Plenocaris plena

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Small bivalved arthropod with a short carapace

Reconstruction of Plenocaris plena.

Taxonomy:

Kingdom: Animalia

Phylum: Arthropoda

Higher Taxonomic assignment: Hymenocarines

Species name: Plenocaris plena

Remarks:

Interpretations of the affinities of Plenocaris have been hindered by the lack of anatomical details of their legs and head. Several similar bivalved arthropods from the early Cambrian of China, Chuandianella (Hou and Bergstrom 1997), Clypecaris (Yang et al. 2016), Ercaicunia (Zhai et al. 2019), Erjiecaris (Fu et al. 2014), Pauloterminus (Hou et al. 2009) and Xiazhuangocaris (Zeng et al. 2020) hint at a potential affinity to the hymenocarine families Waptiidae and Clypecarididae (Yang et al. 2016), based on similar tagmosis and extension of the carapace (Zeng et al. 2020). Plenocaris, these species lack extensive detail on their head conformation and leg morphology, preventing a comprehensive classification.

Described by: Walcott

Description date: 1912

Etymology:

Plenocaris – from the Latin plenus, “full”, and caris, meaning “crab” or “shrimp”

plena – from the Latin plenus, “full”.

Type Specimens: USNM57700 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.

Other species:

Burgess Shale and vicinity: none.

Other deposits: none.

Age & Localities:

Age:

Middle Cambrian, Wuliuan stage, Burgess Shale Formation (approximately 505 million years ago).

Principal localities:

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

History of Research:

Brief history of research:

Plenocaris 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.

Description:

Morphology:

The body of Plenocaris is characterized by a short head and thorax, covered dorsally and laterally by the carapace, and an elongated limbless abdomen. The head bears one pair of antennae, divided into approximately 10 segments, each bearing a spine on its ventral side. The trunk bears between 2 to 4 pairs of legs. The remaining section of the body comprises 13 abdominal segments and ends in one pair of paddle-shaped appendages (caudal rami).

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

Ecology:

Life habits: Nektobenthic, Mobile

Feeding strategies: Deposit feeder

Ecological Interpretations:

Some specimens of Plenocaris have a sediment-filled gut (Whittington 1974), which may indicate a deposit-feeding lifestyle (Briggs and Whittington 1985). However, body legs posterior to the frontal carapace were lacking and therefore the animal might not have been suited for walking on the benthos (Briggs and Whittington 1985). Furthermore, a sediment-filled gut may not always be indicative of deposit-feeding, and similarly-preserved guts may also appear in suspension feeders (Butler et al. 2015). On the other hand, the antenna of Plenocaris bear spinose endites, reminiscent to those in Clypecaris, and similar to this genus, suggesting a potential predatory role (Yang et al. 2016). Swimming and steering were possible thanks to its long body and relatively large tail fan, while the thoracic limbs could have been used to perch while feeding.

References:

ARIA, C. and CARON, J. B. 2017. Burgess Shale fossils illustrate the origin of the mandibulate body plan. Nature, 545: 89–92.
BRIGGS, D. E. G. and WHITTINGTON, H. B. 1985. Modes of life of arthropods from the Burgess Shale, British Columbia. Transactions of the Royal Society of Edinburgh, 76: 149–160.
BRIGGS, D. E. G. and FORTEY, R. A. 1989. The early radiation and relationships of the major arthropod groups. Science, 246: 241–243.
BUTLER, D., CUNNINGHAM, J. A., BUDD, G. E. and DONOGHUE, P. C. J. 2015. Experimental taphonomy of Artemia reveals the role of endogenous microbes in mediating decay and fossilization. Proceedings B, 1–10.
CARON, J. B. and JACKSON, D. A. 2008. Paleoecology of the Greater Phyllopod Bed community, Burgess Shale. Palaeogeography, Palaeoclimatology, Palaeoecology, 258: 222–256.
FU, D., ZHANG, X. and BUDD, G. E. 2014. The first dorsal-eyed bivalved arthropod and its significance for early arthropod evolution. GFF, 136:1: 80–84.
HOU, X.-G. and BERGSTROM, J. 1997. Arthropods of the Lower Cambrian Chengjiang fauna, southwest China. Fossils and Strata No, 45: 1–116.
HOU, X., SIVETER, D. J., ALDRIDGE, R. J. and SIVETER, D. J. 2009. A new arthropod in chain-like associations from the Chengjiang Lagerstätte (Lower Cambrian), Yunnan, China. Palaeontology, 52: 951–961.
VANNIER, J., ARIA, C., TAYLOR, R. S. and CARON, J. B. 2018. Waptia fieldensis Walcott, a mandibulate arthropod from the middle Cambrian Burgess Shale. Royal Society Open Science, 5:172206:
WALCOTT, C. D. 1912. Cambrian geology and paleontology II: Middle Cambrian Branchiopoda, Malacostraca, Trilobita and Merostomata. Smithsonian Miscellaneous Collections, 57: 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: 1–63.
YANG, J., ORTEGA-HERNÁNDEZ, J., LAN, T., HOU, J. B. and ZHANG, X. G. 2016. A predatory bivalved euarthropod from the Cambrian (Stage 3) Xiaoshiba Lagerstätte, South China. Scientific Reports, 6: 27709.
ZENG, H., ZHAO, F.-C., YIN, Z.-J. and ZHU, M.-Y. 2020. A new early Cambrian bivalved euarthropod from Yunnan, China and general interspecific morphological and size variations in Cambrian hymenocarines. Palaeoworld, 30(3): 387–397.
ZHAI, D., ORTEGA-HERNÁNDEZ, J., WOLFE, J. M., HOU, X.-G., CAO, C. and LIU, Y. 2019. Three-dimensionally preserved appendages in an early Cambrian stem-group pancrustacean. Current Biology, 29: 171–177.