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.
[Illustration: 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.