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.赫里昂壳类（helcionelloids）内部的系统发育关系一直难以明确。究其原因之一在于，针对该类群形态变异开展研究的定性方法难以识别清晰的演化模式。而定量研究该类生物则是识别此类模式的可行替代方案。本研究以产自寒武系第二统第3-4阶、由伦尼加尔（Runnegar）命名、本森等（Bengtson et al.）研究的麦金尼螺属（Mackinnonia）两个物种为研究对象，采用形态测量学方法探究其细微形态变异模式，以此作为上述定量研究方法的实例。具体而言，本研究结合椭圆傅里叶分析与多元分析手段，针对麦金尼螺属两个物种的胚壳形态种内与种间变异，以及成体壳的个体发育轨迹变异展开研究。本次研究的化石材料包括：采自南极洲沙克尔顿灰岩与澳大利亚埃杰克斯灰岩的两个喙状麦金尼螺（Mackinnonia rostrata，Zhou & Xiao）化石组合，以及采自格陵兰巴斯特岩组的塔科尼克麦金尼螺（Mackinnonia taconica，Landing & Bartowski）化石组合。本研究结果显示，全部三个化石组合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个体发育序列可显著区分喙状麦金尼螺与塔科尼克麦金尼螺（p<0.05）。尽管不同产地的喙状麦金尼螺化石组合具有共有的定性特征与结构，但本研究方法仍揭示了其显著的种内形态变异；同时本研究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概念的研究语境之下。