Old Earth Ministries Online Earth History Curriculum
Presented by Old Earth Ministries (We Believe in an Old Earth...and God!)
This curriculum is presented free of charge for use by homeschooling families and schools.
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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The phylum was originally called "Polyzoa", but this term was superseded by "Bryozoa" in 1831.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:
Chapter 4 - The Ordovician
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.
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. 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. 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, 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, 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) 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.
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
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
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.
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.
Colony Forms and Composition
End of Reading