Enk. Clarkson et Cm. Taylor, ONTOGENY OF THE TRILOBITE OLENUS-WAHLENBERGI WESTERGARD, 1922 FROM THE UPPER CAMBRIAN ALUM SHALES OF ANDRARUM, SKANE, SWEDEN, Transactions of the Royal Society of Edinburgh. Earth sciences, 86, 1995, pp. 13-34
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.