Page 330 - Introduction to Paleobiology and The Fossil Record

SPIRALIANS 1: LOPHOPHORATES 317

Box 12.6 Bryozoan classifi cation

Class PHYLACTOLAEMATA

• Cylindrical zooids with horseshoe-shaped lophophore. Statoblasts arise as dormant buds. Fresh-

water with non-calcified skeletons. Over 12 genera
• Triassic, possibly Permian to Recent
Class STENOLAEMATA

• Cylindrical zooids with calcareous skeleton. Membraneous sac surrounds each polypide; lopho-
phore protrudes through an opening at the end of the skeletal tube. Marine, with an extensive
fossil record. Contains the following orders: trepostomes (Ordovician–Triassic), cystoporates
(Ordovician–Triassic), cryptostomes (Ordovician–Triassic), cyclostomes (Ordovician–Recent)
and fenestrates (Ordovician–Permian). About 550 genera
• Ordovician (Tremadoc) to Recent

Class GYMNOLAEMATA

• Cylindrical or squat zooids of fixed size with circular lophophore, usually with a calcareous
skeleton. The majority are marine but some are found in brackish and freshwater environments.
Includes the cheilostomes (Jurassic–Recent). Over 650 genera
• Ordovician (Arenig) to Recent

Box 12.7 Module iteration: building a Lego bryozoan

Bryozoan colonies grow by iteration, repeating the same units again and again until the colony is
built. But is this process just a simple addition of individual units (zooids) within the colony? If so,
the opportunity for evolution and morphological complexity would be very limited. There may be
a whole hierarchy of types of modules that are in fact iterated (repeatedly re-evolved). For example,
much more variability will be generated if a branch rather than a zooid is duplicated and attached
to various parts of the colony in various different orientations. Steven Hageman of Appalachian
State University suggested just this in a paper published in 2003: there is a hierarchy of such modules
and those second-order blocks will have a much greater effect on morphological change and evolu-
tion of the colony than simply duplicating the zooids. This can be easily demonstrated by an analogy
with a Lego model. The individual blocks, if iterated, will form only fairly simple patterns, but build
a structure and iterate that and suddenly considerable morphological complexity can be generated
from relatively simple building blocks (Fig. 12.16).

Continued