Biogeography, Time and Place: Distributions, Barriers and by Willem Renema

By Willem Renema

Biogeography considers the distribution of organic devices over a variety of scales. The devices variety from genotypes, populations and species to households and better taxa. strategies will be neighborhood, corresponding to the isolation on islands as a result of sea-level fluctuations, or large-scale tectonic procedures that separates continents and creates oceans. In all approaches time is a vital issue and by means of combining information on fresh styles with paleontological info the knowledge of the distribution of extant taxa will be more desirable. This quantity makes a speciality of speciation as a result of isolation in island-like settings, and the evolution of large-scale variety because the results of origination, upkeep and extinction.

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Back onto the stem, but the crown bent upward for feeding (Fig. 29). This interpretation was rejected by Schmidt (1934), who favoured an upright stem with a bent-down crown during times of rest. The crown would have been raised at a right angle into the current for feeding. Schmidt even thought that Senariocrinus Fig. 31. Holopus alidis. Side view of complete individual with closed arms, dredged from a depth of 460–470 m off the Loyaute´ Islands. -P. Bourseau; from Bourseau et al. ) ϫ3. CRINOID FORM AND FUNCTION Fig.

27) and also some articulates (Fig. 24). Calyxes with fixed brachials typically require extra plates (interradials or interbrachials) to fill in the area between adjacent rays (Figs. 27, 38). Extra plates are also typically present in the posterior interradius2 and the reader is referred to the Treatise (Ubaghs 1978) for further details about these plates or about modifications from the standard plating described here. Calyx shapes such as bowls, urns, cones and hands appear to be sensible constructions for food processing, but the very specialized bilateral recumbent constructions and the fists merit special attention and a short discussion.

THE ARMS The arms represent the food-gathering parts of the crinoid. The ultimate food-collecting structures are the tube feet; hence some crinoids can function quite effectively despite the presence of only one arm or in the absence of any arms (Fig. 35). Although food gathering is the primary function of crinoid arms, it is by no means their only function. Arms are important for respiration and locomotion in some taxa, either swimming as in some comatulids or crawling as in comatulids and even in some stalked isocrinids (Messing 1985; Baumiller et al.

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