Unranked clade (stem group arthropods)
Carnarvonia venosa was originally described as a Malacostracan crustacean (Walcott, 1912) based on a single specimen, but its affinities are unclear.
Carnarvonia – from Mount Carnarvon (3,040 m), a peak in Yoho National Park. The peak was named by Alexander Burgess in 1900 in honour of Lord Henry Herbert Carnarvon (1831-1890), colonial secretary.
venosa – from the Latin vena, “vein,” referring to the vascular markings on the carapace.
Holotype – USNM 57719 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 Raymond Quarry on Fossil Ridge.
Brief history of research:
This taxon was designated by Walcott in 1912 to describe a single specimen of a bivalved carapace that he believed was from a malacostracan crustacean. Simonetta and Delle Cave (1975) added a possible second specimen, which was later synonymized with Perspicaris recondita by Briggs (1977). The crustacean affinity has been questioned by Conway Morris (1979) and Jones and McKenzie (1980). Vannier et al. (1997) described the supposed vascular structures and compared them with modern malacostracan crustaceans.
Carnarvonia venosa has a carapace with two semi-circular and non-mineralized valves preserved flat on the shale, and joined along a straight dorsal ridge (hinge line). The outer margin of the carapace has a smooth outline. There are two globular and raised circles in mirrored arrangement on both valves, which Walcott (1912) and Vannier et al. (1997) interpret to be adducted muscle scars (i.e., muscles attaching the carapace to the body of the animal), as well a pair of potential eye sockets (Vannier et al., 1997). Perhaps the most striking feature preserved is a network of vascular-like elements which have left a clear imprint into the inner side of the soft-carapace and now appear as raised, branching canals. The specimen is approximately 10 cm by 9 cm, but no evidence of body structures, such as thorax, abdomen or limbs have been preserved.
Carnarvonia is extremely rare. Only a single specimen was originally described.
Based on similarities to other bivalved carapaces in the Burgess Shale, it is assumed that the carapace of Carnarvonia venosa was an external covering of an arthropod body. If the circular structures are muscle attachment scars, it may indicate that some movement, such as opening and closing of the valves, was possible. The vascular system is comparable to swimming malacostracans and might suggest a nektonic mode of life. A further description of the life habits is impossible without more complete specimens.