Some fossils display no apparent symmetry and such a fossil would be referred to Part III of the key. If this fossil had internal transverse partitions “A” would be eliminated. If the fossil was not a coiled fossil “B” would also be eliminated and we would proceed directly to “C”—uncoiled fossils. If the specimen is a branching twig-like fossil, numbers 1, 2, and 3 would be eliminated and the specimen referred to number 4 (Branching twig-like fossils). Should the specimen have evenly distributed relatively large openings with radial longitudinal partitions or septa, the specimen is probably a colonial coral (“b” under number 4 on the key). The “a” part of number 4 would be eliminated because the coral had large openings and radial longitudinal septa.
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.