Cambrian Chordates and Early Fin Evolution

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.