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Show - \\\\\''\ Clues to the Early Prehistory of Utah PALEONTOLOGISTS READ THE ROCKS ALMOST AS EASILY AS YOU READ YOUR BOOKS BY JAMES H MADSEN. JR THE EARLIEST RECORD The title " clues to the early prehistory of Utah" may be read in different ways. To the archaeologist it suggests human activity and the discoverv of associated artifacts dating back from about 200 to 15.000 years ago. But to the paleontologist it brings to mind the fossil remains of the plants and animals that inhabited Utah during the last 600 million years - long before the first human habita-tion. The full story of such an immense span of time would be much too long for this maga-zine. So, let's omit the first three- fourths of the earth's history and begin our storv a mere one billion vears ago. Little organic evidence can be found to reconstruct the life of Pre-cambrian times, because those organisms had no hard parts to be preserved as fossils. Hnwever. a formation that outcrops in Big Cottonwood Canyon east of Salt Lake City - the Cottonwood Tillite - does provide an SKULL AND MANDIBLE OF THE DINOSAUR ALLOSAURUS FRAGILIS, BASED ON A DRAWING BY SANDIE KESLER. important clue to Utah's climate a billion years ago. At least part of Utah was covered by glacial ice during the end of the Precam-brian period. How can we tell? The record of the rocks can be read as easily as the pages of a book by the paleontologist or geologist who uses one of the most basic, important, and logical concepts in the earth sciences: the p m t is the key to the past. This phrase means that when we compare the results of modern gla-ciation with the record in the rocks of Big Cottonwood Canyon, for example, we can conclude that the same causes produced iden-tical results. Another important principle in under-standing the record of the rocks is the law of superposition. This law states that the oldest formations were deposited first and. there-fore. are on the bottom of the stack. Later de-position of sedimentary units or strata are each one on top of the other. The latest. or most recent. is on the very top - providing, of course. that the layers have not been dis-turbed by faulting or folding. THE APPEARANCE OF FOSSILS In Big Cottonwood Canyon the rocks over-lying those of glacial origin record more changes over the next several hundred mil-lion years. There we see e monotonous se-quence of drabcolored limestones and quartz-ites that indicates a transition in deposition and environment from an emergent land sur-face covered with glaciers ( very cold and lacking fossils of any kind) to a shallow in-land sea with a few recognizable fossils. A period of more or less continuous deposition in which the greater part of western Utah was rapidly subsiding is indicated by the lime-stones. This sinking allowed flooding by shal-low marine seas, an episode that was more or less continuous for the next 150 f million years. The fossiliferous gray and buff limestones are the lithified ( changed to stone) products of the lirney ooze deposited on the bottom of such an ocean. Much of Utah was covered by one of western America's great oceans for millions of years. Reference has been made to an ocean. and limestone is definitely a very good due. but what else? A closer look at the rock out-crops often reveals a variety of fossils that become noticeably more complex from the bottom to the top of a thick rock sequence. Near the bottom are trilobites ( see fig. 11. Those remnants of past life are commonly found in the limestones of western Utah. They provide an excellent clue to the paleon-tologist about the depth and, most importantly. the age of the sediments around them. which in this case are approximately 500 million years old. A little farther up in the stack of lime-stones deposited some 400 million years ago you can find the shell- like remains of Utah's first fish with tongue- twister names like Cepholaspis, Protuspis. and how about this one - CardipeItis ( see fig. 21. Farther up yet in the pile of liillestones one may see other signs of change. Remember the law of super-position? The limestones begin giving way to shale. meaning that the oceans are either withdrawing, becoming shallow, or a com-bination of the two. One of the best places to FIG. 2. DORSAL SHIELDS OF PRIMITIVE DEVONIAN FISH FROM THE LAKESIDE MOUNTAINS WEST OF GREAT SALT LAKE: ABOVE, CARDIPEL TIS: BELOW, PROTASPIS. t- lti. 3. FUSSIL PLANTS FRVM THE MISSISSIPPIAN SHALES WEST OF UTAH LAKE. view the evidence of this change is west of Utah Lake. There, in several clay quarries, are beautifully preserved remains of Mississip pian plants that are approximately 300 mil-lion years old ( see fig. 31. The fossil leaves tell us that a land area was near and the water shallow. Although the fossil and rock record can be read fairly well in the rocks within a dis-tance of 25 miles from Salt Lake and Utah valleys, there are specific localities in Utah that have a more complete and clearer record - but back to our story. Following a relatively brief period of emergence, Utah goes under water again, and the rocks once more record the evidence of past life in a marine environment with ani-mals such as brachiopods and cephalopods. The trilobites are quite rare in this part of the record. Evidence is strong that more success-ful forms may have outhunted or outcompeted them for the necessities of life. THE FIRST DYING TIME Finally, this lengthy episode of many oceans is marked by great changes in the rocks, most dramaticalIy illustrated by their color. The gray limestones give way to buffs, yellows, oranges, and shades of red. In terms of radical differences in life forms this may be called the end of one of several great dying times. The trilobites are gone, all extinct. Many kinds of brachiopods are gone, extinct too, and only a relatively few familiar ones survive. Most of the brachiopods were re-placed by similar forms, their cousins the pelecypods, the more successful animals that we commonly call clams. Why were some animals more successful than others? Let's take a closer look at the cause and effect of very close competition. Brachiopods and pelecypods are fairly simi-lar as far as environmental needs are con-cerned. They are both bivalves and have similar food and oxygen requirements. But there are two significant differences. First, the shells of the brachiopod are opened and closed by two complex sets of muscles. But the shells of the pelecypod are closed by a single set of adductor muscles and opened automatically by an elastic pad when those muscles are relaxed - a remarkable conser-vation of energy. Second, the feeding appara-tus of the brachiopod is relatively complex and sensitive. In contrast, that of the pelecy-pod is simple and less sensitive to chemical and other contaminants. The case seems straightforward, although oversimplified here: all things being environmentally equal, the pelecypod had a clear edge in surviving matic change took place that produced what one geologist calls the " great sandpile." Dur-ing this span of time huge accumulations of wind- blown sand were deposited in eastern and southern Utah. They make up many of the scenic rock formations seen in Canyonlands and Arches National Parks. THE DINOSAURS APPFAR In the next few pages of Utah's rock- record is one of the most dramatic sequences in the whole story - the dinosaur chapter. There are few places in the entire world where this - part of geologic time is as well illustrated. '- Much of the eastern half of Utah was a play- 4. - OF A PELECY WD{ LEFT) AND A ground for dinosaurs. A brief part of their BRACHIOPOD ( R1GHT). life and times is well documented in the rocks or competing successfully against the brachi* pod when things got tough. Back to the main story. Where the rocks are reddish they indicate an emergent Utah with, perhaps, only restricted, shallow water areas. Good examples of these red rocks some 200 million years of age are seen in the mouth of Parleys Car~ yuri or1 the east side of Salt Lake Valley and in many other scenic areas of Utah. Although there are no really good organic remains of ancient life in these sediments there is the next best thing - trace fossils. Trace fossils are the tracks and trails of former Utah residents. Such traces are really quite common in Utah all through our state's rock record. Some of the oldest are thought to have been made by trilobites ( fig. 1). Others from the red beds mentioned above were made by amphibians and early dinosaurs and their crocodilian cousins ( fig. 5). Those of numer-ous, later dinosaurs that ambled about in Carbon and Emery counties ( fig. 6) are locally well known to residents and visitors in east-central Utah. In fact, a world's record track that measured 48 inches in length was col-lected from one of the many coal mines in Carbon and Emery counties and is now on exhibit in a museum in Pittsburgh, Pennsyl-vania. An accurate cast of this giant footprint may be seen in the Utah Museum of Natural History ( fig. 7) at the University of Utah in Salt Lake City, a good place to see exhibits of Utah's other " very senior" citizens. Foilowing the deposition of the red beds there was a brief interlude when another shallow marine sea invaded Utah once more. It then withdrew suddenly as a dramatic cli- CALLED PTEROSAURS. FIG. 6. REWILE TRACK FROM NEAR DEVIL'SSLIDE of Dinosaur National Monument east of Ver-nal, Uintah County, and south of Price in Emery County at the Cleveland- Lloyd Dino-saur Quarry. Following this first dinosaur episode the marine seas invaded Utah one last time, leav-ing the remains of familiar forms such as fossil clams, oysters, and sharks' teeth, not to mention the coiled shells of cephalopods ( fig. 8). The shelled cephalopods are survived by modern counterparts such as the pearly nautilus, the octopus, and the squid. Finally, with numerous fluctuations, the oceans withdrew from the region for the last time. Then the great thicknesses of vegetation began accumulating that would become the coal of today. A second group of dinosaurs might have been seen frolicking in east- central Utah not suspecting that they, as many other groups of the past, were about to make their last appearance on the Utah scene. Although these last gigantic saurian resi-dents of Utah left few bones, their tracks are common in the coal mines of east- central Utah They walked across the vast thicknesses of vegetation leaving footprints that were rapidly filled with sand and clay that con-tinued to accumulate to great depths. In time the vegetation was changed to coal by the great pressures and temperatures generated at depth. The sand was changed to sandstone and the clays to shale. Today, as miners re-move the coal they expose the tracks which often ornament the ceiling or back of the coal mines. THE SECOND DYING TIME The accumulation of vegetation that was to become coal, the last withdrawal of the great oceans, and the rumbling that signaled the onset of a time of mountain building and severe climate change. - these three events heraIded another great dying time that would remove all of the dinosaurs from the scene forever. The period of mountain building and climate change that erased the dinosaurs once and for all was followed by worldwide changes to seasonality that could best be tolerated by endotherms ( warm- blooded ani-mals). So it was that the furry animals we call mammals and the feathered ones we call birds became the dominant, land- living, verte-brate animals. As mammals became more widespread and diverse there was another invasion of the land by a body of water. This time the water was fresh, however, forming an im-mense lake that almost surrounded the young Uinta Mountains. The fossil record of the Green River lake system is as perfectly pre-served as could ever be expected with such diverse fossils as bird tracks, many kinds of fish, bird feathers and bones, flowers, leaves, turtles, numerous mammals, lizard skins, mollusks, and crocodilians. This period rep-resented the last, well- documented chapter as far as fossils are concerned. Even though the sedimentary record continues right up to the present, no more exceptionally significant fossil accumulations have been found in Utah. FIG. 7. DINOSAURTRACK FROM AN EMERY COAL I I MINE. THE LAST CHAPTER Still, one last chapter was recorded, Being the last, it is most easily read in the bands of sand and gravel that rim the Utah and Salt Lake valleys as we11 as many of the Great Basin valleys to the west. These are the beach and near- shore deposits of ancient Lake Bon-neville, a remnant of which survives now as North America's only inland salt sea, Great Salt Lake. This was the last, and therefore the most recent, time of great change, a time when glaciers capped the Uinta Mountains canyons to the east of Salt Lake. North Arneri- CEPHALOPOO GULCH EAST OF THE UNIVERSITY ca was in a real deep freeze, and the animals OF UTAH-of this region suffered one last dying time. Familiar forms like deer, coyote, bobcat, cougar, porcupine, ground squirrel, skunk, weasel, and bison survived. The saber- tooth cat, ground sloth, horse, camel, and mam-moth could not tolerate the change from an FIG.^. FOSSIL BIRD TRAFROCM NEEAR %'-= SOLDIERS SUMMIT. arctic climate to one that is comfortable to us who now survive in temperate North I America. What of the fossil record that we have read? What is to become of the fossils that illusbate each page? If the fossils are taken away it will be as if the pictures are cut out of our book. If the pictures are gone, who can I understand the pages? Who will want to read the pages? As we witness the continuing destruction of well- known fossil beds in Utah, we wonder how much longer they will last. Whose grand- I children will be able to hike the trails of Utah and see trilobites, bird tracks, fossil fish, I dinosaur bones, and petrified wood? I think my gandchiidren will still be able to see some of these clues to Utah's prehistory, How about yours? Mr. Madsen is the state paleontolopist of Utah. |
