Elrathina cordillerae (ROM 53273). Complete individual; a presumed carcass with free cheeks in place (coated with ammonium chloride sublimate to show details). Specimen length = 24 mm. Specimen dry – direct light. Mount Stephen Trilobite Beds on Mount Stephen.
© Royal Ontario Museum. Photo: Jean-Bernard Caron
Trilobites are, by a large margin, the most diverse of extinct arthropod classes, encompassing more than 2000 species, although the biomineralization of their cuticle is likely largely responsible for their much better-known diversity. A recent probabilistic analysis of trilobite relationships suggests that many of the diagnostic traits used for the taxonomy of the main groups remain highly variable across lineages (Paterson, Edgecombe & Lee, 2019), which explains long-standing difficulties in resolving the trilobite evolutionary tree. The kinship of trilobites relative to other arthropod groups has been a historical dilemma also (Cotton & Braddy, 2004; Aria, Caron & Gaines, 2015; Paterson, 2020). Antennules and some other characters, on the one hand, would ally them with mandibulate arthropods (the Gnathomorpha or Antennulata hypothesis), while others would place them within the chelicerate lineage (the Arachnomorpha hypothesis). Recent phylogenetic results have brought increased support to Arachnomorpha, but there is still no consensus on the question (Aria, 2022).
Elrathina – unspecified.
cordillerae – in reference to the Western Cordillera (Canadian Rocky Mountain ranges), derived from the Spanish cordilla, the diminutive of cuerda, meaning “cord.”
Burgess Shale and vicinity: Elrathina parallela, E. brevifrons, E. spinifera, and E. marginalis have been described from similar stratigraphic horizons at nearby sites on Mount Field, Mount Stephen, and Mount Odaray.
Other deposits: Other species of Elrathina have been reported from the Cambrian of North America and Greenland.
The Walcott Quarry on Fossil Ridge. The Trilobite Beds and additional localities on Mount Stephen.
E. cordillerae was originally described under the genus name Conocephalites in Rominger’s 1887 publication on trilobites from Mount Stephen. In 1888 Walcott reallocated the species to Ptychoparia where it remained until Charles Resser, Walcott’s former assistant at the United States National Museum, established the new replacement genus Elrathina (Resser, 1937). Other workers have subsequently suggested that Elrathina is indistinguishable from Ptychoparella (see Blaker and Peel, 1997).
Species of Elrathina, along with those of the corynexochid Bathyuriscus, were found to be very abundant in a narrow interval of Middle Cambrian rocks throughout western North America, forming the basis of the Bathyuriscus-Elrathina Zone erected by Charles Deiss (1940).
Hard parts: adult dorsal exoskeletons average about 2 cm long. The semicircular cephalon is about one-third the length of the entire dorsal shield, bordered by a well-defined narrow rim, and with rounded genal angles. Weak transverse eye ridges extend to the small eyes, which are located just forward of cephalic mid-length. The slightly anteriorly narrowing glabella is rounded in front and exhibits three pairs of shallow lateral furrows; the pre-glabellar field is about the same width as the narrow anterior rim. The long, tapering thorax with a narrow axial lobe contains between 17 and 19 straight-sided segments, flexed gently downwards a short distance from the rounded tips. The tiny elliptical pygidium usually features two segments.
Unmineralized anatomy: rare specimens from the Walcott Quarry on Fossil Ridge retain tantalizing evidence of soft parts, including a pair of slender uniramous antennae, followed by very delicate looking biramous limbs beneath the cephalon, thorax and pygidium. These and other individuals of E. cordillerae are occasionally associated with a dark stain adjacent to the exoskeleton, presumably representing fluidized decay products.
Relatively common on Fossil Ridge and locally very abundant in the Walcott Quarry, where it represents about 25% of all trilobites collected (Caron and Jackson, 2008).
Like similar-looking ptychoparioid trilobites, E. cordillerae may be interpreted as a fully mobile, epibenthic deposit (particle) feeder adapted to very low oxygen levels.