Family EXECHONELLIDAE Harmer, 1957

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Introduction

The family Exechonellidae was introduced by Harmer (1957) solely for Exechonella Canu & Bassler. A fairly wide range of genera has since been assigned to the family, which now includes Triporula Canu & Bassler (1927) and Anarthropora (Ryland 1982). These are ancient genera, with an extensive fossil and Recent distribution.

In 1966, Cheetham noted that the type species of Exechonella was the European Miocene Cyclicopora (?) grandis Duvergier (1921), thus correcting previous citations of the Australian Miocene species Hiantopora magna MacGillivray (1895).

The principal characteristics of the family are the large, usually encrusting zooids, which develop by intrazooidal budding, and the umbonuloid frontal shield ontogeny, which leaves a number of large, open foramina in the calcification. In Exechonella, this development has been observed in living colonies and involves growth of the partially calcified umbonuloid fold above the fully developed frontal membrane and operculum. The parietal muscles contract slightly as growth proceeds, allowing the shield enough space to overarch the membrane (Cook, 1985). The calcified wall bordering each foramen may form a funnel, or be raised to form a unilaterally curved spinous process. The peristome is sometimes elongated and flaring (Harmer 1957). Zooids communicate through groups of septular pores, and small adventitious avicularia often develop from one or more of the marginal frontal septular pores. In Exechonella antillea the tentacle crown is very large and bell-shaped when everted, and includes up to 28 tentacles. Brooding is inferred to be in interior ovisacs, and there is a little evidence that this may be associated with enlarged zooids (Cook 1985).

In Anarthropora and Triporula, the frontal foramina are numerous and zooids communicate through a well-developed circlet of small pore-chambers at the base of the vertical walls. In Triporula there is occasionally a distal avicularium, and always a pair of lateral oral avicularia. Like the proximal oral avicularium of Anarthropora, these are all derived from one or more marginal frontal septular pores. In contrast, the distal oral avicularium of Anarthropora has a subrostral chamber which replaces a pore chamber. Brooding in interior ovisacs is known in Triporula, where the brooding zooids are not otherwise dimorphic, and is inferred for Anarthropora (Hayward & Ryland 1979; Cook 1985).

The genus Exechonella has representatives from tropical and sub-tropical shallow waters throughout the world, and is often associated with coral reefs and shell substrata. In Australia, the Miocene species Exechonella magna, first described from Victoria by MacGillivray (1895) has since been found living in shelf waters off southern Australia (Wass & Yoo, 1983). Exechonella tuberculata, described from Port Phillip Heads by MacGillivray as Lagenipora in 1883, has spinous frontal processes; its distribution extends to Ceylon and the Philippines. Hayward & Ryland (1995) have recently described a new species, E. ampullacea from Heron Island. Exechonella has a fossil record extending from the Eocene of Europe and North America, and to the Miocene of Australia (Cook 1985). At least one erect, quadriserial fossil species, with very large zooids, and raised, tubular frontal foramina, also occurs from the Miocene of South Australia and Victoria. It was described as 'Tubucellaria marginata' by MacGillivray in 1895. Exechonella and Actisecos were revised by Cáceres-Chamizo et al. (2017), who split the type series, assigning part to a new species Exechonella variperforata Cáceres-Chamizo, Sanner, Tilbrook & Ostrovsky, 2017 and synonymised the name with a non-Australian species.

The type species of Triporula, T. stellata (Smitt 1873), occurs in shallow shelf, tropical and subtropical waters from the eastern Pacific and both sides of the Atlantic. The Australian Triporula biarmata is closely related to T. stellata and was first described by Waters (1882) as a variety of an unrelated European fossil species (Brown 1952). T. biarmata has a Miocene to Recent distribution in Victoria and South Australia. It occurs from Port Phillip Heads (as Microporella stellata MacGillivray 1883; Cook 1967), and across the Great Australian Bight to southeastern Western Australia at shallow shelf depths (Wass & Yoo 1983). An unnamed species of Triporula was reported from the Eocene of the Indian Ocean by Labracherie & Segal (1975).

The type species of Anarthropora is the Recent A. monodon (Busk 1860). 'This is a little-known and rarely reported species' occurring from 'coarse grounds' according to Hayward & Ryland (1999). It lives at a wide range of depths (from 1-1094 metres) from Norway, the Faroes, the Channel Island and the Aegean (Hayward 1974). However, Wass & Yoo (1983) listed a wide range of localities from South Australia to Perth in Western Australia where A. monodon was present, from deeper shelf depths (66-178 metres). The Australian population appears to be referrable to A. voigti, which was described as a Tertiary fossil from southwestern Victoria by Brown (1958). It resembles A. monodon, but has elongated peristomes and additional, acute avicularia arising from lateral marginal frontal septulae. This species has been found in further samples of Recent shelf sediments off southwestern Australia, in water depths of about 100-190 m, and as fossils from the Miocene of Bairnsdale and Curdies Creek in Victoria.

 

Diagnosis

Colony usually encrusting, well calcified, forming sheets, occasionally multilaminar. Zooids very large, with thickened, sometimes tubular peristomes. Frontal shield umbonuloid with large, rounded foramina, sometimes with spinous processes, and usually marginally thickened or tubular. Avicularia not common, if present, marginal or sutural, usually small. Ovicell absent, brooding in interior ovisacs of undifferentiated zooids.

 

General References

Brown, D.A. 1952. The Tertiary cheilostomatous Polyzoa of New Zealand. London : Trustees of the British Museum (Natural History) 405 pp.

Brown, D.A. 1958. Fossil cheilostomatous Polyzoa from south-west Victoria. Memoirs of the Geological Survey of Victoria 20: 1-90

Busk, G. 1860. Zoophytology. Shetland Polyzoa. Collected by Mr.Barlee (Continued and concluded). Quarterly Journal of Microscopical Science 8: 213-214

Cáceres-Chamizo, J.P., Sanner, J., Tilbrook, K.J. & Ostrovsky, A.N. 2017. Revision of the Recent species of Exechonella Canu & Bassler in Duvergier, 1924 and Actisecos Canu & Bassler, 1927 (Bryozoa, Cheilostomata): systematics, biogeography and evolutionary trends in skeletal morphology. Zootaxa 4305(1): 1–79

Cáceres-Chamizo, J.P., Sanner, J., Tilbrook, K.J. & Ostrovsky, A.N. 2017. Revision of the Recent species of Exechonella Canu & Bassler in Duvergier, 1924 and Actisecos Canu & Bassler, 1927 (Bryozoa, Cheilostomata): systematics, biogeography and evolutionary trends in skeletal morphology. Zootaxa 4305(1): 1–79

Canu, F. & Bassler, R.S. 1927. Classification of the cheilostomatous Bryozoa. Proceedings of the United States National Museum 69(14): 1-42

Cheetham, A.H. 1966. Cheilostomatous Polyzoa from the Upper Bracklesham Beds (Eocene) of Sussex. Bulletin of the British Museum (Natural History) 13(1): 1-115

Cook, P.L. 1967. Polyzoa (Bryozoa) from West Africa. The Pseudostega, the Cribrimorpha and some Ascophora Imperfecta. Bulletin of the British Museum (Natural History) Zoology 15: 321-351

Cook, P.L. 1985. Bryozoa from Ghana. A preliminary survey.Bryozoa from Ghana. A preliminary survey. Annales du Musée Royal de l'Afrique Centrale. Série in Octavo. Sciences Zoologiques 238: 1-315

Duvergier, J. 1921. Note sur les Bryozoaires du Néogène de l'Aquitaine. Actes de la Société Linnéene de Bordeaux 72((2e fasc.)): 145-182

Harmer, S.F. 1957. The Polyzoa of the Siboga Expedition. Part 4. Cheilostomata Ascophora II. Siboga-Expéditie Report 28D: 641-1147

Hayward, P.J. 1974. Studies on the cheilostome bryozoan fauna of the Aegean island of Chios. Journal of Natural History 8: 369-402

Hayward, P.J. & Ryland, J.S. 1979. British Ascophoran Bryozoans. pp. 1-312 in Kermack, D.M. & Barnes, R.S.K. (eds). Synopses of the British Fauna. n.s London : Academic Press for the Linnean Society Vol. 14 pp.

Hayward, P.J. & Ryland, J.S. 1995. Bryozoa from Heron Island, Great Barrier Reef. 2. Memoirs of the Queensland Museum 38(2): 533-573

Hayward, P.J. & Ryland, J.S. 1999. Cheilostomatous Bryozoa. Part 2. Hippothoidea - Celleporoidea. pp. 1-416 in Barnes, R.S.K. & Crothers, J.H. (eds). Synopses of the British Fauna. n.s Shrewsbury : Field Studies Council Vol. 14.

Labracherie, M., & Sigal, J. 1975. Les Bryozoaires cheilostomes des formations Eocene Inferieur du Site 246 (crosiere 25, Deep Dea Drlling Project). Documents des Laboratoires de Géologie de la Faculté des Sciences de Lyons HS3: 449-466

Macgillivray, P.H. 1883. Descriptions of new or little-known Polyzoa. Part 2. Transactions and Proceedings of the Royal Society of Victoria 19: 130-138

Macgillivray, P.H. 1895. A monograph of the Tertiary Polyzoa of Victoria. Transactions of the Royal Society of Victoria ns 4: 1-166

Smitt, F.A. 1873. Floridan Bryozoa collected by Count L.F. de Pourtales, Part 2. Kongliga Svenska Vetenskaps-Academiens Nya Handlingar, Stockholm 11: 1-83

Wass, R.E. & Yoo, J.J. 1973. Stratigraphic ranges of some bryozoan species in the Tertiary of South Eastern Australia. University of Sydney, unpublished reports in the geological sciences 3: 1-13

Wass, R.E. & Yoo, J.J. 1983. Cheilostome Bryozoa from the Southern Australian Continental Shelf. Australian Journal of Marine and Freshwater Research 34: 303-354

Waters, A.W. 1882. Fossil cheilostomatous Bryozoa from Mount Gambier, South Australia. Quarterly Journal of the Geological Society of London 38: 257-276

 

History of changes

Published	As part of group	Action Date	Action Type	Compiler(s)
13-Dec-2017	EXECHONELLIDAE Harmer, 1957	09-Nov-2017	MODIFIED
25-Mar-2014	BRYOZOA Ehrenberg, 1831	25-Mar-2014	MODIFIED	Dr Robin Wilson (NMV) Elizabeth Greaves (NMV)
		12-Feb-2010	(import)