153
The semicircular arrangement of spines may also indicate
a relationship between Calloporidae and Cribrilinidae
(see Ostrovsky and Taylor 2004 , 2005a ). In the course of the
further evolution of ovicells, the structure of spines in cribrilinids changed considerably – they lost their basal joints and
became fl attened. Thus, mural and ooecial spines transformed
into costae. The scutum protecting the frontal wall in many
species of Candidae is a good example of how spines can
fl atten to become a kind of shield (Silén 1977 ).
Theoretically, cribrilinids could have inherited ovicells
from their ancestors according to two possible scenarios:
(1) ovicells of early cribrimorphs could have been inherited
from one or more calloporids that had ooecia with a horseshoe arrangement of spines (as in Distelopora spinifera );
(2) in Tricephalopora saltdeanensis (Cribrilinidae) the ooecial surface appears to be implicitly costate (Lang 1922 , pl. 1,
fi g. 7), appearing to retain traces of fused costae. These are
not arranged in a horseshoe pattern (as in Leptocheilopora )
but “linearly” (as in some Stichomicropora ). If these traces
are indeed left by fused costae, then cribrilinids, having
inherited the linear/arched arrangement of ooecial spines
from calloporids, evolved the horseshoe arrangement independently (as did calloporids and monoporellids).
In some species of the latter two families, the number of
spines was reduced to two and the remaining spines became
fl attened and enlarged. In this way cribrilinids also independently underwent reduction in spine number to a single pair.
On the one hand, not only fossil but also some Recent cribrilinids (genera Figularia and Puellina ) possess costate
ooecia, indicative of their origin (reviewed in Ostrovsky 2002 ).
On the other hand, ovicells with bilobate ooecia (in cribrimorph genera Puellina , Figularia , Filaguria , Corbulipora
and Euthyroides ) are structurally more or less identical to
those of the calloporids Wilbertopora and Valdemunitella .
Moreover, the development of the ooecium from two originally independent ooecial halves/folds (demonstrated in
Corbulipora and E . episcopalis and suspected in Puellina
and Figularia ) closely resembles ovicellogenesis in Wilbertopora and Valdemunitella (Gordon 1986 ; Ostrovsky and
Taylor 2005b ; Ostrovsky, unpublished data).
The presence of both costate and bivalved ooecia within
the same genus (as in Figularia and Puellina ) is especially
remarkable. In this context, the transformation from spinose to bilobate ooecia in cribrilinids could be imagined to
result from: (1) reducing the number of spines to two, their
fl attening and enlargement (as probably occurred in calloporids), or (2) fusion of spines and formation of the left and
right ooecial halves. Judging from the external appearance
of the ooecium in the Cretaceous cribrilinid Leptocheilopora
sp. 2 (Fig. 2.26B, D ), the two-lobed ovicells of cribrimorphs may have evolved by fusion of spines, as happened
in spinocysts of more-advanced cribrilinids such as
Cribrilina (see Ostrovsky and Taylor 2005a ). Fusion of
buds of forming zooids is well-known in Gymnolaemata
(Jebram 1978 ); as long as they are not calcifi ed, cystid
walls can merge cuticular and cellular layers. Finally, both
above variants could be realized in different cribrimorph
groups (Ostrovsky et al. 2009a ).
To summarize, the two-lobed ooecium seems to have
originated independently in Calloporidae, Monoporellidae
and Cribrimorpha. However, the evidence that this structure
resulted from spine fusion is present only in cribrilinids
(Ostrovsky and Taylor 2005a ), and it is not known if this
variant is basic. In the course of subsequent evolution (and in
parallel with calloporids), both sides of the two-lobed ooecium in some cribrilinids fused to form a unitary ooecium
with a common communication slit ( Cribrilina macropunctata , C. punctata , C . cryptooecium , Ostrovsky, unpublished
data). As in calloporids, the non-paired rudiment of the ovicell fl oor was retained in species with a bilobate ooecium,
whereas the paired rudiment was probably independently
evolved by cribrilinids together with the unitary ooecium
(Ostrovsky and Taylor 2005b ).
2.4.5 Evolution of Ovicells in the Genera
Monoporella and Macropora
The loss of articulation, the fl attening and fusion of ooecial spines and shift in their arrangement from distally
concave to convex, were also characteristic of ovicell evolution in Monoporella (Monoporellidae) (Figs. 2.57 , 2.60C ,
2.61A, C , and 2.62J, M , see also above). In this genus,
ooecial spines are also overgrown by a cryptocystal matrix
(Figs. 2.57D–F and 2.61A, C, D ) (Ostrovsky and Taylor
2005a ). Secondary calcifi cation similarly covers the ooecium in many ascophorans (see Sect. 2.3.2 ), producing
more-robust brood chambers.
Better protection of embryos may be also achieved by
closure of the brood-chamber opening. Early spinose ovicells appear to have been non-cleithral (non-closed), later
transforming into acleithral (closed by the ooecial vesicle)
then cleithral, with the ovicell opening closed by the operculum of the maternal autozooid. Lateral foramina in the
ovicells of some species also became closed, as can be seen
in transverse sections of Monoporella ovicells (Fig. 2.61A, C,
D ); the foramina are plugged by the membranous frontal
wall of each laterally adjacent zooid so that the brood cavity
is isolated from the environment (see also Cheetham and
Cook 1983 , fi g. 72.2). It seems that lateral foramina were
similarly plugged in ovicells of some species of
Stichomicropora , whereas they remained open in others with
a more- developed proximal gymnocyst (compare Fig. 2.55A,
E , and C, D ). This fact may explain why semicircular or
horseshoe arrangements of ooecial spines did not evolve in
monoporellids (Figs. 2.62A–H, J, M ) – in contrast to
2.4 Evolution of Brood Chambers in Cheilostomata
The semicircular arrangement of spines may also indicate
a relationship between Calloporidae and Cribrilinidae
(see Ostrovsky and Taylor 2004 , 2005a ). In the course of the
further evolution of ovicells, the structure of spines in cribrilinids changed considerably – they lost their basal joints and
became fl attened. Thus, mural and ooecial spines transformed
into costae. The scutum protecting the frontal wall in many
species of Candidae is a good example of how spines can
fl atten to become a kind of shield (Silén 1977 ).
Theoretically, cribrilinids could have inherited ovicells
from their ancestors according to two possible scenarios:
(1) ovicells of early cribrimorphs could have been inherited
from one or more calloporids that had ooecia with a horseshoe arrangement of spines (as in Distelopora spinifera );
(2) in Tricephalopora saltdeanensis (Cribrilinidae) the ooecial surface appears to be implicitly costate (Lang 1922 , pl. 1,
fi g. 7), appearing to retain traces of fused costae. These are
not arranged in a horseshoe pattern (as in Leptocheilopora )
but “linearly” (as in some Stichomicropora ). If these traces
are indeed left by fused costae, then cribrilinids, having
inherited the linear/arched arrangement of ooecial spines
from calloporids, evolved the horseshoe arrangement independently (as did calloporids and monoporellids).
In some species of the latter two families, the number of
spines was reduced to two and the remaining spines became
fl attened and enlarged. In this way cribrilinids also independently underwent reduction in spine number to a single pair.
On the one hand, not only fossil but also some Recent cribrilinids (genera Figularia and Puellina ) possess costate
ooecia, indicative of their origin (reviewed in Ostrovsky 2002 ).
On the other hand, ovicells with bilobate ooecia (in cribrimorph genera Puellina , Figularia , Filaguria , Corbulipora
and Euthyroides ) are structurally more or less identical to
those of the calloporids Wilbertopora and Valdemunitella .
Moreover, the development of the ooecium from two originally independent ooecial halves/folds (demonstrated in
Corbulipora and E . episcopalis and suspected in Puellina
and Figularia ) closely resembles ovicellogenesis in Wilbertopora and Valdemunitella (Gordon 1986 ; Ostrovsky and
Taylor 2005b ; Ostrovsky, unpublished data).
The presence of both costate and bivalved ooecia within
the same genus (as in Figularia and Puellina ) is especially
remarkable. In this context, the transformation from spinose to bilobate ooecia in cribrilinids could be imagined to
result from: (1) reducing the number of spines to two, their
fl attening and enlargement (as probably occurred in calloporids), or (2) fusion of spines and formation of the left and
right ooecial halves. Judging from the external appearance
of the ooecium in the Cretaceous cribrilinid Leptocheilopora
sp. 2 (Fig. 2.26B, D ), the two-lobed ovicells of cribrimorphs may have evolved by fusion of spines, as happened
in spinocysts of more-advanced cribrilinids such as
Cribrilina (see Ostrovsky and Taylor 2005a ). Fusion of
buds of forming zooids is well-known in Gymnolaemata
(Jebram 1978 ); as long as they are not calcifi ed, cystid
walls can merge cuticular and cellular layers. Finally, both
above variants could be realized in different cribrimorph
groups (Ostrovsky et al. 2009a ).
To summarize, the two-lobed ooecium seems to have
originated independently in Calloporidae, Monoporellidae
and Cribrimorpha. However, the evidence that this structure
resulted from spine fusion is present only in cribrilinids
(Ostrovsky and Taylor 2005a ), and it is not known if this
variant is basic. In the course of subsequent evolution (and in
parallel with calloporids), both sides of the two-lobed ooecium in some cribrilinids fused to form a unitary ooecium
with a common communication slit ( Cribrilina macropunctata , C. punctata , C . cryptooecium , Ostrovsky, unpublished
data). As in calloporids, the non-paired rudiment of the ovicell fl oor was retained in species with a bilobate ooecium,
whereas the paired rudiment was probably independently
evolved by cribrilinids together with the unitary ooecium
(Ostrovsky and Taylor 2005b ).
2.4.5 Evolution of Ovicells in the Genera
Monoporella and Macropora
The loss of articulation, the fl attening and fusion of ooecial spines and shift in their arrangement from distally
concave to convex, were also characteristic of ovicell evolution in Monoporella (Monoporellidae) (Figs. 2.57 , 2.60C ,
2.61A, C , and 2.62J, M , see also above). In this genus,
ooecial spines are also overgrown by a cryptocystal matrix
(Figs. 2.57D–F and 2.61A, C, D ) (Ostrovsky and Taylor
2005a ). Secondary calcifi cation similarly covers the ooecium in many ascophorans (see Sect. 2.3.2 ), producing
more-robust brood chambers.
Better protection of embryos may be also achieved by
closure of the brood-chamber opening. Early spinose ovicells appear to have been non-cleithral (non-closed), later
transforming into acleithral (closed by the ooecial vesicle)
then cleithral, with the ovicell opening closed by the operculum of the maternal autozooid. Lateral foramina in the
ovicells of some species also became closed, as can be seen
in transverse sections of Monoporella ovicells (Fig. 2.61A, C,
D ); the foramina are plugged by the membranous frontal
wall of each laterally adjacent zooid so that the brood cavity
is isolated from the environment (see also Cheetham and
Cook 1983 , fi g. 72.2). It seems that lateral foramina were
similarly plugged in ovicells of some species of
Stichomicropora , whereas they remained open in others with
a more- developed proximal gymnocyst (compare Fig. 2.55A,
E , and C, D ). This fact may explain why semicircular or
horseshoe arrangements of ooecial spines did not evolve in
monoporellids (Figs. 2.62A–H, J, M ) – in contrast to
2.4 Evolution of Brood Chambers in Cheilostomata