Reconstruction of Branchiocaris pretiosa.
© Marianne Collins
Hymenocarines were early arthropods with bivalved carapaces and mandibles, forming the bulk of the first mandibulates (represented today by myriapods, crustaceans and insects) (Aria & Caron 2017; Vannier et al. 2018). Branchiocaris belongs to the family Protocarididae (eponymous of Protocaris, one of the oldest known Burgess Shale taxa), characterized by lobate carapaces, multisegmented bodies and long, furcate tailpieces. The relationship of Protocarididae within hymenocarines, as well as the relative placement of hymenocarines within early mandibulates is still under investigation (Aria 2022; Izquierdo-López & Caron 2022).
Branchiocaris – from the Greek branchion, “gill,” and the Latin caris, “crab” or “shrimp,” thus, gilled shrimp.
pretiosa – from the Latin pretiosus, “precious” or “attractive.”
Burgess Shale and vicinity: none.
Other deposits: Branchiocaris has also been found in the Cambrian sediments of Utah (Briggs et al. 2008), and a possible second species, Branchiocaris? yunnanensis, has been described from the Lower Cambrian Chengjiang biota (Hou 1987; Yu et al. 2016).
The Walcott and Raymond Quarries on Fossil Ridge. The Tulip Beds (S7) and Collins Quarry on Mount Stephen.
This animal was originally described as Protocaris pretiosa by Charles Resser in 1929, the second species to be placed in the genus established by Charles Walcott in 1884. Resser (1929) considered it to be a phyllocarid crustacean, but Raymond (1935) and Størmer (Størmer 1944) suggested it had affinities to trilobites. A major redescription was undertaken by Briggs (1976), who moved P. pretiosa to a new genus, Branchiocaris, based on differences in carapace outline and telson morphology. While Briggs acknowledged similarities between Branchiocaris and the branchiopod crustaceans, he did not assign this animal to any extant group of arthropods. Hou and Bergström (1997) suggested that Branchiocaris was a calmanostracan branchiopod. A conservative reassessment of the head structures in Branchiocaris led Budd (2002) to suggest that it was instead a stem-lineage euarthropod. Aria and Caron then illustrated the presence of large mandibles in Branchiocaris, alongside Tokummia, making it an early mandibulate (2017).
Like other protocaridids, Branchiocaris’s long, tubular, multisegmented body is largely enclosed in a broad bivalved carapace with ample, lobate corners. Small processes are present medially at the front and rear of both valves. Eyes are very reduced or absent. The very front of the animal bears a bilobed organ covered by an elliptical sclerite. A pair of short, stout, multisegmented antennules are the most anterior appendages. The next pair of appendages are large, round mandibles, followed by modified appendages identified as maxillules and maxillae. The first pair of thoracic limbs are pincers projecting at the front of the animal, and therefore called maxillipeds. Trunk limbs are composed of walking legs and lobate flaps bearing bristles. There are about 45-50 trunk segments, each bearing a pair of limbs, which gradually decrease in size towards the back. Some tergites are fused at the back of the animal, forming a plate, and the tailpiece is a pair of caudal rami, typical of mandibulates.
Branchiocaris is very rare in the Walcott Quarry on Fossil Ridge, where it makes up a negligible percentage (0.008%) of the community (Caron & Jackson 2008). It is more common on Mount Stephen in rocks of the slightly older Glossopleura Zone (Collins et al. 1983).
The presence of pincers suggests Branchiocaris was a nektobenthic predator. However, as in Tokummia and Protocaris, the absence of distinct eyes in the fossils, implying they were either very reduced or absent, indicates that the predatory lifestyle of Protocarididae might have had its own specificity. These bivalved arthropods either relied more heavily on their other sensory organs or were perhaps more passive predators. The trunk appendages are poorly suited for walking, but the wide flaps would have been ideal for propelling the animal through the water column by wave-like movements. This type of swimming may have set up a water current running along the ventral surface of the animal, assisting in the function of the lamellae as gills. The telson probably aided in propulsion and steering while swimming.
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