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Plenocaris plena

A small arthropod with a short head shield

Image of Plenocaris plena. .

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Plenocaris plena (ROM 61144) – Part and counterpart. Complete specimen showing gut contents. Specimen length = 6 mm. Specimen dry – polarized light (top images), wet – polarized light (bottom images). Walcott Quarry.

© Royal Ontario Museum. Photos: Jean-Bernard Caron

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Taxonomy

Kingdom:

Animalia

Phylum:

Arthropoda

Class:

Unranked clade (stem group arthropods)

Affinity:

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.

Species name:

Plenocaris plena

Described by:

Walcott

Description date:

1912

Etymology:

Plenocaris – from the Latin plenus, “full,” and caris, “shrimp.”

plena – from the Latin plenus, “full.”

Type Specimens:

Holotype –USNM57700 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.

Other species:

Burgess Shale and vicinity: none.

Other deposits: none.

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Age

Period:

Middle Cambrian, Bathyuriscus-Elrathina Zone (approximately 505 million years ago).

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Localities

Principal localities:

The Walcott and Raymond Quarries on Fossil Ridge. The Collins Quarry on Mount Stephen.

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History of Research

Brief history of research:

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.

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Description

Morphology:

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.

Abundance:

Plenocaris represent a trivial proportion (0.2%) of specimens counted in the Walcott Quarry (Caron and Jackson, 2008) and is extremely rare elsewhere.

Maximum size:

17 mm

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Ecology

Life habits:

Nektobenthic, mobile

Feeding strategies:

Deposit feeder

Ecological Interpretations:

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.

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References

Bibliography:

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.

Other links:

None

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