QUANTITATIVE ASSESSMENTS AND IMPLICATIONS ON THECAL MORPHOLOGY IN CAMBRIAN EOCRINOIDS FROM GUIZHOU PROVINCE
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Abstract:
Eocrinoids are a representative fossil group of the Cambrian Balang biota and Kaili biota. The Balang Formation and the Kaili Formation, containing Balang biota and Kaili biota, are composed of green-grey mudstone and shale. Eocrinoids, a typical ecological group in outer shelf deep-water facies, are well-preserved in large amounts in middle-upper part of the Balang Formation and the Kaili Formation. Based on measured data of thecal morphology (including landmark data and linear distances) of 105 samples among 4 species: 36 Guizhoueocrinus yui, 29 Protogloboeocrinus yini, 20 Globoeocrinus globulus and 20 Sinoeocrinus lui, we use traditional morphometric method and geometric morphometric method to investigate the morphological variation of thecal morphology of 4 eocrinoid species. The results show that little difference can be found in thecal size. Morphological variation of theca between Protogloboeocrinus yini and Guizhoueocrinus yui is relatively small, they commonly share an amygdaloidal form. However, theca shape between Sinoeocrinus lui and Globoeocrinus globules exhibits a significant variation, which indicates an evolutionary divergence: Compared to eocrinoids from the Balang Formation (Protogloboeocrinus yini and Guizhoueocrinus yui), eocrinoids from the Kaili Formation (Globoeocrinus globulus and Sinoeocrinus lui) develop a wider oral area and larger/smaller part at the bottom of the theca. In addition, in Sinoeocrinus lui, the theca develops a conical shape. However, in Globoeocrinus globulus, the theca develops a global shape. The result indicates that there is a continuous evolutionary trend in early-middle Cambrian eocrinoids. Conical theca and stem can enlarge the feeding area, while spherical thecal and short stem can make individuals adjust to stronger water energy, by attaching to the sea floor or bio-debris more tightly. This study demonstrates that there may occur an ecological niche expansion of eocrinoids in Cambrian Series 2 and Series 3.
