DOI: 10.5061/dryad.vq83bk3qw
title: Data from: The middle Smithian (Early Triassic) ammonoid Arctoceras blomstrandi: Conch morphology and ornamentation in relation to stratigraphy

description: 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 δ13Corg
 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.
description: <i>Arctoceras blomstrandi </i>specimens were measured
 and described following Klug <i>et al.</i> (2015). For each
 specimen, measurements were taken for the largest preserved diameter
 <i>d</i> and corresponding whorl width <i>ww</i>,
 whorl height <i>wh</i> and umbilical width
 <i>uw</i>. 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. <i>In</i> KLUG, C., KORN,
 D., DE BAETS, K., KRUTA, I. and MAPES, R. H. (EDS). <i>Ammonoid
 paleobiology: From anatomy to ecology</i>. Topics in geobiology, 43,
 934 pp. https://doi.org/10.1007/978-94-017-9630-9
description: 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'.