Evolutionary trends in trimerellid brachiopods

Abstract

Non-articulated trimerellides, one minor group among the largest brachiopods, are commonly found in massive monospecific accumulations in Ordovician and Silurian rocks. In this paper, all species of the family Trimerellidae are listed for the first time, and some of them are discussed. They appeared in the Sandbian and became extinct in the Ludlow with an apex of diversification observed, at generic level, during the Katian and at species level, during the Wenlock. Platforms and vaults are the diagnostic morphological characteristics of the trimerellids, and constitute the important basis for generic classification. Individuals with highly raised platforms and deep vaults first appeared in the middle–late Katian, but became the main morphology in the Silurian. Nineteen genera of Trimerellidae are selected in this study, and 30 characters are coded for morphospace and phylogenetic analyses. Based on the morphospace analysis by non-metric multidimensional scaling (NMDS), we have differentiated the morphological characteristics of platforms and vaults into three types (I, II and III). The results of the phylogenetic analysis show a tendency for platforms and vaults to become more prominent in younger genera (from Type I to III). Trimerellids are divided into four morphological groups based on the differences of shell shape or life style. This evolutionary trend of platforms and vaults is reflected in all four morphological groups.