Some of the world’s most famous dinosaur tracks are to be found in the Lower Cretaceous limestones in Somervell County, Texas. These footprints, which are about 110,000,000 years old (Pl. 4), were discovered in the bed of Paluxy Creek near the town of Glen Rose. Large segments of the rock containing these tracks were collected by paleontologists of the American Museum of Natural History in New York City and the Texas Memorial Museum at Austin. Great slabs of limestone were transported to the museums, replaced in their original position, and are now on display as mute evidence of the gigantic size of these tremendous reptiles. Invertebrates also leave tracks and trails of their activities, and these markings may be seen on the surfaces of many sandstone and limestone deposits. These may be simple tracks, left as the animal moved over the surface, or the burrows of crabs or other burrowing animals. Markings of this sort provide some evidence of the manner of locomotion of these organisms and of the type of environment that they inhabited. Coprolites Coprolites are fossil dung or body waste (fig. 1). These objects can provide valuable information as to the food habits or anatomical structure of the animal that made them. [Illustration: Fig. 1. Sketch of a coprolite—fossilized animal excrement.] Gastroliths These highly polished well-rounded stones (fig. 2) are believed to have been used in the stomachs of reptiles for grinding the food into smaller pieces. Large numbers of these “stomach stones” have been found with the remains of certain types of dinosaurs. [Illustration: Fig. 2. Sketch of a gastrolith—the gizzard stone of an ancient reptile.] PSEUDOFOSSILS Among the many inorganic objects formed by nature there are some that bear superficial resemblance to plants or animals. Because they are often mistaken for organic remains, these objects have been called _pseudofossils_, or “false fossils.” Dendrites [Illustration: Fig. 3. Dendrites. These thin branching mineral deposits bear a marked resemblance to plants, hence they are called pseudofossils.] Although these closely resemble the remains of ferns or other plant material (fig. 3), dendrites are actually thin incrustations of manganese dioxide. They are often found along the bedding planes of Cretaceous and Paleozoic (geologic time scale, Pl. 1) limestones in many parts of Texas. [Illustration: Plate 4 Dinosaur tracks in limestone in bed of Paluxy Creek near Glen Rose, Somervell County, Texas. Photograph courtesy of the American Museum of Natural History. Permission to reproduce by R. T. Bird.] Slickensides