Author: Merrill W. FosterPublisher: American Geophysical Union
ISBN: 0875901220
Category : Science
Languages : en
Pages : 192
Book Description
Published by the American Geophysical Union as part of the Antarctic Research Series, Volume 21. The Recent brachiopod faunas from southernmost South America, Antarctica, and the Subantarctic consisting of 21 genera and 37 species are described from new material. New taxa include the genera Manithyris and Bathynanus. Also new are 11 species: Compsothyris ballenyi, Hispanirhynchia? chiliensis, Manithyris rossi, Liothyrella multiporosa, Liothyrella scotti, Bathynanus tenuicostatus, Eucalathis macrorhynchus, Amphithyris hallettensis, Dallina eltanini, Fallax antarcticus, and Magellania? spinosa. Factor analysis was used to study relationships among morphological characters and environmental parameters. In Liothyrella, negative associations were found between beak height and water depth. The size of the pedicle opening within this genus is associated with the width of the hinge plate and the loop; apparently, these associations are related to increased dorsal adjustor muscle size with increased pedicle size. Similar relationships are found in the terebratellids, although here all of the characters are also negatively associated with water depth. Characters negatively associated with water depth may relate to the differing current strength at various depths. Puncta density is positively correlated with water temperature. Examples of brachiopod variation are discussed. Synonyms have resulted from former failure to study large samples and to appreciate the extent of brachiopod variation. A priori valuation of certain characters as being specific is unwarranted, since sibling species may be more similar than different subspecies of the same species. Many Recent and fossil brachiopod genera are too narrow in definition owing to oversplitting or to a narrow conception of monophyly. I advocate somewhat broader and more practical genera for obtaining the maximal information value from such taxa. Brachiopods, contrary to popular belief, are an abundant and viable group in the southern hemisphere faunas. Most brachiopods in the Ross Sea appear to have definite niches and habitats but may overlap geographically where population densities are low. Brachiopods here can be divided roughly into a slope and a shelf assemblage. The greatest species diversity occurs at the seaward edge of the Ross Sea shelf, interpreted as an ecotone effect where two different water types meet. Only South America and Antarctica appear to have or have had direct communication between some elements of their brachiopod faunas. Other similarities between separate southern continents are related to retention of common pre-Cenozoic elements or to chance dispersal of larvae across barriers. Events related to cooling during the late Pliocene or Pleistocene caused reduction of puncta density, shell thickness, and spiculation in the Recent fauna and apparently influenced the present species structure, at least in Liothyrella uva, Gyrothyris mawsoni, and Macandrevia.
