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-shapedForaminifera (orbitoidid) 4. Fossil disk-shaped or button-like; with longitudinal, radial partitions or septaCoral C. Fossil composed of segments or plates: 1. Fossil composed of circular segments, disks, or chambers; when united form cylinder: a. Tapered shellCephalopod b. Non-tapered, segments small and of relatively uniform thickness with hole in center; individual columnals disk-shapedCrinoid 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 armsCrinoid 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 suturesCephalopod 2. Shell without internal partitions or sutures[Gastropod] 3. Shell small; spindle-shaped; resembles wheat grain. Common in [Pennsylvanian] and [Permian] rocksForaminifera (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 rocksPelecypod 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-shapedCoral b. Shell with internal transverse partitions or sutures; tapering at one endCephalopod c. Shell without internal septa or partitions. (1) Shell large, heavy; usually with external longitudinal ribs. Occur only in Cretaceous rocksRudistid (2) Shell small (usually less than 2 inches), tusk-shaped, open at both ends. Rare in Paleozoic and Mesozoic rocksScaphopod 3. Fossil heart-shaped, domed or flattened; radial star pattern on topEchinoid 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 crayfishCrustacean 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 rocksPelecypod (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 valveGastropod 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 specimensCephalopod 2. Shell without transverse internal partitions or suturesGastropod 3. Solid spiral ridge around central axis; resembles a corkscrewBryozoa 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 septaCoral 3. Shell resembles a clam or oyster shell but valve or shell not symmetricalPelecypod (mostly oysters) 4. Branching twig-like fossils: a. Fossils covered with minute pores or openingsBryozoa 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 filmsBryozoa 6. Fossils composed of radiating masses of [polygonal] or circular tubes containing radial septaColonial 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 rocksRudistid 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 rocksSponge