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]).