The large, triangular back plates of Stegosaurus served as a visual display for warding off predators or attracting the attention of a potential mate. The plates were highly vascular and may also have served as heat conductors for raising or lowering the body temperature of the dinosaur.
5. The armored dinosaurs, or Ankylosauria, first appeared in the Middle Jurassic. Some Cretaceous taxa reached lengths of 33 feet (10 m).
6. Armored dinosaurs are divided into two families, the Anky-losauridae and the Nodosauridae. Ankylosaurids had broader, more heavily armored heads and massive tail clubs for protection. Nodosaurids had narrower skulls and lacked tail clubs but were adorned with a variety of spikes, bony knobs and armor plates. Both groups of armored dinosaurs were slow-moving creatures.
7. The ankylosaur for which there is the most evidence is Euop-locephalus ("true armored head"), a moderately large armored dinosaur measuring about 23 feet (7 m) long. The topside of this animal was completely protected by a bony mosaic of scutes from the tip of its nose to its bony tail club.
8. Stegosaurs and ankylosaurs probably used their tail spikes, clubs, and body spikes to defend themselves against predators.
9. The herbivorous stegosaurs and ankylosaurs were equipped with simple but effective jaws and teeth. They were grazing and low-browsing animals, picking plants from ground cover, bushes, and other material at the base of the herbivory.
The Middle and Late Jurassic Epochs were populated by some of the best-known and most spectacular dinosaurs. The gigantic sau-ropods—including Apatosaurus, Diplodocus, and Camarasaurus— were among the first of the long-necked titans discovered and described based on fossils found when American fossil hunters entered the rich fossil beds of western North America during the 1870s. For many years, those very first specimens symbolized everything that was dinosaurian in the public's view. The discovery of sauropods was soon followed by the excavation of large and equally spectacular predatory dinosaurs such as Allosaurus. Then the first specimen of Stegosaurus was found, further detailing a distantly remote time when animals the size of small buildings walked the Earth and fought for their survival with size, girth, teeth, talons, and spikes in seemingly unending varieties.
The early discovery of sauropods, large theropods, and stego-saurs created an impression of dinosaurs as monstrous, stupid, and lumbering lizards. Since that time, the continued discovery and study of many more dinosaur specimens have provided a wealth of new data and interpretations. The scientific view of dinosaurs as living creatures has changed greatly during the past 150 years. Once considered docile and inactive like their distant, "cold-blooded" reptilian relatives, dinosaurs now are thought to have been active, energetic creatures that used a variety of thermoregulatory schemes to maintain a constant body temperature. Close examination of the fossil evidence coupled with observations about the living habits of extant creatures have also provided a better informed view of the probable lifestyles of dinosaurs.
By the end of the Jurassic Period, many important trends in the evolution of dinosaurs had taken place. Sauropods represented extremism in the size of land animals, with equally elegant metabolic and thermoregulatory schemes to maintain active lifestyles. Theropods were evolving along several different paths, some large and some small. Other creatures, such as Archaeopteryx, began the earliest experimentation of dinosaurs with powered flight, presaging the rise of birds. Among the ornithischians, the plated and armored dinosaurs developed sophisticated jaws and teeth that represented advances in the abilities of herbivorous dinosaurs to chew their food. Equally compelling was their armor plating and weaponry, traits that would persist in their descendants throughout the remaining years of the Mesozoic that followed.
By the end of the Jurassic Period, several evolutionary trends were in motion that would be played out by dinosaurs in the second half of the Mesozoic Era. Sauropods would continue their domination of the high-browsing herbivory, spreading and diversifying in numerous ways to all corners of the globe. Theropods would proliferate with great variation in small, medium, and large sizes, leading to the largest terrestrial predators of all time, such as Tyrannosau-rus. Equally important, however, was the evolution of a small line of theropods that led directly to powered flight and the emergence of birds. The ornithischians would expand in the greatest numbers during the Cretaceous Period. Joining the ankylosaurs would be a wide range of herbivores, including the iguanodontians and horned dinosaurs.
The success of the dinosaurs for so many years is astounding. They existed in one form or another for more than 160 million years. Humans, by comparison, exist today only as a single species that first arose from the evolutionary stock of primates about 2 million years ago. Modern-type humans arose a mere 200,000 years ago. Yet it is not the longevity of the dinosaurs that makes them so popular. When a child is asked why he or she likes dinosaurs, a common reply is that dinosaurs are "big, scary, and dead." Paleontologists are equally passionate about these lost creatures. In the earliest days of dinosaur hunting in the American West, when Edward Drinker Cope first made some of his most startling dinosaur discoveries, he wrote home to his young daughter Julia about his finds:
I have found four new kinds of Laelaps [a dinosaur] which ate meat and several kinds that ate leaves and wood. They were as large as elephants and their teeth are very small, no larger than the end of my little finger. One kind had more than 400 in his mouth at once, of which 100 were in use at once and the rest coming on from below to take their places as soon as they were worn out.
In 1876, when Cope wrote this, one could count the number of known dinosaur genera on the fingers of one's hands. Cope and his rival, Othniel Charles Marsh, were about to open up a wide new window onto the mysteries of prehistoric life. The enthusiasm and relish with which these men attacked their work still resonates in the science of dinosaurs.
MYA EONS ERAS
Approximate Ages of Major Groups of Organisms
65.5 100 150 200 250 300 350 400 450 500 542
Invertebrates Fishes Land Plants Amphibians Reptiles Mammals & Birds Ancestors
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