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Although crinoids are the least understood of living echinoderms, their skeletal remains are among the most abundant and important of fossils. They appeared during the Lower Ordovician and underwent several major radiations during the Paleozoic Era. Crinoids were major carbonate producing organisms during the Paleozoic and Mesozoic. In fact, in many Paleozoic and Mesozoic settings entire carbonate shelves were composed predominantly of crinoidal remains (Ausich 1997). In such examples, the enormous volume of skeletal material controlled the sedimentary environment. Short stratigraphic ranges of many species make them important as at least local biostratigraphic index taxa. The broad morphological diversity of crinoids includes forms characteristic of specific habitats and oceanographic conditions. More than 5000 fossil species have been described.
A persistent, traditional view treats living crinoids as chiefly deep-sea organisms, relicts of their opulent Paleozoic past, holding off final extinction in remote abyssal habitats. This view is generally applied to stalked crinoids, or sea lilies, as typical of the entire group, because they most closely resemble their fossil forebears. It is true that the approximately 80 extant species of stalked crinoids are chiefly restricted to depths greater than 200 m (the shallowest occurs in 100 m). However, 85% of extant crinoids (approximately 540 named species) are unstalked feather stars, or comatulids, the products of a continuing post-Paleozoic radiation (Meyer & Macurda 1977). Comatulids are a monophyletic clade classified within the subclass Articulata (Simms 1988). About 65% of living comatulids occur at shelf depths (<200 m). In the tropical Indo-West Pacific, the richest region, single reefs may support as many as 50 species, almost as many as recorded for any individual fossil assemblage. Here, abundance and diversity reach 115 specimens and 12 species per m2, respectively (Messing 1994). Although far fewer comatulid species exist in cold waters, local abundance may be much greater.
All crinoids are passive suspension feeders. They produce no feeding/respiratory current but, rather, rely on extrinsic, ambient water movement. In extant crinoids, the food-gathering apparatus functions as follows: each featherlike arm that radiates from the central body bears an open ambulacral groove bordered by triads of fingerlike podia, or tube feet, which are terminal extensions of the water vascular system (see figure below). The longest tube foot in each triad, 0.43-0.85 mm in length, is held out at a right angle and flicks passing food particles into the groove. After a food particle is captured by a crinoid, the shortest tube foot wraps it in mucous secretions; ciliary tracts on the groove floor then transport it toward the mouth. In living crinoids, food particle size ranges from about 50 to 400 µm. Diets include a variety of protists (e.g., diatoms and other unicellular algae, foraminiferans, actinopods), invertebrate larvae, small crustaceans, and detrital particles.
Arm and ambulacrum morphology of a living crinoid (from Messing, 1987, copyright © 1987 Charles Messing)