THE FIRST OCCURRENCE OF BLASTOZOAN EXOTHECAL AMBULACRA IN MIDDLE CAMBRIAN LYRACYSTID EOCRINOIDS FROM BRITISH COLUMBIA, CANADA AND GUIZHOU PROVINCE, CHINA

Blastozoans (e.g. eocrinoids, rhombiferans, diploporans, etc.) commonly have a recumbent biserial ambulacral system (with a 2-1-2 arrangement) over the thecal plates with laterally and/or distally attached exothecal biserial brachioles that do the actual feeding from the seawater. In the Middle Cambrian, lyracystid eocrinoids, including Lyracystis Sprinkle and Collins, 2006, from western Laurentia (British Columbia, Burgess Shale), and Balangicystis Parsley and Zhao, 2006, from South China (Guizhou, Kaili Biota), were the first blastozoans to have the recumbent ambulacra extend off the theca as free-standing exothecal structures. Closely placed, straight brachioles branching off the ambulacral plates produce an endotomous arm-like structure similar to that found in some later crinoids. Because it is uncertain if blastozoan feeding structures had a podia-bearing water vascular system, the mode of feeding may have been quite different from that of crinoids. Lyracystids have long stalks and are found in deep- and quiet-water siliciclastics. They may have collected food in open food grooves as part of an essentially horizontal basket-like arrangement in quiet water (near vertical obrution sedimentation), or, in gentle currents, in a modified (tilted) broadly conical basket that captured food in back eddies generated by pass-through currents. This geometry was possible by having the theca and basket at the end of a long, kite string-like stalk. The simple articulations in the exothecal ambulacral plates and brachioles probably precluded the formation of reflexed cones as seen in modern crinoids where the articulations are marked by fossae for ligaments and flexor muscles. This exothecal ambulacral morphology re-occurs in the Late Ordovician eocrinoid Cardiocystites (Bohemia, Shropshire, Morocco) and the diploporan Eumorphocystis (Oklahoma), but these genera occur in higher energy carbonates and coarser siliciclastics. Free-standing ambulacral plates, not brachioles, may have been present in pleurocystitid rhombiferans and seem to be adapted for sweeping along the sediment-water interface. Exothecal ambulacral structures are relatively common (about 15%) in various blastozoan clades, but appear to have been adapted for several sets of environmental conditions.