Unranked clade (stem group arthropods)
Sanctacaris was originally considered to belong to the crown-group chelicerates (Briggs and Collins, 1988), but subsequent analyses have aligned it with the arachnomorphs (Dunlop and Seldon, 1997; Wills et al., 1998; Sutton et al., 2002) or placed it in the stem lineage of the euarthropods (Budd, 2002).
Briggs and Collins
Sanctacaris – from the Latin sanctus, “holy, or saint,” (referring to Santa from the field name “Santa Claws”) and caris, “crab, or shrimp.”
uncata – from the Latin uncata meaning “hooked, or barbed,” referring to the numerous claws.
Holotype – ROM 43502 in the Royal Ontario Museum, Toronto, Canada.
Burgess Shale and vicinity: none.
Other deposits: none.
Middle Cambrian, Bathyuriscus-Elrathina Zone (approximately 505 million years ago).
The Collins Quarry on Mount Stephen.
Brief history of research:
Sanctacaris was first described by Briggs and Collins in 1988, and has since been included in several studies on arthropod relationships (e.g. Briggs and Fortey, 1989; Dunlop and Seldon, 1997; Wills et al. 1998; Sutton et al. 2002; Budd, 2002).
Sanctacaris has a wide head shield with six pairs of head appendages projecting forward that are segmented and branch into two (biramous). The body trunk has eleven divisions, and a wide telson. Its body length ranges from 4.6 cm to 9.3 cm. The head shield is convex in the middle, and has two wide, triangular extensions on either side. A pair of eyes is located on the head shield near the lateral side of the anterior margin. The six biramous head appendages have thin, antenna-like branches in association with a spiny raptorial appendage. The rest of the body is divided into eleven segments. Each segment has a raised central region with wide lateral projections and is associated with one pair of biramous limbs. Each biramous limb consists of a wide flap with a fringe of setae and a thin, segmented walking branch. The telson is wide and paddle-shaped, with a posterior fringe of setae.
Sanctacaris is known from five specimens from Mount Stephen.
Sanctacaris may have lived on or just above the sea floor. The presence of the frontal appendages and eyes indicate that Sanctacaris would have been a free-swimming predator. The broad flaps of the trunk appendages would have been used to propel the animal through the water, with the telson and the lateral projections of the head and body being used to stabilize and steer. The raptorial branches of the biramous head appendages would have served to actively grab prey items, while the antennae-like branches would have been used to sense the environment.