Unranked clade (stem group arthropods)
Although common, Burgessia has not been re-examined since the 1970s, and its phylogenetic placement is uncertain. It has been described as a basal member of the arachnomorphs, a group that includes chelicerates and trilobites (Briggs and Fortey, 1989; Cotton and Braddy, 2004), but see Edgecombe, 2010.
Burgessia – from Mount Burgess (2,599 m), a mountain peak in Yoho National Park. Mount Burgess was named in 1886 by Otto Klotz, the Dominion topographical surveyor, after Alexander Burgess, a former Deputy Minister of the Department of the Interior.
bella – from the Latin bellus, “beautiful.”
Lectotype –USNM57676 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
Burgess Shale and vicinity: none.
Other deposits: none.
Middle Cambrian, Bathyuriscus-Elrathina Zone (approximately 505 million years ago).
The Walcott Quarry on Fossil Ridge.
Brief history of research:
Described by Walcott in 1912, this species was revised by Simonetta (1970) and restudied in detail by Hughes (1975). Phylogenetic analyses suggest a position within the arachnomorph arthropods (Briggs and Fortey, 1989; Wills et al. 1998; Cotton and Braddy, 2004). The function of the posterior spine was analyzed by comparison with modern horseshoe crabs (Limulus) (Lin, 2009).
Burgessia is a small, soft-bodied arthropod capped by a dorsal semicircular carapace and possessing a long terminal spine. The carapace is very thin, appearing gently convex in lateral view, and ranging from 4 to 17 mm in length. The length of the spine is approximately 1.4 times the length of the carapace. There is no evidence of compound eyes. The body bears a pair of flexible antennae at the front and ten pairs of appendages which are segmented and branch into two (biramous), three in the head section and seven in the trunk. The inner branch is a walking leg in all ten pairs, and the outer branch is a flagellum in the three head appendages and a gill branch in the seven trunk appendages. A pair of short appendages that are segmented and non-branching (uniramous) is present on the eighth segment of the trunk. The walking branch of the head and trunk appendages terminate with two or three short claws. The telson (last division) has a single unsegmented posterior spine which is often preserved straight, but is bent in some cases. The digestive system is represented by a pair of conspicuous kidney-shaped gut extensions positioned laterally and connected to a central alimentary canal via primary ducts. The mouth would have been positioned ventrally behind a pair of frontal gut lobes, and the anus was located at the end of the central canal near the base of the telson.
Burgessia is known from at least three thousand specimens from the Walcott Quarry and represents about 2.5% of the community (Caron and Jackson, 2008).
This animal walked along the surface of the mud with its ten pairs of walking legs, and probably used its antennae and cephalic legs to sweep small food particles through the soft sediment to its ventral mouth. The presence of mud in the stomach has been interpreted as evidence for a deposit mode of feeding, but this could be an artifact of preservation. Swimming abilities would have been minimal. The outer gill branches on the trunk appendages are small and would probably have provided an insufficient surface area to support the gas exchange required for prolonged swimming activities. The stiffened posterior spine could have been used to deter predators or escape burial.