139-8 Cambrian Euarthropod Urokodia aequalis Sheds Light on The Origin of Artiopoda Body Plan

Session: Evolution of Life in the Cambrian Seas: Biotic, Biogeochemical, and Sedimentological Contexts, Part II

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Artiopoda is a group of epibenthic euarthropods from Cambrian Lagerstätten, renowned for its distinctive broad dorsal exoskeleton. However, the origin and evolution of this unique body plan remain unclear. Here we re‐examine Urokodia aequalis, one of the minimally investigated euarthropods from the Chengjiang biota. Optical and Micro-CT analyses of both old and new specimens reveal new morphological details of this taxon and revise its taxonomy. Urokodia possesses an elongate body comprising a five-segmented head, a thorax with 13 to 15 tergites, and a three-segmented pygidium with a well‐defined axial region. One pair of ventrally positioned stalked eyes extends beyond the margin of the head shield anterior-laterally. The short anteriormost appendage is uniramous, fleshy, non‐sclerotized, and unsegmented. The following head and thoracic appendages, one pair per segment, are homogeneous and each composed of a long multi‐annular proximal element articulated with a stenopodous branch and a lamellar flap, while the three pairs of pygidial appendages solely consist of lamellar flaps. Cladistic analyses resolved Urokodia as the basal‐most member of the Artiopoda, providing a unique body plan of this clade, an elongate vaulted head-thorax region without lateral extensions followed by a flat pygidial shield with a well define axial region, and offering a hypothesis of the initial origin of trilobation in the pygidium. The new data, in conjunction with the presence of the elongated body plan across major lineages of euarthropods, suggest a convergent evolution of elongated body plan.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7057

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