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.