Trilobite development has been intensively explored during the past decades, but information on ancestral character combinations in the early developmental stages of trilobites remains unknown. Trilobites of the superfamily Olenelloidea are one of the earliest diverging groups. Study of their development coupled with the development of other early diverging trilobite groups can provide information on the ancestral morphology of trilobite early stages. Herein we describe numerous well‐preserved specimens of the olenelloid trilobite Fritzolenellus lapworthi. The earliest stages have circular cephala bearing intergenal spines and lacking genal spines. During subsequent development, morphological changes involve the modification of the cephalic shape from circular to semicircular, expansion of the frontal glabellar lobe, gradual shortening of intergenal spines and origin and prolongation of genal spines. Trunk development of Fritzolenellus suggests that macropleurae and macrospine development are two independent processes and that the origin of the opisthotrunk is linked with the onset of phase 5 of cephalic development. The morphology of the early developmental stages of Fritzolenellus and of some related taxa differs in many aspects from the morphology of equivalent stages of some other members of Olenelloidea. Consequently, two basic morphotypes are recognized during the early development of Olenelloidea: the Fritzolenellus and the Olenellus morphotypes. Comparison with Fallotaspidoidea and Redlichiina indicates that early developmental stages of these taxa share character combinations that are typical for the Fritzolenellus morphotype. Such a comparison suggests that characters defining the Fritzolenellus morphotype are ancestral for Trilobita. The Olenellus morphotype is probably a derived condition within Olenelloidea and might be related to predator deterrence.

中文翻译：

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贝母的胚后发育表明三叶虫早期发育阶段的祖先形态

在过去的几十年中，人们对三叶虫的发育进行了深入的研究，但是关于三叶虫早期发育阶段的祖先特征组合的信息仍然未知。超家族Olenelloidea的三叶虫是最早的分化群体之一。研究它们的发育以及其他早期发散的三叶虫群的发育可以提供有关三叶虫早期祖先形态的信息。在这里，我们描述了许多保存完好的标本的三叶虫类Fritzolenellus lapworthi标本。最早的阶段有圆形的头颈，内生棘突，而缺乏生棘突。在随后的发育过程中，形态学改变包括头状形状从圆形变为半圆形，额叶前额叶扩张，内生棘突和起源的逐渐缩短以及生棘突的延长。Fritzolenellus的躯干发育表明大胸膜和大脊柱发育是两个独立的过程，而opisthotrunk的起源与头部发育的第5期开始有关。贝母发育早期的形态某些相关分类单元的分类与O形纲其他成员的等效阶段的形态在许多方面有所不同。因此，在夹竹桃的早期发育过程中，人们认识到两种基本的形态型：Fritzolenellus和Olenellus形态型。与Fallotaspidoidea和Redlichiina的比较表明，这些类群的早期发育阶段具有Fritzolenellus形态型典型的特征组合。这样的比较表明，定义Fritzolenellus形态型的字符是Trilobita的祖先。该Olenellus形态类型可能是内Olenelloidea派生的条件和可能与捕食者威慑。