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.已知软体动物（Mollusks），尤其是菊石（ammonoids），其壳体常存在显著的形态学种内变异。尽管该现象具有重要学术意义，但相关量化与探究工作却鲜有开展。该现象对分类学及生物多样性分析均至关重要，若未予以考量，任一类群的已描述物种数量均可能远超实际物种总数。早泥盆世（古生代）的早期菊石类普遍受此偏差影响。例如本次研究涉及的摩洛哥东部反阿特拉斯山脉埃尔贝诺菊石（Erbenoceras）层位标本，多数同层同区域的个体间均存在明显形态差异。若过度侧重特定形态特征，几乎可针对每件标本单独建立新物种。 本研究对限定地层间隔（stratigraphic interval）内的近百件此类标本开展测量，并量化了它们的种内变异水平。在多数发育阶段（ontogenetic stages），量化得到的壳体特征存在可变但显著的重叠，本次研究仅区分出两个物种，而非此前在摩洛哥认定的四个。将摩洛哥标本的发育轨迹（ontogenetic trajectories）与其他区域的同期动物群（此前被归为不同物种）进行对比后可见，这些类群所占据的形态空间（morphospaces）存在高度重叠。即便部分壳体参数（conch parameters）的平均值与摩洛哥物种存在显著差异，这些后续认定物种的有效性仍存疑。因此，目前这类松旋早期菊石的有效物种数量可能远高于实际值。在种内变异研究尚不充分的类群开展多样性调查时，必须格外审慎。