Data from: Morphometric analysis of inter- and intraspecific variation in the Cambrian helcionelloid mollusc Mackinnonia

Phylogenetic relationships within the helcionelloids have been difficult to establish. One of the reasons for this is that qualitative approaches to investigating morphological variation in this group have struggled to identify clear patterns. An alternative method of identifying these patterns is to study these organisms quantitatively. Here this approach is exemplified by employing morphometric methods to investigate patterns of subtle morphological variation in two species of Mackinnonia Runnegar in Bengtson et al from Cambrian Series 2, Stages 3–4. Specifically, a combination of elliptical Fourier and multivariate analyses were conducted to study intra- and interspecific variation in protoconch form as well as variation in ontogenetic trajectory of the teleoconch of two species of Mackinnonia. The material used consists of two assemblages of Mackinnonia rostrata (Zhou & Xiao), from the Shackleton Limestone of Antarctica and Ajax Limestone of Australia, and an assemblage of Mackinnonia taconica (Landing & Bartowski) from the Bastion Formation of Greenland. Results of this study show significant (p<0.0001) differences in protoconch shape between all three groups. Ontogenetic sequences of outline curves truncated at successive rugae significantly (p<0.05) discriminate between M. rostrata and M. taconica. These techniques uncovered significant intraspecific morphological variation of disparate assemblages of M. rostrata despite shared qualitative features and structure a conceptual framework for understanding such patterns of variation and put this in the context of the incipient species concept.

海伦壳类（helcionelloids）内部的系统发育关系长期难以确立。究其根源，该类群形态变异研究的定性方法始终难以识别清晰的演化模式。而采用定量研究路径则是破解此类难题的可行方案。本研究以产自寒武系第二统第3-4阶的Mackinnonia Runnegar in Bengtson et al的两个物种为研究对象，通过形态测量学方法探究其细微形态变异模式，以此作为定量研究的应用范例。具体而言，本研究结合椭圆傅里叶分析（elliptical Fourier analysis）与多变量统计分析方法，对两个Mackinnonia物种的胚壳（protoconch）形态的种内与种间变异，以及成体壳（teleoconch）的个体发育轨迹变异展开研究。本次研究的标本包含两件产自南极沙克尔顿灰岩（Shackleton Limestone）与澳大利亚阿贾克斯灰岩（Ajax Limestone）的Mackinnonia rostrata（Zhou & Xiao）组合，以及一件产自格陵兰巴斯蒂安组（Bastion Formation）的Mackinnonia taconica（Landing & Bartowski）组合。研究结果显示，全部三个类群的胚壳形态均存在极显著差异（p<0.0001）。以连续生长纹截断的轮廓曲线所构建的个体发育序列，可显著区分M. rostrata与M. taconica（p<0.05）。本研究方法不仅揭示了尽管M. rostrata的不同类群具有共有的定性特征，但仍存在显著的种内形态变异；同时构建了理解此类变异模式的概念框架，并将其置于初期物种（incipient species）的概念语境下进行探讨。