xviii
2008b). For instance, the data of Helmkampf et al. (2008b) suggest that phylactolaemate bryozoans are more closely related to phoronids than to gymnolaemate bryozoans.
Yet other molecular studies show the Phylactolaemata as the sister group to the clade uniting sister groups Stenolaemata and Gymnolaemata (Fuchs et al. 2009; Hausdorf et al. 2010;
Waeschenbach et al. 2012; Mallatt et al. 2012; see also the cladogram in Todd 2000). Another
combined analysis unites Phylactolaemata and Stenolaemata as sister taxa, making this clade
a sister group to Gymnolaemata (Fuchs et al. 2009). Anstey (1990) found Phylactolaemata to
form a monophyletic group with Stenolaemata, suggesting a sister relationship of this group
with the gymnolaemate order Cheilostomata, however. The third variant of interactions
between the classes was presented by Cuffey (1973), who united phylactolaemates with gymnolaemates, considering this clade as a sister to stenolaemates (see also Cuffey and Blake
1991). At present, bryozoan researchers tend to support the fi rst hypothesis (discussed also in
Gruhl 2008).
Brief Overview of Bryozoa
Bryozoa, predominantly marine epibionts, are active suspension-feeders consuming phytoplankton, bacteria and dead organic matter in diverse habitats from the intertidal zone to hadal
depths exceeding 8,000 m (Ryland 1967, 1970, 1976, 1982, 2005; Kluge 1975; Boardman
et al. 1983; McKinney and Jackson 1989; Taylor 1999; Gordon 2003; Gordon et al. 2009). All
bryozoans are colonial organisms consisting of modules, so-called zooids, which are usually
less than a millimetre long. The pelago-benthic life cycle of Bryozoa includes the formation of
gametes in a hermaphrodite colony, sperm release followed by internal fertilization and development of an exotrophic (planktotrophic) or incubated endotrophic (lecithotrophic or matrotrophic) free-swimming larva, which, when competent, fi nds a place for settlement, attaches to
the substratum and undergoes catastrophic (phylactolaemates excepted) metamorphosis. The
result is the formation of a founder zooid (ancestrula) or group of zooids (ancestrular complex)
that begins to bud the daughter generations of zooids. On attaining maturity, the colony starts
gametogenesis (reviewed in Reed 1991). Budding is traditionally considered as asexual reproduction though in case of colonial organisms it would be more correct to call it colonial growth,
since in these organisms budding is never complete, the colony members remaining physically
interconnected and physiologically dependent throughout their life time. Besides, the zooids
are genetic copies while the colony is a modular organism forming genetically ‘identical’
gametes.
According to the latest estimation, about 6,000 species of extant marine bryozoans and over
15,000 species of extinct bryozoans (Gordon et al. 2009) have been described. These fi gures,
however, are likely to represent as little as one third of the actual diversity of this group (Taylor,
personal communication, 2007).
Traces of boring non-skeletal ctenostome bryozoans (class Gymnolaemata) and fossilized
skeletons of stenolaemate bryozoans are known from marine sediments beginning with the
Early Ordovician (Taylor and Curry 1985; Hu and Spjeldnaes 1991; Todd 2000; Xia et al.
2007; Zhang et al. 2009). Thus, both classes of marine Bryozoa and, according to Todd (2000),
all superfamilies of the order Ctenostomata already existed at that time. A recent report on the
fi nding of Cambrian stenolaemate bryozoans (Landing et al. 2010) is highly dubious. However,
on the basis of the basal position of bryozoans in gene trees relative to brachiopods and molluscs, whose fossilized remains are known from Early Cambrian sediments, Passamaneck and
Halanych (2006) suggested that the origin of Bryozoa dates back at least to the Early Cambrian.
In turn, Buge (1952), Brien (1960) and Emig (1984) argued that bryozoans originated as early
as the Precambrian (see also Hyman 1959). Fossil statoblasts (resting buds) of Phylactolaemata
are known from Middle–Late Triassic deposits (Kohring and Pint 2005; Schcerbakov 2008).
Ctenostomata is one of the oldest surviving groups of bryozoans lacking a mineralized
skeleton, traditionally considered as ancestral to all other groups of marine bryozoans (Banta
Introduction
2008b). For instance, the data of Helmkampf et al. (2008b) suggest that phylactolaemate bryozoans are more closely related to phoronids than to gymnolaemate bryozoans.
Yet other molecular studies show the Phylactolaemata as the sister group to the clade uniting sister groups Stenolaemata and Gymnolaemata (Fuchs et al. 2009; Hausdorf et al. 2010;
Waeschenbach et al. 2012; Mallatt et al. 2012; see also the cladogram in Todd 2000). Another
combined analysis unites Phylactolaemata and Stenolaemata as sister taxa, making this clade
a sister group to Gymnolaemata (Fuchs et al. 2009). Anstey (1990) found Phylactolaemata to
form a monophyletic group with Stenolaemata, suggesting a sister relationship of this group
with the gymnolaemate order Cheilostomata, however. The third variant of interactions
between the classes was presented by Cuffey (1973), who united phylactolaemates with gymnolaemates, considering this clade as a sister to stenolaemates (see also Cuffey and Blake
1991). At present, bryozoan researchers tend to support the fi rst hypothesis (discussed also in
Gruhl 2008).
Brief Overview of Bryozoa
Bryozoa, predominantly marine epibionts, are active suspension-feeders consuming phytoplankton, bacteria and dead organic matter in diverse habitats from the intertidal zone to hadal
depths exceeding 8,000 m (Ryland 1967, 1970, 1976, 1982, 2005; Kluge 1975; Boardman
et al. 1983; McKinney and Jackson 1989; Taylor 1999; Gordon 2003; Gordon et al. 2009). All
bryozoans are colonial organisms consisting of modules, so-called zooids, which are usually
less than a millimetre long. The pelago-benthic life cycle of Bryozoa includes the formation of
gametes in a hermaphrodite colony, sperm release followed by internal fertilization and development of an exotrophic (planktotrophic) or incubated endotrophic (lecithotrophic or matrotrophic) free-swimming larva, which, when competent, fi nds a place for settlement, attaches to
the substratum and undergoes catastrophic (phylactolaemates excepted) metamorphosis. The
result is the formation of a founder zooid (ancestrula) or group of zooids (ancestrular complex)
that begins to bud the daughter generations of zooids. On attaining maturity, the colony starts
gametogenesis (reviewed in Reed 1991). Budding is traditionally considered as asexual reproduction though in case of colonial organisms it would be more correct to call it colonial growth,
since in these organisms budding is never complete, the colony members remaining physically
interconnected and physiologically dependent throughout their life time. Besides, the zooids
are genetic copies while the colony is a modular organism forming genetically ‘identical’
gametes.
According to the latest estimation, about 6,000 species of extant marine bryozoans and over
15,000 species of extinct bryozoans (Gordon et al. 2009) have been described. These fi gures,
however, are likely to represent as little as one third of the actual diversity of this group (Taylor,
personal communication, 2007).
Traces of boring non-skeletal ctenostome bryozoans (class Gymnolaemata) and fossilized
skeletons of stenolaemate bryozoans are known from marine sediments beginning with the
Early Ordovician (Taylor and Curry 1985; Hu and Spjeldnaes 1991; Todd 2000; Xia et al.
2007; Zhang et al. 2009). Thus, both classes of marine Bryozoa and, according to Todd (2000),
all superfamilies of the order Ctenostomata already existed at that time. A recent report on the
fi nding of Cambrian stenolaemate bryozoans (Landing et al. 2010) is highly dubious. However,
on the basis of the basal position of bryozoans in gene trees relative to brachiopods and molluscs, whose fossilized remains are known from Early Cambrian sediments, Passamaneck and
Halanych (2006) suggested that the origin of Bryozoa dates back at least to the Early Cambrian.
In turn, Buge (1952), Brien (1960) and Emig (1984) argued that bryozoans originated as early
as the Precambrian (see also Hyman 1959). Fossil statoblasts (resting buds) of Phylactolaemata
are known from Middle–Late Triassic deposits (Kohring and Pint 2005; Schcerbakov 2008).
Ctenostomata is one of the oldest surviving groups of bryozoans lacking a mineralized
skeleton, traditionally considered as ancestral to all other groups of marine bryozoans (Banta
Introduction