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Molaria spinifera

A small arthropod with a long, flexible posterior spine

Image of Molaria spinifera.

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Molaria spinifera (ROM 61131) – Part and counterpart. Complete specimen preserved in lateral view showing appendages. Specimen length = 24 mm. Specimen dry – polarized light (both 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 phylogenetic position of Molaria is uncertain because the taxon is rather poorly known. It may be belong within Lamellipedia, as closely related to Naraoia and the trilobites (Briggs and Fortey, 1989; Wills et al., 1998), or it could also be aligned with the chelicerates (Briggs et al., 1992).

Species name:

Molaria spinifera

Described by:

Walcott

Description date:

1912

Etymology:

Molaria – from Molar Mountain (3,022 m), a peak near the Valley of the Ten Peaks, named by James Hector in 1859 because the shape of the mountain resembles a molar tooth.

spinifera – from the Latin word spinifer, “spine-bearing.”

Type Specimens:

Lectotype –USNM57688 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.

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History of Research

Brief history of research:

This taxon was designated by Walcott in 1912 and revised by Simonetta in 1964. A major re-description was undertaken by Whittington in 1981. Molaria has since been included in several studies of arthropod relationships (e.g. Briggs and Fortey, 1989; Briggs et al., 1992, Wills et al., 1998).

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Description

Morphology:

Body sizes range from 0.8 cm to 2.6 cm (3.6 cm with tail spine). The body consists of a semi-circular head shield, eight body segments and an elongated tail. The head shield is smooth, with no ornamentation or eyes. A pair of tiny antennae extends forward from the anterior of the head, and three pairs of biramous appendages are located under the head shield behind them. These appendages have a large basal segment, to which are attached a walking branch with five segments and an outer lobe with a fringe of lamellae. Posterior to the head, there are eight trunk segments, each with a pair of limbs that are segmented and branch into two (biramous), similar to those on the head. The subcylindrical tail segment is small, and has an elongated, tapering posterior spine extending from it. This posterior spine is segmented and flexible. The trace of the gut can be seen in some lateral specimens.

Abundance:

A total of 111 specimens of Molaria were originally described from Walcott Quarry. This arthropod comprises 0.28% of the Walcott Quarry community (Caron and Jackson, 2008).

Maximum size:

36 mm

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Ecology

Life habits:

Epibenthic, mobile

Feeding strategies:

Deposit feeder

Ecological Interpretations:

The presence of walking limbs in Molaria suggests it lived and moved around on the sea floor, perhaps using its head shield to plough through the sediment. It fed on detritus or scavenged on the sea floor, using the large basal segment with spines to scrape up food and pass it forwards to the backward-directed mouth. Molaria may also have been able to launch itself off of the sea floor for short bursts of swimming, using the branches of its limbs for propulsion and the posterior spine for balance and steering.

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References

Bibliography:

BRIGGS, D. E. G. AND R. A. FORTEY. 1989. The early radiation and relationships of the major arthropod groups. Science, 246: 241-243.

BRIGGS, D. E. G., R. A. FORTEY, M. A. WILLS. 1992. Morphological disparity in the Cambrian. Science, 256: 1670-3.

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

SIMONETTE, A. M. 1964. Osservazioni sugli artropodi non trilobiti della ‘Burgess Shale’ (Cambriano medio). III conributo. Monitore Zoologico Italiano, 72: 215-231.

WALCOTT, C. D. 1912. Middle Cambrian Branchiopoda, Malacostraca, Trilobita and Merostomata. Smithsonian Miscellaneous Collections, 57: 145-228.

WHITTINGTON, H. B. 1981. Rare arthropods from the Burgess Shale, Middle Cambrian, British Columbia. Philosophical Transactions of the Royal Society of London Series B – Biological Sciences, 292: 329-357.

WILLS, M. A., D. E. G. BRIGGS, R. A. FORTEY, M. WILKINSON AND P. H. A. SNEATH. 1998. An arthropod phylogeny based on fossil and recent taxa, p. 33-105. In G. D. Edgecombe (ed.), Arthropod fossils and phylogeny. Columbia University Press, New York.

Other links:

None

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