Galina T. Ushatinskaya
All brachiopods are sessile benthic organisms; in feeding style they are ciliary suspension feeders. Cambrian brachiopods show several types of substrate relationships: pedicle-anchoring, free-lying, cemented epifaunal, infaunal, quasi-infaunal, and interstitial, as well as possibly pseudoplanktic. The earliest brachiopods are known from Early Cambrian carbonates of the Siberian Platform. Lingulates appeared at the beginning of the Tommotian, and calciates arose in the middle Tommotian. Additional lingulate orders appeared during the late Atdabanian in siliciclastic sediments in northern European areas. The acquisition of a mineralized skeleton by brachiopods at the beginning of the Cambrian may have been connected with changes in ocean water chemistry. Differences in diet probably defined distinctions in skeletal composition: lingulates could consume phytoplankton, but calciates preferred animal proteins.
BRACHIOPODS BELONG to the subkingdom Eumetazoa and are characterized by two unique features. First, they have an intermediate protostomian-deuterostomian embryology. It is likely that brachiopods separated from other Bilateralia prior to protostomian-deuterostomian differentiation (Malakhov 1976, 1983). Second, the Brachiopoda is the only phylum that produces both calcium carbonate and phosphatic shells.
By the second half of the twentieth century, the brachiopod systematics developed by Huxley (1869) became widely accepted (Sarycheva 1960; Williams and Rowell 1965). This subdivided brachiopods into two classes, Articulata and Inarticulata, based on presence or absence of valve articulation, respectively. Therefore, the Inarticulata included brachiopods with calcium carbonate shells, as well as those with phosphatic shells. During the last decades, studies of Recent brachiopods have revealed that forms possessing shells of different composition and microstructure are also distinguished in their embryology and molecular phylogeny (e.g., Williams 1968;