TRACES OF ORGANISMS
Fossils consist not only of plant and animal remains but of any evidence of their existence. In this type of fossilization there is no direct evidence of the original organism, rather there is some definite indication of the former presence of some ancient plant or animal. Objects of this sort normally furnish considerable information as to the identity or characteristics of the organism responsible for them.
Molds and Casts
Many shells, bones, leaves, and other forms of organic matter are preserved as molds and casts. If a shell had been pressed down into the ocean bottom before the [sediment] had hardened into [rock], it may have left the impression of the exterior of the shell. This impression is known as a mold ([Pl. 2]). If at some later time this mold was filled with another material, this produced a [cast]. This cast will show the original external characteristics of the shell. Such objects are called external molds if they show the external features of the hard parts ([Pl. 2], fig. 2) and internal molds ([Pl. 2], fig. 3) if the nature of the inner parts is shown.
Molds and casts are to be found in almost all of the fossil-bearing rocks of Texas, and they make up a large part of most [fossil] collections. It is particularly common to find fossil clams and snails preserved by this method. This is primarily because their shells are composed of minerals that are relatively easy to dissolve, and the original shell material is often destroyed.
PLATE 3
Silicified Brachiopods
All specimens from [Permian] limestones of the Glass Mountains, Brewster County, Texas
Figures— 1, 2. Avonia sp., ×2. [Ventral] and side view of two pedicle valves showing long slender spines. 3. Avonia sp., ×6. Young specimen showing attachment ring at apex. 4-6. Muirwoodia multistriatus Meek, ×4. Respectively, side and ventral view of pedicle [valve] and [dorsal] view of brachial valve. 7-9. “Marginifera” opima Girty. Respectively, ventral and side view of pedicle valve showing long stout spines (×4) and interior of brachial valve showing muscle scars and brachial ridges (×2). 10-13. Aulosteges tuberculatus R. E. King, ×4. Respectively, side and interior view of brachial valve showing muscle scars; ventral view of pedicle valve showing brush of attachment spines on ears; and ventral view of a young pedicle valve. 14. Avonia sp., ×4. Ventral view of a specimen with long spines. 15, 16. Avonia subhorrida (Meek), ×2. Ventral view of a pedicle valve and dorsal view of a brachial valve showing spines on both. 17. Avonia signata (Girty), ×2. Dorsal view of a large specimen showing hairlike spines on brachial valve. 18-20. Prorichthofenia permiana (Shumard). Respectively, side and [posterior] view of pedicle valve (×4) and interior of dorsal valve (×2) showing anchor spines and interior spines of the brachial valve. 21. Heteralosia hystricula (Girty), ×2. Cluster of individuals attached to a large Marginifera. Photograph courtesy of Dr. G. A. Cooper, U. S. National Museum.
Tracks, Trails, and Burrows
Many animals have left records of their movements over dry land or the sea bottom. Some of these, such as footprints ([Pl. 4]), indicate not only the type of animal that left them but often provide valuable information about the animal’s environment.
Thus, the study of a [series] of dinosaur tracks would not only indicate the size and shape of the foot but also provide some information as to the weight and length of the animal. In addition, the type of [rock] containing the track would help determine the conditions under which the dinosaur lived.
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.
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.
Fig. 2. Sketch of a [gastrolith]—the gizzard stone of an ancient reptile.