132
between the bases of fl attened spines and has a drop- like,
oval or rounded shape (Fig. 2.10C, F ); elliptic foramina are
located on the sides of the brood chamber between the lower
surface of the costae and the ovicell fl oor (Fig. 2.10D ); the
main proximal opening of the ovicell is a low, broad arch
(Fig. 2.10E ). The gaps/slits and openings between the
ooecium- forming spines and costae in these species suggest
that water was able to enter the brood cavity (Ostrovsky and
Taylor 2004 , 2005a ).
Signifi cantly, spinose and costate ooecia are characteristic
of some other fossil and Recent cheilostomes too. In the cribrimorph genus Leptocheilopora (Upper Cretaceous), hyperstomial ovicells consist of costae homologous to those of the
frontal shield (Lang 1921 ; Larwood 1962 ; Ostrovsky and
Taylor 2005a ). Edges of ooecial costae are closely adjoined
and their bases are arranged in an elongated semicircle
(horseshoe) (Figs. 2.26A, B, D , 2.60D , and 2.62O ) similar to
the calloporids Distelopora spinifera and Unidistelopora
krauseae (Fig. 2.62P ). In one specimen, the costal edges
were all gently sinuous and tightly apressed (Fig. 2.26D ).
Costae are also used for construction of ooecia in
Bellulopora bellula , but it is doubtful that they are homologous to cribrilinid costae. They are more likely to be kenozooids, judging from the fact that their cavities communicate
with that of the distal kenozooid (forming the non-calcifi ed
ovicell fl oor) through pores with a cuticular annulus identical
to conventional interzooidal communication pores of
Cheilostomata (Fig. 2.60E ) (Ostrovsky and Taylor 2005a ).
Thus, the example of Belluloporidae indicates that cheilostome spines may originally have been zooid polymorphs as
was suggested by Silén ( 1942 , 1977 ).
Ooecia constructed from spines and costae are also
characteristic of fossil and Recent representatives of the
families Monoporellidae ( Stichomicropora , Monoporella )
and Macroporidae ( Macropora ) (Figs. 2.55 , 2.56 , 2.57 ,
2.58 , 2.60A–C , and 2.61 ). In species of the extinct genus
Stichomicropora , ooecial spine bases are arranged in a straight
line or gently curving (concave or convex) arch (Fig. 2.62A–H, J )
as the in calloporids Distelopora bipilata and D . langi
(Fig. 2.62I, L ). In Monoporella , the ooecium is constructed
from several costae or just two broad costae (Figs. 2.57 , 2.60C ,
2.61A, C, D , and 2.62J, M ). If several costae, their bases are
arranged in a gently curving arch as in the Distelopora species
mentioned above; if two broad costae, ooecial structure is
similar to that in the calloporid Gilbertopora (Fig. 2.62K ).
In Macropora (Figs. 2.58 , 2.61B, E , and 2.62Q ), spine bases
are arranged in an elongated semicircle (horseshoe), as in the
calloporids Distelopora spinifera and U . krauseae (Fig. 2.62P )
and cribrilinids of the genus Leptocheilopora (Fig. 2.62O )
(Ostrovsky and Taylor 2005a ; Gordon and Taylor 2008 ).
Similarities in ooecia are important for reconstructing evolutionary transformation of brooding structures within related
bryozoan groups (see Sects. 2.4.3 , 2.4.4 , and 2.4.5 ).
2.3.1.2 Structure and Development
of Hyperstomial Ovicells
in Wilbertopora
Distelopora and Gilbertopora , with spinose ooecia, are stratigraphically somewhat younger than confamilial Wilbertopora
with a hood-like ooecium (Upper Albian–Lower Cenomanian)
(Cheetham 1954 , 1975 ; Cheetham et al. 2006 ) – the earliest
cheilostome genus known to possess ovicells.
The hyperstomial ooecia of Wilbertopora (Figs. 2.11A
and 2.12D–F ) are formed by the distal zooid, whether an
autozooid, kenozooid or avicularium. In some species,
primitive avicularia may also initiate the formation of ooecia
by the distal zooid, whether an autozooid or avicularium
(Fig. 2.12E ). The ooecial roof consists of two lobes adjoining each other along the midline, similar to the arrangement
in Gilbertopora and often forming a low longitudinal crest
(Fig. 2.12D ). The coelomic cavities of the lobes and their
adjoining walls apparently do not merge, as indicated by
ooecia fractured along the medial suture (Fig. 2.12F ). The
bases of the lobes are rather narrow (Figs. 2.12A and 2.62K ).
The fl oor of the brood chamber is rather deeply depressed in
the proximal area of the gymnocyst of the daughter zooid
(Ostrovsky and Taylor 2005b ).
As in Recent calloporids, ovicells are formed at the
periphery of the colony, close to its growing edge
(Fig. 2.11A ). Brood chambers are always arranged in groups,
with the youngest ovicells positioned distally. Ovicellogenesis
starts in the developing autozooid long before its cystid is
completed. The fi rst indication of ooecium formation is
calcifi cation of the proximal part of the frontal wall of the
distal autozooid. Calcifi cation starts from the transverse wall
between maternal and distal zooids, spreads distally and
forms, contrary to Recent calloporids, a simple narrow plate
with a rounded edge (Fig. 2.11B–D ). The shape of the plate
indicates that it could have been surrounded by the arched
ooecial fold of the frontal wall. This membranous outgrowth
is not preserved in fossils, but has been described in living
calloporids (Ostrovsky and Schäfer 2003 ; Ostrovsky et al.
2003 ). Nevertheless, it is also possible that instead of an
ooecial fold, two soft outgrowths, predecessors of ooecial
lobes, were formed (see below). Calcifi cation continues
to expand centrifugally, bordered by two lateral slits
(Fig. 2.11E ) (see also Cheetham 1975 , p. 553). The resulting gymno cystal ovicell fl oor is concave (Fig. 2.11E–H ).
In zooids without brood chambers, the proximal gymnocyst
is fl at or only slightly concave (Ostrovsky and Taylor 2005b ).
As calcifi cation continues, the lateral slits gradually
decrease in length and become separated from one another
(Fig. 2.11F ), as a consequence of which the common ooecial
fold (if it existed at all) would have been transformed into
two hollow symmetrical outgrowths, the future ooecial lobes.
As noted above, they could also form somewhat earlier.
Each lobe communicates with the proximal part of the distal
2 Cheilostome Brood Chambers: Structure, Formation, Evolution
between the bases of fl attened spines and has a drop- like,
oval or rounded shape (Fig. 2.10C, F ); elliptic foramina are
located on the sides of the brood chamber between the lower
surface of the costae and the ovicell fl oor (Fig. 2.10D ); the
main proximal opening of the ovicell is a low, broad arch
(Fig. 2.10E ). The gaps/slits and openings between the
ooecium- forming spines and costae in these species suggest
that water was able to enter the brood cavity (Ostrovsky and
Taylor 2004 , 2005a ).
Signifi cantly, spinose and costate ooecia are characteristic
of some other fossil and Recent cheilostomes too. In the cribrimorph genus Leptocheilopora (Upper Cretaceous), hyperstomial ovicells consist of costae homologous to those of the
frontal shield (Lang 1921 ; Larwood 1962 ; Ostrovsky and
Taylor 2005a ). Edges of ooecial costae are closely adjoined
and their bases are arranged in an elongated semicircle
(horseshoe) (Figs. 2.26A, B, D , 2.60D , and 2.62O ) similar to
the calloporids Distelopora spinifera and Unidistelopora
krauseae (Fig. 2.62P ). In one specimen, the costal edges
were all gently sinuous and tightly apressed (Fig. 2.26D ).
Costae are also used for construction of ooecia in
Bellulopora bellula , but it is doubtful that they are homologous to cribrilinid costae. They are more likely to be kenozooids, judging from the fact that their cavities communicate
with that of the distal kenozooid (forming the non-calcifi ed
ovicell fl oor) through pores with a cuticular annulus identical
to conventional interzooidal communication pores of
Cheilostomata (Fig. 2.60E ) (Ostrovsky and Taylor 2005a ).
Thus, the example of Belluloporidae indicates that cheilostome spines may originally have been zooid polymorphs as
was suggested by Silén ( 1942 , 1977 ).
Ooecia constructed from spines and costae are also
characteristic of fossil and Recent representatives of the
families Monoporellidae ( Stichomicropora , Monoporella )
and Macroporidae ( Macropora ) (Figs. 2.55 , 2.56 , 2.57 ,
2.58 , 2.60A–C , and 2.61 ). In species of the extinct genus
Stichomicropora , ooecial spine bases are arranged in a straight
line or gently curving (concave or convex) arch (Fig. 2.62A–H, J )
as the in calloporids Distelopora bipilata and D . langi
(Fig. 2.62I, L ). In Monoporella , the ooecium is constructed
from several costae or just two broad costae (Figs. 2.57 , 2.60C ,
2.61A, C, D , and 2.62J, M ). If several costae, their bases are
arranged in a gently curving arch as in the Distelopora species
mentioned above; if two broad costae, ooecial structure is
similar to that in the calloporid Gilbertopora (Fig. 2.62K ).
In Macropora (Figs. 2.58 , 2.61B, E , and 2.62Q ), spine bases
are arranged in an elongated semicircle (horseshoe), as in the
calloporids Distelopora spinifera and U . krauseae (Fig. 2.62P )
and cribrilinids of the genus Leptocheilopora (Fig. 2.62O )
(Ostrovsky and Taylor 2005a ; Gordon and Taylor 2008 ).
Similarities in ooecia are important for reconstructing evolutionary transformation of brooding structures within related
bryozoan groups (see Sects. 2.4.3 , 2.4.4 , and 2.4.5 ).
2.3.1.2 Structure and Development
of Hyperstomial Ovicells
in Wilbertopora
Distelopora and Gilbertopora , with spinose ooecia, are stratigraphically somewhat younger than confamilial Wilbertopora
with a hood-like ooecium (Upper Albian–Lower Cenomanian)
(Cheetham 1954 , 1975 ; Cheetham et al. 2006 ) – the earliest
cheilostome genus known to possess ovicells.
The hyperstomial ooecia of Wilbertopora (Figs. 2.11A
and 2.12D–F ) are formed by the distal zooid, whether an
autozooid, kenozooid or avicularium. In some species,
primitive avicularia may also initiate the formation of ooecia
by the distal zooid, whether an autozooid or avicularium
(Fig. 2.12E ). The ooecial roof consists of two lobes adjoining each other along the midline, similar to the arrangement
in Gilbertopora and often forming a low longitudinal crest
(Fig. 2.12D ). The coelomic cavities of the lobes and their
adjoining walls apparently do not merge, as indicated by
ooecia fractured along the medial suture (Fig. 2.12F ). The
bases of the lobes are rather narrow (Figs. 2.12A and 2.62K ).
The fl oor of the brood chamber is rather deeply depressed in
the proximal area of the gymnocyst of the daughter zooid
(Ostrovsky and Taylor 2005b ).
As in Recent calloporids, ovicells are formed at the
periphery of the colony, close to its growing edge
(Fig. 2.11A ). Brood chambers are always arranged in groups,
with the youngest ovicells positioned distally. Ovicellogenesis
starts in the developing autozooid long before its cystid is
completed. The fi rst indication of ooecium formation is
calcifi cation of the proximal part of the frontal wall of the
distal autozooid. Calcifi cation starts from the transverse wall
between maternal and distal zooids, spreads distally and
forms, contrary to Recent calloporids, a simple narrow plate
with a rounded edge (Fig. 2.11B–D ). The shape of the plate
indicates that it could have been surrounded by the arched
ooecial fold of the frontal wall. This membranous outgrowth
is not preserved in fossils, but has been described in living
calloporids (Ostrovsky and Schäfer 2003 ; Ostrovsky et al.
2003 ). Nevertheless, it is also possible that instead of an
ooecial fold, two soft outgrowths, predecessors of ooecial
lobes, were formed (see below). Calcifi cation continues
to expand centrifugally, bordered by two lateral slits
(Fig. 2.11E ) (see also Cheetham 1975 , p. 553). The resulting gymno cystal ovicell fl oor is concave (Fig. 2.11E–H ).
In zooids without brood chambers, the proximal gymnocyst
is fl at or only slightly concave (Ostrovsky and Taylor 2005b ).
As calcifi cation continues, the lateral slits gradually
decrease in length and become separated from one another
(Fig. 2.11F ), as a consequence of which the common ooecial
fold (if it existed at all) would have been transformed into
two hollow symmetrical outgrowths, the future ooecial lobes.
As noted above, they could also form somewhat earlier.
Each lobe communicates with the proximal part of the distal
2 Cheilostome Brood Chambers: Structure, Formation, Evolution