Page 312 - Introduction to Paleobiology and The Fossil Record

SPIRALIANS 1: LOPHOPHORATES 299
linguliformeans, craniiformeans and rhyn-
chonelliformeans (Fig. 12.1a–f; Box 12.1). In
contrast to the bivalves, where the right valve
is a mirror image of the left, the plane of sym- including Craniops and the bizarre trimerel-
metry in brachiopods bisects both valves per-
pendicular to the plane along which the valves
open, or the commissure. The larger of the
two valves is generally the ventral or pedicle
valve; in many brachiopods the fl eshy stalk or
pedicle pokes through the apex of this valve
and attaches the animal to the substrate. The
pedicle can vary from a thick, fl eshy stalk to
a bunch of delicate, thread-like strands, which
can anchor the brachiopod in fi ne mud. Some
extinct brachiopods lost their pedicles during
ontogeny and adopted a free-living mode of
life, lying recumbent on or partially in the
sediments on the seafl oor. The dorsal or bra-
chial valve contains the extendable food-gath- dorsal sockets, and the valves were opened
ering organ or lophophore together with its
supports. A number of types of lophophore
have evolved (Fig. 12.1 g). The earliest growth
stage, the trocholophe, is an incomplete ring
of filaments, still retained by the pedomorphic
(see p. 146) microbrachiopod Gwynia. By the
schizolophe stage a bilobed outline has devel- Obolellata, the Kutorginata, the Strophome-
oped, which probably characterized many of
the smaller Paleozoic taxa. The more complex
plectolophe, ptycholophe and spirolophe
styles are characteristic of the articulated
brachiopods.
The linguliformeans (see Fig. 12.1a, b) have
organophosphatic shells with pedicles that
either emerge between both valves or through
an opening called the foramen. The shells
develop from a planktotrophic, or plankton-
feeding, larval stage, and linguliformeans are
characterized by an alimentary tract ending in
an anus. In the lingulates, the opening and
closing of the valves is achieved by a complex
system of muscles and the pedicle emerges
between both valves. Withdrawal of the soft
parts posteriorly causes a space problem that
can force the valves apart; relaxation allows
the animal to expand again forwards allowing
the valves to close. The paterinates are the
oldest group of brachiopods, appearing in the
lowest Cambrian Tommotian Stage. Although
linked to the other linguliformeans on the
basis of an organophosphatic shell substance,
the shell structure of the group is quite differ- type morphs with increasing strengths of
ent and the shells have true interareas, del- currents (Fig. 12.5). Moreover a number of
thyria and notothyria and apparently had a
functional diductor muscle system.
The craniiformeans (see Fig. 12.1c) include
a diverse, yet probably monophyletic, group
of morphologies centered on Crania but
lids. The shells consist of organocarbonate
and the animal developed separate dorsal and
ventral mantle lobes after the settlement of
the larvae on the seabed during a nektoben-
thonic stage.
The rhynchonelliformeans (see Fig. 12.1d–
f) have a pair of calcitic valves that contain a
fibrous secondary layer, with variable convex-
ity, hinged posteriorly and opening anteriorly
along the commissure. The mantle lobes are
fused posteriorly, where the interareas are
secreted; their margins form the hinge between
the ventral and dorsal valves. Articulation
was achieved by a pair of ventral teeth and
and closed by opposing diductor and adduc-
tor muscle scars. In the majority of rhyn-
chonelliformeans, the valves were attached to
the substrate by a pedicle, emerging through
a foramen in the delthyrial region. The sub-
phylum contains fi ve classes, the Chileata, the
nata and the Rhynchonellata. Already by the
Early Cambrian, representatives of four of
the five classes were present. However the
two latter classes, containing respectively over
1500 and 2700 genera, dominated Phanero-
zoic brachiopod faunas.
Brachiopods possess both planktotrophic
and lecitotrophic larvae. The planktotrophic
stage may have been the most primitive,
spending some time in the plankton, whereas
lecitotrophic larvae lurking in the benthos
may have developed at least twice. This
obviously has important consequences for
brachiopod dispersion. Since many linguli-
formeans are widespread it is assumed they
had planktotrophic larvae in contrast to the
more endemic rhynchonelliformeans with
possible lecitotrophic larvae (Fig. 12.4).
Brachiopod shells can be very variable in
shape. A single species can even mimic the
outlines of a range of different orders. For
example specimens of Terebratalia transversa
from around the San Juan islands, western
USA, show Spirifer-, Atrypa- and Terebratula-
brachiopods, such as the strophomenides,
especially the productoids, may markedly