157
perhaps inherited from umbonulomorph ancestors
(Fig. 2.64A2 ). As described above in umbonulomorphs, the
early establishment of the frontal shield and hypostegal coelom may have resulted in reduction of the basal part of the
ooecium, origination of the “double disc” stage and corresponding changes in communication structures. This trend
is easily traceable in Smittina – ooecial and visceral coeloms communicate via an arched slit in S . antarctica with a
calloporiform ooecium, while all other studied species of
the genus have a central pore in combination with either a
calloporiform or lepralielliform ooecium; correspondingly,
the simple gymnocystal part of the ovicell fl oor is developed
to a different degree in Smittina , as in umbonulomorph
Rhamphostomella (Ostrovsky, unpublished data). The combined lepralioid frontal shield and lepralielliform ooecium,
found in Smittinidae and Bitectiporidae inter alia, may have
evolved in this way (Fig. 2.64F ).
Another combination is that of the lepralioid frontal
shield and the escharelliform ooecium (Fig. 2.64G ), found in
some cheilostome families (see Sect. 2.3.2 ). If we accept that
the lepralioid frontal shield evolved from an umbonuloid
precursor and the escharelliform ooecium evolved from a
calloporiform precursor, then we may suggest that the
lepralioid/escharelliform combination could have evolved
from (1) early lepraliomorphs with a calloporiform ooecium
(Fig. 2.64E ) or (2) umbonulomorphs with an escharelliform
ooecium (Fig. 2.64D ).
The above hypothetical scenarios of ooecium evolution in
lepraliomorphs do not contradict Gordon and Voigt’s ( 1996 )
and Gordon’s ( 2000 ) ideas about the polyphyletic origin of
this morphological grade. Moreover, the fact that there are
different variants of ooecial structure among lepraliomorphs
may indicate that lepralielliform and/or escharelliform
ooecia could have been inherited from different umbonuloid
ancestors that also possessed them.
The microporelliform ooecium and the variant described
in Fenestrulina are found only in the Schizoporelloidea
(e.g. Microporellidae, Pacifi cincolidae, Schizoporellidae,
Myriaporidae, Porinidae). These variants may demonstrate
stages in the transformation of the lepralielliform ooecium.
Ooecial structure in Fenestrulina may be interpreted as transitional between lepralielliform and microporelliform
(Fig. 2.65 ) ( Fenestrulina and “microporelliform” taxa have a
single initial calcifi cation of the ovicell fl oor). Fenestrulina
and Microporella , exhibiting two variants of ooecial structure, belong to the same family Microporellidae.
As with the escharelliform ooecium (Fig. 2.64C, D and
above), the presumed transition from calloporiform to microporelliform may have occurred through reduction of ectooecial calcifi cation, fusion of entooecium with the frontal
shield, and consequent loss of communication between ooecial and visceral coeloms but establishment of communication between ooecial and hypostegal coeloms (Fig. 2.65 ).
In Fenestrulina , with its intermediate structure of ooecium,
the latter coeloms are separated, as indicated by an ooecial
communication pore and calcareous ectooecial thickening
around the base of the vertical part of the entooecium
(Figs. 2.1 , 2.45 , and 2.65B ; see also Nielsen 1981 ). Later in
evolution, the entooecium fuses with the calcifi ed wall of the
lepralioid frontal shield of the distal zooid via several calcifi ed bars (Figs. 2.43E, F and 2.65B ). Further modifi cation
towards the microporelliform ooecium may have led to the
establishment of the connection between ooecial and hypostegal coeloms and loss of the communication pore. The calcifi ed wall of the frontal shield partly fuses with the ovicell
fl oor via knob-like outgrowths, while the entooecium thickens
as a consequence of overgrowth by the calcareous matrix of
the frontal shield (Fig. 2.65C ).
As in the vast majority of cheilostomes with calloporiform and lepralielliform ooecia, those of Fenestrulina are
formed at the periphery of the colony, possibly indicating a
connection between these structural variants. As the microporelliform ooecium evolved, calcifi cation of the entooecium began to proceed independently of that of the distal
frontal shield. Thus, in some families (Microporellidae,
Schizoporellidae) ooecia are formed several zooid rows distant from the colony periphery. In contrast, ooecia begin
their formation on peripheral zooids in the Pacifi cincolidae
and Porinidae (which have the same ooecial structure)
in association with the proximal part of the developing
frontal shield.
2.4.8 Major Trends in the Evolution
of Cheilostome Ovicells
The origination of new ooecial variants and new patterns of
ovicellogenesis were accompanied by a number of additional
changes characteristic of the evolution of brooding structures. These changes occurred independently in different
cheilostome families, though in some cases they may be
indicative of relatedness among distant groups.
2.4.8.1 Integration of Ovicell-Forming Zooids
A major trend in the evolution of brooding in Cheilostomata
was the integration of maternal (egg-producing) and distal
(ooecium-producing) zooids (sometimes reduced to kenozooidal ooecia) as or within a special morphofunctional module –
a “colonial organ” of reproduction or cormidium in the
terminology of Beklemishev ( 1969 ). A close connection
between these two zooids is ensured not only morphologically but also hormonally, resulting in a high degree of
synchronization of their development and functioning
(oogenesis, oviposition, brooding). In such a cormidium the
ooecium (formed by the daughter zooid) plays the role of
the protective capsule and the ooecial vesicle (formed by the
2.4 Evolution of Brood Chambers in Cheilostomata
perhaps inherited from umbonulomorph ancestors
(Fig. 2.64A2 ). As described above in umbonulomorphs, the
early establishment of the frontal shield and hypostegal coelom may have resulted in reduction of the basal part of the
ooecium, origination of the “double disc” stage and corresponding changes in communication structures. This trend
is easily traceable in Smittina – ooecial and visceral coeloms communicate via an arched slit in S . antarctica with a
calloporiform ooecium, while all other studied species of
the genus have a central pore in combination with either a
calloporiform or lepralielliform ooecium; correspondingly,
the simple gymnocystal part of the ovicell fl oor is developed
to a different degree in Smittina , as in umbonulomorph
Rhamphostomella (Ostrovsky, unpublished data). The combined lepralioid frontal shield and lepralielliform ooecium,
found in Smittinidae and Bitectiporidae inter alia, may have
evolved in this way (Fig. 2.64F ).
Another combination is that of the lepralioid frontal
shield and the escharelliform ooecium (Fig. 2.64G ), found in
some cheilostome families (see Sect. 2.3.2 ). If we accept that
the lepralioid frontal shield evolved from an umbonuloid
precursor and the escharelliform ooecium evolved from a
calloporiform precursor, then we may suggest that the
lepralioid/escharelliform combination could have evolved
from (1) early lepraliomorphs with a calloporiform ooecium
(Fig. 2.64E ) or (2) umbonulomorphs with an escharelliform
ooecium (Fig. 2.64D ).
The above hypothetical scenarios of ooecium evolution in
lepraliomorphs do not contradict Gordon and Voigt’s ( 1996 )
and Gordon’s ( 2000 ) ideas about the polyphyletic origin of
this morphological grade. Moreover, the fact that there are
different variants of ooecial structure among lepraliomorphs
may indicate that lepralielliform and/or escharelliform
ooecia could have been inherited from different umbonuloid
ancestors that also possessed them.
The microporelliform ooecium and the variant described
in Fenestrulina are found only in the Schizoporelloidea
(e.g. Microporellidae, Pacifi cincolidae, Schizoporellidae,
Myriaporidae, Porinidae). These variants may demonstrate
stages in the transformation of the lepralielliform ooecium.
Ooecial structure in Fenestrulina may be interpreted as transitional between lepralielliform and microporelliform
(Fig. 2.65 ) ( Fenestrulina and “microporelliform” taxa have a
single initial calcifi cation of the ovicell fl oor). Fenestrulina
and Microporella , exhibiting two variants of ooecial structure, belong to the same family Microporellidae.
As with the escharelliform ooecium (Fig. 2.64C, D and
above), the presumed transition from calloporiform to microporelliform may have occurred through reduction of ectooecial calcifi cation, fusion of entooecium with the frontal
shield, and consequent loss of communication between ooecial and visceral coeloms but establishment of communication between ooecial and hypostegal coeloms (Fig. 2.65 ).
In Fenestrulina , with its intermediate structure of ooecium,
the latter coeloms are separated, as indicated by an ooecial
communication pore and calcareous ectooecial thickening
around the base of the vertical part of the entooecium
(Figs. 2.1 , 2.45 , and 2.65B ; see also Nielsen 1981 ). Later in
evolution, the entooecium fuses with the calcifi ed wall of the
lepralioid frontal shield of the distal zooid via several calcifi ed bars (Figs. 2.43E, F and 2.65B ). Further modifi cation
towards the microporelliform ooecium may have led to the
establishment of the connection between ooecial and hypostegal coeloms and loss of the communication pore. The calcifi ed wall of the frontal shield partly fuses with the ovicell
fl oor via knob-like outgrowths, while the entooecium thickens
as a consequence of overgrowth by the calcareous matrix of
the frontal shield (Fig. 2.65C ).
As in the vast majority of cheilostomes with calloporiform and lepralielliform ooecia, those of Fenestrulina are
formed at the periphery of the colony, possibly indicating a
connection between these structural variants. As the microporelliform ooecium evolved, calcifi cation of the entooecium began to proceed independently of that of the distal
frontal shield. Thus, in some families (Microporellidae,
Schizoporellidae) ooecia are formed several zooid rows distant from the colony periphery. In contrast, ooecia begin
their formation on peripheral zooids in the Pacifi cincolidae
and Porinidae (which have the same ooecial structure)
in association with the proximal part of the developing
frontal shield.
2.4.8 Major Trends in the Evolution
of Cheilostome Ovicells
The origination of new ooecial variants and new patterns of
ovicellogenesis were accompanied by a number of additional
changes characteristic of the evolution of brooding structures. These changes occurred independently in different
cheilostome families, though in some cases they may be
indicative of relatedness among distant groups.
2.4.8.1 Integration of Ovicell-Forming Zooids
A major trend in the evolution of brooding in Cheilostomata
was the integration of maternal (egg-producing) and distal
(ooecium-producing) zooids (sometimes reduced to kenozooidal ooecia) as or within a special morphofunctional module –
a “colonial organ” of reproduction or cormidium in the
terminology of Beklemishev ( 1969 ). A close connection
between these two zooids is ensured not only morphologically but also hormonally, resulting in a high degree of
synchronization of their development and functioning
(oogenesis, oviposition, brooding). In such a cormidium the
ooecium (formed by the daughter zooid) plays the role of
the protective capsule and the ooecial vesicle (formed by the
2.4 Evolution of Brood Chambers in Cheilostomata