Data from: Morphological analysis of phylogenetic relationships among extant rhynchonellide brachiopods

Rhynchonellida is the stratigraphically oldest and phylogenetically most basal of the extant rhynchonelliform brachiopod orders, yet phylogenetic relationships among rhynchonellides are poorly known. The fourteen named rhynchonellide superfamilies (four of which have extant representatives) were defined primarily on the basis of features of the dorsal cardinalia, particularly crural morphology, but their homology and polarity have not been investigated rigorously. Superfamily monophyly is unclear, as is the evolution of several distinctive rhynchonellide morphological features, such as crura. The purpose of this study is to investigate the phylogenetic relationships among extant rhynchonellide genera using skeletal characters, and to compare the results with the current classification, elucidating the evolution of morphological features in the process. We completed parsimony-based and Bayesian analyses using fifty-eight characters of the interior and exterior of the shell that vary among the nineteen extant genera. Our results are readily interpretable with respect to the classification, and indicate that Hemithiridoidea, Dimerelloidea, and (in some analyses) Pugnacoidea appear to be monophyletic. Species classified in Dimerelloidea and Pugnacoidea, and in certain cases Hemithiridoidea, each form derived subclades that evolve from within a paraphyletic Norelloidea at the base of each subclade. Raduliform crura appear to be the most basal, phylogenetically; five other crural morphologies evolve from the raduliform state. However, morphological characters currently uniting genera in rhynchonellide superfamilies are not clearly diagnostic and exhibit a relatively high degree of homoplasy overall, suggesting that consistency with the classification may be based on a false sense of confidence in rhynchonellide morphology to clearly elucidate evolutionary relationships. Published molecular phylogenetic hypotheses conflict with the morphological topologies, further supporting this possibility. The evolutionary trends among diagnostic characters of Recent rhynchonellides appear to reflect successive juvenilization in adult morphology in several subclades, suggesting that heterochrony may have played an important role in the evolution of the group.

小嘴贝目（Rhynchonellida）是现生铰合腕足类（rhynchonelliform brachiopod）各目中地层年代最古老、系统发育位置最基部的类群，但目前学界对小嘴贝类群间的系统发育关系仍知之甚少。已命名的14个小嘴贝总科（其中4个尚有现生代表类群）的界定主要基于背主突起（dorsal cardinalia）的形态特征，尤其是腕茎（crura）的形态，但学界尚未对这些类群的同源性与极性开展严谨的研究。总科的单系性尚不明确，而小嘴贝类群若干标志性形态特征（如腕茎）的演化过程同样模糊不清。本研究旨在利用骨骼性状分析现生小嘴贝属间的系统发育关系，并将分析结果与现行分类体系进行比对，以阐明该类群形态特征的演化历程。本研究选取19个现生小嘴贝属的58个壳内外可变性状，完成了基于简约法与贝叶斯法的系统发育分析。本研究结果与现行分类体系具有较好的适配性，分析结果显示半贝总科（Hemithiridoidea）、双壳贝总科（Dimerelloidea）以及部分分析中得到的拳贝总科（Pugnacoidea）均呈现单系性。归属于双壳贝总科、拳贝总科，以及部分情况下归属于半贝总科的物种，各自形成衍生的亚支，这些亚支均起源于每个亚支基部的并系诺雷贝总科（Norelloidea）类群。系统发育上，齿舌型腕茎为最基部的性状，其余5种腕茎形态均由该性状演化而来。然而，当前用于界定小嘴贝总科内各属的形态性状并不具备明确的诊断性，整体呈现出较高的同塑现象。这表明，学界对基于小嘴贝形态特征明确阐明演化关系的信心，或许是一种误判，而本研究结果与现行分类体系的一致性可能正是源于此。已发表的分子系统发育假说与本研究得到的形态拓扑结构存在冲突，进一步佐证了上述可能性。现生小嘴贝类群的诊断性状演化趋势，在若干亚支中呈现出成体形态持续幼态化的特征，这表明异时性演化或许在该类群的演化过程中发挥了重要作用。