Pterotheca (Gastropoda) from the Telychian (Silurian) Xiushan formation of South China: Taxonomy, paleoecology and paleogeography

Pterotheca Salter, 1853 is an unusual but readily identifiable bellerophontoid gastropod that occurs in the Upper Ordovician through to the Llandovery of the lower Silurian in many parts of North America and Europe. Recently, a large collection of Pterotheca was obtained for the first time from the Xiushan Formation of middle Telychian (Llandovery) age in the Hunan Province of South China. This is also the first record of the genus in the low-latitude peri-Gondwanan region. Based on the collection, two new species of Pterotheca, i.e. P. yongshunensis n. sp. and P. xiushanensis n. sp., were identified and described herein. The morphologic analysis suggests that close relatives of these new species may be Pterotheca species from the Telychian of Scotland. The new species show continuous variations of marginal apex to submarginal apex, implying that one of the Pterotheca species may be ancestral to the Devonian Aspidotheca Spriesterbach, 1919. The Pterotheca species from South China possibly lived a slowly crawling life on a soft substrate, feeding on algae and/or detritus, and were adapted to a shallow-water setting with substantial terrigenous input. Given that all the known Silurian Pterotheca species occurred in siliciclastic settings and most of which represent sea-level fall and lowstand periods, we demonstrate that geographic isolation and enhanced ocean circulation during the early Silurian regression facilitated the speciation of Pterotheca globally, the connection of sea pathway during the Rhuddanian transgression after the end-Ordovician glaciation could have led to the primary dispersal of Silurian Pterotheca.

Methods
All the Pterotheca specimens from South China are measured by Image J and/or CorelDRAW 2021® on photographs, noting that the skewed specimens which are possibly related taphonomic deformation are measured on retro-deformed images using CorelDRAW 2021®, where the shell is symmetrical along the median axis (marked with * after Specimen No. in the file).