gastropods. The more advanced articulate brachiopods appeared in the
Devonian and became important stratigraphic markers for the period. Rocks
from the Cambrian and Devonian periods contain brachiopod fossils and
wave marks, indicating that some ancient forms inhabited the shore areas.
Modern forms, which number about 260 species, inhabit warm ocean bot-
toms from a few feet to more than 500 feet deep. Some rare types thrive at
depths approaching 20,000 feet.
Brachiopod shells are lined on the inside of the valves with a membrane
called a mantle.This encloses a large central cavity that holds the lophophore,
which functions in food gathering. Projecting from a hole in the valve is a
muscular stalk called a pedicel by which the animal is attached to the seabed.
The structure of the valves aids in the identification of various brachiopod
species.The shells come in a variety of forms, including ovoid, globular, hemi-
spherical, flattened, convex-concave, or irregular. The surface is smooth or
ornamented with ribs, grooves, or spines. Growth lines and other structures
show changes in form and habit that offer clues to brachiopod history.
The bryozoans were tiny coral-like colonies, with an encrusting, branch-
ing, or fanlike structure.They were a major group of marine fauna that witnessed
a marked change from early to late Paleozoic.The stony bryozoans, which were
particularly abundant in the Ordovician and Silurian, declined to insignificance
by the Devonian.The lacy forms greatly diversified and dominated all bryozoan
groups, reaching a peak in the Devonian and Carboniferous. Delicate, twiglike
bryozoans were also common but declined significantly in the Permian, with
only a few groups surviving the extinction at the end of the period.
The tabulate corals that dominated the Ordovician and Silurian were
not nearly as abundant in the Devonian. The tetracorals, which had fourfold
asymmetry with septa arranged in quadrants, were an important group of reef-
building corals that reached their peak during the Devonian. They built the
reef that forms the Falls of the Ohio River at Louisville, Kentucky.The tetra-
corals were still common in the Carboniferous but declined significantly in
the Permian, as the seas in which they thrived receded.They possibly gave rise
to the hexacorals, with six-sided sepia. Most tabulate corals and tetracorals
failed to survive the Permian extinction.
The glass sponges, with an interlocking gridwork of siliceous spicules,
were very common in the shallow Devonian seas. They consisted of glasslike
fibers of silica intricately arranged to form a beautiful network. These hard
skeletal structures are generally the only parts of sponges preserved as fossils.
The great success of the sponges along with organisms such as diatoms that
extract silica directly from seawater to construct their skeletons explains why
today’s ocean is largely depleted of this mineral.
The conodonts (Fig. 96), which are bony appendages of a possible leech-
like animal that had the appearance of jawbones, show their greatest diversity
DEVONIAN FISH