Order: Fenestrida (Elias & Condra, 1957)
Family: Fenestellidae, Phylloporinidae
Early Ordovician – Late Permian (?Triassic)
Fenestrida is an extinct order of stationary epifaunal suspension feeders occurring mainly in bioclastic packstones and fossiliferous quartz wacke.
Characteristics of the Order
- Colonies of this order are generally erect, delicate, and take on reticulate or pinnate forms.
- Can also take on cone shaped, fan-like, or complex folded sheet forms.
- Bilaterally symmetrical, polarized shape.
- Branches usually bear two rows of apparently monomorphic autozooids distal to bifurcations and three rows of zooids for a variable distance proximal to bifurcations.
- Zooidal apertures located on convex surface of fenestrate mesh.
- Basal shape of zooids elongate pentagonal or quadrangular.
- Individual zooids typically measure less than one millimeter in size.
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- Colonies erect, composed of narrow unilaminate branches; short autozooids commonly with hemisepta, but basal diaphragms in very few; primary zooidal without communication pores, typically granular; heterozooids (gonozooids, nanozooids, and various others) in some; extrazooidal skeleton extensive, several times autozooidal skeleton in volume, consisting of laminae penetrated by small granular rods. Lower Ordovician to Upper Permian or Triassic. (Although Admiratella is reported from the Cretaceous)
Frank K. McKinney (1994)
- Fenestrate Bryozoa usually grew into erect fans, cones, or complexly folded sheets. Some, however, grew into unusual and highly characteristic colony forms that contrasted with the usual fan shaped to complexly folded sheets. The most conspicuous of the unusual colony forms was developed by the genus Archimedes, with erect colonies of regularly spiraled meshwork radiating from a central,
heavily calcified axis. Archimedes was especially abundant on quiet-water marine bottoms of eastern North America during the later parts of the Mississippian (Chesterian; Visean-early Namurian). In addition, some Carboniferous and Permian deposits yield unusual, free-lying fenestrate bryozoans that have heavily calcified, U- or V-shaped, massive proximal and lateral margins.
Taylor & Curry (1985):
- The Fenestrata are regarded as a distinct order of stenolaemate bryozoans in the revised Treatise on invertebrate paleontology (Boardman et al. 1983), although some bryozoologists argue for their retention as a suborder of the Cryptostomata (Blake in Boardman et al. 1983). Most fenestrates are readily recognizable by their reticulate or pinnate colonies with zooecial apertures opening on one side of the branches only. Fenestrate bryozoans peaked in diversity during the late Paleozoic when members of the families Fenestellidae and Acanthocladiidae dominated the majority of bryozoans faunas. However, the origins of the order may be traced back to the Ordovician. Hitherto the earliest described fenestrate bryozoans dates from the middle Ordovician.
Treatise on Invertebrate Paleontology, Part G (Revised Bryozoa) (1983):
- In Paleozoic stenolaemates skeletal indications of inferred brood chambers have been reported in a few taxa of two orders, the Cystoporata (see Utgaard, this revision) and the Fenestrata (e.g. Tavener-Smith, 1966; Startton, 1975). In both orders the inflated chambers are skeletal blisters attached to outer ends of zooecia, similar in position to generally larger brood chambers of most post-Paleozoic tubuliporates.
- In at least some taxa of the order Fenestrata, the encrusting wall of the basal disc is reportedly not calcified from inside the disc (Tavener-Smith, 1969a; Gautier, 1972). As reconstructed, a circular flap of ectodermal epithelium (Tavener-Smith, 1969a, p. 295) projected form the aperture of the basal disc and folded over so that the flap rested on the exterior cuticle of the outer surface of the disc. A calcified layer was then deposited on the outer surface of the disc by the ectodermal epithelium of the flap.
- It should be made clear that in this reconstruction, the hypothesized flap has to be a complete exterior membranous wall enclosing body cavity. It is assumed, therefore, that the folding places the exterior cuticles of the basal disc and flap back to back (questioned by Gautier, 1972). The skeletal wall of the disc is here interpreted to be an exterior wall, equivalent to the basal colony wall folded over on top of the basal disc in a lichenoporid (left basal side, Fig. 25). The distal neck of the ancestrula and skeletons of subsequent zooids and extrazooidal structures of fenestrates are interior in origin, surrounded by epidermis and body cavity on all sides.
- The Cryptostomata was subdivided first through recognition of the order Fenestrata by Ellias and Condra (1957) and then by the recognition of the order Rhabdomesonata by Shishova (1968). These changes left only the Ptilodictyoidea and the Timanodictyoidea in the Cryptostomata. The Fenestrata of Elias and Condra was based largely upon the presence of a “colonial plexus,” expressed as a clear granular calcite layer. Also important in the concept of the Fenestrata were inferred homologies between the colonial plexus and the cyclostomate common bud. Shishova (1968), in considering the fenestellids, further emphasized zooecial shape, budding pattern, microstructure, the presence of peristomes, and in some cases, lunaria and ovicells. The Rhabdomesonata of Shishova was based primarily upon zoarial form, zooecial shape, and budding pattern. Shishova did not consider the three traditional cryptostomate suborders to be closely related.
- The new ordinal concepts of Fenestrata and Rhabdomesonata have not been universally accepted. For example, following the classification of Astrova and Morozova (1966), Tavener-Smith and Williams (1972) considered the three major groups to be suborders. Cuffey (1973), in a numerical taxonomic study, retained the unified Cryptostomata. Within the order, he concluded that the rhabdomesines and ptilodictyines were closer to each other than either were to fenestellines. He recognized two suborders with the rhabdomesines and ptilodictyines as infraorders in one and the fenestellines alone in the other.
- Blake (1975, 1980) argued for retention of the three traditional branches as suborders within an order Cryptostomata because of morphological similarities among the Ordovician members of the Arthrostylidae (Rhabdomesoidea), Phylloporinidae (Fenestelloidea) and some genera of the Rhinidictyidae and Stictoporellidae (Ptilodictyoidea).
Elias & Condra (1957):
- 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 homologus 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 in Cyclostomata. In both there is little secondary calcareous deposition over the primary wall inside the zooecial chambers, but by far the greatest secondary discovery deposit is added from the outside. This deposition forms thick laminated crust.
- In view of these observations, it seems inevitable to conclude that the sclerenchyma in Fenestrata is of the same nature as that in the group of living Cyclostomata represented by the genus Hornera, as that in Paleozoic Trepostomata, and perhaps as that in some Cheilostomata (see Fig. 11).
- If this conclusion is accepted, then it follows that the sclerenchyma in Fenestrata (and in Trepostomata) apparently was secreted and deposited in the same manner as that in the living Hornera and related Cyclostomata; this means that it was secreted by the ectoderm that stretched externally over the whole zoarium, not by a special “capillary system.” In fact, no equivalent of the latter is known in Hornera or any living bryozoans that have the same or similar secondary calcification of the wall.
- Furthermore, the laminated tissue in the walls of some Trepostomata, shown in detailed sketches and mentioned in descriptions by Cumings and Galloway (1915), seems to differ from that in Fenestrata by a much denser spacing of the smaller laminae: circa 1 micron thick in Bythopora gracilis in which the granules of the laminae are “larger than in most other forms.” Our comparative observations on some Trepostomata from the Cincinnatian of Ohio confirm those by Cumings and Galloway. In the “dark zones” the fine laminae are commonly spaced with no intervals of ground substance. In the lighter-colored zones the laminae are separated by narrow intervals, which usually average laminal thickness. Some laminae are made of small dashlike bars, or even shorter “granules,” just as Cumings and Galloway show on their sketches; other, perhaps most, laminae remain continuous for long stretches, as seen in both longitudinal and transverse sections of the zoaria. Hence it is the closer spacing of thinner laminae and weaker development of the enclosing structureless substance that distinguishes the wall of Trepostomata from that in Fenestrata.