Page 328 - Introduction to Paleobiology and The Fossil Record

SPIRALIANS 1: LOPHOPHORATES 315
lophophore equipped with a ring of 8–100
tentacles – a major organizational jump from
the cnidarians. The bryozoan lophophore is
constructed differently from those of the bra- setae project downwards from specialized
chiopods and phoronids and it may be a
mistake to think that all three groups are
closely related just because they possess cili- a lifestyle can be traced back to the Late Cre-
ated feeding organs. Individual zooids are
enclosed by a gelatinous, leathery or calcare- called lunulitiforms, evolved their regular
ous exoskeleton, usually in the form of slender
tubes or box-like chambers called zooecia.
The primary function of most zooids is the
capture of food, but some are specialists in
defense, reproduction or sediment removal;
the bryozoan colony thus functions as a well-
organized unit.
Morphology: Bowerbankia
The genus Bowerbankia is a relatively simple
bryozoan useful for illustrating the general
anatomy of bryozoan zooids (Fig. 12.15).
Each living zooid is enclosed by a body wall
or cystid. The lophophore, with its beating
cilia, extends outwards from the zooid and
comprises a ring of 10 tentacles, directing
food to a central mouth leading into a U-
shaped gut; the feces finally exit out through
an anus. A funiculus extends along the stolon
connecting all the zooids. This is thought to
be a homolog of the blood vessels found in
other animals. The individual zooids are her-
maphrodites, developing eggs and sperm at
different times; the eggs are usually fertilized
in the tentacle sheath, developing later into
trochophore larva.
Ecology: feeding and colonial morphology
Feeding strategies of bryozoans have had a
major influence on the style of colony growth.
Feeding behavior patterns are correlated with
the shape of the colony and the size of the
zooids. Bryozoan colonies can grow in a
variety of modes from encrusting runners,
uniserial or multiserial branches that split,
and sheets where growth occurs around the
entire margin, to more erect type forms that
have complex three-dimensional morpholo-
gies (Box 12.7). Many elegant forms have
evolved such as the bush- and tree-like trepo- the biserially-arranged zooids. Archimedes,
stomes of the Paleozoic, the spiral Archimedes
and vase-shaped Fenestrella, in both of which
the entire colony may have acted like a sponge.
But bryozoan colonies can also move. For
example, colonies of Selenaria can scuttle
across the seafloor. Stilt-like appendages or
zooids and as the setae move in waves, the
colony is transported across the seabed. Such
taceous when free-living colonies, the so-
shape, without interference from adjacent
objects on the seafl oor.
Zooid size can give important clues about
environment and particularly water tempera-
ture. Increased ranges of seasonal variation in
temperature seem to be correlated with an
increased amount of variation in the size of
zooids in the colony (O’Dea 2003). It is not
clear why there is this relationship, but nev-
ertheless zooid size may also be a useful envi-
ronmental proxy.
Evolution: main fossil bryozoan groups
The oldest bryozoans in the fossil record
occur in the Tremadocian Stage of the Lower
Ordovician, but it is very likely that primitive,
soft-bodied bryozoans existed during the
Cambrian but have not been fossilized; numer-
ous families of bryozoans are found in the
succeeding Floian Stage. The Stenolaemata
dominated Paleozoic bryozoan faunas (Fig.
12.17). The trepostomes or stony bryozoans
commonly had bush-like colonies with pris-
matic zooecia having polygonal apertures.
The group diversified during the Ordovician
to infiltrate the low-level benthos. Genera
such as Monticulipora, Prasopora and
Parvohallopora are typical of Ordovician
assemblages.
The cryptostomes, although originating
during the Early Ordovician, were more abun-
dant during the Mid and Late Paleozoic as the
trepostomes declined; in some respects the
group forms a link with the net-like fenes-
trates that were particularly common in the
Carboniferous (Fig. 12.18). Fenestella, itself,
may be in the form of a planar mesh, cone or
funnel. The branches of the colony are con-
nected by dissepiments; rectangular spaces or
fenestrules separate the branches that contain
however, has a meshwork wound around a
screw-shaped central axis. Richard Cowen
and his colleagues (University of California)