ONTOGENY OF THE TRILOBITE OLENUS-WAHLENBERGI WESTERGARD, 1922 FROM THE UPPER CAMBRIAN ALUM SHALES OF ANDRARUM, SKANE, SWEDEN

The upper Cambrian trilobite Olenus wahlenbergi Westergard, 1922 occur s abundantly in stinkstone concretions in the Alum Shales at Andrarum, in Skane, southern Sweden (Olenus/Homagnostus obesus Zone). All growt h stages from anaprotaspis onwards are represented, though the early s tages are comparatively uncommon and complete specimens are rare. Fine details of structure are preserved, suitable for scanning electron mi croscopic study. The protaspis has five axial rings, which in the adul t become divided transversely and reduced to three, and the preglabell ar field expands markedly in the holaspis. The librigena broadens duri ng development and the eye, with its widely separated lenses is retain ed on the librigena throughout meraspid development. The visual surfac e dehisces during ecdysis from the early holaspid onwards, due to empl acement of the ocular suture. The hypostome apparently changes from a conterminant to a natant condition in the early holaspis, which is con sistent with the growth of the preglabellar held at this stage. The tr ansitory pygidium has a slightly serrated margin throughout developmen t, but becomes entire when all thoracic segments have been liberated. Cuticular sculpture changes dramatically throughout ontogeny. Large (T ype A) tubercles appear on the fixigena in early meraspides; these bec ome prominent and coalesce by the end of meraspid development. They ar e abruptly replaced by smaller (Type B) tubercles in the early holaspi d, which link to form a rough network in mature specimens. Comparable Type A tubercles in early meraspid librigenae give rise to a strong po lygonal caecal sculpture in holaspids, while a radial sculpture on the broadening anterior border develops independently. The significance o f these cuticular changes remains obscure. A median occipital organ is described for the first time in the Olenidae.