Fossil identification keys may be useful in helping the beginning collector identify specimens. The collector compares a fossil with the key description and eliminates those characters that do not fit the specimen.
The key used in this handbook is based primarily on symmetry—the orderly arrangement of the parts of an object with reference to lines, planes, or points. The shape of the shell or body, presence or absence of coiling, and presence or absence of body partitions are also useful criteria in identifying fossils. To use the key the beginner should know something about symmetry. Two major types of symmetry are used in this key.
1. Radial symmetry—the symmetrical repetition of parts around an axis. This is the symmetry of a wheel, and any vertical section through the center of the object divides it into symmetrical halves (fig. 4a).
2. Bilateral symmetry—the symmetrical duplication of parts on each side of a plane (fig. 5). The plane divides the object into two halves that are mirror images of each other. This is the symmetry of a plank.
It should be noted that many objects may have both kinds of symmetry. For example: A cone when viewed from the top has radial symmetry and when viewed from the side shows bilateral symmetry (fig. 4a, b).
Fig. 4. Types of symmetry in a fossil coral. (a) Radial symmetry. (b) Bilateral symmetry.
Once a tentative identification has been made from the key, pictures and descriptions of this fossil group are examined to establish a more precise identification. It should be remembered that keys are not perfect, and the collector should not expect to be able to identify every specimen with this key.
IDENTIFICATION KEY TO MAIN TYPES OF INVERTEBRATE FOSSILS
(Instructions on pages 23-25 for use of key)
- I. Fossils displaying radial symmetry—symmetrical repetition of parts around a central axis
- A. Fossil tapering, cylindrical, cone-shaped:
- 1. Fossil with longitudinal radial partitions or septa; cone-shaped
- Coral
- 2. Shell with transverse septa or sutures; tapering at one end
- Cephalopod
- 3. Shell without internal septa or partitions:
- a. Shell large, heavy; usually with external longitudinal ribs. Occur only in Cretaceous rocks
- Rudistid
- b. Shell small (usually less than 2 inches long), tusk-shaped, open at both ends. Rare in Paleozoic and Mesozoic rocks
- Scaphopod
- B. Fossil disk-shaped or flattened dome-shaped:
- 1. Fossil with radiating star pattern on top
- Echinoid
- 2. Fossil subconical to hemispherical, dome-shaped; base concave or flat; minute pits or pores covering surface; typically small (less than 3 inches across)
- Bryozoa
- 3. Fossil small (less than ½ inch); generally disk-shaped
- Foraminifera (orbitoidid)
- 4. Fossil disk-shaped or button-like; with longitudinal, radial partitions or septa
- Coral
- C. Fossil composed of segments or plates:
- 1. Fossil composed of circular segments, disks, or chambers; when united form cylinder:
- a. Tapered shell
- Cephalopod
- b. Non-tapered, segments small and of relatively uniform thickness with hole in center; individual columnals disk-shaped
- Crinoid stem
- 2. Fossil composed of many-sided plates:
- a. Bud-shaped fossil of 13 wedge-shaped plates
- Blastoid
- b. Cup-shaped fossil of many curved plates surrounded by branching arms
- Crinoid
- II. Fossils displaying bilateral symmetry—symmetrical duplication of parts on each side of a plane
- A. Fossil coiled in a single plane:
- 1. Shell divided by internal transverse partitions or sutures
- Cephalopod
- 2. Shell without internal partitions or sutures
- Gastropod
- 3. Shell small; spindle-shaped; resembles wheat grain. Common in Pennsylvanian and Permian rocks
- Foraminifera (fusulinids)
- B. Fossil not coiled:
- 1. Shells or valves similar to clams:
- a. Plane of symmetry parallel to hinge; equivalved
- Pelecypod
- b. Plane of symmetry (almost bilaterally symmetrical) at right angles to hinge line; mostly inequivalved; strongly ribbed. “Scallop-like” with “ears.” Rare in Paleozoic rocks
- Pelecypod
- c. Plane of symmetry at right angles to hinge line; inequivalved; not “scallop-like” and without “ears.” Larger valve commonly has an opening in beak. Common in Paleozoic rocks
- Brachiopod
- 2. Fossil tapering, cylindrical, cone-shaped:
- a. Fossil with internal longitudinal, radial septa or partitions; cone-shaped
- Coral
- b. Shell with internal transverse partitions or sutures; tapering at one end
- Cephalopod
- c. Shell without internal septa or partitions.
- (1) Shell large, heavy; usually with external longitudinal ribs. Occur only in Cretaceous rocks
- Rudistid
- (2) Shell small (usually less than 2 inches), tusk-shaped, open at both ends. Rare in Paleozoic and Mesozoic rocks
- Scaphopod
- 3. Fossil heart-shaped, domed or flattened; radial star pattern on top
- Echinoid
- 4. Fossil segmented:
- a. Fossil divided into 3 lobes; may be curled up. Not found in Mesozoic or Cenozoic rocks
- Trilobite
- b. Fossil flattened or elongate; resembles shrimp, crab, or crayfish
- Crustacean
- III. Fossils displaying no apparent symmetry
- A. Shell without transverse internal partitions or sutures:
- 1. Shell coiled like ram’s horn, low spired, opening of shell very large; surface has concentric ridges. Shell has two valves; smaller, flattened valve not often found. In Texas found only in Cretaceous rocks
- Pelecypod
- (Note: Some Paleozoic gastropods, “2,” closely resemble larger valve of these pelecypods)
- 2. Shell tightly coiled; most have higher spire than “1.” Opening of shell smaller than “1”; shell not as rough as “1” and has only one valve
- Gastropod
- B. Coiled fossils; coiling not in one plane:
- 1. Shell with transverse internal partitions or sutures:
- a. Partitions always smooth; thick shelled; loosely and irregularly coiled, usually in large compact masses of many individual shells. Occur only in Cretaceous rocks
- Caprinid
- b. Partitions (sutures) usually wrinkled; relatively thin shelled; mostly regularly and tightly coiled; occur as separate individual specimens
- Cephalopod
- 2. Shell without transverse internal partitions or sutures
- Gastropod
- 3. Solid spiral ridge around central axis; resembles a corkscrew
- Bryozoa
- C. Uncoiled fossils:
- 1. Fossil resembles a narrow saw blade; typically found as thin film of carbon. Not found in Mesozoic or Cenozoic rocks
- Graptolite
- 2. Fossil irregularly cone-shaped; longitudinal radial partitions or septa
- Coral
- 3. Shell resembles a clam or oyster shell but valve or shell not symmetrical
- Pelecypod (mostly oysters)
- 4. Branching twig-like fossils:
- a. Fossils covered with minute pores or openings
- Bryozoa
- b. Fossils with evenly distributed, relatively large openings with longitudinal radial partitions or septa
- Colonial coral
- 5. Lace-like fossils; occur as thin sheets or films
- Bryozoa
- 6. Fossils composed of radiating masses of polygonal or circular tubes containing radial septa
- Colonial coral
- 7. Irregular fossils; typically cylindrical with rough surface:
- a. Fossil has large axial opening and thick wall; usually has external longitudinal ribs. Occurs only in Cretaceous rocks
- Rudistid
- b. Fossil solid with no large axial opening; surface with small pits or pores (fewer than in Bryozoa). In Texas, occurs most commonly in Pennsylvanian and Permian rocks
- Sponge
