Data from: Intraspecific variability through ontogeny in early ammonoids

Mollusks in general and ammonoids in particular are known to display a sometimes profound morphological intraspecific variability of their shell. Although this phenomenon is of greatest importance, it has rarely been investigated and quantified. It is especially crucial for taxonomy and incidentally for biodiversity analyses to account for it, because otherwise, the number of described species might exceed that of actual species within any group. Early ammonoids (Early Devonian, Paleozoic) typically suffer from this bias. For instance, most specimens from the same layer and the same region (e.g., the Erbenoceras beds of the Moroccan eastern Anti-Atlas studied here) differ morphologically from each other. Depending on the importance given to certain morphological characters, therefore, one could create a new species for almost every specimen. In this study, we measured nearly 100 such specimens from a restricted stratigraphic interval and quantified their intraspecific variability. There is a variable but strong overlap of the quantified shell characters at most ontogenetic stages, and only two species are here separated rather than the four previously recognized in Morocco. When ontogenetic trajectories of the Moroccan specimens are compared with coeval faunas from other regions (assigned to other species), a strong overlap between the morphospaces occupied by these taxa becomes apparent. The justification of some of these latter species is thus questionable even if their mean values in some conch parameters differ considerably from the mean values of the Moroccan species. Hence, the number of currently valid species of these loosely coiled early ammonoids is probably much too high. Extreme caution must therefore taken when examining the diversity of groups in which the intraspecific variability is poorly known.众所周知，软体动物整体类群，尤其是菊石（ammonoids），其壳体常存在程度不一的显著形态学种内变异（intraspecific morphological variability）。尽管该现象具有极高的研究价值，但目前对其的研究与量化工作仍较为匮乏。在分类学研究中，该现象尤为关键，在生物多样性分析中亦具有重要参考意义；若未将其纳入考量，任意类群的已描述物种数量均可能远超实际物种数。古生代泥盆纪早期的原始菊石类群，便是受此偏差影响的典型代表。例如本研究针对摩洛哥东部反阿特拉斯山脉的埃尔贝诺菊石（Erbenoceras）地层展开采样，同层位、同区域产出的多数标本在形态上均存在显著差异。因此，若对特定形态特征赋予不同权重，几乎可针对每一件标本建立新物种。本研究从限定地层区间（stratigraphic interval）内采集了近100件此类标本，对其形态学种内变异进行了量化分析。在多数个体发育阶段（ontogenetic stages），量化得到的壳体特征虽存在一定波动，但重叠度极高；本研究仅识别出2个有效物种，而非此前摩洛哥地区报道的4个。将摩洛哥标本的个体发育轨迹与其他区域对应年代的化石群（此前被划归为不同物种）进行对比后可见，这些类群所占据的形态空间（morphospace）存在高度重叠。尽管部分此类域外物种的壳体参数（conch parameters）均值与摩洛哥物种存在显著差异，但其分类学合理性仍存疑。因此，当前这类松旋壳型早期菊石的有效物种数量极有可能被高估。在研究种内变异尚不明确的类群多样性时，需保持极高的审慎态度。