Until the Early Cretaceous Epoch, the world's flora was dominated by ferns and gymnosperms—seed plants whose seed embryos are not protected by a fruit, cone, or other body. Gymnosperms first appeared in the late Paleozoic Era and became dominant during the first half of the Mesozoic Era. They are still represented today by more than 600 known species of conifers (evergreen trees), cy-cads, gnetophytes, and Ginkgo, none of which have flowers or fruits. Gymnosperms are typically tough and hearty. Their woody pulp, thick bark, branches, and needles or frondlike leaves are difficult to chew. Herbivorous dinosaurs of the Jurassic Period—including the sauropods, stegosaurs, and ankylosaurs—developed jaws, teeth, and digestive systems capable of extracting nutrition from the likes of evergreens, cycads, and other tough gymnosperms. The animals' basic digestive strategy was to minimize chewing of the food in the mouth and to use a fermentation process in the stomach to slowly extract nourishment from the nutritionally stingy gymnosperms.
Gymnosperms were successful at surviving in a Jurassic world with a moderately warm and arid climate. They relied only on wind to carry pollen to their seeds. Pollinated gymnosperm seeds grew slowly, and when fully grown they often took the form of tall trees in species such as ginkgos and conifers. Slow growth and height discouraged consumption by herbivores. Sauropod dinosaurs adapted by growing taller to reach the ever-more-lofty canopies of gymno-sperms and also developed consumption habits that allowed them to eat fairly constantly in order to derive enough sustenance from the nutritionally stingy conifers and cycads.
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THINK ABOUT IT
The polar regions of today's world are the coldest and harshest on the planet. Most kinds of organisms would not survive for long if left to fend for themselves above the Arctic Circle or in Antarctica. The polar regions of the Earth were not always so uninhabitable, however, and there is growing evidence that a wide variety of dinosaurs lived within the polar circles of the Mesozoic.
Even though the middle latitudes of the Earth were uniformly warm during the Mesozoic Era, temperatures at the poles would have been somewhat cooler, even without the presence of ice caps. Studies of fossil plants and associated oxygen isotope studies of polar sediments have been carried out to determine the average annual temperatures of the Mesozoic polar regions. Results suggest that the North Pole had a mean average temperature between 36° and 46°F (2° and 8 °C) and the South Pole about 50 °F (10 °C)—not tropical temperatures by any means, but not below freezing, either. Another factor affecting life on the extreme ends of the planet would have been prolonged periods of darkness and cooler temperatures still during winter.
The idea that dinosaurs could have lived at the relatively cool poles of the Earth was virtually unthinkable 50 years ago because of the widespread belief that their metabolism was more like that of cold-blooded modern reptiles than that of birds or mammals. The work of paleontologists to collect fossils in these regions during the past 20 years has led to a change of thinking. Not only did dinosaurs colonize the poles by at least 190 million years ago, but fragmentary remains have now been identified there for nearly all major branches of the dinosaur evolutionary tree, with the notable and interesting exception of sauropods.
"Polar" dinosaurs—as defined by paleontologists Thomas Rich (Museum of Victoria, Australia); Roland Gangloff (University of Alaska); and William Hammer (Augustana College, Illinois)—are defined as those dinosaurs "that lived within the polar circles of their time, not necessarily within the current polar circles." This means that their fossils are sometimes found on landmasses that have since drifted to the fringes of the ancient polar circles, such as Australia and New Zealand in the south and Alaska, Russia, and the Canadian Yukon in the north.
One of the most productive fossil sites for polar dinosaurs is found on the banks of the Colville River in northeast Alaska. Evidence of polar dinosaurs is usually scant. The most complete dinosaur from any polar locality was found at the Matanuska Formation of south-central Alaska in 1995 and consisted of about a quarter of the animal. Bones from the foot, limbs, and tail were enough to convince paleontologist Anne Pasch of the University of Alaska that what had been found was a specimen of a hadrosaur—a duck-billed dinosaur. Dating of the fossil sediments was made easier by the presence of sea creatures such as ammonites, the age of which can be fixed at about 90 million years ago. That's about 10 million years older than other hadrosaurs from North America and suggests that the Alaskan duckbill might be linked to early hadrosaurs from Asia. Finding the bones of terrestrial animals in marine deposits is not so unusual, although the specimens are usually spotty and incomplete. Pasch speculated that the hadrosaur died on the shore of an ancient ocean and "floated out to sea, probably as a bloated carcass. It eventually sank to the bottom and was buried in fine black mud along with shells and other sea creatures" found with its bones.
Most main groups of dinosaurs are represented by fossil evidence from regions that would have been polar during the Mesozoic. In the Northern Hemisphere, compelling evidence of hadrosaurs, horned dinosaurs, large and small theropods, sauropods, and plated and armored dinosaurs is found in the northern reaches of Alaska, Canada, and Siberia. In the Southern Hemisphere, polar dinosaurs are represented by specimens of armored dinosaurs, small ornithopods, hadrosaurs, prosauropods, sauro-pods, large and small theropods, and possible horned dinosaurs. These remains are not alone and are often found with fossils of other creatures
from the polar neighborhood such as crocodilians, amphibians, pterosaurs, birds, and small mammals.
The presence of polar dinosaurs cannot be denied but raises questions about their lifestyle, metabolism, and thermoregulation. Chief among these questions is whether the presence of dinosaurs in cooler regions of the world is evidence of a more active, energetic thermoregulatory metabolism, or whether there was more to the story. Australian paleontologists Thomas Rich and Patricia Vickers Rich, who have done much to advance knowledge of polar dinosaurs of the Southern Hemisphere, speculate that some small dinosaurs may have actually burrowed into the ground to protect themselves against the chill of the long winter nights. Another plausible idea is that some dinosaur groups migrated to the south toward the poles during seasonally warmer periods and returned toward the Equator when the winter chill set in. That would have been possible given the configuration of connected landmasses during much of the Mesozoic. Another clue to dinosaur survival in colder climates might also be related to the possible use of feathers as a form of body insulation—at least for small theropods for which such body coverings have been found.
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The dominance of gymnosperms diminished during the Cretaceous Period with the rise of flowering plants—the angiosperms. Angiosperms were characterized by a new reproductive life cycle that quickened their ability to grow, breed, and disperse. Angio-sperms utilize flowers to attract pollinating animals, such as insects, and also encase their seeds in fruits that, when separated from the plant, can aid in dispersal of seeds. The innovations of flowers to aid in pollination and fruits to protect the embryo contributed to the rapid success and spread of flowering plants. The oldest known
angiosperm dates from the Early Cretaceous Period, about 125 million years ago. Found in the same Chinese fossil region that contains exciting fossils of early marsupial and placental mammals, feathered dinosaurs, and birds, this primitive early example of a flowering
plant had paired stamens (the pollen-producing parts of a plant) and multiseeded fruits, although it may have lacked flowers.
Angiosperms quickly became a favorite food of dinosaurs. Flowering plants reproduced much more quickly than gymnosperms; angiosperms constantly replenished a landscape that could become heavily browsed by hungry dinosaurs. The ability of angiosperms to grow rapidly and disperse widely allowed them to diversify into hundreds of species by the end of the Cretaceous Period. The
importance of the angiosperms to the evolution of dinosaurs cannot be understated. The Cretaceous Period is known for an explosion of new lines of ornithischians—duck-billed, armored, and horned dinosaurs in particular—that developed specialized adaptations for chewing and consuming the wider assortment of vegetation available to them, including the recently evolved flowering plants and gymnosperms. Those special anatomical features will be explored in Section Three of Last of the Dinosaurs, in the discussion of ornithischians of the Cretaceous Period.
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