The functional head of the Cambrian radiodontan (stem-group Euarthropoda) Amplectobelua symbrachiata

Abstract

BackgroundSegmental 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.ResultsIn 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.ConclusionsThe 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.

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 ‘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 . , ).…”

“…An enlarged/elongated endite is present in the equivalent position (first podomere of the distal articulated region) in the amplectobeluid taxa Amplectobelua and Ramskoeldia (Cong et al . , ). However, Laminacaris bears unpaired auxiliary spines on this endite (and all other endites), while in Amplectobelua and Ramskoeldia the auxiliary spines on this endite are paired (Hou et al .…”

“…; Cong et al . , ). If the endite on podomere 7 is in fact larger than on podomere 5 (three podomeres distal to the shaft), Laminacaris would bear additional similarities with Amplectobelua and Ramskoeldia , in which the endite on podomere 8 is larger than that on podomere 6 (also three podomeres distal to the shaft) (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 . , ).…”