The middle Smithian (Early Triassic) ammonoid Arctoceras blomstrandi: Conch morphology and ornamentation in relation to stratigraphy

The ammonoid genus Arctoceras (Hyatt, 1900) occurs across all palaeolatitudes, and is a key genus for middle Smithian (Early Triassic) biostratigraphic correlations at a global scale. In this study, intraspecific variations in conch morphology, ornamentation and allometry are examined in relation to stratigraphic position. Arctoceras is the most abundant ammonoid genus in the middle Smithian of Svalbard, Arctic Norway. Originally, seven Arctoceras species were described from Svalbard, but their practical use in fossil identification was questionable. Later, as the importance of intraspecific variation was recognized, six of the Arctoceras species from Svalbard were declared junior synonyms of Arctoceras blomstrandi (Lindström, 1865). Yet, the variations in A. blomstrandi conch morphology remain poorly quantified and the dependence on stratigraphic position unknown. Our research quantifies the intraspecific variation in conch morphology, ornamentation and allometry in relation to stratig...