Open Peer Review on Qeios Cambrian Chordates and Early Fin Evolution Mark McMenamin 1 1 Mount Holyoke College Funding: No specific funding was received for this work. Potential competing interests: No potential competing interests to declare. Abstract An enigmatic bilaterian from the Chengjiang biota, Shenzianyuloma yunnanense, is assigned to an enigmatic Cambrian group, the vetulicolians. In Shenzianyuloma, a clear notochord can be observed, as can cone-in-cone myomeres. Unlike other vetulicolians and Yunnanozoon, Shenzianyuloma lacks arthropod-like segmentation in the posterior part of its body. Removal of matrix from the Shenzianyuloma holotype has revealed a broader, more laterally flattened tail. Also present in the freshly prepared area is evidence that Shenzianyuloma possessed a possible ventral fin ray box, comparable to that of Amphioxus and Branchiostoma, implying that Shenzianyuloma had an even greater top-to-bottom tail width than hitherto known. A new tail reconstruction for Shenzianyuloma shows greater lateral area than in previous reconstructions, indicating a more efficient posterior organ of propulsion. A possible dorsal fin spine can also now be recognized in Shenzianyuloma. The revised reconstruction of Shenzianyuloma provides insight into early chordate fin evolution, and is sufficiently unique to require the erection of ••• n. fam. within Class Vetulicolida. Early bilaterians such as yunnanozoans and Shenzianyuloma are crucial for assessing the morphological innovations, and combination of traits, associated with the appearance of vertebrates. Mark A. S. McMenamin 1* 1 Department of Geology and Geography, Mount Holyoke College, South Hadley, Massachusetts, USA. Email address: mmcmenam@mtholyoke.edu * Corresponding author Keywords: early chordates, fin evolution, vetulicolians, Vetulicolida, Shenzianyuloma, ••• n. fam., Nesonektris, notochord, ventral fin ray box. Introduction It has been recently remarked that vertebrate evolution is “a puzzle that researchers will continue to explore, being driven by a curiosity to understand our own origins” (Hockman, 2022). The puzzle has a long history, dating at least as far back Qeios, CC-BY 4.0 · Article, November 10, 2022 Qeios ID: GX4RC3 · https://doi.org/10.32388/GX4RC3 1/7