(the saurischian Ceratosaurus)
Pelvic girdle bird-hipped dinosaur
(the ornithischian Lesothosaurus)
When dinosaurs evolved an upright, 2-legged posture, they had to evolve a new attachment site for the muscles that swung the powerful hindlimbs forward. Lizard-hipped dinosaurs (top) evolved a downward and forward extension of the pubis bone to which the muscles attached. In bird-hipped dinosaurs (right) the muscles were attached to either a forward extension of the ilium, or, in later types, to a new "pre-pubic" extension of the pubis.
Pelvic girdle Ilium Ischium was first explored by parties from the American Museum of Natural History during the years 1922 to 1925. Ironically, it was the search for fossil humans, rather than fossil reptiles, that led to the exploration of the region.
This ancient dinosaur graveyard yielded an array of Late Cretaceous beasts, including Protoceratops, the ancestor of the North American horned dinosaurs, together with its nests and eggs (see p. 165). Other finds included the saurischian dinosaurs Oviraptor (one specimen preserved with a clutch of Protoceratops' eggs, see p. 112), Vei-ociraptor and Saurornithoides (see p. 113).
Dinosaurs have been found in South America, Africa, Australia and even New Zealand. But, as yet, the southern hemisphere has not produced any dinosaur faunas of comparable richness to those of the northern hemisphere.
Perhaps the most famous locality is Tendaguru in Tanzania, East Africa, where a wealth of Late Jurassic dinosaurs was found, similar to those of the Morrison Formation in the western USA.
Another African site, discovered in 1987 in Niger, south of the Sahara, promises to yield rich dinosaur remains. Initial work has unearthed the bones of Camarasaurus, one of the giant, plant-eating sauropods (see p. 129).
Although the dinosaurs of the southern hemisphere are less well known than those of the north, there is little doubt that, in reality, they were just as diverse. Both "lizard-" and "bird-hipped" dinosaurs evolved in the Triassic, when all the continents of the world were still joined together in one super-landmass called Pangaea (see pp. 10-11). Any land animal, including dinosaurs, could have spread throughout the world.
The continents were still interconnected throughout the Jurassic, and it was not until the Late Cretaceous that there is evidence of different, isolated dinosaur faunas. By then, there were 2 separate areas of land in the northern hemisphere — Euramerica and Asiamerica (see pp. 10-11). For some reason, many new types of Late Cretaceous dinosaur evolved in Asiamerica. The hadrosaurs, ornith-omimids, saurornithoids, tyrannosaurs and protoceratopids all started life on that continent. As a result, more "old-fashioned" types of dinosaur, such as the iguanodonts, persisted for longer in Euramerica, removed from the competition of their new relatives.
About 300 genera of dinosaur have so far been described, of which about 55 percent are "lizard-hipped" sauris-
chians, and the rest are "bird-hipped" ornithischians. Only 7 percent of the known dinosaurs come from Triassic rocks (nearly all are saurischians), and 28 percent from Jurassic rocks. The remaining 65 percent come from Cretaceous rocks, and three-quarters of these have been found only in the Late Cretaceous.
It is significant that at the end of the Cretaceous, the flowering plants underwent an explosive evolution, diversifying into many new types which adapted to niches all over the world. This new source of food may well have led to the evolution of new types of dinosaur.
For many years, biologists assumed that dinosaurs, like their living reptilian relatives and the ancestral amphibians, were cold-blooded, or ectothermal. But recently it has been suggested that they may have been warm-blooded, or endo-thermal, like modern birds and mammals.
Cold-blooded animals rely on the sun as their main source of heat and energy. Warm-blooded animals, in contrast, rely on energy derived from their food. Living warm-blooded animals maintain a high body temperature (on average, between 97- 106T/36 4TC), and therefore use energy at a high rate — about 12 times the rate of a living cold-blooded animal. Even if a dinosaur such as Bra-chiosaurus had eaten 24-hours a day, it could not have produced enough energy to fuel its massive body at such a rate.
Paleontologists believe that dinosaurs had a lower body temperature than modern warm-blooded animals. Since they lacked an insulating covering of hair or feathers (which allows modern warm-blooded animals to keep warm when the environmental temperature drops), dinosaurs must have relied on the environmental temperature remaining relatively stable. This would have made them vulnerable to any major climatic change, and it may have been such a reliance that led to their extinction, and replacement by the less vulnerable, well-insulated, furry mammals.
The disappearance of the dinosaurs worldwide at the end of the Mesozoic Era is the best-known mass-extinction event of the prehistoric world. But other extinctions took place at or about the same time. The flying pterosaurs, and also the marine ichthyosaurs, plesiosaurs and mosasaurs, all disappeared at or near the end of the Cretaceous. And several marine invertebrate groups suffered the same fate — ammonities, certain bivalves and many tiny organisms of the plankton.
Since modern methods of dating rocks is only accurate to within a few hundreds of thousands of years (see p. 13), it is difficult to be certain whether the terrestrial and marine extinctions of the Cretaceous took place at the same time. It seems that the vertebrate extinctions happened gradually. For example, the ichthyosaurs became extinct before the end of the Cretaceous. Also, the plesiosaurs, pterosaurs and dinosaurs may have been becoming less common toward the end of that period. These facts suggest that some gradual change in the environment was taking place worldwide, rather than the popular notion of a sudden cataclysmic event that wiped out huge numbers of creatures.
Sea levels dropped comparatively rapidly during the Late Cretaceous. As a result, the average air temperature dropped, and the climate became more variable worldwide. This could have been the cause of the gradual decline of the pterosaurs and dinosaurs, especially if the dinosaurs' method of regulating their body temperature depended partly on the equability of the environment.
In contrast, the extinction of many types of microscopic marine invertebrates seems to have happened suddenly and simultaneously, at the end of the Cretaceous. They disappeared at precisely the time that the rocks show an abnormally high concentration of a number of normally rare metals, such as iridium, osmium and rhodium. This enriched layer has been found in some 50 different localities, ranging from North America and Europe to New Zealand, and in deep-sea sediments of the Pacific.
The American geologists Luis and Walter Alvarez pointed out that these rare metals are present in the same proportions as in meteorites. They suggest that an enormous meteorite, some 6 ml/10 km in diameter, hit our planet at the end of the Cretaceous period, some 65 million years ago. The impact would have thrown up a mass of débris, which would have darkened the skies for many years, affecting plant life. This could well have caused climatic changes that led to the mass-extinction of both the marine plankton and the terrestrial vertebrates, which were already on the decline.
If the meteorite had impacted in areas of calcium carbonate (as in the Cretaceous seabed) great amounts of carbon dioxide would have been released into the atmosphere. This could have increased the average temperature worldwide so much that dinosaurs, which relied on a warm, steady, climate, may have expired of heat.
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