Moult stages and dimorphism of Early Cambrian bradoriids from Xichuan, Henan, China
Title:
Moult stages and dimorphism of Early Cambrian bradoriids from Xichuan, Henan, China
Author:
Zhang, X. -G.
Appeared in:
Alcheringa
Paging:
Volume 11 (1987) nr. 1 pages 1-19
Year:
1987
Contents:
Carapaces or valves of 6 species of bradoriids are described from the Early Cambrian (Canglangpu Stage) Shuigoukou Formation in Xichuan, Henan, China. They comprise four genera, including two new genera: Ovaluta gen. nov. is established for some species found in China which were previously described under the name of Aluta; Phaseolella gen. nov., with distinctive carapace features, should be referred to phosphatocopines. The five new species proposed are Monasterium bucerum, Houlongdongella xichuanensis, Ovaluta henanensis, Phaseolella dimorpha, and P. curvata. Abundant bradoriids, isolated by etching limestone with dilute acid, permitted detailed study of their ontogeny. The sexual dimorphism of Phaseolella dimorpha, which may be the earliest (Lower Cambrian) record so far known in Bradoriida, is demonstrated by the growth curve in a scatter diagram. Growth of this species is isometric in juvenile stages but anisometric in pre-adult-adult stages. Nevertheless, separate moult stages could not be distinguished during its ontogeny. In contrast, 9 moult stages can be recognized in Houlongdongella xichuanensis, at a growth rate of about 1.40, which suggests upon moulting an increase in volume of 2.74 times that of the previous instars. Clearly, some Cambrian bradoriids, at least, are closely related to ostracodes on such evidence. Although poorly known, the Cambrian bradoriid fauna of Xichuan seems most closely related to that of Queensland.
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