SIZE MATTERS:MORPHOLOGY, FUNCTIONAL MORPHOLOGY AND PALEOECOLOGY OF ASCOCYSTITES (ASCOCYSTIDA, EOCRINOIDEA) FROM THE UPPER ORDOVICIAN OF MOROCCO

More than 13 nearly complete specimens of the ascocystitid eocrinoid Ascocystites (Katoua Formation, U. Ordovician, Msissi-Alnif area, Morocco) were recently obtained by the Smithsonian. The genus also known from the M. Ord. of France, and Portugal and U. Ord. of Bohemia. In addition to the Bohemian type material in Prague, 130 specimens from the Ouine-Inirne (M. Ord) and the Izzeguirene (U. Ord) formations of eastern Anti-Atlas are housed in French museums. The Kataoua specimens are the largest known specimens of the genus and size range of this material enables us to reconstruct a reasonably accurate generic ontogeny. The sac-like mature theca is composed of hundreds of hexagonal plates, each with a raised six-rayed ornament; some ornament rays aligned to form long longitudinal (2 to 8) semi-rigid spiraling ribs that originate at the basal plate and end at the oral disk. Thecal plates are without respiratory structures. Uniserial brachioles are approximately equal to the height of the theca in length up to 50 mm and then lengthen only slightly and appear to number from ca. 5 to 10 (TH = <20 mm), 20 (TH= 20-30), 30 (TH = 30-100) and 55 (TH = >100). Brachiole widths (BW) are fairly consistently 0.5 mm until TH = ca. 60 mm and BW can go up to ca. 1 mm suggesting that food gathering capacity increases in late ontogenetic stages. Stem is gently tapering, uniserial, with several intercalate series of ossicles. There is no evidence for an attachment disk and attachment was probably by distal stem piercement into the sediment and possibly by excretion of a bioglue (?collagen) mimicking (L. to M. Cam.) gogiids. Stem length is usually about equal to thecal height, up to ca. 40 mm above which stem growth is minimal. Morphology indicates that the base of the theca in mature specimens was generally about 40 mm above the seafloor: theca and brachioles streamed near vertically mimicking Cambrian gogiids. Brachiole food grooves are about twice the depth/width of Cam. eocrinoids (gogiids) suggesting a wider size range of food (both microorganisms and organic particulate matter). The semi-rigid theca facilitated oxygen exchange on its surface. Peristaltic motion of the gut and probable flexible muscular fields between the spiraling thecal ridges moved internal body fluids past the internal surfaces of the thecal plates to facilitate oxygen exchange.