The last common ancestor of all living arthropods had biramous postantennal appendages, with an endopodite and exopodite branching off the limb base. Morphological evidence for homology of these rami between crustaceans and chelicerates has, however, been challenged by data from clonal composition and from knockout of leg patterning genes. Cambrian arthropod fossils have been cited as providing support for competing hypotheses about biramy but have shed little light on additional lateral outgrowths, known as exites. Here we draw on microtomographic imaging of the Cambrian great-appendage arthropod Leanchoilia to reveal a previously undetected exite at the base of most appendages, composed of overlapping lamellae. A morphologically similar, and we infer homologous, exite is documented in the same position in members of the trilobite-allied Artiopoda. This early Cambrian exite morphology supplements an emerging picture from gene expression that exites may have a deeper origin in arthropod phylogeny than has been appreciated.

所有现存节肢动物的最后一个共同祖先都有双侧触角后附属物，有一个内足和外足从四肢基部分出。然而，甲壳类动物和螯合物之间这些枝条同源性的形态学证据受到来自克隆组成和敲除腿部图案基因的数据的挑战。寒武纪节肢动物化石被认为为关于 biramy 的竞争假设提供了支持，但几乎没有阐明额外的横向生长，即出口。在这里，我们利用寒武纪大附肢节肢动物Leanchoilia 的显微断层成像在大多数附属物的底部揭示一个以前未被发现的出口，由重叠的薄片组成。在与三叶虫相关的节肢动物的成员中，在同一位置记录了一种形态相似且我们推断同源的 exite。这种早期寒武纪出口形态补充了基因表达的新图景，即出口在节肢动物系统发育中的起源可能比人们所了解的更深。