Article: Brachidia of late Ordovician and Silurian eospiriferines (Brachiopoda) and the origin of the spiriferides
Rong Jia-Yu and Zhan Ren-Bin
The brachidium is revealed for the first time in the Late Ordovician Eospirifer praecursor, the earliest known eospiriferine and probably the ancestor of the entire Spirifer group. The adult stages of the species possess a spiralium directed ventro-laterally with a pair of small jugal processes without a jugum. The brachidium of the other five species of Eospirifer (including E. radiatus (Sowerby), the type species) and three of Striispirifer (including S. plicatellus (Linnaeus)) from the Llandovery, Wenlock or Ludlow in South China, Kazakhstan, England, Sweden and Canada are also revealed and reconstructed, demonstrating the same type of brachidium as in E. praecursor. The structure of the early brachidial growth stage of E. praecursor is similar to that of the adult stages of the early atrypoid Cyclospira bisulcata (Emmons). This, coupled with the evidence of cardinalia, is of significance in the evaluation of eospiriferine origins. A shallow-water environment is proposed for the origination of the Spiriferida. Eospirifer, a progenitor and Lazarus genus, probably originated in East and Central Asia in the mid Ashgill; it disappeared in the late Ashgill, but survived the latest Ordovician mass extinction, recovered in Asia and Australia in the Early-Mid Llandovery, and did not spread to Europe and America until the later Llandovery (about the sedgwickii Biozone). A striated cardinal process is also seen for the first time in the topotype of E. radiatus, indicating that its presence may not be of fundamental significance in the generic classification of the eospiriferine group.