155
mental differences indicate that ooecia (and ovicells in general) could have evolved at least fi ve times: in Scrupariidae
(from a pair of distal kenozooids), Thalamoporellidae (from
a pair of frontal outgrowths of the fertile autozooid),
Alysidiidae (from two to several distal kenozooids),
Bellulopora (from kenozooidal costae) and Calloporidae
(from articulated mural spines). Ooecia constructed of spines
(the latter variant) were obviously inherited by monoporellids and cribrimorphs. Reductions in the number and fl attening of spines, the acquisition of the distally convex
arrangement of spine bases, loss of articulation, fusion of
costae and immersion of the ovicell fl oor apparently occurred
independently within Calloporoidea, Monoporelloidea
(Mono porellidae and Macroporidae) and Cribrilinoidea, all
of these trends being expressed in them to varied degrees
(Ostrovsky and Taylor 2005a ).
Given that spinose and costate ooecia are the ancestral
structural variant, further evolution resulted in fi rst, bilobate
and then unitary (complete) calloporiform ooecia (see
Sect. 2.3.2 ). An example of such a transition to unitary ooecia is provided by fossil and Recent calloporids. Wilbertopora
(Albian–Cenomanian) and Gilbertopora (Cenomanian) are
characterized by bilobate ooecia and a pair of communication openings, while Callopora , which evolved in the
Cenomanian and survived until the present, has a complete
ooecium and a common communication slit (later reduced
to a pore). In Recent calloporid genera such as Alderina ,
Callopora , Concertina , Crassimarginatella , Corbulella ,
Copidozoum , Retevirgula , Leptinatella and Bryocalyx (see
Canu and Bassler 1933 ; Prenant and Bobin 1966 ; Harmelin
1973a ; Gordon 1986 ; Tilbrook 1998 ; Cook and Bock 2000 ),
ovicells have a medial suture or a keel, demonstrating different degrees of fusion of ooecial lobes (summarized in
Ostrovsky 2002 ). For instance, the ooecial base is complete
(with no traces of the paired origin) in Concertina and
Bryocalyx , whereas the proximal edge is bilobate. In
Corbulella maderensis a short medial keel is retained on the
inner ooecial surface. In Callopora lineata and Tegella unicornis there is instead a medial groove in the proximal ooecial rim. In Recent Valdemunitella ooecia are bilobate, with
narrow bases and a pair of communication slits as in confamilial Wilbertopora from the Middle Cretaceous. The ooecial rudiment (initial calcifi cation of the ovicell fl oor) is
single in species with bilobate ooecia and paired in those
with complete ooecia (see Sect. 2.3.2 ).
A similar transition from bilobate to complete ooecia presumably occurred among cribrimorph cheilostomes. Species
of Figularia , Euthyroides and Corbulipora have bilobate
ooecia with lateral communication slits and a single ooecial
rudiment, which is very similar to that in the calloporid
Wilbertopora (Ostrovsky and Taylor 2005b ). Ovicells in
most Recent cribrimorphs (e.g. Membraniporella , Cribrilina ,
Puellina , Collarina , Reginella ) and some early fossil cribrimorphs (e.g. Pliophloea , Anaptopora , Monoceratopora ,
Lagynopora , Castanopora ) have a more or less expressed
medial suture and/or keel, indicative of fusion of ooecial
halves (summarized in Ostrovsky 2002 ). Sometimes the
medial suture is mostly visible at the inner ooecial surface
( Cribrilina annulata ) (Ostrovsky 1998 ). Thus, traces of
paired ooecial structure have been retained throughout bryozoan evolutionary history. At the same time, some cribrilinids have a complete ooecium, a common communication
slit and a paired rudiment of the ovicell fl oor ( Cribrilina
cryptoecium , C . punctata ) (see also Sect. 2.4.4 ).
Thus, the most advanced ooecial morphology (unitary)
appears to have been acquired independently in Calloporidae
and Cribrilinidae. As the latter family is considered ancestral
to the former (Silén 1942 ; Gordon 2000 ), this trend may be
regarded as exemplifying parallelism.
The calloporiform ooecium co-occurs with all known
types of frontal wall – simple anascan (malacostegan), cryptocystal (coilostegan), spinocystal (cribrimorph), gymnocystal
ascophoran (hippothoomorph), and umbonuloid and lepralioid ascophoran – in which a relatively wide area of proximal
gymnocyst does not prevent the formation of an arch-like
ooecial outfold. A narrow ooecial base of lepralielliform
ooecia forming on a “wide” proximal gymnocyst is known
only in bugulids and the causes of this modifi cation are
uncertain. It is clear only that these ooecia evolved in
Bugulidae independently from advanced ascophorans with a
similar narrow ooecial base.
The transition from a calloporiform to an escharelliform
ooecium may have fi rst occurred in a coilostegan. Taxonomically, its lineage would presumably have been within the
calloporidae (see, for instance, Voigt 1991 ), in which there
was a gradual expansion of the cryptocyst beneath the membranous frontal wall (reviewed by Silén 1942 ). In contrast,
the calcifi cation of the ectooecium shows varying degrees of
reduction. The evolution of the escharelliform ooecium in
microporids was accompanied by fusion of the entooecium
with the cryptocyst and the establishment of direct communication of ooecial and hypostegal coeloms. The loss of
ectooecial calcifi cation and fusion of the ooecial fl oor with
the zooidal cryptocyst (Fig. 2.63B, C ) resulted in closure of
the ooecial communication slit once the ooecial fold was
formed (Fig. 2.63D ). In this situation, ooecial epithelia could
remain viable only if ooecial and hypostegal coeloms were
united. All stages of the calloporiform–escharelliform transition are found in the Microporidae (Fig. 2.63A, D ; see also
Figs. 2.33 and 2.34 ), with the less-derived calloporiform
condition occurring in Micropora . For instance, the ectooecium in the majority of species in the ancient families
Microporidae and Onychocellidae (Microporoidea) is mostly
uncalcifi ed (as a rule, only the proximal rim is calcifi ed)
(Figs. 2.33 , 2.34 , and 2.63 ). In many species ooecia also have
direct communication with the hypostegal coelom of the distal zooid, and the ovicell fl oor is fused with its cryptocyst
(Figs. 2.34 and 2.63D ). Genera such as Onychocella and
2.4 Evolution of Brood Chambers in Cheilostomata
mental differences indicate that ooecia (and ovicells in general) could have evolved at least fi ve times: in Scrupariidae
(from a pair of distal kenozooids), Thalamoporellidae (from
a pair of frontal outgrowths of the fertile autozooid),
Alysidiidae (from two to several distal kenozooids),
Bellulopora (from kenozooidal costae) and Calloporidae
(from articulated mural spines). Ooecia constructed of spines
(the latter variant) were obviously inherited by monoporellids and cribrimorphs. Reductions in the number and fl attening of spines, the acquisition of the distally convex
arrangement of spine bases, loss of articulation, fusion of
costae and immersion of the ovicell fl oor apparently occurred
independently within Calloporoidea, Monoporelloidea
(Mono porellidae and Macroporidae) and Cribrilinoidea, all
of these trends being expressed in them to varied degrees
(Ostrovsky and Taylor 2005a ).
Given that spinose and costate ooecia are the ancestral
structural variant, further evolution resulted in fi rst, bilobate
and then unitary (complete) calloporiform ooecia (see
Sect. 2.3.2 ). An example of such a transition to unitary ooecia is provided by fossil and Recent calloporids. Wilbertopora
(Albian–Cenomanian) and Gilbertopora (Cenomanian) are
characterized by bilobate ooecia and a pair of communication openings, while Callopora , which evolved in the
Cenomanian and survived until the present, has a complete
ooecium and a common communication slit (later reduced
to a pore). In Recent calloporid genera such as Alderina ,
Callopora , Concertina , Crassimarginatella , Corbulella ,
Copidozoum , Retevirgula , Leptinatella and Bryocalyx (see
Canu and Bassler 1933 ; Prenant and Bobin 1966 ; Harmelin
1973a ; Gordon 1986 ; Tilbrook 1998 ; Cook and Bock 2000 ),
ovicells have a medial suture or a keel, demonstrating different degrees of fusion of ooecial lobes (summarized in
Ostrovsky 2002 ). For instance, the ooecial base is complete
(with no traces of the paired origin) in Concertina and
Bryocalyx , whereas the proximal edge is bilobate. In
Corbulella maderensis a short medial keel is retained on the
inner ooecial surface. In Callopora lineata and Tegella unicornis there is instead a medial groove in the proximal ooecial rim. In Recent Valdemunitella ooecia are bilobate, with
narrow bases and a pair of communication slits as in confamilial Wilbertopora from the Middle Cretaceous. The ooecial rudiment (initial calcifi cation of the ovicell fl oor) is
single in species with bilobate ooecia and paired in those
with complete ooecia (see Sect. 2.3.2 ).
A similar transition from bilobate to complete ooecia presumably occurred among cribrimorph cheilostomes. Species
of Figularia , Euthyroides and Corbulipora have bilobate
ooecia with lateral communication slits and a single ooecial
rudiment, which is very similar to that in the calloporid
Wilbertopora (Ostrovsky and Taylor 2005b ). Ovicells in
most Recent cribrimorphs (e.g. Membraniporella , Cribrilina ,
Puellina , Collarina , Reginella ) and some early fossil cribrimorphs (e.g. Pliophloea , Anaptopora , Monoceratopora ,
Lagynopora , Castanopora ) have a more or less expressed
medial suture and/or keel, indicative of fusion of ooecial
halves (summarized in Ostrovsky 2002 ). Sometimes the
medial suture is mostly visible at the inner ooecial surface
( Cribrilina annulata ) (Ostrovsky 1998 ). Thus, traces of
paired ooecial structure have been retained throughout bryozoan evolutionary history. At the same time, some cribrilinids have a complete ooecium, a common communication
slit and a paired rudiment of the ovicell fl oor ( Cribrilina
cryptoecium , C . punctata ) (see also Sect. 2.4.4 ).
Thus, the most advanced ooecial morphology (unitary)
appears to have been acquired independently in Calloporidae
and Cribrilinidae. As the latter family is considered ancestral
to the former (Silén 1942 ; Gordon 2000 ), this trend may be
regarded as exemplifying parallelism.
The calloporiform ooecium co-occurs with all known
types of frontal wall – simple anascan (malacostegan), cryptocystal (coilostegan), spinocystal (cribrimorph), gymnocystal
ascophoran (hippothoomorph), and umbonuloid and lepralioid ascophoran – in which a relatively wide area of proximal
gymnocyst does not prevent the formation of an arch-like
ooecial outfold. A narrow ooecial base of lepralielliform
ooecia forming on a “wide” proximal gymnocyst is known
only in bugulids and the causes of this modifi cation are
uncertain. It is clear only that these ooecia evolved in
Bugulidae independently from advanced ascophorans with a
similar narrow ooecial base.
The transition from a calloporiform to an escharelliform
ooecium may have fi rst occurred in a coilostegan. Taxonomically, its lineage would presumably have been within the
calloporidae (see, for instance, Voigt 1991 ), in which there
was a gradual expansion of the cryptocyst beneath the membranous frontal wall (reviewed by Silén 1942 ). In contrast,
the calcifi cation of the ectooecium shows varying degrees of
reduction. The evolution of the escharelliform ooecium in
microporids was accompanied by fusion of the entooecium
with the cryptocyst and the establishment of direct communication of ooecial and hypostegal coeloms. The loss of
ectooecial calcifi cation and fusion of the ooecial fl oor with
the zooidal cryptocyst (Fig. 2.63B, C ) resulted in closure of
the ooecial communication slit once the ooecial fold was
formed (Fig. 2.63D ). In this situation, ooecial epithelia could
remain viable only if ooecial and hypostegal coeloms were
united. All stages of the calloporiform–escharelliform transition are found in the Microporidae (Fig. 2.63A, D ; see also
Figs. 2.33 and 2.34 ), with the less-derived calloporiform
condition occurring in Micropora . For instance, the ectooecium in the majority of species in the ancient families
Microporidae and Onychocellidae (Microporoidea) is mostly
uncalcifi ed (as a rule, only the proximal rim is calcifi ed)
(Figs. 2.33 , 2.34 , and 2.63 ). In many species ooecia also have
direct communication with the hypostegal coelom of the distal zooid, and the ovicell fl oor is fused with its cryptocyst
(Figs. 2.34 and 2.63D ). Genera such as Onychocella and
2.4 Evolution of Brood Chambers in Cheilostomata