Fossil Gallery

Home > Fossil Gallery > Elrathia

Elrathia permulta

A small, rare, and poorly known trilobite

Image of Elrathia permulta.

Get Adobe Flash player

Elrathia permulta (USNM 65517) – Holotype. Complete individual; a presumed carcass with free cheeks in place. Specimen length = 26 mm. Specimen dry – direct light. Walcott Quarry.

© Smithsonian Institution – National Museum of Natural History. Photo: Jean-Bernard Caron

Media 1 of 2 for Elrathia permulta Photo
Media 2 of 2 for Elrathia permulta Photo

Taxonomy

Kingdom:

Animalia

Phylum:

Arthropoda

Class:

Trilobita (Order: Ptychopariida)

Affinity:

Trilobites are extinct Euarthropoda, probably stem lineage representatives of Mandibulata (which includes crustaceans, myriapods, and hexapods) (Scholtz and Edgecombe, 2006).

Species name:

Elrathia permulta

Described by:

Walcott

Description date:

1918

Etymology:

Elrathia – unspecified.

permulta – from the Latin per, “very much”, and multus, “many”.

Type Specimens:

Holotype –USNM65517 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.

Other species:

Burgess Shale and vicinity: none.

Other deposits: other species occur, sometimes abundantly, elsewhere in the Cambrian of North America and Greenland.

Back to top

Age

Period:

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

Back to top

Localities

Principal localities:

The Walcott Quarry on Fossil Ridge. Elrathia sp. has been reported from localities on Mount Stephen.

Back to top

History of Research

Brief history of research:

The concept of Elrathia permulta is quite confused. Walcott named the species Ptychoparia permulta in 1918 and illustrated two specimens, one clearly designated as the type. Resser (1937) noted that the illustrated specimens were quite different, moved both to Walcott's 1924 genus Elrathia, and proposed the name Elrathia dubia for the second species. Unfortunately, he based this on the original type of permulta; Rasetti (1951) declared dubia invalid, returned the type specimen to Elrathia permulta, and designated the other of Walcott's specimens as a paratype of Ehmaniella burgessensis. The holotype of permulta, however, lacks many of the diagnostic characters of Elrathia, and Robison (1964) suggested it represents a new genus.

Back to top

Description

Morphology:

Hard parts: the adult dorsal exoskeleton is up to 25 mm long, with a large semicircular cephalon occupying about one-third the total length. The cephalon is bordered by a rounded rim and broad inner furrow; genal angles are produced into sharp triangular spines extending back to the fourth thoracic segment. There is a relatively long field between the narrow, tapered, and anteriorly rounded glabella and the frontal rim. Eyes are small and transverse eye ridges are very weak. Three pairs of shallow lateral furrows mark the glabella. The thorax comprises 14 segments, and tapers back more rapidly over the posterior half to a small rounded pygidium. The surface of the exoskeleton is variably granulate.

Unmineralized anatomy: not known.

Abundance:

Rare in the Walcott Quarry on Fossil Ridge, and elsewhere.

Maximum size:

25 mm

Back to top

Ecology

Life habits:

Epibenthic, mobile

Feeding strategies:

Deposit feeder

Ecological Interpretations:

E. permulta may, like similar small ptychoparioid trilobites, be interpreted as a mobile, epibenthic deposit (particle) feeder adapted to low oxygen levels.

Back to top

References

Bibliography:

RASETTI, F. 1951. Middle Cambrian stratigraphy and faunas of the Canadian Rocky Mountains. Smithsonian Miscellaneous Collections, 116 (5): 277 p.

RESSER, C. E. 1937. Third contribution to nomenclature of Cambrian trilobites. Smithsonian Miscellaneous Collections, 95(22): 29 p.

ROBISON, R. A. 1964. Late Middle Cambrian faunas from western Utah. Journal of Paleontology, 38:510-566.

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. 1918. Cambrian Geology and Paleontology IV. Appendages of trilobites. Smithsonian Miscellaneous Collections, 67(4): 115-216.

WALCOTT, C. D. 1924. Cambrian and Lower Ozarkian trilobites. Smithsonian Miscellaneous Collections, 75(2): 53-60.

Other links:

http://www.trilobites.info/ordptychopariida.htm

Back to top