Hymenocarines were early arthropods with bivalved carapaces and mandibles, forming the bulk of the first mandibulates (represented today by myriapods, crustaceans and insects) (Aria and Caron 2017; Vannier et al. 2018). In many hymenocarines, including Fibulacaris, determining the exact number and types of appendages on their head remains difficult, which hinders a detailed understanding of the evolutionary history of this group. Fibulacaris most probably belongs to the family Odaraiidae, a group of hymenocarines with highly multisegmented bodies, reduced or absent antennae and highly multisegmented legs.
Fibulacaris – from a “fibula”, a type of brooch, the latin caris, meaning “crab” or “shrimp”
nereidis – from the Greek mythological creatures known as Nereids, the daughters of Nereus, given the similarities of Fibulacaris to the Burgess Shale odaraiid Nereocaris (Legg et al. 2012).
Burgess Shale and vicinity: None
Other deposits: None
Marble Canyon, Tokumm Creek.
Several specimens of Fibulacaris nereidis were discovered at the Marble Canyon site in 2014 and nicknamed “epsilon-arthropod” based on the characteristic shape of its carapace. The majority of specimens were discovered at Mount Whymper and Tokumm Creek sites during the expeditions of 2016 and 2018, sometimes referred as “safety-pin”. Its genus and species were later described in 2019 (Izquierdo-López and Caron 2019).
Fibulacaris is generally small, with most specimens measuring around 1 cm. It has a distinct bivalved carapace enclosing its body laterally, covering up to two-thirds of its entire length. The dorsal side of the carapace is dome-shaped with a small crest that runs across the entire length, and a small spinose process on its posterior side. The frontal side of the carapace bends ventrally into a highly elongated spine, almost as long as the carapace itself. The ventral margins of the carapace are thicker, and end with a small process posteriorly on both sides. One pair of pedunculate eyes protrudes from the notches formed between the carapace and the spine. Other details about its head remain unknown, but antennae are either absent or highly reduced. The anterior side of the body is bent posteriorly, so that the eyes are facing backward. The body is multisegmented, subdivided into 30 segments, with each segment bearing limbs subdivided into two branches (biramous). Its tail has two small appendages shaped like a paddle (caudal rami).
Fibulacaris is rare at the Marble Canyon site, but very abundant (with more than 100 specimens) along Tokumm Creek.
Fibulacaris was likely a nektobenthic suspension feeder (Izquierdo-López and Caron 2019). Its gut is sometimes preserved as a three-dimensional structure, a type of preservation that has been associated with deposit feeders (Legg and Caron 2014). However, Fibulacaris’ carapace extends through its ventral side, indicating that this arthropod was not able to walk on surfaces and obtain organic material from the sediment, like a deposit feeder. Extant branchiopod crustaceans, such as many water fleas (Cladocera), have carapaces similar to that of Fibulacaris. Using their limbs, they generate small water currents carrying organic particles that pass through their limbs and carapace. Fibulacaris, could have used a similar suspension-feeding strategy. Given that the dorsal side of Fibulacaris was covered by its carapace, and that its eyes were facing towards the back of its body, it has been suggested that it was swimming upside down (Izquierdo-López and Caron 2019), as fairy shrimps do (Anostraca) (Fryer 2006). This way, Fibulacaris would have had capture organic particles falling from the water column, while being protected from predators from its back thanks to the carapace, from its ventral and posterior side thanks to the spine.