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 stratigraphy of the Svalbard Arctoceras. The results support the assignment of all Arctoceras morphotypes from Svalbard to a single species A. blomstrandi. The new data allow for an updated species description and opens for the use in biostratigraphy of the endmember morphology A. blomstrandi var costatus. We document consistent changes in both conch morphology and ornamentation in the studied stratigraphic interval with a distinct shift towards more evolute and ornate conchs in the top. The trends seen in the strength of ornamentation are partly explained by a covariation with the conch morphology, as wider and more evolute conchs tend to be more ornate (Buckman’s law of covariation). The middle Smithian was characterized by a thermal maximum and low δ¹³C_org values which shifted towards less negative values in the late Smithian and culminated in a positive carbon isotope excursion at the Smithian–Spathian boundary. The most marked shift in the conch morphology and allometric development of A. blomstrandi coincides with the onset of the positive carbon isotope excursion at the end of the middle Smithian, but predates the mid late Smithian cooling of the sea surface.

Arctoceras blomstrandi specimens were measured and described following Klug et al. (2015). For each specimen, measurements were taken for the largest preserved diameter d and corresponding whorl width ww, whorl height wh and umbilical width uw. Where the conch was not complete on both sides, the half width was measured on the intact side and the full whorl width inferred. All measurements are available in the online material. In some damaged specimens the best measurements were obtained not on the outermost whorl, but on a younger part of the conch (measurements noted with a ‘*’ in the online dataset). Specimens with diameters below ~10 mm were not included in analyses because of the low measuring accuracy. Qualitative characterization of shell ornamentation (i.e. ribs) is also considered for each specimen (weak/strong/absent). When a specimen was too damaged, the ornamentation was characterised as ‘unknown’.

The material is curated at the Paläontologisches Institut und Museum, Universität Zürich, Switzerland, and figured specimens have received catalogue numbers of this institution.

KLUG, C., KORN, D., LANDMAN, N. H., TANABE, K., DE BAETS, K. and NAGLIK, C. 2015. Chapter 1: Describing ammonoid conchs. 3–24. In KLUG, C., KORN, D., DE BAETS, K., KRUTA, I. and MAPES, R. H. (EDS). Ammonoid paleobiology: From anatomy to ecology. Topics in geobiology, 43, 934 pp. https://doi.org/10.1007/978-94-017-9630-9

Specimens marked in red in the data set are not used in analyses for the manuscript 'Hansen et al.: The middle Smithian (Early Triassic) ammonoid Arctoceras blomstrandi: Conch morphology and ornamentation in relation to stratigraphy'.