Reconstruction of Branchiocaris pretiosa.
© Marianne Collins
Branchiocaris is considered to represent either a stem-lineage euarthropod (Budd, 2002; 2008) or a primitive branchiopod crustacean (crown-group arthropod) (Resser, 1929; Hou and Bergström, 1997; Briggs et al., 2008), perhaps closely related to Marrella (Briggs et al., 1992; Wills et al., 1998).
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).
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 (1944) suggested it had affinities to trilobites.
A major redescription was undertaken by Briggs (1976), who removed P. pretiosa to a new genus, Branchiocaris, based on differences in carapace outline and telson morphology. While Briggs (1976) 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, while other phylogenetic analyses placed it close to Marrella (Briggs et al., 1992; Wills et al., 1998). A reassessment of the head structures in Branchiocaris led Budd (2002, 2008) to suggest that it is a stem-lineage euarthropod, belonging to a clade with Odaraia, Fuxianhuia, Perspicaris and Canadaspis.
Branchiocaris has a prominent bivalved carapace covering most of the narrowly-segmented body, which has many trunk limbs and a distinctive two-bladed telson. It ranges in total body length from 7 cm to over 9 cm. The sub-oval carapace has a straight dorsal hinge line with a pointed spine at each corner. The carapace surface is smooth except for a narrow border along the margin, consisting of 150-200 small, irregularly spaced and elongated pits.
The bivalved carapace covers most of the head and the anterior part of the body, extending partly over the lateral side of the animal. The frontmost structure of the head is a semicircular anterior sclerite, behind which two pairs of cephalic appendages are located. The foremost pair consists of stout but highly flexible antennae that attach towards the front of the upper surface (anterodorsally) and taper gradually towards the rear. Each antenna is made up of at least 20 segments. Behind the antennae is a pair of claw-like (subchelate) appendages. Each has at least seven segments, the last of which is elongated into a spine with a central furrow.
There is no evidence of eyes in this animal. The body trunk had at least 44 divisions or segments, and appendages were attached to a ridge running along the ventrolateral side of the animal. The trunk segments bear appendages that consist of a wide triangular flap, bearing gill blades (lamellae) and hair-like bristles (setae), and a short proximal area with up to seven segments. The posterior end of the body terminates in a large segment, or telson, that is sub-oval and has two broad lanceolate processes extending from its ventral surface. Traces of the alimentary canal are visible along the length of some specimens, as either a highly reflective dark stain or a raised ridge.
Branchiocaris is very rare in the Walcott Quarry on Fossil Ridge, where it makes up a negligible percentage (0.008%) of the community (Caron and Jackson, 2008). It is more common on Mount Stephen in rocks of the slightly older Glossopleura Zone (Collins et al., 1983).
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. The subchelate frontal appendages were most likely use to grasp food material from the sea floor and pull it towards the mouth. The antennae presumably served a sensory function. Since Branchiocaris apparently lacks eyes, it seems unlikely to be an active predator. It was probably a deposit feeder, swimming just above the sea floor gathering food.
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