Origination and diversification of Devonian ambocoelioid brachiopods in South China

Abstract

The superfamily Ambocoelioidea is an important group of the Brachiopoda in the Devonian, both globally and in South China. In the Devonian, this group is also most diverse compared with that in other periods. Based on features of cardinal process and cruralium, three types of cardinalia are recognised, including Ambocoelia-type, Emanuella-type, and Rhyncospirifer-type. Our study shows that the Rhynchospirifer-type cardinalia is a distinct character that distinguishes them from the rest of the Ambocoeliidae; therefore, the Rhynchospiriferinae is re-elevated to the family rank. Guangxiispirifer of the previous Ambocoeliinae is reassigned to the Rhynchospiriferidae while Ambothyris, Choperella, Crurispina, Diazoma, Emanuella, Ilmenia, Ilmospirifer, Ladjia, Moravilla, and Zhonghuacoelia of the previous Rhynchospiriferinae are transferred to the Ambocoeliidae (= previous Ambocoeliinae). Three earliest ambocoelioids in South China are Ambothyris, Amboglossa, and Prolazutkinia in the upper Emsian; their emergences were likely associated with the global Upper Zlichov Event. The upper Emsian occurrences of Amboglossa and Prolazutkinia are their FADs (first appearance datum) globally. These two genera probably gave rise to the Rhynchospiriferidae and the Lazutkiniidae, respectively. After an initial gradual increase in generic richness, the Ambocoelioidea reached the highest diversity (10 genera including four endemic genera) in the late Eifelian and Early Givetian in South China. From then on, the diversity gradually decreased till the end of the Devonian (two genera) and was least affected by the Frasnian–Famennian Event. The Ambocoelioidea is a strongly facies-controlled group. Their temporal and spatial distributions show a close relationship with sea level changes. Ambocoeliids and rhynchospiriferids were adapted to different environments. The former inhabited deeper and partly dysoxic environments, therefore showed greater capabilities to migrate and survive extinctions. On the other hand, the rhynchospiriferids dwelled in a reef-related, high energy, and oxic environments, which were probably more sensitive to environmental changes; they also showed high diversity and endemism and more easily experienced rapid and regional extinctions. Three species representing the three types of cardinalia, Ambocoelia yidadeensis sp. nov., Ambothyris panxiensis, and Rhynchospirifer sp., are described.