Abstract

Ribbing became an increasingly common feature of pentamerid shells, especially the family Stricklandiidae, during the later Llandovery (Early Silurian). The thin shells of stricklandiids from the Llandovery (Aeronian-Telychian) equatorial carbonate rampshelf of Anticosti Island reveal at least three ribbing designs in various genera: 1) radially arranged ribs. e.g., Costistricklandia; 2) divergent, chevron-like ribs, e.g., Ehlersella; and 3) diagonally criss-cross, mutually intersecting divaricate ribs, as in Kulumbella and Microcardinalia (Chiastodoca) new subgenus. Weakly divaricate species previously assigned to Kulumbella, Stricklandia or Microcardinalia from Anticosti Island, South China and the Baltic region, are assigned to the new subgenus Chiastodoca (type Stricklandinia salterii Billings, 1868), and they are suggested to have evolved their divaricate ribbing from smooth forms, independently from the earlier genus Kulumbella, in the same sense that radial or divergent ribbing was developed separately in a number of pentamerid and stricklandiid lineages in the late Llandovery and Wenlock. In Anticosti, the divaricate genus Kulumbella, originally described from Siberia, appeared, without a known ancestral form, in the Anticosti Basin (as K. xacta n.sp.), and became extinct equally suddenly in the upper part of the Gun River Formation (mid-Aeronian). This sudden arrival of immigrant taxa, originally appearing earlier in other areas, is matched in other brachiopod groups and corals in the Anticosti section. Weakly divaricate ribbing in Chiastodoca is characteristic of the early Telychian (Ferrum Member, Jupiter Formation), and radial ribbing is typical of the late Telychian genus Costistricklandia (Pavillon Member, Jupiter Formation).

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