EXERCISE 8 (Bryozoans & Arthropods).docx - EXERCISE 8...

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EXERCISE 8 Bryozoans and Arthropods 8.1. Introduction to Bryozoa Bryozoans are one of the most diverse and abundant groups of fossils in shallow marine carbonate and mixed carbonate/clastic rocks that accumulated during the Ordovician through Permian Periods. After a period of low diversity and uncommon occurrence during the early Mesozoic, bryozoans became an important constituent of the marine fossil record again during the Middle Jurassic. Following the Late Jurassic to Early Cretaceous origin and radiation of a new, rapidly evolving group (the Cheilostomata), bryozoans have been diverse and widespread in the seas up to the present, locally reaching the abundance that they had during the Paleozoic and even exceeding their previous diversity. Bryozoans are exclusively colonial , their colonies have grown as simple to elaborate forms that may be reduced to three groups: encrusting, erect, and free-living (Fig. 8.1). The purposes of this exercise are: 1. To examine carefully a single polypide and enclosing body wall of a feeding zooid in a Recent bryozoan colony; and 2. To examine skeletal morphology of representatives of the higher taxa of bryozoans with calcified skeletons. Fig. 8.1. Basic groups of bryozoan growth forms. Free-living bryozoans are represented by a cap-shaped form that has been locally common from Late Cretaceous to the Recent. 8.1.1. Individual modules of the colonies The basic unit or module of a bryozoan colony is termed a zooid . It consists of a polypide , which both contains and is situated within a coelom, a few organs connected to the polypide, and an enclosing body wall. The polypide is organized into two parts, the lophophore (a set of ciliated tentacles, each of which contains a central coelom) and a U-shaped gut (Fig. 8.2). The body wall may be entirely organic or may be 8. 1 Bryozoans & Arthropods
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reinforced by calcite, mixed calcite and aragonite, or aragonite skeleton. The skeletal parts of a zooid are called the zooecium . The skeletal part of a colony is termed the zoarium , and it consists of the collective zooecia plus any skeleton deposited by the colony between the zooecia. Fig. 8.2. Autozooids of bryozoans, with polypide retracted on the right and with polypide protruded to the feeding position on the left (modified from McKinney & Jackson, 1989). 8.1.2 Classification Marine bryozoans are distributed among the classes Stenolaemata and Gymnolaemata. Stenolaemates are usually divided into four or five orders depending on whether the Fenestrata are recognized separately from the Cryptostomata, as is done in this laboratory manual, and gymnolaemates are divided into two orders (Table 8.1). The gymnolaemates are studied almost exclusively from their surface morphology, but study of stenolaemates usually involves oriented sections cut through the colony (Fig. 8.3). Representatives of each of the orders of mineralized bryozoans are illustrated in Figs. 8.4–8.7. Zooidal shape and orientation includes: 1. Shape of the feeding zooids: a) Box- or sac-shaped; b) Conical (distinctly tapered; length < 4x width); c) Tabular (length > 4x width).
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  • Fall '19
  • Crustacean, cephalon

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