New radiodonts with gnathobase‐like structures from the Cambrian Chengjiang biota and implications for the systematics of Radiodonta

Abstract

A recent description of paired gnathobase-like structures (GLSs) in the head region of the radiodont Amplectobelua symbrachiata raised the question of whether these appendicular structures are more widely spread within Radiodonta, putative lower stem-group euarthropods. Here we describe a new genus of Radiodonta, Ramskoeldia gen. nov., that also bears GLSs. Its two new species, Ramskoeldia platyacantha sp. nov. and R. consimilis sp. nov., are distinguished based on the morphology of their frontal appendages. The presence of three pairs of GLSs associated with reduced segments posterior to the head and the detailed morphological similarities of the GLSs suggest that Ramskoeldia is closely related to Amplectobelua Hou et al., and they are classified together in the revised Family Amplectobeluidae. Other diagnostic characters of this family include the lack of a radiallyarranged oral cone, instead sharing mouthparts composed of smooth and tuberculate plates, and a frontal appendage with three podomeres in the shaft and prominent larger endites on podomeres 4 and 8. Due to its lack of GLSs and the different morphology of its mouthparts, membership of Lyrarapax Cong et al., in Amplectobeluidae cannot be confirmed. Appraisal of available evidence indicates that the morphology of the feeding structures, including frontal appendages, the mouth apparatus, and GLSs, serves as a fundamental source of characters in the classification of radiodonts.

A new radiodont (stem Euarthropoda) frontal appendage with a mosaic of characters from the Cambrian (Series 2 Stage 3) Chengjiang biota

“…) being employed to describe the proximal podomere(s) of the frontal appendage (see also Cong et al . , ) and ‘ventral endite’ used in place of ‘ventral spine’ as it describes any inward projection (including spines) that are presumably used in feeding. The ‘distal articulated region’ consists of those podomeres distal to the shaft region, characterized by well‐defined podomere boundaries and usually a consistent pattern of arrangement of the ventral endites (see also Cong et al .…”

“…The early Cambrian Chengjiang biota from Yunnan, China is well‐known for a diverse assemblage of radiodonts, with five genera and seven species documented so far. Members of both Anomalocarididae ( Anomalocaris saron Hou et al ., ) and Amplectobeluidae ( Amplectobelua symbrachiata Hou et al ., , Ramskoeldia consimilis Cong et al ., and R. platyacantha Cong et al ., ) are well represented, while several other species cannot be assigned with confidence to any existing families, such as Cucumericrus decoratus Hou et al . , Lyrarapax unguispinus Cong et al ., and L. trilobus Cong et al ., (Cong et al .…”

“…The Family Amplectobeluidae has also been given a character‐based diagnosis (Cong et al . ), its members characterized by three pairs of gnathobase‐like structures with nearly identical morphology, and frontal appendages with three shaft podomeres and an enlarged endite on podomere 4 (Cong et al . , ).…”

“…, Lyrarapax unguispinus Cong et al ., and L. trilobus Cong et al ., (Cong et al . ). Several types of sclerotized carapace from the Chengjiang biota have been described as the remains of hurdiids, some of which were named as a new genus Tauricornicaris Zeng et al ., .…”

“…However, articulated specimens of the same sclerites indicate that Tauricornicaris is not radiodont, but rather the tergites of a more crownward euarthropod (Cong et al . ). Here we describe a new taxon from the Chengjiang biota showing a unique mix of frontal appendage characters, of which the elongated endites bearing auxiliary spines confined to one side are shared with Hurdiidae.…”