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The AAPG/Datapages Combined Publications Database

Grand Junction Geological Society

Abstract


Paleontology and Geology of the Dinosaur Triangle, 1987
Pages 45-54

Remains of Ancient Life in Cretaceous Rocks of the Dinosaur Triangle

Robert G. Young

Abstract

Cretaceous rocks of the Dinosaur Triangle were formed in diverse environments; thus the numbers and varieties of plant and animal remains found in a unit depend on whether it formed in a floodplain, coastal plain, deltaic, swamp, lagoonal, beach, or marine environment. The oldest unit present (Cedar Mountain/Burro Canyon Formation) was formed in piedmont, lacustrine, and floodplain environments. Many plant fossils are known, but only an occasional dinosaur bone is present. Numerous eggshell fragments and “gizzard stones” suggest dinosaurs must have been common.

The Dakota Sandstone formed in coastal plain, lagoonal, swamp, deltaic, and beach environments where plants were sufficiently abundant to form carbonaceous shales and coals. Many animals must have been present, but most of their remains were probably dissolved by swamp waters. Some brackish-water mollusks and many trails (ichnofossils) suggest abundant invertebrates.

The thickest of the stratigraphic units is the Mancos Shale, which formed in a marine environment. It contains abundant remains of foraminifera, gastropods, pelecypods, cephalopods, crinoids, fish, and even a few bones of marine reptiles (mosasaurs and plesiosaurs). Many of these forms are poorly preserved and disappear quickly on exposure.

At the top of the Cretaceous is the Mesaverde Group, which consists of two parts. The lower part, formed in transitional environments near the sea, is rich in coal and carbonaceous shale, Consequently, plant remains are common and there are pelecypod shells, turtle shells, crocodile remains, dinosaur footprints, and even a dinosaur nest complete with eggs. The upper part formed on floodplains at some distnace from the sea. Sandstones, representing the fills of old river channels, have yielded a few remains of fish, turtles, crocodiles, and dinosaurs, and some clams and snails.


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