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Helmetia expansa

A swimming arthropod with a wide, leaf-like carapace

Image of Helmetia expansa.

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Helmetia expansa (ROM 61038) – Part and counterpart. Nearly complete specimen in dorsal view showing parts of the gut. Specimen length = 130 mm. Specimen dry – polarized light (top row), wet – direct light (bottom row). Raymond Quarry.

© Royal Ontario Museum. Photos: Jean-Bernard Caron

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Taxonomy

Kingdom:

Animalia

Phylum:

Arthropoda

Class:

Unranked clade (stem group arthropods)

Affinity:

Helmetia is extremely rare and poorly known, so its phylogenetic affinity is uncertain. Some researchers have suggested it may be aligned with the arachnomorphs, as a sister group to the trilobites (Edgecombe and Ramsköld, 1999; Cotton and Braddy, 2004). If this is confirmed, Helmetia probably represents a stem group of the Mandibulata, which includes crustaceans, myriapods, and hexapods (Scholtz and Edgecombe, 2006).

Species name:

Helmetia expansa

Described by:

Walcott

Description date:

1918

Etymology:

Helmetia – from Helmet Mountain (3,138 m) in Kootenay National Park, named by Joseph Scattergood in 1900 because of the helmet shape of the peak.

expansa – from the Latin expansa, “spread out or wide,” referring to the broad body.

Type Specimens:

Lectotype –USNM83952 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.

Other species:

Burgess Shale and vicinity: none.

Other deposits: Helmetia? fastigata from the Jince Formation in the Czech Republic (Chlupáč and Kordule 2002).

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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

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History of Research

Brief history of research:

Helmetia expansa was first described by Walcott in 1918, and has not been re-examined since, except for brief reviews by Conway Morris et al. (1992) and Briggs et al. (1994). It has been included in several phylogenetic analyses of the arachnomorphs (e.g. Edgecombe and Ramsköld, 1999; Cotton and Braddy, 2004).

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Description

Morphology:

The most complete specimen of Helmetia is approximately 19 cm long and has a broad, flat and thin dorsal exoskeleton. The body consists of a head shield, six thoracic segments, and a large tail shield. The head shield has a concave frontal margin with two prominent spines at each corner. An oval structure with two large spots sits in the center of the head shield anterior margin. Two eyes are preserved as reflective spots behind the oval structure on either side. The six body segments are relatively thin but broad, and have a single spine projecting from their back corners. The tail shield tapers rapidly to a point, with two posteriorly-directed spines on the sides. The central region of the body has paired reflective muscle attachment scars near the anterior, and a series of large filamentous structures.

Abundance:

Helmetia expansa is extremely rare at the Walcott Quarry and the Raymond Quarry.

Maximum size:

190 mm

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Ecology

Life habits:

Nektonic, mobile

Feeding strategies:

Suspension feeder

Ecological Interpretations:

Helmetia was likely free-swimming, as indicated by the lack of walking limbs, the flat carapace and the presence of filamentous appendages. The shape of the carapace might have limited sinking, but the body is not streamlined, suggesting it was a slow swimmer. It is unlikely that it actively pursued swimming prey, because there are no apparent appendages or other modifications for predation or scavenging. It may have been a suspension feeder.

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References

Bibliography:

BRIGGS, D. E. G., D. H. ERWIN AND F. J. COLLIER. 1994. The fossils of the Burgess Shale. Smithsonian Institution Press, Washington D. C.

CHLUPÁČ, I. AND V. KORDULE. 2002. Arthropods of Burgess Shale type from the Middle Cambrian of Bohemia (Czech Republic). Bulletin of the Czech Geological Survey, 77: 167-182.

CONWAY MORRIS, S., H. B. WHITTINGTON, D. E. G. BRIGGS, C. P. HUGHES. AND D. L. BRUTON. 1992. Atlas of the Burgess Shale. Palaeontological Association, London.

COTTON, T.J. AND S. J. BRADDY. 2004. The phylogeny of arachnomorph arthropods and the origin of the Chelicerata. Transactions of the Royal Society of Edinburgh-Earth Sciences, 94: 169-193.

EDGECOMBE, G. D. AND L. RAMSKÖLD. 1999. Relationships of Cambrian Arachnata and the systematic position of Trilobita. Journal of Paleontology, 73: 263-287.

SCHOLTZ, G. AND G. D. EDGECOMBE. 2006. The evolution of arthropod heads: reconciling morphological, developmental and palaeontological evidence. Development Genes and Evolution, 216: 395-415.

WALCOTT, C. D. 1918. Geological explorations in the Canadian Rockies. Explorations and fieldwork of the Smithsonian Institution in 1917. Smithsonian Miscellaneous Collections, 68: 4-20.

Other links:

None

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