(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