A possible case of inverted lifestyle in a new bivalved arthropod from the Burgess Shale.

Alejandro Izquierdo-López,Jean-Bernard Caron

doi:10.1098/rsos.191350

Abstract

The origin of the arthropod carapace, an enlargement of cephalic tergites, can be traced back to the Cambrian period. However, its disparity and evolution are still not fully understood. Here, we describe a new ‘bivalved’ arthropod, Fibulacaris nereidis gen. et sp. nov., based on 102 specimens from the middle Cambrian (Wuliuan Stage) Burgess Shale, Marble Canyon area in British Columbia's Kootenay National Park, Canada. The laterally compressed carapace covers most of the body. It is fused dorsally and merges anteriorly into a conspicuous postero-ventrally recurved rostrum as long as the carapace and positioned between a pair of backwards-facing pedunculate eyes. The body is homonomous, with approximately 40 weakly sclerotized segments bearing biramous legs with elongate endopods, and ends in a pair of small flap-like caudal rami. Fibulacaris nereidis is interpreted as a suspension feeder possibly swimming inverted, in a potential case of convergence with some branchiopods. A Bayesian phylogenetic analysis places it within a group closely related to the extinct Hymenocarina. Fibulacaris nereidis is unique in its carapace morphology and overall widens the ecological disparity of Cambrian arthropods and suggests that the evolution of a ‘bivalved' carapace and an upside-down lifestyle may have occurred early in stem-group crustaceans.

Highlights

The study of Cambrian fossils, in particular, those from Burgess Shale and Orsten-type deposits, has been vital to the understanding of the origin and relationships among the main royalsocietypublishing.org/journal/rsos R
We examined a total of 102 specimens, collected from talus slopes or in situ from two localities in the Burgess Shale fossil localities in Kootenay National Park (British Columbia, Canada): 11 specimens were obtained from the locality of Marble Canyon [34] from two expeditions in 2012 and 2014
Fibulacaris nereidis contributes to the increasing morphological, functional, ecologic and taxonomic diversity of bivalved arthropods known from the Cambrian period

Summary

Introduction

The study of Cambrian fossils, in particular, those from Burgess Shale and Orsten-type deposits, has been vital to the understanding of the origin and relationships among the main royalsocietypublishing.org/journal/rsos R. Though, the arthropod head shield or carapace has hardly been discussed in character evolution reconstructions or as a part of the cephalic conformation debates. This might be owing to the fact that the embryological origin and development of the head shield or carapace has been subject to different interpretations [9,10,11]. The carapace can cover the whole cephalon (e.g. cephalic shield in nauplius larvae), the cephalothorax (e.g. some Decapoda, Notostraca), or even the whole body (e.g. Laevicaudata, Spinicaudata, Ostracoda) [9,10,14]. Crustaceans with reduced or absent carapaces (i.e. tanaidaceans, isopods, anostracans) appeared relatively late in crustacean evolution (e.g. Lepidocaris) [16,17,18] and some arthropods with carapaces from the upper Cambrian (e.g. Rehbachiella) have already postulated as basal branchiopods [19], indicating an early origin of the trait

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