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.

穿孔贝目（Rhynchonellida）是现生绞合腕足动物（rhynchonelliform brachiopod）各目中地层学上最古老、系统发育地位最基干的类群，但目前学界对穿孔贝类群内部的系统发育关系仍知之甚少。已命名的14个穿孔贝超科（其中4个尚存现生代表）的界定主要基于背主腕骨（dorsal cardinalia）的形态特征，尤其是腕突（crura）的形态，但这些类群的同源性与性状极性尚未得到严格的研究验证。超科级别的单系性尚不明确，而穿孔贝类群多个标志性形态特征（如腕突）的演化路径也有待厘清。本研究旨在利用壳体内外的形态性状，探究现生穿孔贝属间的系统发育关系，并将分析结果与现行分类方案进行对比，以此阐明该类群形态特征的演化历程。本研究基于19个现生属的58个可变壳体内外形态性状，完成了简约法分析（parsimony-based analysis）与贝叶斯分析（Bayesian analyses）。研究结果与现行分类方案具有较好的可解释性，结果显示半贝超科（Hemithiridoidea）、双壳贝超科（Dimerelloidea），以及部分分析中的拳螺贝超科（Pugnacoidea）均呈现单系性特征。归类于双壳贝超科、拳螺贝超科，以及部分分析中归入半贝超科的物种，各自形成衍生亚支，且这些亚支均起源于各分支基部的并系类群诺瑞贝超科（Norelloidea）。齿舌形腕突（raduliform crura）在系统发育上应为最基干的性状状态，其余5种腕突形态均由该状态演化而来。但当前用于界定穿孔贝超科属类群的形态性状，其分类诊断性并不明确，整体呈现出较高的趋同演化（homoplasy）水平，这表明学界对穿孔贝形态特征能够清晰阐明演化关系的信心，或许建立在一种错误的认知之上。已发表的分子系统发育假说与本次形态学拓扑结构存在冲突，进一步佐证了这一可能性。现生穿孔贝类群的诊断性状演化趋势，似乎在多个亚支中体现为成体形态的持续幼态化，这表明异时性（heterochrony）可能在该类群的演化过程中发挥了重要作用。