The Burgess Shale

Mollisonia symmetrica

A poorly-known arthropod with seven thoracic segments

Mollisonia symmetrica (USNM 57659) – Holotype. Exoskeleton preserved in dorsal view. Specimen length = 48 mm. Specimen dry – direct light (with different angles of low angle light). Trilobite Beds on Mount Stephen.

© SMITHSONIAN INSTITUTION – NATIONAL MUSEUM OF NATURAL HISTORY. PHOTOS: JEAN-BERNARD CARON

Taxonomy:

Kingdom: Animalia
Phylum: Arthropoda
Class: Unranked clade (stem group arthropods)
Remarks:

The affinity of Mollisonia has not been considered in detail because the appendages are unknown and the specimens are poorly preserved.

Species name: Mollisonia symmetrica
Described by: Walcott
Description date: 1912
Etymology:

Mollisonia – from Mount Mollison (2,952 m), southwest of Field in British Columbia, named by Joseph H. Scattergood in 1898 after the Mollison sisters, who managed some of the Canadian Pacific Railway hotels in the Rocky Mountains.

symmetrica – from the Greek syn, “plus,” and metron, “measure,” referring to the symmetrical nature of the body.

Type Specimens: Holotype –USNM57659 (M. symmetrica) in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
Other species:

Burgess Shale and vicinity: A possible new species from Stanley Glacier (Caron et al., 2010).

Other deposits: M. sinica from the Middle Cambrian Kaili Formation of southwest China (Zhang et al., 2002).

Age & Localities:

Period:
Middle Cambrian, Bathyuriscus-Elrathina Zone (approximately 505 million years ago).
Principal localities:

The Walcott Quarry on Fossil Ridge. A few specimens potentially representing different species are known from the Trilobite Beds on Mount Stephen and Stanley Glacier.

History of Research:

Brief history of research:

Two definite species of Mollisonia were originally described by Walcott (1912), M. symmetricaand M. gracilis, based on one specimen each. A second specimen of M. gracilis discovered in 1925 and preserved with soft-tissues was removed from the genus and renamed Houghtonites gracilis by Raymond (1931). A third possible species, Mollisonia? rara, was described from several fragmentary specimens. Simonetta and Delle Cave (1975) restudied these specimens and synonymized Mollisonia? rara with M. symmetrica. Specimens of M. symmetrica have also been found in the Wheeler Formation and Spence Shale of Utah (Robison, 1991; Briggs et al. 2008).

Description:

Morphology:

The body of M. symmetica is elongated and symmetrical, with a convex dorsal surface. The rounded head shield has two anterior projections and several pairs of central oval structures. The body consists of seven thoracic segments that are all of equal width. The tail shield is rounded and of similar shape to the head shield, but with three posterior projections.

Abundance:

One specimen of M. symmetrica is known from the Mount Stephen Trilobite beds. A few dozen specimens are known from the Walcott Quarry where itrepresents less than 0.04% of the community (Caron and Jackson, 2008).

Maximum Size:
79 mm

Ecology:

Life habits: Mobile, Nektobenthic
Feeding strategies: Unknown
Ecological Interpretations:

Mollisonia symmetrica is too poorly known to allow detailed studies of its ecology but comparisons with a related form called Hougthonites gracilis suggest a nektobenthic lifestyle.

References:

BRIGGS, D. E. G., B. S. LIEBERMAN, J. R. HENDRICKS, S. L. HALGEDAHL AND R. D. JARRARD. 2008. Middle Cambrian arthropods from Utah. Journal of Paleontology, 82(2): 238-254.

CARON, J.-B. AND D. A. JACKSON. 2008. Paleoecology of the Greater Phyllopod Bed community, Burgess Shale. Palaeogeography, Palaeoclimatology, Palaeoecology, 258: 222-256.

CARON, J.-B., R. GAINES, G. MANGANO, M. STRENG AND A. DALEY. 2010. A new Burgess Shale-type assemblage from the “thin” Stephen Formation of the Southern Canadian Rockies. Geology, 38: 811-814.

RAYMOND, P. E. 1931. Notes on invertebrate fossils, with descriptions of new species. Bulletin of the Museum of Comparative Zoology, Harvard University, 55: 165-213.

ROBISON, R. A. 1991. Middle Cambrian biotic diversity: Examples from four Utah Lagerstätten, p. 77-98. In A. Simonetta and S. Conway Morris (eds.), The Early Evolution of Metazoa and the Significance of Problematic Taxa. Cambridge University Press, Cambridge.

SIMONETTA, A. M. AND L. DELLE CAVE. 1975. The Cambrian non-trilobite arthropods from the Burgess shale of British Columbia: A study of their comparative morphology, taxonomy and evolutionary significance. Palaeontographia Italica, 69: 1-37.

WALCOTT, C. D. 1912. Cambrian Geology and Paleontology II. Middle Cambrian Branchiopoda, Malacostraca, Trilobita and Merostomata. Smithsonian Miscellaneous Collections, 57(6): 145-228.

ZHANG, X., Y. ZHAO, R. YANG AND D. SHU. 2002. The Burgess Shale arthropod Mollisonia(M. sinica new species): New occurrence from the Middle Cambrian Kaili fauna of southwest China. Journal of Paleontology, 76: 1106-1108.

Other Links:

None