Anomalocaridids and the origin of arthropods: the view from Chengjiang

The classification of Radiodonta is primarily based on the morphology of their frontal appendages, a main feeding structure of this iconic group of mostly Cambrian stem‐group euarthropods. However, recent progress in the description and revision of radiodont taxa, particularly drawing on their frontal appendages, has exposed morphological variation that challenges reliable identification of higher‐level groupings. Here we describe a new taxon of Radiodonta, Laminacaris chimera gen. et sp. nov., from the Cambrian Series 2, Stage 3, Chengjiang biota of China, based on its unique frontal appendage morphology. Laminacaris is distinctive for its combination of characters shared by hurdiids and other early Cambrian radiodont families. Elongated, possibly unpaired endites on two proximal podomeres that bear small distally‐directed auxiliary spines oriented perpendicular to the long axis of the endite, are comparable with the elongated endites and their auxiliary spines of all known Cambrian members of Hurdiidae. In contrast, endites on more distal podomeres are similar to some species of Anomalocaris, and the dorsal spines at the distal end resemble those of Amplectobelua. The mosaic characters in the frontal appendage of Laminacaris chimera may capture morphology close to the divergence between the major radiodont groups.

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 radially‐arranged 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.

The frontal appendage is the main feeding structure and the most well-known part of radiodontans, and thus has been used as the main source of characters in the classification of these putative stem-group euarthropods. The endites (or ventral/inner spines), normally considered as paired on each podomere, are of particular value in taxonomy and autecology. Here we show that, in all known taxa of Amplecotobeluidae, the paired endites on each podomere are conspicuously different in size, contradicting the traditional idea that they are always of the same size. Such asymmetry between the paired endites suggests that the frontal appendage might have had the ability to rotate to some degree. We further argue that the paired endites were particularly differentiated in most taxa of Hurdiidae, forming one row of comb-like endites and one row of ‘lateral spines’. This is in contrast to Anomalocarididae, where paired endites were ventral, symmetrical, and equal in shape and size. This differentiation in the size of paired endites in different clades indicates divergence of feeding strategy in early radiodontans, which was accompanied by the differentiation of other feeding structures (such as mouthparts and gnathobase-like structures). Such discoveries offer new characters for discerning the main clades of radiodontans.

Background: Segmental composition and homologies of the head of stem-group Euarthropoda have been the foci of recent studies on arthropod origins. An emerging hypothesis suggests that upper-stem group euarthropods possessed a three-segmented head/brain, including an ocular segment (protocerebrum) followed by the deutocerebrum with associated antennae/raptorial limbs and the tritocerebrum, while in the lower stem, head structures of Radiodonta are wholly associated with the protocerebrum and its preceding part. However, this hypothesis is incompletely tested because detailed knowledge on the head components of radiodontans is patchy, and informative articulated specimens are lacking for many taxa. Amplectobelua symbrachiata is the most common radiodontan species in the Chengjiang biota (ca. 520 Ma), normally known as isolated frontal appendages. Here we present detailed descriptions of new articulated specimens that elucidate the morphology and function of its head structures, and discuss their implications for hypotheses about euarthropod cephalic organisation. Results: In addition to a central oval head shield, A. symbrachiata also bears a pair of P-elements connected by an elongated rod. The mouth consists of sets of smooth and tuberculate plates, in contrast to the typical radial oral cones of other radiodontans. Previously identified ‘palm-like teeth’ are located external to the mouth in the posterior head region, and are interpreted as segmental gnathobase-like structures (GLSs) associated with at least three reduced transitional flaps in a one (pair)-to-one (pair) pattern, consistent with an appendicular nature. Comparisons with other panarthropods show that GLSs are morphologically similar to the mandibles and other gnathobasic mouthparts of euarthropods, as well as to the jaws of onychophorans, indicating their functional integration into the feeding activities of A. symbrachiata. Conclusions: The functional head of A. symbrachiata must include the reduced transitional segments (and their associated structures), which have been identified in several other radiodontans. This functional view supports the idea that the integration of segments (and associated appendages) into the head region, probably driven by feeding, occurred along the euarthropod stem-lineage. However, the number of reduced transitional segments varies between different groups and it remains uncertain whether GLSs represent proximal or distal parts of appendages. Our study is the first description of appendicular structures other than the frontal appendages in the functional head region of radiodontans, revealing novel feeding structures in the morphological transition from the lower- to the upper- stem-group of Euarthropoda.

The recently described radiodontan Lyrarapax unguispinus Cong et al., 2014 from the Chengjiang biota (Cambrian Series 2, Stage 3) highlighted a new morphological type of frontal appendage and unique mouth structures, a functional combination reinforcing the diversification of feeding strategies of radiodontans during the early Cambrian. Here we describe Lyrarapax trilobus n. sp. from the same fossil Konservat-Lagerstätte. The new species differs from L. unguispinus in the morphology and distribution of endites on the frontal appendage and the strengthening structure of the body flaps. The two species resemble each other in body shape (pattern of flap size), neck segment number, cephalic plates, and most importantly a mouth characterized by concentric wrinkled furrows. The latter confirms that a soft mouth without sclerotized plates is a real feature of Lyrarapax and supports the idea that oral structures provide valid diagnostic characters within Radiodonta.