© Royal Ontario Museum. Photo: Jean-Bernard Caron
Trilobites are extinct euarthropods, probably stem lineage representatives of the Mandibulata, which includes crustaceans, myriapods, and hexapods (Scholtz and Edgecombe, 2006).
Ehmaniella – modification of Ehmania, a trilobite genus name coined in 1935 by C. E. Resser to honour Philip Ehman (Montana) for his geological assistance.
burgessensis – from the Burgess Shale.
Burgess Shale and vicinity: Ehmaniella waptaensis Rasetti, 1951.
Other deposits: other species have been reported from elsewhere in the Cambrian of North America.
The Walcott, Raymond and Collins Quarries on Fossil Ridge. The Trilobite Beds on Mount Stephen, and smaller localities on Mount Odaray.
Walcott illustrated two Burgess Shale trilobite specimens in establishing Ptychoparia permulta in 1918. Resser (1937) saw that the two individuals belonged in different species, but erroneously used Walcott’s clearly designated primary type of permulta to found the new combination Elrathia dubia. Rasetti (1951) declared Resser’s dubia invalid, left the original type of permulta in Elrathia, and employed Walcott’s other specimen as a paratype of a new species (burgessesnsis), which he assigned to Resser’s 1937 genus Ehmaniella. Ehmaniella waptaensis, also described by Rasetti in 1951, is nearly indistinguishable.
Hard parts: adult dorsal exoskeletons may reach 2.8 cm long. The semicircular cephalon is about one-third the length of the dorsal shield, bordered by a well-defined rounded rim; wide free cheeks often show anastomosing ridges and carry short, sharp genal spines. Strong transverse eye ridges extend to relatively large eyes, which are located at or behind cephalic mid-length. The bluntly rounded glabella tapers evenly forward and bears three pairs of shallow lateral furrows; the pre-glabellar field is short. A thorax of thirteen parallel-sided segments has a barrel-shaped outline and a rather broad axial lobe. The short, wide, rounded triangular pygidium usually shows 4 or 5 axial rings with corresponding pleurae. The surface of the exoskeleton is variably granulate.
Unmineralized anatomy: rare specimens of Ehmaniella from the Walcott Quarry and above on Fossil Ridge preserve a pair of slender uniramous antennae (Walcott, 1918; Rudkin 1989). These are sometimes associated with a dark stain adjacent to the exoskeleton, presumably representing fluidized decay products.
Relatively common on Fossil Ridge and locally abundant in the Walcott Quarry (fourth most common trilobite with about 400 specimens observed, only 13 of which are E. waptaensis, Caron and Jackson, 2008).
Like similar-looking ptychoparioid trilobites, Ehmaniella may be interpreted as a fully mobile, epibenthic deposit (particle) feeder.
CARON, J.-B. AND JACKSON, D. A. 2008. Paleoecology of the Greater Phyllopod Bed community, Burgess Shale. Palaeogeography, Palaeoclimatology, Palaeoecology, 258:222-256.
RASETTI, F. 1951. Middle Cambrian stratigraphy and faunas of the Canadian Rocky Mountains. Smithsonian Miscellaneous Collections, 116 (5): 277 p.
RESSER, C. E. 1935. Nomenclature of some Cambrian trilobites. Smithsonian Miscellaneous Collections, 95(22): 29 p.
RESSER, C. E. 1937. Third contribution to nomenclature of Cambrian trilobites. Smithsonian Miscellaneous Collections, 93(5): 46 p.
RUDKIN, D. M. 1989. Trilobites with appendages from the Middle Cambrian Stephen Formation of British Columbia. 28th International Geological Congress, Washington, D.C. July 9-19, 1989. Abstracts: 2-729.
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.