Data from: Millipede taxonomy after 250 years: classification and taxonomic practices in a mega-diverse yet understudied arthropod group

BACKGROUND: The arthropod class Diplopoda is a mega-diverse group comprising >12,000 described millipede species. The history of taxonomic research within the group is tumultuous and, consequently, has yielded a questionable higher-level classification. Few higher-taxa are defined using synapomorphies, and the practice of single taxon descriptions lacking a revisionary framework has produced many monotypic taxa. Additionally, taxonomic and geographic biases render global species diversity estimations unreliable. We test whether the ordinal taxa of the Diplopoda are consistent with regards to underlying taxonomic diversity, attempt to provide estimates for global species diversity, and examine millipede taxonomic effort at a global geographic scale. METHODOLOGY/PRINCIPAL FINDINGS: A taxonomic distinctness metric was employed to assess uniformity of millipede ordinal taxa. We found that ordinal-level taxa are not uniform and are likely overinflated with higher-taxa when compared to related groups. Several methods of estimating global species richness were employed (Bayesian, variation in taxonomic productivity, extrapolation from nearly fully described taxa). Two of the three methods provided estimates ranging from 13,413–16,760 species. Variations in geographic diversity show biases to North America and Europe and a paucity of works on tropical taxa. CONCLUSIONS/SIGNIFICANCE: Before taxa can be used in an extensible way, they must be definable with respect to the diversity they contain and the diagnostic characters used to delineate them. The higher classification for millipedes is shown to be problematic from a number of perspectives. Namely, the ordinal taxa are not uniform in their underlying diversity, and millipedes appear to have a disproportionate number of higher-taxa. Species diversity estimates are unreliable due to inconsistent taxonomic effort at temporal, geographic, and phylogenetic scales. Lack of knowledge concerning many millipede groups compounds these issues. Diplopods are likely not unique in this regard as these issues may persist in many other diverse yet poorly studied groups.研究背景：节肢动物门倍足纲（Diplopoda）是一个高度多样的类群，已描述的马陆（millipede）物种超过12000种。该类群的分类学研究历史颇为曲折，因此催生了存疑的高阶分类系统。鲜有高阶类群以共有衍征（synapomorphies）进行定义，仅开展单类群描述却缺乏修订性框架的研究范式催生了大量单型类群。此外，分类学与地理层面的采样偏差导致全球物种多样性估算结果可信度不足。本研究旨在验证倍足纲的目级类群在内在分类多样性方面是否一致，尝试估算全球物种多样性，并从全球地理尺度审视马陆类群的分类学研究投入。 研究方法与主要发现：本研究采用分类学区分度指标（taxonomic distinctness metric）评估马陆目级类群的均一性。结果显示，目级类群并不均一，相较于其他近缘类群，其高阶类群数量大概率存在高估。本研究采用多种方法估算全球物种丰富度，包括贝叶斯法（Bayesian）、分类学产出变异法以及基于近完全描述类群的外推法。三种方法中的两种得出的估算结果范围为13413至16760种。地理多样性分布分析显示，研究偏向北美与欧洲，针对热带类群的研究极为匮乏。 结论与意义：若要以可扩展的方式应用分类类群，需先明确其所包含的多样性以及用于界定它们的鉴别特征。多项证据表明，马陆类群的高阶分类系统存在诸多问题。具体而言，目级类群的内在多样性并不均一，且马陆类群的高阶类群数量比例失调。由于分类学研究投入在时间、地理以及系统发育尺度上均存在不一致性，物种多样性估算结果可信度不足。对众多马陆类群的认知匮乏进一步加剧了这些问题。倍足纲类群在这方面并非特例，此类问题可能普遍存在于其他诸多多样性丰富但研究不足的类群中。