At certain localities in north and central Texas the Woodbine sands of Upper Cretaceous age (geologic time scale and geologic map, Pls. 1, 10) contain large numbers of shark and fish teeth (Pl. 37), fish scales and vertebrae. The remains of these vertebrates are unusually well preserved and are prized by both amateur and professional collectors.
Calcareous Remains
Hard parts composed of calcite (calcium carbonate) are very common among the invertebrates. This is particularly true of the shells of clams, snails, and corals. Many of these shells have been preserved with little or no evidence of physical change (Pl. 2).
Phosphatic Remains
The bones and teeth of vertebrates and the exoskeletons of many invertebrates contain large amounts of calcium phosphate. Because this compound is particularly weather resistant, many phosphatic remains (such as the fish teeth in the Woodbine sands) are found in an excellent state of preservation.
Siliceous Remains
Many organisms having skeletal elements composed of silica (silicon dioxide) have been preserved with little observable change. The siliceous hard parts of many microfossils and certain types of sponges have become fossilized in this manner (Pl. 14).
Chitinous Remains
Some organisms have an exoskeleton (outer body covering) composed of chitin, a material that is similar to finger nails. The fossilized chitinous exoskeletons of arthropods and other organisms are commonly preserved as thin films of carbon because of their chemical composition and method of burial.
ALTERED HARD PARTS OF ORGANISMS
The original hard parts of an organism normally undergo great change after burial. These changes take place in many ways, but the type of alteration is usually determined by the composition of the hard parts and where the organism lived. Some of the more common processes of alteration are discussed below.
Carbonization
This process, known also as distillation takes place as organic matter slowly decays after burial. During the process of decomposition, the organic matter gradually loses its gases and liquids leaving only a thin film of carbonaceous material (Pl. 2, fig. 7). This is the same process by which coal is formed, and large numbers of carbonized plant fossils have been found in many coal deposits.
In Texas the carbonized remains of plants, fish, and certain invertebrates have been preserved in this manner, and some of these carbon residues have accurately recorded even the most minute structures of these organisms.
Petrifaction or Permineralization
Many fossils have been permineralized or petrified—literally turned to stone. This type of preservation occurs when mineral-bearing ground waters infiltrate porous bone, shell, or plant material. These underground waters deposit their mineral content in the empty spaces of the hard parts making them heavier and more resistant to weathering. Some of the more common minerals deposited in this manner are calcite, silica, and various compounds of iron.
Replacement or Mineralization
This type of preservation takes place when the original hard parts of organisms are removed after being dissolved by underground water. This is accompanied by almost simultaneous deposition of other substances in the resulting voids. Some replaced fossils will have the original structure destroyed by the replacing minerals. Others, as in the case of certain silicified tree trunks, may be preserved in minute detail.
Although more than 50 minerals have been known to replace original organic structures, the most frequent replacing substances are calcite, dolomite (a calcium magnesium carbonate), silica, and certain iron compounds.
Replacement by calcareous material
Calcareous replacement occurs when the hard parts of an organism are replaced by calcite, dolomite, or aragonite (a mineral which is composed of calcium carbonate but which is less stable than calcite). The exoskeletons of many corals, echinoderms, brachiopods, and mollusks have been replaced in this manner.
Replacement by siliceous material
When the original organic hard parts have been replaced by silica the fossil is said to have undergone silicification, and this type of replacement often produces a very high degree of preservation. This is particularly true of the silicified Permian (geologic time scale, Pl. 1) fossils from the Glass Mountains in Brewster County. These fossils are embedded in limestone which must be dissolved in vats of acid, and after the enclosing rock has been dissolved the residue yields an amazing variety of perfectly preserved invertebrate fossils (Pl. 3).
Silicified Cretaceous fossils have been recovered from the Edwards limestone of central Texas. The silicified fauna is restricted to a few scattered localities, each of which may yield many unusually well-preserved fossils.
Replacement by iron compounds
Several different iron compounds have been known to replace organic matter. Many Texas limestones contain fossil snails and clams which have had their original shell material replaced by iron compounds such as limonite, hematite, marcasite, or pyrite. Certain of the fossiliferous Tertiary sandstones of the Texas Gulf Coast area contain large amounts of glauconite which commonly replaced organic material.
In some areas entire faunas have been replaced by iron compounds. Such is the case in the famous “Pyrite Fossil Zone” of the Pawpaw formation (Lower Cretaceous) in Tarrant County. The fossils in this part of the formation are very small or “dwarfed” and have been replaced by limonite, hematite, or pyrite. Ammonites, clams, snails, and corals are particularly abundant at this locality.
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.
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
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.
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
These are striations that are produced when rock surfaces move past each other while being fractured. Slickensides may superficially resemble certain of the Pennsylvanian coal plants of Texas.
Since slickensides are commonly at an angle to the bedding plane and plant remains lie parallel to the bedding plane, the two are usually easily distinguished.
Concretions
Many shales and sandstones contain hardened masses of minerals and rock that are often mistaken for fossils. These masses, called concretions, are usually found weathered out of the surrounding rock and may assume the shape of bones, flowers, vegetables, turtles, etc. Although these concretions do not represent organic remains, it is sometimes possible to find true fossils inside them.