Geoscience Reference
In-Depth Information
How many skeletal elements?
One
Two
Many
Is the open end of
the shell subdivided
by radial walls?
(valves)
Bilateral
symmetry of a
row of transverse
elements
Five-fold
symmetry around
central axis; may
be secondarily
modified
Yes
No
Is there any shelly
evidence for the
body not having
occupied all of the
shell interior (e.g.
in internal moulds)?
Plane of symmetry
cuts through
both valves (few
exceptions)
(trilobed body
with head-shield,
trunk and
tailpiece
No arms
or stalk
free-living
Arms and
stalk; usually
attached to
sediment
Plane of symmetry
passes between
valves (some
common exceptions)
Yes
No
(radial septa)
(septa with
complex sutures;
shell also
usually coiled
in one plane)
(shell usually
coiled
asymmetrically
into a cone-
shaped spiral)
Coral
Ammonoid
Gastropod
Brachiopod
Bivalve
Trilobite
Echinoid
Crinoid
Figure A5.1
A simple scheme for the identifi cation of invertebrate fossils based on symmetry.
