Trilobita (Order: Ptychopariida)
Trilobites are extinct Euarthropoda, probably stem lineage representatives of Mandibulata (which includes crustaceans, myriapods, and hexapods) (Scholtz and Edgecombe, 2006).
Elrathia – unspecified.
permulta – from the Latin per, “very much”, and multus, “many”.
Holotype – USNM 65517 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
Burgess Shale and vicinity: none.
Other deposits: other species occur, sometimes abundantly, elsewhere in the Cambrian of North America and Greenland.
Middle Cambrian, Bathyuriscus–Elrathina Zone (approximately 505 million years ago).
The Walcott Quarry on Fossil Ridge. Elrathia sp. has been reported from localities on Mount Stephen.
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.
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.
Rare in the Walcott Quarry on Fossil Ridge, and elsewhere.
E. permulta may, like similar small ptychoparioid trilobites, be interpreted as a mobile, epibenthic deposit (particle) feeder adapted to low oxygen levels.