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Figure 12.16 The modular construction of a colony using Lego blocks: complex forms are generated by iteration of higher order modular units. (From Hageman 2003.)

Stenolaemata | Gymnolaemata

Figure 12.17 Stratigraphie ranges and absolute abundances of the main bryozoan groups. Geological period abbreviations are standard, running from Ordovician (O) to Neogene (N). (From Taylor, 1985.)

Stenolaemata | Gymnolaemata

Figure 12.17 Stratigraphie ranges and absolute abundances of the main bryozoan groups. Geological period abbreviations are standard, running from Ordovician (O) to Neogene (N). (From Taylor, 1985.)

Figure 12.18 Some bryozoan genera: (a) Rhabdomeson (Carboniferous cryptostome), (b) Rectifenestella (Carboniferous fenestrate), (c) Fistulipora (Carboniferous cystopore), (d) Penniretepora (Carboniferous fenestrate), (e) Archimedes (Carboniferous fenestrate), (f) Archaeofenestella (Silurian fenestrate), (g) Lunulites (Cretaceous cheilostome), (h) Castanapora (Cretaceous cheilostome). Magnification approximately x30 (a), x15 (b, c), x1 (d-f), x5 (g), x20 (h). (a-c, courtesy of Patrick Wyse Jackson; d-h, from Taylor 1985.)

Figure 12.18 Some bryozoan genera: (a) Rhabdomeson (Carboniferous cryptostome), (b) Rectifenestella (Carboniferous fenestrate), (c) Fistulipora (Carboniferous cystopore), (d) Penniretepora (Carboniferous fenestrate), (e) Archimedes (Carboniferous fenestrate), (f) Archaeofenestella (Silurian fenestrate), (g) Lunulites (Cretaceous cheilostome), (h) Castanapora (Cretaceous cheilostome). Magnification approximately x30 (a), x15 (b, c), x1 (d-f), x5 (g), x20 (h). (a-c, courtesy of Patrick Wyse Jackson; d-h, from Taylor 1985.)

and elsewhere. The trepostomes, however, lingered on until the Late Triassic.

The cyclostomes have tube-shaped zooecia and often grew as branching tree-like colonies or alternatively encrusting sheets or ribbons. The first representatives of the order are known in Lower Ordovician rocks, but the group peaked during the mid-Cretaceous in spectacular style, with a diversity of over 70 genera. Many genera such as Stomatopora, consisting of a series of bifurcating, encrusting branches, have very long stratigraphic ranges; moreover Stomatopora may have pursued an opportunist life strategy, rapidly spreading their zooids over hard surfaces.

The Gymnolaemata are represented in the fossil record by two orders, the ctenostomes and the cheilostomes. The ctenostomes first appeared in the Early Ordovician and many genera have since pursued boring and encrusting life strategies. Penetrantia and Terebri-pora are borers whereas the modern genus

Bowerbankia has an erect colony with semi-spirally arranged zooecia clustered around a central branch. The cheilostomes, however, dominate the class and are most diverse of all the bryozoan groups (Box 12.8). Cheilostomes typically have polymorphic zooids, adapted for different functions, which are usually linked within the highly integrated colony. This advanced group appeared during the Late Jurassic; they are particularly common in shallow-water environments of the Late Cretaceous and Paleogene of the Baltic and Denmark. Lunulites, for example, is discoidal and free-living, whereas Aechmella is an encrusting form often associated with sea urchins.

Ecology and life modes_

Virtually all bryozoans are part of the sessile benthos, mainly occurring from the sublitto-

Box 12.8 Competition and replacement in cyclostome and cheilostome clades: what really happened at the KT boundary?

Perhaps one of the most obvious changes in bryozoan faunas through time involves the relative decline of the cyclostomes and the diversification of the cheilostomes leading up to the Cretaceous-Tertiary (KT) boundary. Since both groups occupied similar ecological niches and are comparable morphologically, many workers have assumed that the cyclostomes, originating during the Ordovi-cian and diversifying in the Cretaceous, were outcompeted by the cheilostomes at the end of the Cretaceous. However Scott Lidgard (Field Museum of Natural History, Chicago) and his colleagues have analyzed this transition in detail and the results are far from conclusive (Lidgard et al. 1993). Both groups continued to participate together in bryozoan communities during the Cenozoic and much of the apparent decline in the cyclostome numbers may be due to the greater diversification or expansion of the cheilostomes that began to dominate these assemblages in the Cenozoic. Perhaps this expansion had already been seeded in the Jurassic, when the poor and sporadic bryozoan fauna provided the ecological space for the expansion of the cheilostomes. A detailed statistical study based on generic-range data from Sepkoski's database (McKinney & Taylor 2001) has confirmed that origination within the cheilostome clades was the driving force behind the apparent takeover by this group (Fig. 12.19).

See http://www.blackwellpublishing.com/paleobiology/.

Figure 12.19 Distribution of (a) cyclostome and (b) cheilostome bryozoans across the Mesozoic-Cenozoic boundary: the cheilostomes suffered the heaviest losses while the erect genera of both groups suffered more than the encrusters. (Replotted from McKinney & Taylor 2001.)

ral zone to the edge of the continental shelf at depths of about 200 m. Nevertheless a few intertidal forms are known, while some bryo-zoans have been dredged from depths of over 8 km in oceanic trenches; moreover numerous species have been recorded from the hulls of ships. Most species are sensitive to substrate types, turbulence, water depth and temperature together with salinity. The shape of colonies can be very plastic, adapting to environmental conditions, with erect, treelike colonies varying their branch thickness according to depth. In addition spines may be induced by high current velocities or by the presence of predators (Taylor 2005). Bryozoans are thus typical facies fossils exhi biting marked ecophenotypic variation (Box 12.9).

Bryozoans have successfully pursued several different life modes. Encrusting, erect, unattached or rooted phenotypes all reflect adaptive strategies in response to ambient environmental conditions. Shallow-water colonies, particularly in the subtidal zone, are and were dominated by encrusting, erect, rooted and free-living forms. But deeper-water environments, over 1 km deep, are characterized by mainly attached and rooted forms. Nevertheless bryozoan colonies have occasionally formed reefs or bryoherms, particularly during the mid-Silurian and Carboniferous.

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