166
Alysidium . In fact, Thalamoporella ovicells are unlike those
of any other cheilostome, raising the question (similar to
Alysidiidae) of the taxonomic relatedness of the family to
the rest of the Flustrina. An independent origin of
Thalamoporellidae is also supported by the fact that
Thalamoporella ovicells contain several embryos at a time.
Also in T. evelinae , zygotes are transferred to the brood
chamber with the help of an intertentacular organ, which is
predominantly characteristic of gymnolaemate broadcasters. These plesiomorphic characters indicate that thalamoporellids evolved directly from malacostegans (see also
Ostrovsky and Porter 2011 ).
The presumed relatedness of Thalamoporellidae and
Steginoporellidae (Harmer 1926 ; Gordon 2000 ) further
complicates the situation. Both families appeared in the
Middle Eocene and have a well-developed cryptocyst, but
steginoporellids brood embryos in internal brood sacs.
As shown above, this incubation type in cheilostomes is
secondary, its origin having been accompanied by the loss
of ovicells. Can it be, then, that Thalamoporellidae is
ancestral to Steginoporellidae?
Harmer ( 1926 ) compared the bivalved ovicells of
Thalamoporella with the brood chambers of Alysidium ,
based on their external appearance. Hastings ( 1941 ), having
found as many as seven embryos in the ovicell of Scruparia
chelata , compared its multiple incubation and “bivalved”
ovicells with these features in Thalamoporella . A third
argument in favour of the relatedness of Thalamoporella and
Scruparia is the external appearance of their larvae (Marcus
1939 ; discussed in Zimmer and Woollacott 1977 ).
Nevertheless, zooidal and ooecial structure in Scruparia and
Thalamoporella are very different; inter alia, the lobes of the
bipartite ooecium, have a different structure and origin in
these two taxa.
Finally, Hyman ( 1959 ) suggested that the bivalved ovicells
of Scruparia , Thalamoporella , Alysidium and Catenicula
were modifi ed spines and considered them as kenozooids.
Mawatari ( 1973a ) held the same view concerning Scruparia .
The ooecial valves in Scruparia , Alysidium and, possibly,
Catenicula are indeed kenozooids budded from the maternal
zooid, whereas in Thalamoporella they are outgrowths of the
frontal wall of the maternal zooid.
The structure and development of the brood chambers
discussed in this section indicate that they evolved independently and that their resemblance to the bipartite ooecia of some calloporids and cribrimophs is a result of
convergent evolution. Accordingly, the Thalamoporellidae
(plus Bellulopora and Tendridae) are removed from the
suborder Flustrina and separate suborders designated for
them (see Appendix II for diagnoses). An additional study
is required to confi rm if the Alysidiidae deserve a similar
status, which is highly likely. Note too, that further evidence
of the independent origin of brooding in Tendra and
Thalamoporella may be the intertentacular organ, presumably inherited from their non-brooding ancestors
(Ostrovsky and Porter 2011 ) (see also Sect. 1.3.9 ).
2.5
Conclusions
The various types of brood chambers found in living and fossil Cheilostomata vividly exemplify the evolution of these
structures in this order. The differences in their morphology
and the pattern of their distribution in the Cheilostomata
show that chambers for incubation of the embryo evolved in
this group at least seven times – in Aeteidae, “ Carbasea ”
indivisa , Scrupariidae, Thalamoporellidae, Calloporidae,
Tendridae, Bellulopora and possibly Alysidiidae. The inevitable conclusion is that the Flustrina (=Neocheilostomina),
as currently conceived, is polyphyletic. Some of these
brooding structures underwent considerable modifi cation
in the course of further evolution, probably associated with
enhancement of their protective function. All of this, as
well as the broad distribution of brood chambers within the
order, points to the paramount role of parental care in the
evolutionary success of Cheilostomata. Taylor ( 1988 ),
who in general tended to think that brooding cheilostomes
were monophyletic, nevertheless noted that Aetea ,
Scruparia and Eucratea could have evolved brooding
independently of other “neocheilostomes”. This suggestion was supported by Ostrovsky and Taylor ( 2005a ). I
agree with my respected colleague that bryozoans that
evolved brooding independently play a relatively unimportant role in the overall taxonomic diversity of
Cheilostomata. Nevertheless, the early idea that “other
types of larval brooding … are likely to be secondarily
derived from the ovicellar brooding” (Taylor and Larwood,
1990 , p. 224) cannot be correct.
In conclusion, it should be noted that an important feature of brood-chamber evolution is the abundance of parallelisms and convergence, which hampers the search for
phylogenetic connections between the taxa within this
order. As for the phylum Bryozoa as a whole, my data convincingly show that the brooding structures of cheilostomes evolved independently from those in other bryozoan
orders and classes. Therefore, the hypothesis that incubation chambers in the various orders of Bryozoa are homologous (Silén 1944 ) is erroneous (see Sect. 2.4.2 ). On the
other hand, the presence of external membranous brood
sacs in some primitive cheilostomes (e.g. Aetea ) may indicate either their relatedness to some brooding ctenostomes
(Jebram 1992 ).
2 Cheilostome Brood Chambers: Structure, Formation, Evolution
Alysidium . In fact, Thalamoporella ovicells are unlike those
of any other cheilostome, raising the question (similar to
Alysidiidae) of the taxonomic relatedness of the family to
the rest of the Flustrina. An independent origin of
Thalamoporellidae is also supported by the fact that
Thalamoporella ovicells contain several embryos at a time.
Also in T. evelinae , zygotes are transferred to the brood
chamber with the help of an intertentacular organ, which is
predominantly characteristic of gymnolaemate broadcasters. These plesiomorphic characters indicate that thalamoporellids evolved directly from malacostegans (see also
Ostrovsky and Porter 2011 ).
The presumed relatedness of Thalamoporellidae and
Steginoporellidae (Harmer 1926 ; Gordon 2000 ) further
complicates the situation. Both families appeared in the
Middle Eocene and have a well-developed cryptocyst, but
steginoporellids brood embryos in internal brood sacs.
As shown above, this incubation type in cheilostomes is
secondary, its origin having been accompanied by the loss
of ovicells. Can it be, then, that Thalamoporellidae is
ancestral to Steginoporellidae?
Harmer ( 1926 ) compared the bivalved ovicells of
Thalamoporella with the brood chambers of Alysidium ,
based on their external appearance. Hastings ( 1941 ), having
found as many as seven embryos in the ovicell of Scruparia
chelata , compared its multiple incubation and “bivalved”
ovicells with these features in Thalamoporella . A third
argument in favour of the relatedness of Thalamoporella and
Scruparia is the external appearance of their larvae (Marcus
1939 ; discussed in Zimmer and Woollacott 1977 ).
Nevertheless, zooidal and ooecial structure in Scruparia and
Thalamoporella are very different; inter alia, the lobes of the
bipartite ooecium, have a different structure and origin in
these two taxa.
Finally, Hyman ( 1959 ) suggested that the bivalved ovicells
of Scruparia , Thalamoporella , Alysidium and Catenicula
were modifi ed spines and considered them as kenozooids.
Mawatari ( 1973a ) held the same view concerning Scruparia .
The ooecial valves in Scruparia , Alysidium and, possibly,
Catenicula are indeed kenozooids budded from the maternal
zooid, whereas in Thalamoporella they are outgrowths of the
frontal wall of the maternal zooid.
The structure and development of the brood chambers
discussed in this section indicate that they evolved independently and that their resemblance to the bipartite ooecia of some calloporids and cribrimophs is a result of
convergent evolution. Accordingly, the Thalamoporellidae
(plus Bellulopora and Tendridae) are removed from the
suborder Flustrina and separate suborders designated for
them (see Appendix II for diagnoses). An additional study
is required to confi rm if the Alysidiidae deserve a similar
status, which is highly likely. Note too, that further evidence
of the independent origin of brooding in Tendra and
Thalamoporella may be the intertentacular organ, presumably inherited from their non-brooding ancestors
(Ostrovsky and Porter 2011 ) (see also Sect. 1.3.9 ).
2.5
Conclusions
The various types of brood chambers found in living and fossil Cheilostomata vividly exemplify the evolution of these
structures in this order. The differences in their morphology
and the pattern of their distribution in the Cheilostomata
show that chambers for incubation of the embryo evolved in
this group at least seven times – in Aeteidae, “ Carbasea ”
indivisa , Scrupariidae, Thalamoporellidae, Calloporidae,
Tendridae, Bellulopora and possibly Alysidiidae. The inevitable conclusion is that the Flustrina (=Neocheilostomina),
as currently conceived, is polyphyletic. Some of these
brooding structures underwent considerable modifi cation
in the course of further evolution, probably associated with
enhancement of their protective function. All of this, as
well as the broad distribution of brood chambers within the
order, points to the paramount role of parental care in the
evolutionary success of Cheilostomata. Taylor ( 1988 ),
who in general tended to think that brooding cheilostomes
were monophyletic, nevertheless noted that Aetea ,
Scruparia and Eucratea could have evolved brooding
independently of other “neocheilostomes”. This suggestion was supported by Ostrovsky and Taylor ( 2005a ). I
agree with my respected colleague that bryozoans that
evolved brooding independently play a relatively unimportant role in the overall taxonomic diversity of
Cheilostomata. Nevertheless, the early idea that “other
types of larval brooding … are likely to be secondarily
derived from the ovicellar brooding” (Taylor and Larwood,
1990 , p. 224) cannot be correct.
In conclusion, it should be noted that an important feature of brood-chamber evolution is the abundance of parallelisms and convergence, which hampers the search for
phylogenetic connections between the taxa within this
order. As for the phylum Bryozoa as a whole, my data convincingly show that the brooding structures of cheilostomes evolved independently from those in other bryozoan
orders and classes. Therefore, the hypothesis that incubation chambers in the various orders of Bryozoa are homologous (Silén 1944 ) is erroneous (see Sect. 2.4.2 ). On the
other hand, the presence of external membranous brood
sacs in some primitive cheilostomes (e.g. Aetea ) may indicate either their relatedness to some brooding ctenostomes
(Jebram 1992 ).
2 Cheilostome Brood Chambers: Structure, Formation, Evolution