Larval Ecology And Brachiopod Distribution

General brachiopod paleoecology has often been discussed (e.g., Lochman 1956; Sa-rycheva 1960; Ivanova 1962; Rudwick 1965, 1970; Gorjansky 1969; Rowell and Krause 1973; McKerrow 1978; Percival 1978; Williams and Lockley 1983; Bassett 1984; Holmer 1989; Popov etal. 1989; Wright and McClean 1991; Popov 1995), and the following information is based on this research.

All brachiopods are passive suspension feeders that use the lophophore, a variously looped or coiled extension of the mesocoelom, for water and food uptake. Chuang (1959) showed that the alimentary system of Lingula unguis contains fermenters that allow the animal to digest phytoplankton. The remaining Recent lingulates do not differ from it in this respect. In contrast, Recent calciates lack such fermenters and feed, mainly, on bacterial aggregates and dissolved nutrient matter (Atkins 1960; McCammon 1969; Rhodes and Thompson 1993).

All brachiopods belonging to sessile benthos spread at the larval stage. Recent cal-ciates have a lecithotrophic larva that is free-living from several hours to 1-2 days. After that, the larva settles on a substrate. Recent lingulates possess a planktotrophic larva that floats in the water column for from several days to a month (Malakhov 1976). In some cases, under unfavorable conditions, a complete morphogenesis is observed, and a pedicle and a lophophore, bearing numerous cirri, are formed that appear just like those in anchored animals (Zezina 1976). These features allow lin-gulates exclusive facilities for migration. Many Recent lingulates are widespread in shallow waters of the Indian and western Pacific oceans, and Pelagodiscus atlanticus (order Discinida) is a cosmopolitan species. Some Cambrian species of the Paterinida,

Acrotretida, and Lingulida orders have a wide, sometimes global distribution. This phenomenon may be due to a protracted pelagic phase (Jablonski 1986; Rowell 1986). Indirect evidence for this is that larval shell sizes do not exceed one-third to one-fifth of the whole shell (figures 16.3A,B,D). In addition, the larval shell surface of all acrotretids and most Cambrian lingulids bears numerous minute pits (figures 16.3C,G). Many researchers ascribe this feature to a vesicular structure of the periostracum or to an entirely organic larval shell (Biernat and Williams 1970; Williams and Curry 1991). Popov et al. (1982) regarded the pitted microornament of the umbonal area in acrotretids as a negative impression of the entirely organic larval shell and suggested that the acrotretid shell acquired mineralization only after settlement. In any case, such a structure may increase buoyancy and probably is an adaptation to a pelagic lifestyle.

Prolongation of the pelagic larval stage might serve to enhance the distribution of some acrotretids. The acceleration in foramen development might indirectly substantiate such a suggestion (Popov and Ushatinskaya 1992). Early Cambrian Linnarssonia possessed only a vestigial delthyrium on the posterior margin of the larval shell. Development of this delthyrial opening into a foramen occurred after the settlement of the animal. Later genera, Homotreta and Hadrotreta, had a well-defined delthyrium that turned into a foramen soon after settlement. Several genera (Neotreta, Quadri-sonia, Angulotreta, Rhondellina) had larval shells with well-developed foramens (figures 16.3I,J). Some Middle Cambrian paterinids (Paterina, Micromitra, Dictyonina) from the Siberian Platform had larval valves of about one-half to one-third of the entire shell size, although usually the size of larval valves was about one-fifth to one-tenth of the adult shell. This phenomenon probably also indicates prolongation of the larval stage.

Was this article helpful?

0 0

Post a comment