Gastropods from the Late Triassic Nayband Formation (Iran), their relationships to other Tethyan faunas and remarks on the Triassic gastropod body size problem

摘要

A collection of marine gastropods from the Late Triassic (Rhaetian) Nayband Formation of Iran near Esfahan consists of 67 specimens representing 11 species (two species are new: Cryptaulax? hautmanni and Rhynchocerithium esfahanense). Species richness, diversity indices as well as rarefaction analysis suggest a comparatively low diversity. The gastropod fauna is dominated by two large caenogastropod species: Anulifera binodosa and Pseudokatosira? seminodosa which are both as high as 100 mm. The genus Anulifera is also abundant in the Dachsteinkalk of the Northern Calcareous Alps and has been reported from Southeast Asia. The size distribution pattern of gastropods in the Nayband Formation is compared with the size distribution of a collection from the - Carnian Cassian Formation (N Italy, Southern Alps). The gastropods from the Cassian Formation are smaller than two centimetres. However, the Cassian gastropod fauna is not necessarily dwarfed or stunted because a small size is not unusual for gastropods and even the majority of extant gastropod species is smaller than 1-2 cm. This size range is also present in some Early Triassic gastropod faunas and this fact detracts from the significance of the so-called "Lilliput Effect". However, the absence of very large (> 5-10 cm) gastropods in Early Triassic faunas seems to be a correct observation ("Gulliver Absence Effect").