A TALE OF TWO EOCRINOID ASSEMBLAGES: POSSIBLE EXTREME PAEDOMORPHOSIS IN A MIDDLE CAMBRIAN (SERIES 3) GOGIID FROM LAURENTIA

Gogiids from Series 2, Balang Fauna, Guizhoueocrinus yui and basal Series 3, Kaili Biota, Sinoeocrinus lui and Globoeocrinus globulus from the South China (Yangtze) Plate, display complex ontogenies where juvenile (3 substages), mature (commonly 3 substages) and gerontic stages are clearly discernible. Thecal height (TH=) is the metric and characters such as position, shape and composition of sutural pores, number and arrangement of thecal plates, number and pattern of brachioles and composition and plate differentiation in the stalk determine stage/substage. While not well documented, most Cambrian gogiids in Laurentia also appear to have undergone a fairly complex ontogeny. An undescribed gogiid (“Gogiid nsp”) from the Spence Shale (Mid Series 3, Wellsville Mountains, northern Utah) has exceptional and somewhat different morphology. The theca is devoid of sutural pores except in large specimens (TH=ca. 20 mm) where they appear densely scattered under the ambulacral plates. Thecal plates are essentially hexagonal and arranged in 6-8 alternating circlets. Brachioles are long and straight with a 1-1-1 juvenile configuration (6-8 brachioles) and mature specimens with a “2-1-2” configuration (9-13 brachioles). Stalk plates are small with rounded upper surfaces and are undifferentiated throughout its length. Together, these characters observed in large specimens are similar to those (except for brachiole number and configuration) that characterize middle and late juvenile stages of Guizhoueocrinus and Sinoeocrinus. Mature and gerontic stages are missing and resulting in an apparent extreme case of paedomorphosis. Causation of this prolonged juvenile morphology may be response to energetic shallower water settings with higher oxygen content.