156
Aechmella (Onychocellidae) had this type of ooecium as
early as in the Cenomanian (Voigt 1989 ). The genus
Micropora evolved at the same time, but there is currently no
information about the ooecium in any Cenomanian species.
Presumably it was calloporiform, with a calcifi ed ecto- and
entooecium and the ooecial coelom connected with the
visceral cavity of the distal zooid (as in Recent Micropora
gracilis ) (Fig. 2.63A ).
There is no obvious reason why the ectooecium would
have trended towards reduced calcifi cation (see Sect. 2.4.8 ).
There may have been a shift in the locus of the calcium
carbonate deposition consequent upon evolution of the
coilostegan cryptocyst – the more CaCO 3 is deposited into
an enlarging cryptocyst, the less it is deposited into the
ectooecium, which would make sense energetically.
The endotoichal ovicells of Cellarioidea are structurally
similar to the ooecia of Microporidae and Onychocellidae.
Common features include a lack of ectooecial calcifi cation,
communication of ooecial and hypostegal coeloms and fusion
of the entooecium with the cryptocyst of the distal zooid(s)
(compare Figs. 2.34 and 2.39 ). Endotoichal ovicells were
probably formed by immersion of the ovicellar brood cavity in the colony (see Sect. 2.3.2 ), which is one of the major
trends in the evolution of brooding structures in Cheilostomata.
Another important aspect of endotoichal ovicell evolution was
the development of the ooecial vesicle, which formed a sac
inside the brood cavity. That this sac is a modifi ed ooecial
vesicle is evidenced by the presence of the sclerite and numerous muscle bundles within it. These considerations are supportive of the origin of the endotoichal ovicell within
Microporoidea, including the evolution of Cellarioidea
(known since the Santonian) from an ancestor within
Microporidae (known since the Cenomanian). The specifi c
hypothesis that Cellaria evolved from Micropora (Banta et al.
1997 ) is supported by a comparison of ooecial structure.
The primitive calloporiform ooecium is found in umbonulomorph and lepraliomorph ascophorans. It is the basic
ooecial type from which escharelliform and lepralielliform
variants evolved in ascophorans. As mentioned earlier in
this chapter, the latter two variants are also both found in
umbonulomorphs (including the family Lepraliellidae) and
lepraliomorphs.
According to the least-contradictory and best-supported
hypothesis, the lepralioid frontal shield repeatedly evolved
from umbonuloid precursors. The umbonuloid shield itself
apparently originated when frontal (adventitious) kenozooids overgrew the zooidal spinocyst of cribrimorphs
(Fig. 2.64A1 ). Kenozooids like these have been found in
cribrimorphs from the Cretaceous (including the Santonian)
to the Holocene and the present day (Gordon and Voigt 1996 ;
Gordon 2000 ). Thus, in accord with this hypothesis, umbonulomorph ancestors would have been cribrilinoidean taxa
with a calloporiform ooecium (Figs. 2.64A2, C ) inherited by
the early umbonulomorphs. For instance, the combination of
a calloporiform ooecium and umbonulomorph frontal shield
exists in some Recent Arachnopusiidae.
It is likely that the early progressive development of
frontal kenozooids and the formation of hypostegal coelom
(derived from the laterally expanded kenozooidal coelom) of
the frontal shield infl uenced the formation of the ooecial
fold, thus reducing the size of the ooecial base. In the calloporiform ooecium the ooecial fold starts its formation around
the simple gymnocystal fl oor of the future brood chamber,
whereas in the lepralielliform variant formation of the fold
begins much earlier, with the ovicell fl oor placed (partially or
completely) above the horizontal part of the ectooecium and
the frontal shield (compare Figs. 2.22 and 2.41 ). The “double
disc” developmental stage characteristic of the latter variant
is in fact a somewhat more compact version of the ooecial
fold of calloporids and cribrimorphs (compare Figs. 2.18 and
2.40 ). Reduction of the ooecial base infl uenced the shape and
size of the communication pores – a central pore was formed
instead of an arched slit. Expansion of the frontal kenozooids
accompanied by the diminution of the ooecial base resulted
in coordinated development of the umbonulomorph frontal
shield and the lepralielliform ooecium (Fig. 2.64B ), characteristic of some Recent species from the families Arachnopusiidae, Lepraliellidae, Bryocryptellidae and Umbonulidae.
In the latter family, species of Rhamphostomella exhibit this
reduction to varying degrees (Fig. 2.41 ). It may be additionally supposed that the kenozooids that formed the umbonuloid shield overgrew not only the cribrimorph spinocyst but
also the ooecial base and the ooecium itself, giving rise to
secondary calcifi cation.
Paralleling the transformation in the anascan family
Microporidae, the escharelliform variant in umbonulomorphs
presumably evolved from a calloporiform ooecium
(Fig. 2.64C ). This would have involved a reduction of ectooecial calcifi cation and fusion of the basal part of the entooecium (ovicell fl oor) with the proximal part of the calcifi ed
wall of the frontal shield. The combined umbonulomorph
frontal shield and escharelliform ooecium thus emerged
(Fig. 2.64D ). The ooecial coelom began to communicate with
the hypostegal coelom of the distal zooid, and the communication canal between the ooecium and the visceral coelom
was closed (with few exceptions, see Sect. 2.3.2 ). Among others, this type of ooecium characterizes modern species of
Lepraliellidae and Romancheinidae (Figs. 2.35 and 2.36 ).
According to Gordon and Voigt ( 1996 ) and Gordon
( 2000 ), the lepraliomorph frontal shield (whether pseudoporous or centrally imperforate) originated by progressive
reduction of the umbonuloid component by the distal expansion of the proximal part of the frontal shield (gymnocyst
concealed by transformed frontal kenozooids) and ascus
formation. Some lepraliomorph cheilostomes (few smittinids, see below) have calloporiform ooecia (Fig. 2.64E ),
2 Cheilostome Brood Chambers: Structure, Formation, Evolution
Aechmella (Onychocellidae) had this type of ooecium as
early as in the Cenomanian (Voigt 1989 ). The genus
Micropora evolved at the same time, but there is currently no
information about the ooecium in any Cenomanian species.
Presumably it was calloporiform, with a calcifi ed ecto- and
entooecium and the ooecial coelom connected with the
visceral cavity of the distal zooid (as in Recent Micropora
gracilis ) (Fig. 2.63A ).
There is no obvious reason why the ectooecium would
have trended towards reduced calcifi cation (see Sect. 2.4.8 ).
There may have been a shift in the locus of the calcium
carbonate deposition consequent upon evolution of the
coilostegan cryptocyst – the more CaCO 3 is deposited into
an enlarging cryptocyst, the less it is deposited into the
ectooecium, which would make sense energetically.
The endotoichal ovicells of Cellarioidea are structurally
similar to the ooecia of Microporidae and Onychocellidae.
Common features include a lack of ectooecial calcifi cation,
communication of ooecial and hypostegal coeloms and fusion
of the entooecium with the cryptocyst of the distal zooid(s)
(compare Figs. 2.34 and 2.39 ). Endotoichal ovicells were
probably formed by immersion of the ovicellar brood cavity in the colony (see Sect. 2.3.2 ), which is one of the major
trends in the evolution of brooding structures in Cheilostomata.
Another important aspect of endotoichal ovicell evolution was
the development of the ooecial vesicle, which formed a sac
inside the brood cavity. That this sac is a modifi ed ooecial
vesicle is evidenced by the presence of the sclerite and numerous muscle bundles within it. These considerations are supportive of the origin of the endotoichal ovicell within
Microporoidea, including the evolution of Cellarioidea
(known since the Santonian) from an ancestor within
Microporidae (known since the Cenomanian). The specifi c
hypothesis that Cellaria evolved from Micropora (Banta et al.
1997 ) is supported by a comparison of ooecial structure.
The primitive calloporiform ooecium is found in umbonulomorph and lepraliomorph ascophorans. It is the basic
ooecial type from which escharelliform and lepralielliform
variants evolved in ascophorans. As mentioned earlier in
this chapter, the latter two variants are also both found in
umbonulomorphs (including the family Lepraliellidae) and
lepraliomorphs.
According to the least-contradictory and best-supported
hypothesis, the lepralioid frontal shield repeatedly evolved
from umbonuloid precursors. The umbonuloid shield itself
apparently originated when frontal (adventitious) kenozooids overgrew the zooidal spinocyst of cribrimorphs
(Fig. 2.64A1 ). Kenozooids like these have been found in
cribrimorphs from the Cretaceous (including the Santonian)
to the Holocene and the present day (Gordon and Voigt 1996 ;
Gordon 2000 ). Thus, in accord with this hypothesis, umbonulomorph ancestors would have been cribrilinoidean taxa
with a calloporiform ooecium (Figs. 2.64A2, C ) inherited by
the early umbonulomorphs. For instance, the combination of
a calloporiform ooecium and umbonulomorph frontal shield
exists in some Recent Arachnopusiidae.
It is likely that the early progressive development of
frontal kenozooids and the formation of hypostegal coelom
(derived from the laterally expanded kenozooidal coelom) of
the frontal shield infl uenced the formation of the ooecial
fold, thus reducing the size of the ooecial base. In the calloporiform ooecium the ooecial fold starts its formation around
the simple gymnocystal fl oor of the future brood chamber,
whereas in the lepralielliform variant formation of the fold
begins much earlier, with the ovicell fl oor placed (partially or
completely) above the horizontal part of the ectooecium and
the frontal shield (compare Figs. 2.22 and 2.41 ). The “double
disc” developmental stage characteristic of the latter variant
is in fact a somewhat more compact version of the ooecial
fold of calloporids and cribrimorphs (compare Figs. 2.18 and
2.40 ). Reduction of the ooecial base infl uenced the shape and
size of the communication pores – a central pore was formed
instead of an arched slit. Expansion of the frontal kenozooids
accompanied by the diminution of the ooecial base resulted
in coordinated development of the umbonulomorph frontal
shield and the lepralielliform ooecium (Fig. 2.64B ), characteristic of some Recent species from the families Arachnopusiidae, Lepraliellidae, Bryocryptellidae and Umbonulidae.
In the latter family, species of Rhamphostomella exhibit this
reduction to varying degrees (Fig. 2.41 ). It may be additionally supposed that the kenozooids that formed the umbonuloid shield overgrew not only the cribrimorph spinocyst but
also the ooecial base and the ooecium itself, giving rise to
secondary calcifi cation.
Paralleling the transformation in the anascan family
Microporidae, the escharelliform variant in umbonulomorphs
presumably evolved from a calloporiform ooecium
(Fig. 2.64C ). This would have involved a reduction of ectooecial calcifi cation and fusion of the basal part of the entooecium (ovicell fl oor) with the proximal part of the calcifi ed
wall of the frontal shield. The combined umbonulomorph
frontal shield and escharelliform ooecium thus emerged
(Fig. 2.64D ). The ooecial coelom began to communicate with
the hypostegal coelom of the distal zooid, and the communication canal between the ooecium and the visceral coelom
was closed (with few exceptions, see Sect. 2.3.2 ). Among others, this type of ooecium characterizes modern species of
Lepraliellidae and Romancheinidae (Figs. 2.35 and 2.36 ).
According to Gordon and Voigt ( 1996 ) and Gordon
( 2000 ), the lepraliomorph frontal shield (whether pseudoporous or centrally imperforate) originated by progressive
reduction of the umbonuloid component by the distal expansion of the proximal part of the frontal shield (gymnocyst
concealed by transformed frontal kenozooids) and ascus
formation. Some lepraliomorph cheilostomes (few smittinids, see below) have calloporiform ooecia (Fig. 2.64E ),
2 Cheilostome Brood Chambers: Structure, Formation, Evolution