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

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.

A Burgess Shale mandibulate arthropod with a pygidium: a case of convergent evolution

“…A group of hymenocarines informally known as ‘odaraiids’ ( Nereocaris , Odaraia , Fibulacaris ) has been postulated to exist based on the absence or reduction of frontal appendages (i.e. antennae) and a highly multisegmented body with homonomous limbs (Aria & Caron 4; Izquierdo‐López & Caron 41). Other common traits include the absence of an abdominal somite (although it is present in Nereocaris ; Legg et al .…”

“…Other common traits include the absence of an abdominal somite (although it is present in Nereocaris ; Legg et al . 50; Legg & Caron 48) and a wide cephalothoracic carapace with ventral margins that can enclose the body (absent in Fibulacaris nereidis ; Izquierdo‐López & Caron 41; Aria & Caron 4). Pakucaris appears closely related to multiple odaraiids (e.g.…”

“…8) (e.g. Fibulacaris ) (Izquierdo‐López & Caron 41), sharing multiple morphological traits (e.g. multisegmented elongated body, poorly sclerotized segmented limbs, paddle‐shaped limbs used for metachronal feeding currents) that can be associated with particle feeding (Fryer 27).…”

“…4B) per side. A black trace running along the ventral margin of the pygidium in morphotype B (Figs 4A, B, 6) is reminiscent of darker tissues observed within the rostrum in Fibulacaris and the cephalic spines in Marrella (see Izquierdo‐López & Caron 41, fig. s1).…”

“…antennae), could potentially point to a sub‐mandibulate group within Hymenocarina, the ‘odaraiids’ (e.g. Nereocaris , Odaraia , Fibulacaris ) (Aria & Caron 4; Izquierdo‐López & Caron 41). Even when considering only the Hymenocarina, bivalved arthropods not only show wide morphological disparity (Aria 1), but also encompass a broad range of ecological niches, including scavenging, suspension feeding and deposit feeding.…”