Death or Living Assemblage? The Middle Permian Discinid Brachiopods in the Kamiyasse Area, Southern Kitakami Mountains, Northeastern Japan

Abstract

Linguliformean brachiopods often exhibit a clustered occurrence. Because there has been no detailed analysis of whether these clusters represent death or living assemblages, we exemplified the burial processes of the Middle Permian discinid brachiopods from the Hoso-o Formation with the systematic descriptions of Orbiculoidea verum sp. nov. and Discinidae gen. et sp. indet. There were three types of discinid occurrences: 1) articulated Orbiculoidea in a ventral-side-down orientation, 2) disarticulated valves of both species in a convex-up orientation, and 3) disarticulated ventral valves of the Orbiculoidea in a convex-down orientation. The articulated specimens occurred in laminated mudstone and must have been autochthonous, while the disarticulated specimens with a convex-up orientation were para-autochthonous, resulting from turbiditic or storm flow. The specimens with convex-down ventral valves always occurred at the base of the sandstone beds with their ventral apexes slightly penetrating the underlying mudstone. This is the argument for an autochthonous ventral valve, whereby only the dorsal valve was transported after death. The distribution of epibionts suggests that the life posture of the Orbiculoidea was a ventral-side-down orientation, with its anterior half slightly higher than the sediment-water interface, which was advantageous for the feeding flow and the clearance of sediments around the posterior margin. Although dense monospecific assemblages of Orbiculoidea occur only as disarticulated valves, the exclusive occurrence of articulated individuals in the underlying mudstone suggest that the animal inhabited the specific environments opportunistically, which has never been explored in other fossilised organisms.