Alatoconchids: Giant Permian bivalves from South China

Alatoconchidae, a unique bivalve family from the Permian, are characterized by a giant body size (up to 100 cm long), and unusual shell form with wing-like flanges and coarse prismatic outer layer of the shell wall. Palaeogeographically, they are restricted to low-latitude Tethyan and Panthalassan regions. However, the clams have not been previously been reported from South China even though this is one of the main, eastern Palaeotethyan terranes. Here we show that these giant clams were in fact widely distributed in South China, but they have been previously misidentified as phylloid algae. The present study verifies their occurrence in the region using reconstruction of shell forms from a series of transverse sections, as well as their characteristic shell microstructure. The alatoconchids occur commonly as coquina beds. Over thirty occurrences, ranging from the early Kungurian to probably the latest Capitanian in time, have been found. Based on detailed lithological and microfacies observation at more than ten fossil localities, most alatoconchid-bearing horizons (ABH) are autochthonous deposits, confined to medium- to thick-bedded limestones (often wackestone) of shallow water carbonate platforms. Two occurrences from an intra-platform basin are allochthonous deposits shed from the nearby carbonate platform by gravity flows, and featured by completely disintegrated fragments. Autochthonous and condensed accumulation of the fossil materials with great lateral persistence imply a gregarious habit of the clam, although slightly reworking might be involved. Its absence from high energy conditions, as well as the completeness of shell preservation, suggests it was not a reef dweller. To obtain gigantism in the tropical oligotrophic environment, alatoconchids may have harboured photosymbionts like present-day tridacnids, facilitated by its transparent shell texture, although this notion is challenged by Asato et al. (2017). Their association with calcareous algae and corals in the ABH indicates that the optimum belt of the clams is likely within the euphotic zone, but below that of calcareous green algae judging from the lithological succession and skeletal grain association.