Ordovician – Recent
Stenolaemata is a class of extant, stationary, epifaunal suspension feeders.
Characteristics of the Class
- Feeding zooids with interior walls.
- Interior walls vertical.
- Frequently, the internal walls are elongated and enclose tubular, conical or sac–shaped body cavities.
- The vertical elongation occurs parallel to the direction of zooid growth.
- Generally, the zooids open up at large angles in relation to the surface of the colony.
- Zooecial aperture is approximately the same size as living area.
- Tentacles protrude through a pore-like opening.
- A membrous sac surrounds lophophore and gut.
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Treatise on Invertebrate Paleontology (1983):
- The Stenolaemata are considered to make up one of the three classes of the phylum Bryozoa. Members of the class are characterized by feeding zooids with complete interior vertical walls that are commonly elongated to enclose tubular, conical, or sac-shaped body cavities. Vertical walls are elongated parallel to the direction of zooidal growth Vertical walls of all zooids have skeletal layers, as do basal and frontal walls where they occur. In most taxa, zooids open at high angles to colony surfaces, and zooecial apertures are comparable in area to cross sections of living chambers. Zooecial apertures and the terminal membranous orficial walls that cover them in living colonies are transverse to zooidal length. Tentacles are protruded through circular porelike orifices by the action of a membranous sac that surrounds the lophophore and gut in recent stenolaemates.
- Extended Information:
- The Class Stenolaemata produced virtually all of the vast accumulation of fossil bryozoans from the Early Ordovician into the Early Cretaceous, a time interval lasting nearly 400 million years. During that interval the class Gymnolaemata is represented by a few scattered species of ctenostomates beginning in the Ordovician and of ctenostomates beginning in the Ordovician and of cheilostomates beginning in the Jurassic. Stenolaemates are the most abundant fossil group in many rock units throughout the stratigraphic column and the continuity of their stratigraphic occurrences is comparable to that of other major groups of fossils. During the Late Cretaceous, the stenolaemates began to lose their predominance within the phylum to the class Gymnolaemata. Stenolaemate numbers and diversity have apparently been on a slow decline since the Cretaceous. Stenolaemates can be found living in large numbers, however in many marine communities.
- The class includes four to six orders, depending on the classification used. Five orders are recognized here. The Trepostomata, Cystoporata, Cryptostomata and Fenestrata all appeared during the Ordovician, all were prolific at times during the Paleozoic Era, and all are generally considered to have become extinct during or just after the Permian. The Tubuliporata (formerly the Cyclostomata) also appeared in the Ordovician, but remained unimportant in numbers and diversity until the Mesozoic and Cenozoic eras, when they occurred in large numbers
- Unfortunately, the Paleozoic and post-Paleozoic taxa have been studied using different preparation techniques and taxonomic characters. The present literature tests neither the assumed Permian and Triassic extinction of Paleozoic stocks, nor the generally accepted monophyletic origin of the post-Paleozoic Tubuliporata. One of the questions of highest priority to improved understanding of the class Stenolaemata is the piecing together of its evolutionary history across the Paleozoic-Mesozoic boundary, using modern taxonomic procedures and as many taxonomic characters as are available.
- Stenolaemate bryozoans apparently have been entirely marine throughout their history. In Paleozoic rocks, their numbers are largest in calcareous shales, mudstones, and some limestones. Colonies that grew erect are commonly preserved broken but unscattered in shales and mudstones, indicating little or no transportation after death.
- Growth habits of colonies of many species of bryozoans have long been assumed to be modified significantly by different environments. A thorough review of the literature of stenolaemate ecology and paleoecology was published by Duncan (1957). Experimental studies are just beginning to emphasize the effects of different environments on colony growth habits and correlated changes of internal morphology with in the same species of living stenolaemates.
- Details of skeletal structures are commonly well preserved in fossil stenolaemates of all ages and provide many taxonomic characters that can be inferred to be genetically controlled. Skeletal structures furnish evidence of modes of growth, functional morphology, and intra- and intercolony morphologic variation, especially where their relationships with soft parts can be inferred with confidence.
- A surprising number of indications or actual fragmentary remains of soft parts occur throughout the fossil record of the stenolaemates and some very general comparisons can be made with the complete soft parts of modern species. Unfortunately, the soft parts of most modern species and their growth and functional relationships with skeletal counterparts are poorly known. For example, recent sectioning of a few randomly selected taxa has revealed four different morphologies affecting the protrusion of tentacles. Only one of these had previously been reported. Most of the character states derived from soft parts that are assumed to be characteristic of the order Tubiliporata are known from relatively few species and therefore should be investigated further.
- Independent, apparently genetically controlled taxonomic characters within colonies that are carefully collected from vertical sequences commonly show transitional changes. Not enough of these detailed studies have been published, however, to demonstrate many evolutionary patterns and detailed morphologic trends. Unfortunately, the study of stenolaemate bryozoans has not been advanced enough for a general realization of their potential value in applied problems of ecology, zoogeography and biostratigraphy.
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.