Geology Reference
In-Depth Information
stomata) or aragonite (Cheilostomata). Differences in
skeletal mineralogy may control diagenetic pathways
of bryozoan cold-water carbonates (Bone and James
1993).
Bone 1991; James et al. 1993). These differences are
used in interpreting the water energy, substrate and tem-
perature conditions of ancient bryozoan-bearing car-
bonates. Differences in colony growth forms are used
as paleobathymetrical proxies (for shallow and deep
shelf environments) and indicators of substrate stabil-
ity (Hageman et al. 1998). This approach is successful
in the study of Cenozoic bryozoan carbonates (Herbig
1991) and sometimes also Mesozoic carbonates, but
has limitations in the study of Paleozoic bryozoans
(Kelly and Horrowitz 1987), see also Pl. 85/3.
Classification:
Modern Bryozoa include the marine
classes Stenolaemata and Gymnolaemata and the fresh-
water class Phylactolaemata, the first two with skel-
etons, the last without a skeleton.
The
Stenolaemata
are characterized by elongated tu-
bular or conical zooids and tube-like zooecia. The class
covers the wealth of bryozoans found in limestones of
Ordovician through the mid-Cretaceous age. By the
Late Cretaceous the stenolaemates were exceeded in
diversity by the gymnolaemates but continue to flour-
ish in marine communities. The Stenolaemata include
the orders
Trepostomata
(Early Ordovician to Trias-
sic),
Cystoporata
(Early Ordovician to Triassic),
Cryptostomata
(Early Ordovician to Late Permian),
Fenestrata
(Early Ordovician to Triassic), and
Tubuli-
porata
(Early Ordovician to Holocene).
The
Gymnolaemata
are the morphologically most
varied class and exhibit box-shaped zooecia. It includes
the orders
Ctenostomata
(uncalcified, Ordovician to
Holocene) and
Cheilostomata
(calcified, Late Jurassic
to Holocene). The greater part of the gymnolaemate
diversity has evolved since the mid-Cretaceous.
Carbonate production:
Recent bryozoans are com-
mon in sediments of the continental shelves, but also
occur in reefal environments (Cuffey 1972). Bryozoan
bioclasts are abundant constituents of modern and an-
cient cool-water shelf carbonates contributing to the
formation of carbonate particles of all grain sizes. Grain
sizes of bryozoan carbonates are strongly controlled
by skeletal architecture, as shown by the bryozoan
sands, muds and gravels on the southern Australia
Lacepede Shelf.
Skeletal architecture is related to the strength and
disintegration potential of bryozoan skeletons (Cheet-
ham and Thomsen 1981). A significant number of mod-
ern bryozoans live attached to sessile, benthic inverte-
brate hosts, which are not or only slightly calcified and
leave no or little body fossil record. Bryozoan sedi-
ment production from epizoans on ephemeral substrates
may have been important in the Tertiary (Hageman et
al. 2000).
Bryozoans can contribute to Cenozoic shelf depos-
its with a quantity of up to 95% (e.g. off New Zealand).
Huge areas of the Pacific deep shelf bottom between
western Australia, New Zealand and the Chatham Is-
lands extending about 6000 km are dominated by bryo-
zoan sediments. This '
bryomol facies
' is a characteris-
tic feature of non-tropical carbonates (see Sect. 12.2).
Bryozoan-rich microfacies types are common in plat-
form and ramp carbonates throughout the geological
past, particularly in Paleozoic shelf limestones and may
contribute to the formation of reefs (e.g. Permian Zech-
stein reefs; Pl. 85/11; Early Tertiary Danian reefs, Pl.
147/2, 3).
Ecology:
Most colonies are attached to hard material
such as rock, shells or sediment grains. Intertidal and
upper subtidal encrusters are common on the under-
sides of hard substrates. Other habitats of encrusting
bryozoans are long-lived stable hard substrates (reefs,
rock walls). Bryozoans adhere also to flexible soft sub-
strates, e.g. sea weeds and algal fronds. The majority
of bryozoan habitats is found in areas of low sedimen-
tation.
Paleoenvironmental significance of bryozoans:
Bryozoans are excellent paleoenvironmental and paleo-
climatic indicators particularly for temperate and cold-
water carbonates. The presence or absence of bryozo-
ans, diversity and abundance, zooid morphology, colo-
nial plasticity and growth form as well as reef-like struc-
tures provide information on the physical controls on
the habitat. Paleoenvironmental parameters, such as as
temperature, salinity, water energy, character of the sub-
strate, and sedimentation rate may be indicated by bryo-
zoans (Smith 1995).
Modern cyclostome and cheilostome bryozoans show
distinct zonation of growth form assemblages and spe-
cial growth-form morphologies depending on the en-
vironment (Stach 1936; Nelson et al. 1988; James and
Distribution:
Bryozoans are known since the Ordovi-
cian. Five of six known Paleozoic orders became ex-
tinct in the Triassic, but the Cyclostomata persisted from
the Ordovician to the present, dominating many fau-
nas in the Mesozoic. The Cheilostomata are the other
important modern marine group. They arose in the Late
Jurassic and became dominant in the Late Cretaceous
and Cenozoic.
