An Early Triassic (Dienerian) microgastropod assemblage from the Salt Range, Pakistan and its implication for gastropod recovery from the end-Permian mass extinction

Abstract

A single limestone sample from the Early Triassic (late Dienerian) of the Salt Range in Pakistan has yielded numerous small gastropods (smaller 2 mm), few bivalves and abundant ammonoids. The limestone is interpreted as a tempestite. Many of the gastropods are extremely well-preserved including protoconch preservation and micro-ornaments. Five species of gastropods are present: (1) isolated larval shells representing a neritimorph taxon of the family Trachyspiridae, (2) early juvenile and larval specimens of the caenogastropod Strobeus pakistanensis, (3) the new caenogastropod species Coelostylina hydrobifomis Nutzel & Ware, (4) the new heterobranch genus and species Eographis microlineata Ntitzel & Ware representing the earliest occurrence of the extant family Hyalogyrinidae (Valvatoidea), (5) the heterobranch Sinuarbullina sp., a genus which originates in the Early Triassic and has also been reported from China, Spitsbergen and the US. Three genera (trachyspirid. Strobeus and Coelostylina) represent survivors from the end-Permian mass extinction whereas Eographis and Sinuarbullina have their earliest known occurrence in the Early Triassic. Eographis is the first Early Triassic genus for which coaxial larval heterostrophy is shown. Together with the Devonian genus Palaeocarboninia and the Carboniferous genus Heteroaclisina, it witnesses a high age of Ectobranchia (living families Hyalogyrinidae, Valvatidae. Comirostridae and Xylodisculidae). This supports the assumption that this group is the first extant offshoot of the Heterobranchia. The studied assemblage is strongly dominated by Strobeus pakistanesis. The small size of the gastropods reflects their ontogenetic stage (early juvenile and larval shells) as well as a small adult size in some of the species. Most of the present gastropods had planktotrophic larval development providing further evidence against the hypothesis that the end-Permian mass extinction event selected against larval planktotrophy. Failure to grow to a larger size or to perform metamorphosis suggests unfavourable living conditions in the source community probably due to oxygen deficiency prior to transport by storm. Size sorting is seen as less likely because the limestone is poorly sorted with relatively large ammonoids being present in considerable numbers. The studied sample yielded also three bivalve taxa amongst them two species representing the genera Leptochondria and Crittendenia.