Data from: Ontogenetic trajectories of septal spacing in Early Jurassic belemnites from Germany and France, and their palaeobiological implications

Based on well-preserved belemnites, the ontogenetic trajectories of septal spacing between succeeding chambers were analysed. In the examined species (Passaloteuthis laevigata, Parapassaloteuthis zieteni and Pseudohasitites longiformis) that come from Buttenheim, Germany, and Lixhausen, France, the ontogenetic trajectories of septal spacing follow exponentially increasing trends with no decreasing phase of septal crowding during the earliest ontogenetic stage. The absence of a decreasing trend at the earliest ontogenetic stage is a unique character in contrast with those in modern cuttlefish and ancient and modern nautiloids, in which the decreasing trends are related to hatching events. These ontogenetic septal spacing trends suggest that the belemnite hatchlings had only a protoconch with no chamber. These belemnite hatchlings with no chamber and therefore small embryonic shell diameter are similar to those of ammonoids. Significant difference in a statistical test that compared the protoconch size between the two localities, might suggest that there was limited transportation at the embryonic stage, although it could also just indicate differences in regional environmental conditions, age and/or degree of time averaging which might differ between the examined taxa.

本研究基于保存完好的箭石（belemnites）标本，对依次形成的气室之间的隔壁间距的个体发育轨迹展开了分析。研究涉及的物种包括产自德国巴滕海姆（Buttenheim）与法国利克桑（Lixhausen）的Passaloteuthis laevigata、Parapassaloteuthis zieteni以及Pseudohasitites longiformis，其隔壁间距的个体发育轨迹呈指数增长模式，且在个体发育最早阶段未出现隔壁拥挤的递减阶段。与现代乌贼、古生代及现代鹦鹉螺类截然不同，本研究类群在个体发育最早阶段不存在递减趋势，而后者的此类递减趋势与孵化事件密切相关。上述隔壁间距的个体发育趋势表明，箭石幼体仅具有原壳（protoconch），未形成气室。这些未形成气室、因而胚胎壳直径较小的箭石幼体，与菊石类的幼体特征相似。对两个产地的原壳尺寸进行的统计学检验显示出显著差异，这或许意味着胚胎阶段的搬运作用较为有限；不过该结果也可能仅反映了研究类群之间存在的区域环境条件、形成时代以及/或者时间平均程度的差异。