Intraspecific variation of Svalbardiceras spitzbergensis (Frebold) from the Early Triassic (Spathian) of Spitsbergen

Abstract

The ammonoid species Svalbardiceras spitzbergensis from the latest Early Triassic (Spathian, subrobustus Zone) shows a remarkable intraspecific variation. The more than 400 specimens studied come from one fossiliferous carbonate nodule layer within a sequence of mudstones and clayey siltstones of the Vikinghøgda Formation (Vendomdalen Member), mainly collected in the Sassendalen and Agardhbugta areas in central and eastern Svalbard, respectively. The preservation of the ammonoids is very good, usually with preserved shells and complete body chambers. Morphologically they range from compressed, more or less smooth platycones, with narrow umbilicus, to wide umbilicate, depressed forms with straight, prominent ribs. The morphological variation is continuous: all specimens belong to a single assemblage, and are variants of one biospecies. This phenomenon, which is quite common in Boreal Triassic ammonoid faunas, has in the past led to serious taxonomic oversplitting. Furthermore, this new example presents a serious challenge to current ideas about the close correlation between mode of life and shell morphology of ammonoids.