Reassessment of the Order Trinucleida (Trilobita)

Trinucleoids have been considered as a superfamily of the Order Asaphida, based on: a pre-occipital glabellar tubercle, a cephalic median suture in most ancestral forms, and the presence of broadly similar globular protaspid larvae in the life cycle. Recent discoveries cast doubts on the monophyly of the two last characters. This study provides a phylogenetic analysis whose main purpose is to test the affiliation of trinucleoids. The topology of the strict consensus tree obtained indicates that trinucleoids share a more recent common ancestor with members of the Order Ptychopariida than they do with any asaphid. The stratigraphically higher and most derived trinucleoids are well defined and easy to recognize. Their basal representatives can also be identified with confidence, in spite of substantial variation in the diagnostic characters of the group. These early trinucleoids are similar enough to the typical basal ‘ptychopariid’ bauplan to suggest ancestry within that diffuse and poorly defined trilobite order. Indeed, Ptychopariida is presently regarded as being paraphyletic. We consider that raising trinucleids to ordinal status, by proposing the Order Trinucleida, will make it easier to decipher higher level relationships among trilobites. This action, sustained by improved morphological and ontogenetic information of Cambro-Ordovician trilobites, reveals convergences of the three characters that have been used to support the inclusion of trinucleids within Asaphida. Based on this new information, we propose six distinct morphotypes of commutavi protaspis and, as a probable sensorial organ, that the cephalic axial tubercle migration might be caused by an increase in the height of the glabella and thus may not bear an important systematic signal. Moreover, the tree topology provided herein offers a preliminary overview of evolution within the order. Alsataspididae and Raphiophoridae are paraphyletic, suggesting that more investigation is needed to define them clearly. Dionidae appears to be more closely related to Raphiophoridae than to Trinucleidae, suggesting that the character state of ‘a perforated fringe’ evolved more than once.

三瘤虫类（Trinucleoids）曾被归为等称虫目（Asaphida）下的一个超科，其分类依据包括：枕前鞍部结节（pre-occipital glabellar tubercle）、多数祖先类群具有头部中缝线，以及生命周期中存在形态高度相似的球状原幼虫（globular protaspid larvae）。近期的发现对上述最后两个分类特征的单系性（monophyly）提出了质疑。本研究开展系统发育分析（phylogenetic analysis），核心目的为验证三瘤虫类的分类归属。所得严格合意树（strict consensus tree）的拓扑结构显示，三瘤虫类与褶颊虫目（Ptychopariida）类群的最近共同祖先，要近于其与任何等称虫类的共同祖先。地层层位更高、特化程度最深的三瘤虫类类群界定清晰，易于识别。尽管该类群的鉴别特征存在显著变异，其基干代表类群仍可被可靠鉴定。这些早期三瘤虫类与典型基干‘褶颊虫类’的身体构型（bauplan）高度相似，暗示其隶属于这个弥散且界定模糊的三叶虫（trilobite）目。事实上，褶颊虫目目前被认为是并系群。我们认为，将三瘤虫科提升至目级分类地位、建立三瘤虫目（Trinucleida），将更便于厘清三叶虫类的高阶类群亲缘关系。该修订基于寒武纪-奥陶纪（Cambro-Ordovician）三叶虫更完善的形态学与个体发育学信息，其结果揭示了曾被用于将三瘤虫科归入等称虫目的三个特征存在趋同演化现象。基于这一新证据，我们提出了六种独特的commutavi原幼虫形态型；同时，头部轴部结节（cephalic axial tubercle）或可作为潜在感觉器官，其迁移可能由鞍形头甲（glabella）高度增加所致，因此该特征并不具备重要的分类学信号（systematic signal）。此外，本文提供的树拓扑结构初步展现了该目内的演化脉络。阿尔塔萨皮达科（Alsataspididae）与光尾虫科（Raphiophoridae）均为并系群，提示需开展更多研究以明确其分类界定。狄翁虫科（Dionidae）与光尾虫科的亲缘关系较三瘤虫科更近，这表明‘具穿孔边缘’的特征状态曾多次独立演化。