Article: A Middle Cambrian chelicerate from Mount Stephen, British Columbia

A recently discovered arthropod, Sanctacaris uncata gen. et sp. nov., from the Glossopleura Zone, Stephen Formation of Mount Stephen, British Columbia belongs in the Chelicerata. The head shield is wider than long, convex axially, and extends laterally into two flat triangular projections. It bears at least six pairs of biramous appendages. The first five are similar, increasing in size posteriorly and arranged with their inner rami in a raptorial array of inwardly facing, segmented, spinose limbs, accompanied by antenna-like, presumably sensory, outer rami. The outer ramus of the sixth appendage is also antenna-like, but the inner is short and terminates in a fringe of radiating spines. The eyes are at the front of the head shield. The trunk has eleven tergites, each with a convex axis and projecting pleurae. The corresponding somites of the first ten each bear a pair of biramous appendages with an inner segmented spinose ramus and an outer lamellate ramus, fringed with long setae, which functioned in swimming and respiration. The wide flat telson is adapted for stabilizing and steering.Sanctacaris displays characters which are all derived for some member of the chelicerates. These include: 1, at least six pairs of appendages (the first five raptorial) on the head shield; 2, a cardiac lobe; 3, the division of the body into tagmata comparable to the prosoma and opisthosoma of merostomes; and 4, the anus at the rear of the last trunk segment. Such a combination is unique to the chelicerates. The apparent lack of chelicerae, an advanced character present in all other chelicerates, is consistent with the primitive biramous appendages on both the head and trunk. It places Sanctacaris in a primitive sister group of all other chelicerates.Sanctacaris demonstrates that chelicerates, although rare, were present in Middle Cambrian seas. Moreover, even at this early stage of chelicerate evolution, Sanctacaris had the number and type of head appendages that are found in modified form in the eurypterids and xiphosurids, the major Palaeozoic groups that succeeded it.