ANKYROIDS IN SHALLOW WATER SILICEOUS SANDS:VIZCAINOCARPUS FROM THE IMFOUT SYNCLINE IN MOROCCO'S WESTERN MESETA (UPPER ARENIG, ORDOVICIAN)

In-life orientation of Ankyroida (Stylophora, Echinodermata: Upper Cambrian to Lower Pennsylvanian) figures prominently in the debate over the echinoderm or calcichordate affinities of these animals. The Middle Arenigian ankyroid Vizcainocarpus aff. dentiger Ruta has been found in significant numbers (100+ specimens) in shelf deposits in the Imfout Syncline of Morocco’s western meseta. Instead of the usual quiet water organic shales, silty carbonates and limestones commonly inhabited by ankyroids these specimens were clearly living in shallow water fine sands. Specimens in this unusual setting orientate themselves with the convex face up and aulacophore extending into the paleocurrent: the aulacophore draped slightly convexly over the surface. Styloid blades and spines on the proximal end of the distal aulacophore project into the coarse substrate. This rheophylic orientation has been long suspected but with little supporting evidence. Flume studies have demonstrated this orientation is stable even in 30cm/sec current range. Ubaghs (1979) opined that the distal aulacophore was extended near vertically over the convex upwards face of the theca and fed in the water column. This orientation is unlikely because flume studies on 1:1 scale models show that it is highly unstable even in low current velocities. In the calcichordate (Jefferies, 1967) model specimens are commonly depicted as having the convex face of the theca buried in the substrate and the stem (=aulacophore) extended into the substrate (Sutcliffe et al., 1999). Bedding clearly indicates that this facing is not the case and ankyroid in-life orientation is clearly not in keeping with the calcichordate model.