Carnarvonia venosa (USNM 57719) – Holotype. Complete valves attached along the hinge line showing the network of vascular-like elements as mirror-like structures, dorsal view. Specimen length = 100 mm. Specimen dry – direct light. Raymond Quarry.
© Smithsonian Institution – National Museum of Natural History. Photo: Jean-Bernard Caron
Hymenocarines were early arthropods with bivalved carapaces and mandibles, forming the bulk of the first mandibulates (represented today by myriapods, crustaceans and insects) (Aria and Caron 2017; Vannier et al. 2018).
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.
Burgess Shale and vicinity: none.
Other deposits: none.
The Raymond Quarry in Fossil Ridge, Tokumm Creek.
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. Based on our current understanding of early arthropod evolution, Carnarvonia most probably belongs to the bivalved arthropod group Hymenocarina (Aria and Caron 2017; Vannier et al. 2018), or stands among its close relatives.
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 a 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 as 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 on the inner side of the soft carapace and now appear as raised, branching canals. No evidence of body structures, such as thorax, abdomen or limbs have been found, so far.
Carnarvonia is extremely rare. Only two specimens are known.
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.
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