ORIGINAL HARD PARTS OF ORGANISMS
Almost all plants and animals possess some type of hard parts which are capable of becoming fossilized. Such hard parts may consist of the shell material of clams, oysters, or snails, the teeth or bones of vertebrates, the exoskeletons of crabs, or the woody tissue of plants. These hard parts are composed of various minerals which are capable of resisting weathering and chemical action, and fossils of this sort are relatively common.
Many of the [fossil] mollusks found in the [Tertiary] and [Cretaceous] rocks of Texas have been preserved in this manner. In some of the specimens the original shell material is so well preserved that the iridescent mother-of-pearl layer of the shell is found virtually intact. This type of preservation is less common, however, in the older rocks of the State.
PLATE 2
Types of [Fossil] Preservation
Figures— 1. Internal mold of a Texas [Cretaceous] [ammonite] (×½). 2. Internal and external molds of gastropods and pelecypods in Cedar Park limestone member of the Walnut clay of Comanchean age (×½). Specimen from quarry near Cedar Park, Williamson County, Texas. 3. Internal mold of a Texas Cretaceous [pelecypod] (×½). 4. [Fossil] worm tubes on mold of a Cretaceous ammonite (×½). 5. Petrified or permineralized mammal bone of [Tertiary] age (×½). 6. Internal mold (steinkern) of a typical Texas Cretaceous [gastropod] (×½). 7. Carbon residue of a Tertiary fish (×¼).
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.