Perspicaris dictynna

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A small, swimming arthropod with large eyes on stalks

Outlines of Perspicaris dictynna (top right) and Perspicaris recondita (bottom) approximately to scale.

Perspicaris recondita (ROM 61113) – Part and counterpart. Complete specimen showing appendages and gut contents. Specimen length = 49 mm. Specimen dry – direct light (top images), dry – polarized light (bottom row). Walcott Quarry.

Perspicaris recondita (USNM 114255) – Holotype. Complete specimen showing antennae and eyes and gut contents. Specimen length = 66 mm. Specimen dry – polarized light (left), wet – direct light (right). Walcott Quarry.

Perspicaris dictynna (USNM 189280) – Holotype (Part and counterpart). Complete specimen showing appendages below the carapace which has been partially prepared out (left image). Specimen length = 18 mm. Specimen dry – polarized light. Walcott Quarry.

Taxonomy:

Kingdom: Animalia

Phylum: Arthropoda

Higher Taxonomic assignment: Unranked clade (stem group arthropods)

Species name: Perspicaris dictynna

Remarks:

Perspicaris is closely related to Canadaspis, but the phylogenetic position of this group is debated, with most considering them to be phyllocarid crustaceans (Briggs, 1977; Briggs and Fortey, 1989; Wills et al. 1998). However, a basal position within the stem-lineage euarthropods has also been proposed (Budd, 2002).

Described by: Simonetta and Delle Cave

Description date: 1975

Etymology:

Perspicaris – from the Latin perspicax, “sharp-sighted,” and caris, “crab, or shrimp,” thus, a sharp-sighted shrimp.

dictynna – from Dictynna, an alternate name for the Cretan goddess Britomartis, who was caught in a fisherman’s net when she threw herself into the sea to escape the pursuit of King Minos.

Type Specimens: Holotypes –USNM189280 (P. dictynna) andUSNM114255 (P. recondita) in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.

Other species:

Burgess Shale and vicinity: P. recondita from Walcott Quarry, Fossil Ridge.

Other deposits: ?P. dilatus and ?P. ellipsopelta from the Wheeler, Pioche, Marjum and Bloomington Formations of Utah and Nevada (Robison and Richards, 1981; Lieberman, 2003).

Age & Localities:

Age:

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

Principal localities:

The Walcott Quarry on Fossil Ridge. The Tulip Beds (S7) and the Trilobite Beds on Mount Stephen, Mount Odaray and Mount Field.

History of Research:

Brief history of research:

Specimens of Perspicaris were first described by Walcott (1912) as Hymenocaris and then moved to several species of Canadaspis, including C. dictynna, by Simonetta and Delle Cave (1975). A major re-examination of the material by Briggs (1977) led him to erect the new genus Perspicaris and designate the two Burgess Shale species. Perspicaris has been included in several studies of arthropod relationships (e.g. Briggs and Fortey, 1989; Wills et al. 1998; Budd, 2002).

Description:

Morphology:

Perspicaris had a bivalved carapace, with prominent eyes and an abdomen that extends beyond the carapace. The valves of the carapace were suboval with an anterior taper, and attached to each other along a straight hinge line. Immediately behind the large, oval eyes with stalks, which projected forward from the bivalved carapace, there was a pair of stout, segmented antennae. The body trunk consisted of ten thorax segments, bearing flattened appendage flaps, followed by seven abdomen segments, and a forked tail. The trace of the gut is sometimes preserved due to sediment infill. P. dictynna is distinguished from P. recondita by having a more elongated and spiny tail, with P. dictynna being smaller (maximum length 2.9 cm) than P. recondita (maximum length 6.6 cm).

Abundance:

P. dictynna and P. recondita are relatively common in the Walcott Quarry (<0.1% of the community, Caron and Jackson, 2008).

Maximum Size:

29 mm

Ecology:

Life habits: Nektonic, Mobile

Feeding strategies: Deposit feeder, Carnivorous

Ecological Interpretations:

The large eyes and flap-like appendages (with no walking branch) suggest that Perspicarisswam in the water column. The delicate antennae were likely sensory, as opposed to being used to manipulate food items. The sediment-filled gut in some specimens indicates that Perspicaris may have been a deposit feeder, but the large eyes could also indicate that it was a scavenger or even a predator.

References:

BRIGGS, D. E. G. 1977. Bivalved arthropods from the Cambrian Burgess Shale of British Columbia. Palaeontology, 20: 596-612.

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

BUDD, G. E. 2002. A palaeontological solution to the arthropod head problem. Nature, 417: 271-275.

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

LIEBERMAN, B. S. 2003. A new soft-bodied fauna: the Pioche Formation of Nevada. Journal of Paleontology, 77: 674-690.

ROBISON, R. A. AND B. C. RICHARDS. 1981. Larger bivalve arthropods from the Middle Cambrian of Utah. The University of Kansas Paleontological Contributions, 106: 1-28.

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

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.