MORPHOLOGY AND REVISED SYSTEMATICS OF THE SOLUTE MAENNILIA ESTONICA (HOMOSTELEA, ECHINODERMATA) FROM THE UPPER ORDOVICIAN OF ESTONIA

Maennilia estonica Rozhnov and Jefferies 1996 from the Upper Ordovician Vazalemma Formation, in Northern Estonia, was originally described as a stem-chordate, but this restudy confirms that it is a solute echinoderm. It is found in coarse-grained limestone facies deposited between bryozoan-rich bioherms. The unusually large, asymmetrical, flattened theca of Maennilia has tessellately plated upper and lower faces, with a narrow band of tightly sutured plates forming a thin marginal rim. The short proxistele is composed of polymerous rings and the long dististele bears alternating dimerous plates. The well-preserved ambulacral structure emerges from an oral frame, of undetermined plate pattern located at the end of a fold on the rim along the left thecal margin. Although the ambulacral structure is externally similar to a large brachiole, details of its construction are very different. Ambulacrals are evenly paired with paired cover plates comprising each segment, and a large lumen runs the length of the ambulacrum, pierces the ambulacral plates, and opens directly into the thecal cavity. Articulatory surfaces between segments are significantly reduced, and in life rigidity may result, from elevated internal thecal pressure. Increased flexibility and unusual length of the structure support this interpretation. The shallow food groove is overarched by tall, gogiid-like cover plates. There is no evidence for an associated water vascular system (such as pores or podial basins) traversing its length. Three or four rows of quadremerous sleeve plates placed between the oral opening and ambulacral plates, impart increased flexibility proximally, and probably helped elevate the structure into the slipstream over the theca. The madreporite (”sugarloaf plate”) is set into the marginals adjacent to the base of the ambulacrum and suggests that a water vascular system was present and contained entirely within the theca. The distinctly different feeding structure (seen in its entirety in Maennilia and seemingly typical for all solutes) coupled with the asymmetrical theca suggests that solutes are not closely related to, or derived from, echinoderms with basically 2-1-2 or pentameral symmetry (blastozoans and normally pentaradiate echinoderms).