Data from: Ion microprobe measured stable isotope evidence for ammonite habitat and life mode during early ontogeny

Ammonites have disparate adult morphologies indicative of diverse ecological niches, but ammonite hatchlings are small (~1 mm diameter), which raises questions about the similarity of egg incubation and hatchling life mode in ammonites. Modern Nautilus is sometimes used as a model organism for understanding ammonites, but despite their outward similarities, the groups are only distantly related. Trends in ammonite diversity and extinction vulnerability in the fossil record contrast starkly with those of nautilids, and embryonic shells from Late Cretaceous ammonites are two orders of magnitude smaller than nautilid embryonic shells. To investigate possible environmental changes experienced by ammonite hatchlings, we used secondary ion mass spectrometry (SIMS) to analyze the oxygen and carbon isotope composition of the embryonic shells and early postembryonic whorls of five juveniles of Hoploscaphites comprimus obtained from a single concretion in the Fox Hills Formation of South Dakota. Co-occurring bivalves and diagenetic calcite were also analyzed to provide a benthic baseline for comparison. The oxygen isotope ratios of embryonic shells are more like those of benthic bivalves, suggesting that ammonite eggs were laid on the bottom. Ammonite shell immediately after hatching has more negative δ18O, suggesting movement to more shallow water that is potentially warmer and/or fresher. After approximately one whorl of postembryonic growth, the values of δ18O become more positive in three of the five individuals, suggesting that these animals transitioned to a more demersal mode of life. Two other individuals transition to even lower δ18O values that could suggest movement to nearshore brackish water. These data suggest that ammonites, like many modern coleoids, may have spawned at different times of the year. Because scaphites were one of the short-term K-Pg survivors, it is possible that this characteristic allowed them to develop a broader geographic range and, consequently, a greater resistance to extinction.

菊石（Ammonites）具有迥异的成体形态，反映出其占据了多样的生态位，但菊石孵化幼体体型极小（直径约1毫米），这引发了关于菊石卵孵化过程与幼体生活模式是否相似的疑问。现代鹦鹉螺（Nautilus）常被用作研究菊石的模式生物，但尽管二者外观相似，二者的亲缘关系却极为疏远。化石记录中菊石的多样性变化与灭绝脆弱性趋势，与鹦鹉螺类（nautilids）形成鲜明反差；同时，白垩纪晚期菊石的胚胎壳尺寸比鹦鹉螺类胚胎壳小两个数量级。为探究菊石幼体可能经历的环境变化，我们采用二次离子质谱（secondary ion mass spectrometry, SIMS），对采自南达科他州福克斯山组单个结核中的5枚压扁霍普狭菊石（Hoploscaphites comprimus）幼体的胚胎壳及胚后早期旋壳进行氧、碳同位素组成分析。同时对伴生双壳类与成岩方解石开展分析，以提供用于对比的底栖基准参照。胚胎壳的氧同位素比值与底栖双壳类更为接近，表明菊石的卵产于水底。刚孵化的菊石壳体具有更负的δ¹⁸O值，暗示其迁移至更浅的水环境，这类环境可能温度更高、盐度更低。在经历约一圈胚后生长后，5个个体中的3个的δ¹⁸O值逐渐升高，表明这些菊石转向了更典型的底栖生活模式。剩余2个个体的δ¹⁸O值进一步降低，这可能意味着它们迁移至近岸半咸水环境。上述数据表明，与多数现代鞘亚纲头足类（coleoids）类似，菊石可能存在季节性产卵的差异。由于狭菊石类是白垩纪-古近纪（K-Pg）灭绝事件中的短期残存类群之一，这一特征或许使其能够拥有更广泛的地理分布范围，进而具备更强的灭绝抗性。