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 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.

Methods
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

菊石类（ammonoid）北极菊石属（Arctoceras，Hyatt, 1900）的分布范围涵盖所有古纬度，是全球尺度下早三叠世史密斯阶中期生物地层对比的关键属级单元。本研究针对壳体形态（conch morphology）、壳饰（ornamentation）及异速生长（allometry）的种内变异（intraspecific variation），探讨其与地层位置（stratigraphic position）的关联。北极菊石属是挪威北极圈斯瓦尔巴群岛（Svalbard）史密斯阶中期最优势的菊石类属。最初曾在斯瓦尔巴群岛记述过7个北极菊石属物种，但这些物种在化石鉴定中的实际应用价值存疑。后续随着种内变异的重要性被学界认识，斯瓦尔巴群岛的6个北极菊石属物种被归为北极菊石（Arctoceras blomstrandi，Lindström, 1865）的晚出异名（junior synonyms）。然而，目前对A. blomstrandi壳体形态的变异仍缺乏定量研究，其与地层位置的相关性亦尚不明确。本研究定量分析了斯瓦尔巴群岛北极菊石的壳体形态、壳饰及异速生长的种内变异，并探讨其与地层的关系。研究结果支持将斯瓦尔巴群岛所有北极菊石形态型（morphotypes）归为单一物种A. blomstrandi。本研究获得的新数据可为该物种的重新描述提供依据，并为端元形态（endmember morphology）类型A. blomstrandi var costatus应用于生物地层学研究奠定基础。本研究记录了所研究地层段内壳体形态与壳饰的一致性变化：地层顶部的壳体逐渐向更外卷（evolute）且壳饰更丰富的方向转变。壳饰强度的变化趋势部分可通过与壳体形态的协变（covariation）关系解释：更宽且更外卷的壳体往往壳饰更为丰富（巴克曼协变定律（Buckman’s law of covariation））。史密斯阶中期以热盛期（thermal maximum）和低有机碳同位素δ¹³C_org（δ13Corg）值为特征，至史密斯阶晚期该同位素值向偏负程度减弱的方向变化，并在史密斯阶-斯帕斯阶界线（Smithian–Spathian boundary）处达到正碳同位素偏移（carbon isotope excursion）的峰值。A. blomstrandi的壳体形态与异速生长的最显著转变，恰好与史密斯阶中期末期正碳同位素偏移的起始时间重合，但早于史密斯阶晚期中期的海面冷却事件。

## 研究方法
本研究按照Klug等人（2015）的方法对A. blomstrandi标本进行测量与描述。针对每个标本，测量其保存最大直径d、对应旋环宽度（whorl width）ww、旋环高度（whorl height）wh及脐宽（umbilical width）uw。若壳体两侧保存不全，则在完整一侧测量半旋环宽度，并据此推算完整旋环宽度。所有测量数据均已上传至在线补充材料（online material）。部分受损标本的最优测量数据并非取自最外侧旋环，而是壳体的较年轻部位（在线数据集中标注‘*’的测量值）。直径小于约10 mm的标本因测量精度较低，未纳入分析。此外，本研究还对每个标本的壳饰（即肋纹（ribs））进行定性表征（弱/强/缺失）。若标本受损过于严重，则将其壳饰表征为“未知”。

本研究的标本馆藏于瑞士苏黎世大学古生物研究所与博物馆，已绘图的标本均获得该机构的馆藏编号。

## 参考文献
KLUG, C., KORN, D., LANDMAN, N. H., TANABE, K., DE BAETS, K. 及 NAGLIK, C. 2015. 第1章：菊石壳体描述。3–24页。收录于：KLUG, C., KORN, D., DE BAETS, K., KRUTA, I. 及 MAPES, R. H. (主编). 菊石古生物学：从解剖学到生态学. 地质生物学专题, 第43卷, 共934页. https://doi.org/10.1007/978-94-017-9630-9