154
calloporids and cribrilinids, the lateral openings became
closed by the frontal membranes of the lateral zooids.
Similarly, such openings (two lateral and one distal) were
probably closed in the monoporellid Monoporella multilamellosa (Fig. 2.57A, B ), which had an ooecium of two fl attened,
non-articulated spines (Figs. 2.60C and 2.62M ). Lateral
foramina were probably likewise closed by adjacent frontal
membranes while the distal foramen was closed by that of the
distal zooid. In contrast, lateral and distal ooecial openings in
the calloporid Gilbertopora larwoodi with a similar ooecial
structure most probably remained open (Figs. 2.10C–F , 2.54C ,
and 2.59E ), with water able to enter the brood cavity.
In contrast, in Macropora (Macroporidae) the bases of
ooecial spines are arranged in a horseshoe, while the ooecium
has no lateral foramina (Figs. 2.58 , 2.61B, E , and 2.62Q ). At
the same time, as in some monoporellids, the ooecial costae
of macroporids are overgrown, exteriorly and completely, by
a cryptocystal matrix, i.e. secondary calcifi cation.
Zooidal morphology and especially the well-developed
cryptocyst indicate that a species of Stichomicropora (with
spinose ooecia) could have been ancestral to Micropora
(Microporidae) (with complete ooecia) [both of these genera
evolved in the Cenomanian] or these two genera could have
shared a common ancestor. However, if this were the case,
there should have been species of Micropora with spinose
ooecia, demonstrating a transitional stage to a unitary
ooecium as seen in Calloporidae, Monoporellidae and
Cribrilinidae. So far, no such microporids are known and it is
almost certain that the ancestral microporid inherited a complete ooecium from a calloporid precursor. To note, a
medial suture has been found on the internal surface of
the ooecium in Micropora notialis (Fig. 2.33E ). I therefore formally propose a superfamily Monoporelloidea for
the Monoporellidae (see Appendix II for diagnosis). The
idea that Macropora could have evolved from Micropora
(Banta et al. 1997 ) is not supported by any evidence, since
the former has fundamentally costate ovicells and the latter
has not; Macropora is also a considerably younger genus.
Macropora could have evolved from Monoporella but the
genera are separated by a time interval of 15–17 million
years. At the same time, no Macropora species has the
arched arrangement of ooecial spines and foramina characteristic of Monoporella . Nevertheless, the two genera have
much in common and Macroporidae may provisionally be
included in the Monoporelloidea.
2.4.6 Acanthostegal Brood Chambers of
Tendridae and Ovicells of Bellulopora
The acanthostegal brood chambers of Tendridae appear
to have evolved, as did the calloporid ovicell, by the
modification of periopesial spines in a malacostegan
ancestor. However, whereas the calloporid ovicell originated by differentially inclining of a small group of proximal opesial spines of the distal zooid towards the
maternal autozooid, the tendrid brood chamber involved
all of the periopesial spines of the distal autozooid. These mural
spines are inclined towards the midline of the zooid to
form a frontal shield (Figs. 2.50 and 2.59A ). The uncalcified floor of the acanthostegal chamber in Tendra comprises the membranous frontal wall of the brooding
(distal) zooid, in complete contradistinction to the calcified floor (proximal gymnocyst of the distal zooid) of
calloporid ovicells (Оstrovsky and Taylor 2005a ).
When describing Heteroecium amplectens , Hincks ( 1892 ,
p. 333) quite correctly remarked that its “ribbed roofi ng …
bears a close resemblance in structure of the front wall of the
Cribriline zooecium, and like it has originated in a modifi cation and adaptation of the marginal spines”. Tendrids, like
cribrilinids, have both articulated oral spines and nonarticulated costal spines that form the brood chamber. It is
possible that acanthostegal brood chambers formed from
costae were preceded by similar chambers formed from
articulated mural spines.
The brood-chamber complex of Heteroecium (Figs. 2.51
and 2.59B ), consisting of the maternal zooid and the distal
kenozooid, structurally resembles ooecia formed by the distal kenozooid in Calloporidae, Cribrilinidae, Catenicellidae,
Hippothoidae (e.g. Fig. 1.36B, C ) and some other families.
This means that the trend towards reduction of the distal
zooid, characteristic of these cheilostome groups, is observed
in tendrids as well (see Sect. 2.4.8 ). This trend is also found
in Macropora , in which the ooecium may be formed by the
distal autozooid or the kenozooid (Fig. 2.61B, E ).
Bellulopora ovicells are unique. Their costae are kenozooids (see Sect. 2.3.1 ); the brood-cavity fl oor is uncalcifi ed
(Fig. 2.60E ) and water enters the cavity freely as in fossil
species with primitive ovicells and in Tendra . The ovicell
fl oor may have lost calcifi cation secondarily or is a rudiment
of the membranous frontal wall of the distal zooid. If the latter
is true, then the Bellulopora brood chamber evolved independently of ovicelled cribrimorphs in a manner reminiscent
of Tendridae (from the distal zooid). It is not inconceivable
that Bellulopora and Tendra are related (Оstrovsky and
Taylor 2005a ). It should be noted that the calcifi cation of the
brood- cavity fl oor (homologous to the frontal wall of the
autozooid) appears to be secondary in Heteroecium . It has,
however, retained a small membranous area (Fig. 2.51C, D ),
of uncertain function.
2.4.7 Evolution of the Unitary Ooecium
and Frontal Shield
As discussed above, spinose brood chambers could have
evolved three times in Cheilostomata (in Tendridae,
Calloporidae and Bellulopora ). Also, structural and develop2 Cheilostome Brood Chambers: Structure, Formation, Evolution
calloporids and cribrilinids, the lateral openings became
closed by the frontal membranes of the lateral zooids.
Similarly, such openings (two lateral and one distal) were
probably closed in the monoporellid Monoporella multilamellosa (Fig. 2.57A, B ), which had an ooecium of two fl attened,
non-articulated spines (Figs. 2.60C and 2.62M ). Lateral
foramina were probably likewise closed by adjacent frontal
membranes while the distal foramen was closed by that of the
distal zooid. In contrast, lateral and distal ooecial openings in
the calloporid Gilbertopora larwoodi with a similar ooecial
structure most probably remained open (Figs. 2.10C–F , 2.54C ,
and 2.59E ), with water able to enter the brood cavity.
In contrast, in Macropora (Macroporidae) the bases of
ooecial spines are arranged in a horseshoe, while the ooecium
has no lateral foramina (Figs. 2.58 , 2.61B, E , and 2.62Q ). At
the same time, as in some monoporellids, the ooecial costae
of macroporids are overgrown, exteriorly and completely, by
a cryptocystal matrix, i.e. secondary calcifi cation.
Zooidal morphology and especially the well-developed
cryptocyst indicate that a species of Stichomicropora (with
spinose ooecia) could have been ancestral to Micropora
(Microporidae) (with complete ooecia) [both of these genera
evolved in the Cenomanian] or these two genera could have
shared a common ancestor. However, if this were the case,
there should have been species of Micropora with spinose
ooecia, demonstrating a transitional stage to a unitary
ooecium as seen in Calloporidae, Monoporellidae and
Cribrilinidae. So far, no such microporids are known and it is
almost certain that the ancestral microporid inherited a complete ooecium from a calloporid precursor. To note, a
medial suture has been found on the internal surface of
the ooecium in Micropora notialis (Fig. 2.33E ). I therefore formally propose a superfamily Monoporelloidea for
the Monoporellidae (see Appendix II for diagnosis). The
idea that Macropora could have evolved from Micropora
(Banta et al. 1997 ) is not supported by any evidence, since
the former has fundamentally costate ovicells and the latter
has not; Macropora is also a considerably younger genus.
Macropora could have evolved from Monoporella but the
genera are separated by a time interval of 15–17 million
years. At the same time, no Macropora species has the
arched arrangement of ooecial spines and foramina characteristic of Monoporella . Nevertheless, the two genera have
much in common and Macroporidae may provisionally be
included in the Monoporelloidea.
2.4.6 Acanthostegal Brood Chambers of
Tendridae and Ovicells of Bellulopora
The acanthostegal brood chambers of Tendridae appear
to have evolved, as did the calloporid ovicell, by the
modification of periopesial spines in a malacostegan
ancestor. However, whereas the calloporid ovicell originated by differentially inclining of a small group of proximal opesial spines of the distal zooid towards the
maternal autozooid, the tendrid brood chamber involved
all of the periopesial spines of the distal autozooid. These mural
spines are inclined towards the midline of the zooid to
form a frontal shield (Figs. 2.50 and 2.59A ). The uncalcified floor of the acanthostegal chamber in Tendra comprises the membranous frontal wall of the brooding
(distal) zooid, in complete contradistinction to the calcified floor (proximal gymnocyst of the distal zooid) of
calloporid ovicells (Оstrovsky and Taylor 2005a ).
When describing Heteroecium amplectens , Hincks ( 1892 ,
p. 333) quite correctly remarked that its “ribbed roofi ng …
bears a close resemblance in structure of the front wall of the
Cribriline zooecium, and like it has originated in a modifi cation and adaptation of the marginal spines”. Tendrids, like
cribrilinids, have both articulated oral spines and nonarticulated costal spines that form the brood chamber. It is
possible that acanthostegal brood chambers formed from
costae were preceded by similar chambers formed from
articulated mural spines.
The brood-chamber complex of Heteroecium (Figs. 2.51
and 2.59B ), consisting of the maternal zooid and the distal
kenozooid, structurally resembles ooecia formed by the distal kenozooid in Calloporidae, Cribrilinidae, Catenicellidae,
Hippothoidae (e.g. Fig. 1.36B, C ) and some other families.
This means that the trend towards reduction of the distal
zooid, characteristic of these cheilostome groups, is observed
in tendrids as well (see Sect. 2.4.8 ). This trend is also found
in Macropora , in which the ooecium may be formed by the
distal autozooid or the kenozooid (Fig. 2.61B, E ).
Bellulopora ovicells are unique. Their costae are kenozooids (see Sect. 2.3.1 ); the brood-cavity fl oor is uncalcifi ed
(Fig. 2.60E ) and water enters the cavity freely as in fossil
species with primitive ovicells and in Tendra . The ovicell
fl oor may have lost calcifi cation secondarily or is a rudiment
of the membranous frontal wall of the distal zooid. If the latter
is true, then the Bellulopora brood chamber evolved independently of ovicelled cribrimorphs in a manner reminiscent
of Tendridae (from the distal zooid). It is not inconceivable
that Bellulopora and Tendra are related (Оstrovsky and
Taylor 2005a ). It should be noted that the calcifi cation of the
brood- cavity fl oor (homologous to the frontal wall of the
autozooid) appears to be secondary in Heteroecium . It has,
however, retained a small membranous area (Fig. 2.51C, D ),
of uncertain function.
2.4.7 Evolution of the Unitary Ooecium
and Frontal Shield
As discussed above, spinose brood chambers could have
evolved three times in Cheilostomata (in Tendridae,
Calloporidae and Bellulopora ). Also, structural and develop2 Cheilostome Brood Chambers: Structure, Formation, Evolution