Ordovician – Recent
Cyclostomata is an extinct order of stationary, epifaunal suspension feeders
Characteristics of the Order
- Zooecia consisting of simple calcareous tubes
- Zooecia generally without transverse partitions (diaphragms)
- Plain, rounded, uncontracted aperture, not closed by an operculum
- Walls thin, minutely porous (with pseudopores)
- Zoarium assumes many different forms of growth, from delicate jointed threadlike branches to solid masses 10 or more centimeters in diameter
- Zoarium variable within a genus but fairly constant in a species
Elias & Condra (1957):
- Borg found that in all Cyclostomatous Bryozoa the zoarium originates in a primary disk, “a semicircular hollow formation with calcareous walls.” The common bud originates before this disk is “completely calcified,” and from the common bud the whole zoarium gradually develops. The bud is an approximately cylindrical structure, and “its lateral wall consists of a cuticle, a calcified layer, ectoderm, and mesoderm, thus showing the same structure as the body wall of a zoid. The calcified layer of the wall of the bud is the thickest nearest the primary disk and becomes distally thinner and thinner until it finally ceases at the edge of the terminal membrane of the bud, which is made up of a thin cuticle only with underlying ectoderm and mesoderm,” the structure observed in Crisiidae (Borg, 1926, p. 255); but he found it also typical for all Cyclostomata. As the bud grows in length the calcified layer also “grows through apposition,” new particles being added at the edge of the terminal membrane and on the inner side of the wall of the bud, which results in an increase in thickness of the calcified layer. An important structural character of the calcified layer, revealed by examination in polarized light is “that the minute particles composing it are located in a certain direction; namely, parallel to the longitudinal axis of the bud. The same is true also of the calcified walls of the zoids” (Borg, 1926, p. 254). That Borg does not record that in an adult stage of calcification the particles fuse and make a uniformly oriented continuous crystalline substance seems to indicate that the substance of the common bud between the calcified particles remains largely soft during the life of zoarium, but obviously the oriented calcareous particles may serve as centers for further calcification, during the life and in post-mortem petrifaction, so that eventually they fuse into a uniformly oriented crystalline unit.
- In his matured concept of the zoarium of Crisina, Cyclostomata, and of the part played by the common bud, Borg state, (1942, p. 38-40): “The zoids all originate within the common bud and are, consequently, all covered by its terminal membrane. … These zoids are thus never constricted off from the common bud but remain permanently covered by its terminal membrane just as do the kenozoids. The whole zoarium in Crisina is thus to be considered as much widened and strongly complicated common bud. This easily explains the secretion of calcareous matter from the outside, e.g., the secondary thickening.” The discussed observations indicate that the structure, location, and zoarial extent of the colonial plexus in fenestrate Cryptostomata correspond so well with Borg’s description of the common bud in Cyclostomata that the two structures are considered homologous. If so, this is strong evidence of close relationship between the living Cyclostomata and the late-Paleozoic fenestrate Cryptostomata. The following observations on other colonial structures known in some Bryozoa indicate lesser development of similar structure.
- All Cryptostomata were originally classified in comprehensive suborder Cyclostomata (Ulrich, 1882, p. 149-151). In 1883 Vine removed from Cyclostomata the bifoliate and a few closely allied ramose forms were the last removed from Cyclostomata. The fact that they possess zoarial plexus, apparently homologous to the common bud of Cyclostomata, and the fact that comparable structures are absent in other Cryptostomata suggest a return to Vine’s concept of Cryptostomata and segregation of fenestrate forms into the new order Fenestrata to be added to Borg’s class Stenolaemata at a par with orders Trepostomata and Cyclostomata.
- The colonial plexus in Fenestrata is apparently homologous to the common bud or the colonial bud in the living Cyclostomata. It may be further pointed out that the laminated sclerenchyma of Fenestrata has the same texture and occupies the same position relative to the plexus as the secondary calcification relative to the primary calcareous common bud in Cyclostomata. In both, there is little secondary calcareous deposition over the primary wall inside the zooecial chambers, but by far the greatest secondary deposits is added from the outside. This deposition forms the thick laminated crust.
- It seems permissible to conclude, therefore, that the zooecia in Fenestrata have a common primary wall, another point of similarity between these fossil Bryozoa and the living Cyclostomata. Among living Bryozoa only in Cyclostomata and Phylactolaemata is the primary wall shared by neighboring zooecia (Borg, 1926, p. 192). In Cheliostomata and Ctenostomata each zooecium has its own primary wall.
Treatise on Invertebrate Paleontology, Part G (1953):
- Zooecia consisting of simple calcareous tubes, generally without transverse partitions (diaphragms), with plain, rounded, uncontracted aperture, not closed by an operculum; walls thin, minutely porous (with pseudopores), lacking the more complicated structures developed in Cheilostomata and Trepostomata. Reproduction in an ovicell, consisting of an enlarged single zooecium (gonoecium) with special opening (oeciopore) terminated externally by a peristome-like rim (oeciostome), or an inflation of the zoarial surface (gonocyst) covering several apertures. Appendicular organs wanting. The zoarium assumes many different forms of growth, from delicate jointed threadlike branches to solid masses 10 or more centimeters in diameter, commonly variable in a genus but fairly constant in a species.
- The bryozoans belonging to this order have a simplicity of structure highly characteristic in both zoaria and zooecia. The zooecia are especially simple throughout the order so that the classification is based almost entirely upon their arrangement and method of growth. The recent forms show that this simplicity extends to the polypide and that the larvae of the different families are practically identical. The zooecia are simple, calcareous tubules with plain, uncontracted, usually rounded apertures and with minutely porous walls. The interzooecial spaces may or may not be filled with solid strengthening deposits. Marsupia and appendicular organs are wanting, but the ovicell is present in the form of a large cell or an inflation of the zoarial surface set aside for reproductive purposes.
- The simplest Cyclostomata are comprised of Busk’s families DIastoporidae and Idmoneidae and in the Entalophoridae of Reuss, all of which have numerous fossil species. Each of these families is represented, although somewhat sparsely, in the Russian strata under discussion.
- The much-debated Ceramoporidae and Fistuliporidae are more numerously represented both in genera and species. These families, considered by many authors as tabulate corals but by Ulrich as trepostomatous bryozoans, were doubtfully referred by the latter in 1900 to the Cyclostomata. Further studies have shown the wisdom of this course, and I believe enough evidence can now be assembled to establish the Ceramoporidae and Fistuliporidae as undoubted Cyclostomata. Both have the minutely porous wall structure of typical members of the order, and, in addition, most of their genera show zooecia exactly as in recent forms.