Old Earth Ministries Online Earth History Curriculum

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Chapter 4 - The Ordovician Period

Lesson 24: Ordovician Species In-Depth - Bryozoa

 

     The Bryozoa, also known as Ectoprocta, are a phylum of aquatic invertebrate animals. Typically about 0.5 millimetres (0.020 in) long, they sieve food particles out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia. Most marine species live in tropical waters, but a few occur in oceanic trenches, and others are found in polar waters. One class lives only in a variety of freshwater environments, and a few members of a mostly marine class prefer brackish water. Over 4,000 living species are known. One genus is solitary and the rest colonial.

The phylum was originally called "Polyzoa", but this term was superseded by "Bryozoa" in 1831. Another group of animals discovered subsequently, whose filtering mechanism looked similar, was also included in "Bryozoa" until 1869, when the two groups were noted to be very different internally. The more recently discovered group were given the name Entoprocta, while the original "Bryozoa" were called "Ectoprocta". However, "Bryozoa" has remained the more widely used term for the latter group.

     Scientific classification is as follows:

Kingdom: Animalia

Superphylum:  Lophotrochozoa

Phylum: Bryozoa

Chapter 4 - The Ordovician

 

Lesson 21: Ordovician Overview

Lesson 22: Orogeny

Lesson 23: The Appalachians

Lesson 24: Bryozoa

Lesson 25: Ordovician-Silurian Extinction Event

Test

 

Haeckel Bryozoa

"Bryozoa", from Ernst Haeckel's Kunstformen der Natur, 1904

     Individuals in bryozoan (ectoproct) colonies are called zooids, since they are not fully-independent animals. All colonies contain autozooids, which are responsible for feeding and excretion. Colonies of some classes have various types of non-feeding specialist zooids, some of which are hatcheries for fertilized eggs, and some classes also have special zooids for defense of the colony.

ordovician fossil bryozoa, edrioasteroid Cystaster stellatus
Above: An Upper Ordovician cobble with the edrioasteroid Cystaster stellatus and the thin branching cyclostome bryozoan Corynotrypa. Kope Formation, northern Kentucky.
Bryozoa Fossils
Above: Bryozoan fossils in an Upper Ordovician oil shale (kukersite), northern Estonia.
Bryozoa 
Above:  Bryozoan in Ordovician Limestone, Batavia, Ohio (Picture Source

 

Fossil Record

 

Fossils of about 15,000 bryozoan species have been found. Marine taxa with mineralized skeletons appear in rocks dating from the Arenigian stage of the Early Ordovician period, about 480 million years ago. At this point all the modern orders of stenolaemates were present, and the ctenostome order of gymnolaemates had appeared by the Middle Ordovician, about 465 million years ago. Other types of filter feeders appeared around the same time, which suggests that some change made the environment more favorable for this lifestyle. Fossils of cheilostomates, another order of gymnolaemates, first appear in the Mid Jurassic, about 172 million years ago, and these have been the most abundant and diverse bryozoans from the Cretaceous to the present. Evidence compiled from the last 100 million years show that cheilostomates consistently grew over cyclostomates in territorial struggles, which may help to explain how cheilostomates replaced cyclostomates as the

dominant marine bryozoans. Marine fossils from the Paleozoic era, which ended 251 million years ago, are mainly of erect forms, those from the Mesozoic are fairly equally divided by erect and encrusting forms, and more recent ones are predominantly encrusting. Fossils of the soft, freshwater phylactolaemates are very rare, appear in and after the Late Permian (which began about 260 million years ago) and consist entirely of their durable statoblasts. There are no known fossils of freshwater members of other classes.

   Since all the other phyla that have left fossils are found in Cambrian rocks, it is surprising that the earliest bryozoan fossil dates from the Ordovician, which immediately followed the Cambrian. This suggests that the first bryozoans appeared much earlier and were entirely soft-bodied, and the Ordovician fossils record the appearance of mineralized skeletons in this phylum. The Early Ordovician fossils may also represent forms that had already become significantly different from the original members of the phylum.

 

Description

 

      Bryozoans, phoronids and brachiopods strain food out of the water by means of a lophophore, a "crown" of hollow tentacles. Bryozoans form colonies consisting of clones called zooids that are typically about 0.5 millimetres (0.020 in) long. Phoronids resemble bryozoan zooids but are 2 to 20 centimetres (0.79 to 7.9 in) long and, although they often grow in clumps, do not form colonies consisting of clones. Brachiopods, generally thought to be closely related to bryozoans and phoronids, are distinguished by having shells rather like those of bivalves. All three of these phyla have a coelom, an internal cavity lined by mesothelium. Some encrusting bryozoan colonies with mineralized exoskeletons look very like small corals. However, bryozoan colonies are founded by an ancestrula, which is round rather than shaped like a normal zooid of that species. On the other hand the founding polyp of a coral has a shape like that of its daughter polyps, and coral zooids have no coelom or lophophore. Entoprocts, another phylum of filter-feeders, look rather like bryozoans but their lophophore-like feeding structure has solid tentacles, their anus lies inside rather than outside the base of the "crown" and they have no coelom.

 

Types of Zooid


     All bryozoans are colonial except for one genus, Monobryozoon. Individual members of a bryozoan colony are about 0.5 millimetres (0.020 in) long and are known as zooids, since they are not fully-independent animals. All colonies contain feeding zooids, known as autozooids, and those of some groups also contain non-feeding specialist heterozooids; colony members are genetically identical and co-operate, rather like the organs of larger animals. What type of zooid grows where in a colony is determined by chemical signals from the colony as a whole or sometimes in response to the scent of predators or rival colonies.
     The bodies of all types have two main parts. The cystid consists of the body wall and whatever type of exoskeleton is secreted by the epidermis. The exoskeleton may be organic (chitin, polysaccharide or protein) or made of the mineral calcium carbonate. The body wall consists of the epidermis, basal lamina (a mat of non-cellular material), connective tissue, muscles, and the mesothelium which lines the coelom (main body cavity)  except that in one class, the mesothelium is split into two separate layers, the inner one forming a membranous sac that floats freely and contains the coelom, and the outer one attached to the body wall and enclosing the membranous sac in a pseudocoelom. The other main part of the bryozoan body, known as the polypide and situated almost entirely within the cystid, contains the nervous system, digestive system, some specialized muscles and the feeding apparatus or other specialized organs that take the place of the feeding apparatus.

 

Feeding zooids

 

     The most common type of zooid is the feeding autozooid, in which the polypide bears a "crown" of hollow tentacles called a lophophore, which captures food particles from the water. In all colonies a large percentage of zooids are autozooids, and some consist entirely of autozooids, some of which also engage in reproduction.

 

Avicularia and Vibracula

 

     Some authorities use the term avicularia to refer to any type of zooid in which the lophophore is replaced by an extension that serves some protective function, while others restrict the term to those that defend the colony by snapping at invaders and small predators, killing some and biting the appendages of others. In some species the snapping zooids are mounted on a peduncle (stalk), their bird-like appearance responsible for the term  Charles Darwin described these as like "the head and beak of a vulture in miniature, seated on a neck and capable of movement". Stalked avicularia are placed upside-down on their stalks.
     In vibracula, regarded by some as a type of avicularia, the operculum is modified to form a long bristle that has a wide range of motion. They may function as defenses against predators and invaders, or as cleaners. In some species that form mobile colonies, vibracula around the edges are used as legs for burrowing and walking.

 

Other types of Colonial Zooid

 

     Kenozooids (from Greek κενος meaning "empty") consist only of the body wall and funicular strands crossing the interior, and no polypide. In some species they form the stems of branching structures, while in others they act as spacers that enable colonies to grow quickly in a new direction.
    Spinozooids form defensive spines, and sometimes appear on top of autozooids.  Gonozooids act as brood chambers for fertilized eggs. Some species have miniature nanozooids with small single-tentacled polypides, and these may grow on other zooids or within the body walls of autozooids that have degenerated.


Colony Forms and Composition


     Although zooids are microscopic, colonies range in size from 1 centimeter (0.39 in) to over 1 meter (3.3 ft). However, the majority are under 10 centimeters (3.9 in) across. The shapes of colonies vary widely, depend on the pattern of budding by which they grow, the variety of zooids present and the type and amount of skeletal material they secrete.
Some marine species are bush-like or fan-like, supported by "trunks" and "branches" formed by kenozooids, with feeding autozooids growing from these. Colonies of these types are generally unmineralized but may have exoskeletons made of chitin. Others look like small corals, producing heavy lime skeletons. Many species form colonies which consist of sheets of autozooids. These sheets may form leaves, tufts or, in the genus Thalmoporella, structures that resemble an open head of lettuce.
     The most common marine form, however, is encrusting, in which a one-layer sheet of zooids spreads over a hard surface or over seaweed. Some encrusting colonies may grow to over 50 centimeters (1.6 ft) and contain about 2,000,000 zooids. These species generally have exoskeletons reinforced with calcium carbonate, and the openings through which the lophophores protrude are on the top or outer surface.
     Some freshwater species secrete a mass of gelatinous material, up to 1 meter (3.3 ft) in diameter, to which the zooids stick. Other freshwater species have plant-like shapes with "trunks" and "branches", which may stand erect or spread over the surface. A few species can creep at about 2 centimeters (0.79 in) per day.
     Each colony grows by asexual budding from a single zooid known as the ancestrula, which is round rather than shaped like a normal zooid. This occurs at the tips of "trunks" or "branches" in forms that have this structure. Encrusting colonies grow round their edges. In species with calcareous exoskeletons, these do not mineralize until the zooids are fully grown. Colony lifespans range from one to about 12 years, and the short-lived species pass though several generations in one season.
     Species that produce defensive zooids do so only when threats have already appeared, and may do so within 48 hours. The theory of "induced defenses" suggests that production of defenses is expensive and that colonies which defend themselves too early or too heavily will have reduced growth rates and lifespans. This "last minute" approach to defense is feasible because the loss of zooids to a single attack is unlikely to be significant.

 

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Source: Bryozoa