Bryozoa

Besides these, there were the Bryozoa, a small kind of Mollusk allied to the Clams, and very busy then in the ancient Coral work. They grew in communities, and the separate individuals are so minute that a Bryozoan stock looks like some delicate moss. They still have their place among the Reef-Building Corals, but play an insignificant part in comparison with that of their predecessors.

Atlantic Monthly (April, 1863)

Box 12.5 Tethyan brachiopods in Greenland: a Cretaceous Gulf Stream current?

Brachiopods can give clues about ancient ocean currents. Today, the Gulf Stream runs out from the Caribbean, sweeps up the eastern seaboard of North America, and then detaches from the coast just north of New York and heads across the Atlantic to wrap the shores of Britain and western Europe in warmer-than-expected waters. Has the Gulf Stream always flowed the same way? Some Cretaceous brachiopods give us a clue. David Harper and colleagues (2005) showed how some Early Cretaceous brachiopod faunas from East Greenland were a mix of animals from two ocean provinces, Tethyan (low latitude) and Boreal (high latitude). The Boreal, shallow-water assemblage is dominated by large terebratulids and ribbed rhynchonellids, and occurs adjacent to a fauna containing Tethyan elements, more typical of deeper water, including Pygope (see p. 311). How did these exotic, tropical visitors travel so far north? Harper and colleagues suggested that an Early Cretaceous out-of-Tethys migration was helped by the early and persistent northward track of a proto-Gulf Stream current (Fig. 12.14). These kinds of studies of changing patterns of paleobiogeography through time are critical for understanding modern climate and ocean patterns.

Laurasia And Gondwanaland
Figure 12.14 Tethyan brachiopods in East Greenland: Pygope and the proto-North Atlantic current (arrows), one of its possible migration routes. The star indicates the Lower Cretaceous, East Greenland locality.

Bryozoans are the only phylum in which all species are colonial. Many skeletons are exquisitely designed, but fragment very easily after death. Although relatively common, bryozoans are among the least well-known invertebrates. There are about 6000 living and 16,000 fossil species, and most are marine

(Box 12.6). Superficially resembling the corals and hydroids, the bryozoans ("moss animals") are like minute colonial phoronids (see p. 298) with tiny individuals or zooids, commonly less than 1 mm in diameter. Each zooid is celomate with a separate mouth and anus together and a circular or horseshoe-shaped 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-chiopods and phoronids and it may be a mistake to think that all three groups are closely related just because they possess ciliated feeding organs. Individual zooids are enclosed by a gelatinous, leathery or calcareous 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 hermaphrodites, 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 morphologies (Box 12.7). Many elegant forms have evolved such as the bush- and tree-like trepo-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 setae project downwards from specialized zooids and as the setae move in waves, the colony is transported across the seabed. Such a lifestyle can be traced back to the Late Cretaceous when free-living colonies, the so-called lunulitiforms, evolved their regular shape, without interference from adjacent objects on the seafloor.

Zooid size can give important clues about environment and particularly water temperature. 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 nevertheless zooid size may also be a useful environmental 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; numerous 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 prismatic 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 abundant 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 connected by dissepiments; rectangular spaces or fenestrules separate the branches that contain the biserially-arranged zooids. Archimedes, however, has a meshwork wound around a screw-shaped central axis. Richard Cowen and his colleagues (University of California)

lophophore lophophore

Figure 12.15 Morphology of two living bryozoans: (a) a stenolaemate and (b) a gymnolaemate. (Based on various sources.)

have modeled the feeding strategies of these screw-shaped colonies and other fenestrates. Carboniferous fenestrate colonies usually had inward-facing zooids and probably drew water in through the top of the colony and flushed it out through the fenestrules at the sides. On the other hand, Silurian colonies had outward-facing zooids and sucked in water through the fenestrules, expelling it out of the open top of the colony.

In general both the cryptostomes and fenes-trates outstripped the trepostomes during the Late Paleozoic, many of the fenestrates populating reef environments. Although both groups disappeared at the end of the Permian or soon after, they were still conspicuous members of the Late Permian benthos; both Fenestella and Synocladia form large, vase-shaped colonies in the communities of the Zechstein reef complex in the north of England

Box 12.6 Bryozoan classification

Class PHYLACTOLAEMATA

• Cylindrical zooids with horseshoe-shaped lophophore. Statoblasts arise as dormant buds. Freshwater with non-calcified skeletons. Over 12 genera

• Triassic, possibly Permian to Recent

Class STENOLAEMATA

• Cylindrical zooids with calcareous skeleton. Membraneous sac surrounds each polypide; lophophore 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 evolution 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).

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