Owing to their distinctive appearance, agnostids have been classified either as trilobites, related to Eodiscina, or as stem group “crustaceans” (Müller and Walossek 1987; Cotton and Fortey 2005; Haug et al. 2009). The most recent phylogenetic analysis finds that agnostids form a grouping with trilobites, supported by shared features of the dorsal exoskeleton, such as mineralization, the expression of segmental boundaries, and the form of the thoracic joints (Moysiuk and Caron 2019). More taxonomically inclusive analyses will be needed to determine whether they belong inside or outside the group of true trilobites.
Peronopsis – From the Greek perone, “pin, brooch ” and opsis, “looking like.”
columbiensis – No etymology provided, but presumably in reference to the occurrence of the species in British Columbia, Canada.
Burgess Shale and vicinity: P. montis
Other deposits: other species occur throughout the world in the middle Cambrian.
Mount Stephen, Mount Odaray, Marble Canyon.
Burgess Shale material was originally named Peronopsis columbiensis by Rasetti (1951). Naimark (2012) proposed to reassign the species to the genus Quadragnostus based on published images, but we maintain it here under Peronopsis pending taxonomic restudy of Burgess Shale specimens. Moysiuk and Caron (2019) recently described the appendages, digestive tract, and other soft tissues from exceptionally preserved specimens.
Adult dorsal exoskeletons reach about 20 mm in length. The semicircular cephalon has a narrow marginal rim around the front and sides and rounded genal angles. There are no dorsal eyes and no facial sutures. The narrow glabella comes to an ogival point, with no anterior median furrow; a transverse furrow crosses the glabella near its anterior. A pair of short genal spines are present. Two short thoracic segments carry lateral nodes on the axial rings. A narrowly rimmed pygidium, the same size and general shape as the cephalon, has abruptly angled anterolateral corners and a pair of short, backwards-directed marginal spines posterolaterally. The pygidial axis is broader than the glabella, but of similar outline, with a median tubercle between two transverse furrows. The pointed tip of the axis is separated by a gap from the pygidial rim posteriorly, without a median furrow. A saddle-shaped hypostome is present ventrally, unfused to the headshield. Unmineralized anatomy: The head probably bears six pairs of appendages, including one pair of elongate sensory antennules, two pairs of appendages with oar-like outer branches, and probably three pairs of stout walking limbs with a row of club-like projections. Additional walking limbs were present beneath the thorax (2) and pygidium (probably 4). The digestive tract curves dorsally from the mouth before emitting two pairs of branching gut glands, the first of which is the largest and occupies much of the space below the headshield. Behind this, the cylindrical midgut extends back to the pygidium. The hindgut begins roughly below the pygidial tubercle, and narrows considerably before reaching the anus below the tip of the pygidial axis.
Specimens likely assignable to this species are very common at Tokumm Creek and in the upper levels of the Marble Canyon quarry, where it is the most abundant artiopodan (Nanglu et al. 2020). Peronopsis columbiensis also occurs in notable numbers at Mount Odaray, Mount Stephen, and a few smaller localities (Rasetti 1951).
The mode of life of agnostids has been extensively debated (Fortey and Owens 1999). With the oar-like appendages capable of protruding while the animal was partially enrolled, agnostids certainly appear well-adapted for swimming (Müller and Walossek 1987). Together with their occurrence in mass mortality beds with wide geographic range, this evidence has been proposed to support a pelagic lifestyle (Fortey 1985). However, most specimens at the Burgess Shale are found in unrolled position, suggesting they did not live permanently enrolled. Further, Peronopsis is sometimes found in groups, associated with the remains of other Burgess Shale organisms, where it was potentially feeding on carrion or bacterial films, providing evidence for a benthic habitat. The huge, branching gut glands in the head likely acted as a food storage organ, possibly enabling a feast-and-famine lifestyle. The club-like outgrowths on the walking legs may have functioned in respiration (Moysiuk and Caron 2019).