Morphology of diverse radiodontan head sclerites from the early Cambrian Chengjiang Lagerstätte, south-west China

Recorded in exceptionally preserved Lagerstätten, the radiodontans (including anomalocaridids) are cosmopolitan, soft-bodied, stem-group euarthropods that inhabited Palaeozoic seas. They are notable for their unique morphology, peculiar ecology and basal position in euarthropod evolution. The early Cambrian Chengjiang Lagerstätte has yielded some of the oldest radiodontans, including three unquestionable taxa – Anomalocaris saron Hou et al., 1995, Amplectobelua symbrachiata Hou et al., 1995 and Lyrarapax unguispinus Cong et al., 2014 – based on articulated specimens. However, recent discoveries of the Burgess Shale Hurdia Walcott, 1912 and the Fezouata Aegirocassis benmoulae Van Roy et al., 2015 highlight the presence of another radiodontan group (Hurdiidae) characterized by a tripartite head sclerite complex consisting of one central element and a pair of lateral elements. Here, we describe six morphotypes of possible hurdiid head sclerite elements from the Chengjiang Lagerstätte, some of which are closely associated with other typical radiodontan body parts. These, recognized as central elements, include two new species of the same new genus, Tauricornicaris latizonae gen. et sp. nov. and T. oxygonae gen. et sp. nov., and a former bivalved euarthropod, Zhenghecaris shankouensis Vannier et al., 2006. Another three unnamed sclerite elements are identified as lateral elements. These sclerite elements from Chengjiang provide detailed anatomy, such as mammillary tubercles and soft-tissue reticulate structure, to help understand the morphology and biology of all hurdiid head sclerites. A common single dorsal plate in Anomalocaris saron and Amplectobelua symbrachiata is confirmed. Although the morphologically diverse central and lateral elements of the Hurdiidae and the conserved dorsal plate of the Anomalocarididae + Amplectobeluidae reflect a major distinction in radiodontan body plans, they share a fundamental structure of double layers of cuticle. The Chengjiang specimens not only clarify the morphology and biology of the radiodontan head sclerites in general, but also illuminate the diversity and disparity of radiodontans in their early evolutionary history.http://zoobank.org/urn:lsid:zoobank.org:pub:D0590390-A85A-493A-8529-B2DF64D91169