Title: Dinosaur debates. Subject(s): FOSSILS; PALEONTOLOGY; DINOSAURS -- Classification Source: Rocks & Minerals, Nov/Dec98, Vol. 73 Issue 6, p376, 2p, 1 diagram, 1c Author(s): MATTY, JANE M. Abstract: Discusses how dinosaurs were classified according to fossils discovered by paleontologists. Evolutionary relationships between fossils found in those of reptiles and birds; Types of fossils collected; Paleontologic evidences on the classification of dinosaurs. AN: 1253269 ISSN: 0035-7529 Note: Tucson-Pima Public Library subscribes to this magazine. Database: MasterFILE Elite Section: GEO-CURRENTS: A LOOK AT RECENT GEOLOGICAL NEWS DINOSAUR DEBATES DINOSAURS HAVE FASCINATED people since they were first recognized nearly two hundred years ago. So different from familiar animals living today, so imposing, they kindle the imagination of children and adults alike. In recent years numerous discoveries have transformed our understanding of dinosaurs and generated some heated debate. Were dinosaurs warm-blooded like birds and mammals? Or were they cold-blooded like reptiles? Was Tyrannosaurus rex a fearsome predator? Or was he merely a scavenger stealing meals that others had killed? Are birds descended from dinosaurs? What really caused dinosaurs to become extinct? The mysteries of the dinosaurs exemplify the excitement of science. We are intrigued by the unexplained and work to develop a story from the scanty clues at hand. As more clues are uncovered, the story may change--or be replaced by a new story that explains all the facts. In the case of dinosaurs and other extinct organisms, the clues are few and far between and come to us across a great expanse of time. In addition, paleontologists have estimated that only a fraction of one percent of all species that ever lived has been found in the fossil record. Thus, telling the dinosaurs' story is a challenge indeed. The most familiar fossils are body fossils, which consist of the remains of ancient organisms. In order for the remains of a dinosaur to persist--and be discovered--several uncommon events must take place. The first step in creating a body fossil is burial of the bones. The best tales are told by complete skeletons, which may occur if the carcass is buried quickly (beneath river sediments, windblown sand, or flood deposits, for example). The next step is converting the bone to mineral as the sediment is lithified to rock. This occurs through processes such as permineralization (where mineral material permeates the pores and cavities of bone) and replacement (where bone is replaced by a different mineral substance, such as silica or pyrite). Eventually, the fossil must be exposed (following uplift and erosion of the host rock) and discovered before it is destroyed. Another important group of fossils are trace fossils, such as footprints, trackways, and nests, which can tell us more about the lives and behavior of dinosaurs. This type of evidence must also be buried quickly if it is to be preserved. From this sort of evidence, paleontologists have made remarkable progress toward understanding dinosaurs--their biology, their habitats, and their habits. Yet much of what is reported is based on only one of several interpretations that could explain the facts. New dinosaur species are continually being discovered, allowing scientists to defend, refine, or revise their hypotheses about dinosaurs. A few recent discoveries that are helping to shape current theories about the dinosaurs are described below. Are birds descended from dinosaurs? Most paleontologists are convinced that birds are direct descendants of one group of dinosaurs--the theropods (fig. 1). Some scientists disagree, however, so debate on the subject continues. Data that support this theory include numerous similarities in anatomy and bone structure shared by birds and dinosaurs. In addition, evidence that dinosaurs exhibited certain characteristic bird behaviors, such as nesting and migration, also supports the theory. Several new fossil finds have provided additional clues. For example, a recent discovery in China, that of a well-preserved dinosaur with what appear to be featherlike structures on its back, contributes new fuel to the debate. Opponents of the theory that birds are descended from dinosaurs contend that shared characteristics--such as wings, feathers, and anatomical similarities--may have developed independently in dinosaurs and birds and that such analogous structures are merely coincidental. Were dinosaurs warm-blooded or cold-blooded? The direct evidence for this debate comes from analysis of dinosaurs' bones, and much of the evidence is apparently contradictory (see table). Many dinosaur bones exhibit growth lines, similar to the growth rings found in trees. This feature is characteristic of modern cold-blooded animals, in which growth slows or stops during cold seasons. A recent discovery in Australia, however, suggests that at least one type of dinosaur was very likely warm-blooded. During the Cretaceous Period, southern Australia, where these dinosaurs lived, was located south of the Antarctic Circle and was subject to bitterly cold winters. Yet the bones of these dinosaurs display no growth lines, indicating that they were probably warm-blooded. It may be that different dinosaur species had different physiologies, or even that at least some dinosaurs did not fit into either category but possessed an intermediate type of metabolism. Were the dinosaurs killed by an asteroid impact? There is convincing evidence of a devastating impact near the Yucatan Peninsula at the time of the dinosaurs' extinction (see Geo-Currents in the July/August 1992 issue of Rocks & Minerals). Whether this impact caused the mass extinctions observed at the end of the Cretaceous Period is a more difficult question to answer; some scientists think the impact was merely the final act in an extinction event that was already under way. Recent discoveries on the sea floor provide new data that may help answer the question. Sediment cores collected off the coasts of Florida and New Jersey each contain a definitive transition zone consisting of several parts. The base of the sequence consists of Cretaceous-aged sediments with plentiful marine fossils. Above that is a layer of impact debris, covered, in the case of the Florida core, by remains of the asteroid itself. Overlying these layers is a bed of clay (representing perhaps five thousand years) that is devoid of fossils. These cores present compelling evidence that the effects of this impact were widespread and long lasting. For More Information Enjoyable accounts of the development of theories about dinosaurs include Robert Bakker's popular book, The Dinosaur Heresies (William Morrow and Company, 1986) and T-rex and the Crater of Doom (Princeton University Press, 1997) by Walter Alvarez, one of the original proponents of the impact-extinction hypothesis. Much is available on the Internet, as well; for some reliable information check the National Museum of Natural History's Paleobiology homepage at http://www.nmnh.si.edu/departments/paleo.html, the Dinosaur Society's homepage at http://www.dinosociety.org, and Dino Russ's Lair homepage at http://denr1.igis.uiuc.edu/isgsroot/dinos/dinos(underbar)home.html. Some of the evidence used to infer dinosaur physiology. Legend for Chart: A - Characteristics observed B - Suggested physiology A B Growth of bones: Growth rings -- indicate seasonal slowing of growth Cold-blooded Evidence that bones grew rapidly, then stopped at adulthood Warm-blooded Structure of nasal passages: Lack of complex turbinate structures Cold-blooded Oxygen isotopes in bones: Consistent ratios throughout skeleton Warm-blooded DIAGRAM: Figure 1. Evolutionary relationships among fossil and living reptiles and birds. The dinosaur species formed two main groups: the Ornithischians ("bird-hipped" dinosaurs) and the Sanrischians ("lizard-hipped" dinosaurs). Ornithischian dinosaurs include Stegosaurus, Iguanodon, and Triceratops. Saurischian dinosaurs include the plant-eating Sauropods (such as Apatosaurus) and the meat-eating theropods (such as Tyrannosaurus and Velociraptor). Most paleontologists are convinced that modern birds are descended from theropod dinosaurs. PHOTO (COLOR): Figure 2. Skeleton of Tyrannosaurus rex, on display in ... Figure 2. Skeleton of Tyrannosaurus rex, on display in the Black Hills Institute Fossil Gallery, Hill City, South Dakota. In a recent study it was found that the bones of T. rex faces are shaped in a way that may have allowed good binocular vision, hence good depth perception. Such an adaptation supports the idea that T. rex was a hunter not a scavenger. ~~~~~~~~ By JANE M. MATTY, Geology Department Central Michigan University Mount Pleasant, Michigan 48859 Dr. Jane M. Matty, an environmental geologist at Central Michigan University, welcomes suggestions and news items for this column. _________________ Copyright of Rocks & Minerals is the property of Heldref Publications and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. Source: Rocks & Minerals, Nov/Dec98, Vol. 73 Issue 6, p376, 2p, 1 diagram, 1c. Item Number: 1253269