152
and Unidistelopora (Calloporidae) as well as in several
Stichomicropora species (Monoporellidae). Their ooecia were
represented by a straight or bent row of articulated spines on
the gymnocyst of the distal zooid (Figs. 2.9 , 2.10A, B , 2.54A,
B , 2.55 , 2.59C, D , 2.60A , and 2.62A, B, D, E, G, H, I, L, P ).
The bases of the medial spines of the ooecium are often situated close to or on the mural (opesial) rim of the distal zooid.
Because of this, in Distelopora (as a rule) and in Unidistelopora
(always) the bases of the mural spines of the distal zooid and
those of the medial spines of the ooecium together form an
uninterrupted row (Figs. 2.9B, D , 2.10A, B , and 2.62P ), with
the latter occupying the position of proximal mural spines.
This circumstance is direct evidence for the origin of ooecial
spines – they clearly evolved from mural spines.
Variations in the morphology and arrangement of ooecial
spines throughout the Late Cretaceous demonstrate how
transitions from simple to advanced character states may
have occurred. A distally concave arch formed by the bases
of the ooecial spines represents a less-derived character
state, in essence corresponding to the arrangement of the
usual mural spines in the proximal part of the opesia of the
distal zooid ( Stichomicropora ; Figs. 2.55A–D and 2.62A, B ).
A more-derived character state is when most of the ooecialspine bases are arranged transversely in a more or less
straight line ( Stichomicropora ; Figs. 2.55A, C, D , and 2.62D,
E ). The next step, a distally convex arch of spine bases,
characterizes species of Stichomicropora (Monoporellidae)
(Figs. 2.55D–F and 2.62G, H ) and Distelopora (Calloporidae)
(Figs. 2.9A–E , 2.54A , and 2.62I, L ). It should be stressed
that all three basic stages can be found in a single species –
Campanian Stichomicropora sp. 1, which had articulated
spines similarly to calloporids (Figs. 2.55C, D and 2.62A, D, G )
(Ostrovsky and Taylor 2005a , see also Taylor and
McKinney 2002 ). Finally, a horseshoe arrangement of
ooecial- spine bases is found in the calloporids Distelopora
spinifera (Figs. 2.9F–H and 2.54B ), Unidistelopora krauseae
(Figs. 2.10A, B and 2.62P ) and, sometimes D . bipilata
(Fig. 2.9B ). The monoporellid genus Stichomicropora is, in
fact, younger than most of these calloporids, from which we
may infer that the Calloporidae in the Late Cretaceous would
have included species with ooecia having distally concave
and transverse spine arrangements.
This morphoseries agrees well with the idea that the protective function of the ooecium was enhanced in the course
of evolution. Proximally inclined ooecial spines, their bases
arranged as a gently curving arch or a straight line, formed
the roof of the ooecium, the brood cavity of which opened to
the environment on three sides (Fig. 2.54A ). In contrast, the
horseshoe arrangement resulted in the formation of a cagelike ooecium opening on one side only (Fig. 2.54B ) (see
Ostrovsky and Taylor 2004 , 2005a ). Thus, a shift in ooecialspine arrangement may have been associated with a change
in function, that is, from protection of the membranous
frontal wall of the distal zooid to more effective protection of
developing embryos. This change required some of the
spines to develop directly on the proximal gymnocyst of the
distal zooid, beyond the edge of the opesia. This developmental variant is found, for instance, in the living malacostegan
Villicharixa strigosa (Fig. 2.53B ). Finally, ooecia could
completely lose contact with the opesial rim; in some ovicells of Distelopora bipilata and D . spinifera , even the
medial ooecial spines, usually located near the rim, may be
positioned at some distance from it (Fig. 2.9H ). In this
instance substitute spines occupy the position on the
opesial rim.
Further evolution of ovicells in calloporids was probably
associated with a reduction in the number of ooecial spines
to two, accompanied by their fl attening and enlargement as
well as the loss of articulation. Ooecia of Gilbertopora
larwoodi consist of two costa-like lobes. Apart from the
main ovicell opening, the brood cavity communicates with
the external environment via two lateral foramina and a distal
opening between the basal parts of the lobes (Figs. 2.10C–F ,
2.54C , and 2.59E ). The next stage is represented by complete ooecia with a medial suture, as seen in Wilbertopora
(Figs. 2.11A , 2.12D–F , and 2.54D ).
In addition to the species of Stichomicropora with
articulated spines, similar variants of the position of ooecial
spine bases also occurred in S. ostrovskyi and in the genus
Monoporella with non-articulated ooecial spines being
arranged in distally concave row. (Fig. 2.62C ), across the
daughter zooid (Figs. 2.56C and 2.62F ) or in a distally convex arch (Figs. 2.57C, D and 2.62J ). A reduction in spine
number to two, accompanied by fl attening, also occurred in
both these genera (Figs. 2.57A, B , 2.60C , and 2.62M )
(Ostrovsky and Taylor 2005a ; Taylor and McKinney 2006 ).
Ooecium-forming costae in Macropora (Macroporidae)
and Leptocheilopora (Cribrilinidae) are arranged in a semicircular or horseshoe pattern (Figs. 2.26 , 2.28 , 2.60D , and
2.62N–O, R ) (Ostrovsky and Taylor 2004 , 2005a ).
2.4.4 Evolution of Ovicells in the Family
Cribrilinidae
The existence of ovicells constructed of spines in calloporids
and monoporellids is supportive of a monophyletic origin of
these two groups, with Calloporidae basal. As was mentioned above, the Calloporidae in the Late Cretaceous would
have included species with ooecia having a distally concave
spine arrangement that was supposedly inherited by their
monoporellid descendants. The further evolution of ooecia –
involving a transition to the distally convex spine arrangement, loss of spine articulation, spine fl attening and reduction in number – in both clades was probably independent
(see above).
2 Cheilostome Brood Chambers: Structure, Formation, Evolution
and Unidistelopora (Calloporidae) as well as in several
Stichomicropora species (Monoporellidae). Their ooecia were
represented by a straight or bent row of articulated spines on
the gymnocyst of the distal zooid (Figs. 2.9 , 2.10A, B , 2.54A,
B , 2.55 , 2.59C, D , 2.60A , and 2.62A, B, D, E, G, H, I, L, P ).
The bases of the medial spines of the ooecium are often situated close to or on the mural (opesial) rim of the distal zooid.
Because of this, in Distelopora (as a rule) and in Unidistelopora
(always) the bases of the mural spines of the distal zooid and
those of the medial spines of the ooecium together form an
uninterrupted row (Figs. 2.9B, D , 2.10A, B , and 2.62P ), with
the latter occupying the position of proximal mural spines.
This circumstance is direct evidence for the origin of ooecial
spines – they clearly evolved from mural spines.
Variations in the morphology and arrangement of ooecial
spines throughout the Late Cretaceous demonstrate how
transitions from simple to advanced character states may
have occurred. A distally concave arch formed by the bases
of the ooecial spines represents a less-derived character
state, in essence corresponding to the arrangement of the
usual mural spines in the proximal part of the opesia of the
distal zooid ( Stichomicropora ; Figs. 2.55A–D and 2.62A, B ).
A more-derived character state is when most of the ooecialspine bases are arranged transversely in a more or less
straight line ( Stichomicropora ; Figs. 2.55A, C, D , and 2.62D,
E ). The next step, a distally convex arch of spine bases,
characterizes species of Stichomicropora (Monoporellidae)
(Figs. 2.55D–F and 2.62G, H ) and Distelopora (Calloporidae)
(Figs. 2.9A–E , 2.54A , and 2.62I, L ). It should be stressed
that all three basic stages can be found in a single species –
Campanian Stichomicropora sp. 1, which had articulated
spines similarly to calloporids (Figs. 2.55C, D and 2.62A, D, G )
(Ostrovsky and Taylor 2005a , see also Taylor and
McKinney 2002 ). Finally, a horseshoe arrangement of
ooecial- spine bases is found in the calloporids Distelopora
spinifera (Figs. 2.9F–H and 2.54B ), Unidistelopora krauseae
(Figs. 2.10A, B and 2.62P ) and, sometimes D . bipilata
(Fig. 2.9B ). The monoporellid genus Stichomicropora is, in
fact, younger than most of these calloporids, from which we
may infer that the Calloporidae in the Late Cretaceous would
have included species with ooecia having distally concave
and transverse spine arrangements.
This morphoseries agrees well with the idea that the protective function of the ooecium was enhanced in the course
of evolution. Proximally inclined ooecial spines, their bases
arranged as a gently curving arch or a straight line, formed
the roof of the ooecium, the brood cavity of which opened to
the environment on three sides (Fig. 2.54A ). In contrast, the
horseshoe arrangement resulted in the formation of a cagelike ooecium opening on one side only (Fig. 2.54B ) (see
Ostrovsky and Taylor 2004 , 2005a ). Thus, a shift in ooecialspine arrangement may have been associated with a change
in function, that is, from protection of the membranous
frontal wall of the distal zooid to more effective protection of
developing embryos. This change required some of the
spines to develop directly on the proximal gymnocyst of the
distal zooid, beyond the edge of the opesia. This developmental variant is found, for instance, in the living malacostegan
Villicharixa strigosa (Fig. 2.53B ). Finally, ooecia could
completely lose contact with the opesial rim; in some ovicells of Distelopora bipilata and D . spinifera , even the
medial ooecial spines, usually located near the rim, may be
positioned at some distance from it (Fig. 2.9H ). In this
instance substitute spines occupy the position on the
opesial rim.
Further evolution of ovicells in calloporids was probably
associated with a reduction in the number of ooecial spines
to two, accompanied by their fl attening and enlargement as
well as the loss of articulation. Ooecia of Gilbertopora
larwoodi consist of two costa-like lobes. Apart from the
main ovicell opening, the brood cavity communicates with
the external environment via two lateral foramina and a distal
opening between the basal parts of the lobes (Figs. 2.10C–F ,
2.54C , and 2.59E ). The next stage is represented by complete ooecia with a medial suture, as seen in Wilbertopora
(Figs. 2.11A , 2.12D–F , and 2.54D ).
In addition to the species of Stichomicropora with
articulated spines, similar variants of the position of ooecial
spine bases also occurred in S. ostrovskyi and in the genus
Monoporella with non-articulated ooecial spines being
arranged in distally concave row. (Fig. 2.62C ), across the
daughter zooid (Figs. 2.56C and 2.62F ) or in a distally convex arch (Figs. 2.57C, D and 2.62J ). A reduction in spine
number to two, accompanied by fl attening, also occurred in
both these genera (Figs. 2.57A, B , 2.60C , and 2.62M )
(Ostrovsky and Taylor 2005a ; Taylor and McKinney 2006 ).
Ooecium-forming costae in Macropora (Macroporidae)
and Leptocheilopora (Cribrilinidae) are arranged in a semicircular or horseshoe pattern (Figs. 2.26 , 2.28 , 2.60D , and
2.62N–O, R ) (Ostrovsky and Taylor 2004 , 2005a ).
2.4.4 Evolution of Ovicells in the Family
Cribrilinidae
The existence of ovicells constructed of spines in calloporids
and monoporellids is supportive of a monophyletic origin of
these two groups, with Calloporidae basal. As was mentioned above, the Calloporidae in the Late Cretaceous would
have included species with ooecia having a distally concave
spine arrangement that was supposedly inherited by their
monoporellid descendants. The further evolution of ooecia –
involving a transition to the distally convex spine arrangement, loss of spine articulation, spine fl attening and reduction in number – in both clades was probably independent
(see above).
2 Cheilostome Brood Chambers: Structure, Formation, Evolution