Reconstruction of Plenocaris plena.
© MARIANNE COLLINS
The affinity of Plenocaris is poorly known. It has been assigned to the Class Malacostraca (Whittington, 1974), but no phylogenetic analysis of this species has been carried out.
Plenocaris – from the Latin plenus, “full,” and caris, “shrimp.”
plena – from the Latin plenus, “full.”
Burgess Shale and vicinity: none.
Other deposits: none.
The Walcott and Raymond Quarries on Fossil Ridge. The Collins Quarry on Mount Stephen.
Plenocairs was first described by Walcott (1912) as Yohoia plena. Whittington (1974) invalidated Y. plena, upgrading it to its own genus, Plenocaris plena, leaving Y. tenuis as the only species of Yohoia.
The body of Plenocaris is elongated and consists of a head region and 13 body segments ending in a paddle-shaped telson. The head bears a pair of simple antennae. The trunk segments 2 to 4 bear a pair of uniramous, elongate appendages whereas the other segments have no appendages. A carapace is loosely connected to the rest of the body and dorsally and laterally covers the head region and anterior of trunk.
Plenocaris represent a trivial proportion (0.2%) of specimens counted in the Walcott Quarry (Caron and Jackson, 2008) and is extremely rare elsewhere.
Some specimens of Plenocaris have a sediment-filled gut, suggesting that this animal lived near the benthos and was a deposit feeder. In the absence of swimming appendages, swimming and steering was only possible thanks to its long trunk and large lobate tail.
WALCOTT, C. D. 1912. Cambrian Geology and Paleontology II. Middle Cambrian Branchiopoda, Malacostraca, Trilobita and Merostomata. Smithsonian Miscellaneous Collections, 57(6): 145-228.
WHITTINGTON, H. B. 1974. Yohoia Walcott and Plenocaris n. gen. arthropods from the Burgess Shale, Middle Cambrian, British Columbia. Geological Survey of Canada Bulletin, 231: 1-27.
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