The ontogeny of Parabolinella panosa (the Family Olenidae) from the uppermost Furongian (Upper Cambrian) of the Rabbitkettle Formation, MacKenzie Mountains, northwestern Canada, is described. The protaspides are characterized by a highly convex lateral profile, parallel-sided axial furrows, and three pairs of fixigenal spines. Protaspid morphologies of eight olenid species, including P. panosa, demonstrate that the Olenidae, a widely accepted monophyletic group, displays surprisingly disparate morphologies during the protaspid period. The olenid protaspides show morphologic differences according to oxygenation conditions; the olenid protaspides from poorly oxygenated environments are smaller in size, and have a spindle-shaped axis, distinct anterior pits, and a smaller protopygidium, but lack anterior and mid-fixigenal spine pairs, while the other protaspides which lived in better oxygenated condition are larger, have three pairs of fixigenal spines and a larger protopygidium, and lack distinct anterior pits. Olenimorph form is retained by most, if not all, olenid holaspides, even by those which inhabited better oxygenated conditions, suggesting greater morphological plasticity in the protaspid period.

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