Tuzoia_leg measuremetns from The problematic Cambrian arthropod Tuzoia and the origin of mandibulates revisited

The origin of mandibulates, the hyperdiverse arthropod group that includes pancrustaceans and myriapods, dates back to the Cambrian. Bivalved arthropod groups such as hymenocarines have been argued to be early mandibulates, but many species are still poorly known, and their affinities remain uncertain. One of the most common and globally distributed Cambrian bivalved arthropods is Tuzoia. Originally described in 1912 from the Burgess Shale based on isolated carapaces, its full anatomy has remained largely unknown. Here, we describe new specimens of Tuzoia from the Canadian Burgess Shale (Wuliuan, Cambrian) showcasing exceptionally preserved soft tissues, allowing for the first comprehensive reconstruction of its anatomy, ecology and evolutionary affinities. The head bears antennae and differentiated cephalic appendages. The body is divided into a cephalothorax and a homonomous trunk bearing ca 10 pairs of legs with heptopodomerous endopods and enlarged basipods, and two pairs of caudal rami form a tail fan. These traits suggest that Tuzoia swam along the seafloor and used its spinose legs for predation or scavenging. Tuzoia is retrieved by a Bayesian phylogenetic analysis as an early mandibulate hymenocarine lineage, exemplifying the rapid diversification of this group in open marine environments during the Cambrian Explosion.

有颚类（mandibulates）是物种多样性极高的节肢动物类群，涵盖泛甲壳类（pancrustaceans）与多足类（myriapods），其起源可追溯至寒武纪时期。此前学界曾提出，膜虾类（hymenocarines）等双瓣壳节肢动物（bivalved arthropod）类群可能属于早期有颚类，但该类群多数物种仍研究不足，其演化亲缘关系尚未明确。吐卓虾（Tuzoia）是寒武纪最为常见且全球分布最广的双瓣壳节肢动物类群之一。该物种最初于1912年依据布尔吉斯页岩（Burgess Shale）中分离得到的孤立壳瓣被正式命名，但直至此前，其完整解剖结构仍几乎未被认知。本研究报道了产自加拿大寒武纪伍里安阶布尔吉斯页岩的吐卓虾新标本，其软组织保存极为精美，这使得我们首次能够全面重建其解剖结构、生态习性与演化亲缘关系。该物种头部具触角与分化的头附肢；身体分为头胸部与同律躯干，躯干约具10对步足，其具有七节分节的内肢（heptopodomerous endopods）与膨大的基肢，另有两对尾肢构成尾扇。上述特征表明，吐卓虾可沿海底巡游，并借助其具棘的步足开展捕食或腐食活动。经贝叶斯系统发育分析（Bayesian phylogenetic analysis），吐卓虾被归入早期有颚类的膜虾类支系，这一发现印证了该类群在寒武纪大爆发期间于开阔海洋环境中快速辐射演化的演化模式。