Paper No. 41-10
Presentation Time: 4:15 PM
EARLY GOGIID (ECHINODERMATA) EVOLUTION IN CAMBRIAN STAGES 4 AND 5 ON THE SOUTH CHINA (YANGTZE) PLATE COMPARED TO THAT IN WESTERN LAURENTIA
Gogiid eocrinoids are an important component of Cambrian (upper Stage 4 [Balang Fauna] and basal Stage 5 [Kaili Fauna]) faunas of the South China [Yangtze] Plate, especially in Guizhou Province. Despite their diverse morphology a number of traits are common to all of them and strongly suggest diversification occurred in a limited gene pool and area over a fairly narrow time span. All six of the recognized genera have a 2-1-2 ambulacral pattern with one early (juveniles), two (most mature specimens) or three (late mature Sinoeocrinus) exothecal brachioles branching from the ends of the sessile ambulacral branches. Thecal plates around the ambulacral area, in most genera, have abundant sutural pores, and secondarily may have pores develop above the stalk-thecal interface. Late juvenile to early mature specimens commonly have pores developed over the rest of the theca. The most primitive genus, Protogloboeocrinus has thecal pores that may develop anywhere along the length of a facet and may be asymmetrically divided across the suture. In all other Guizhou gogiids the sutures start at the mid- point of the facet and are evenly to nearly evenly divided across the facet. This more orderly growth pattern produces “sprocket-like” plates that make up large areas of the theca. Two general patterns of stalk morphology diverge from the Protogloboeocrinus- like ?arch-morphotype. First: a shortening and thickening of the stalk by proliferation of small circular platelets accompanied by a large pancake-like attachment disk (Globoeocrinus, Turbanicystis) and second: a lengthening and thinning of the stalk (reduction of platelets per circlet) and by a reduction in the size of the attachment disk (Guizhoueocrinus, Sinoeocrinus) or to a nubbin-like terminus (some specimens of Sinoeocrinus and Balangicystis). The relatively smooth morphological transition between South China subclades suggests that most evolutionary innovation occurred here as compared to the other great gogiid habitation, western Laurentia where some species of Gogia are derived from or are closely related to Guizhou genera: eg Short thick stalks, Globoeocrinus to G. ojenai; long stalks, Sinoeocrinus to G. prolifica, Guizhoueocrinus to G. multibrachiatus, and Balangicystis to Lyracystis. However, most Laurentian gogiids appear to have evolved independently.