Martin Stein

The Stem Crustacean Oelandocaris oelandica Re-Visited

The arthropod Oelandocaris oelandica from the upper Middle Cambrian “Orsten” of Sweden was recently recognized as a member of the early phase of crustacean evolution based on additional morphological detail from new specimens. Here we present a detailed investigation of all available material. It includes the description of a 400 µm long specimen probably representing an early developmental stage. Variation in size correlated with variation of trunk-segment numbers allowed recognition of different instars. The largest specimens do not exceed an estimated length of about 1 mm, indicating that our material may consist only of immature specimens. The characteristic, extremely long antennula of O. oelandica branches into three long rods. It may have served as the major structure to sweep in food, aided by the two subsequent appendages. These and the more posterior limbs were also responsible for locomotion. Minute pores on the outer edges of the posterior limbs and on the trunk tergites possibly contained sensilla originally, which may have served as water-current detectors. The presence of a minute proximal endite only on the third head appendage suggests a rather basal position of this species within Crustacea, because comparable developmental stages of other known stem crustaceans have such an endite on more of their appendages. Reconstruction of O. oelandica and its life attitudes (referred to the largest instar known) benefited from the application of 3D modelling. These helped, e.g., in identifying the combination of the plesiomorphic feeding function of the antennulae and the specialisation of the exopods of the next two appendages as a step toward the development of a sweep-net mode of feeding, one of the key novelties in the evolution of Crustacea. Such a mode of feeding coupled with locomotion of the three anterior appendages is still practiced in the naupliar and metanaupliar phases of many extant eucrustaceans, and even some adults.

A NEW ?LAMELLIPEDIAN ARTHROPOD FROM THE EARLY CAMBRIAN SIRIUS PASSET FAUNA OF NORTH GREENLAND

A new ?lamellipedian arthropod from the Early Cambrian Sirius Passet Fauna of North Greenland

Perissopyge (Trilobita) from the lower Cambrian (Series 2, Stage 4) of North America and Greenland

Abstract The distinctive trilobite Perissopyge phenax was originally described from the Henson Gletscher Formation (Cambrian Series 2, Stage 4) of central North Greenland and reported from the Sekwi Formation of Yukon Territory, Canada. This latter material is illustrated herein for the first time. Perissopyge phenax is also currently described from the Paralleldal Formation of Peary Land, North Greenland, some 100 km east of the type locality, where overlying strata correlate with the middle–upper Toyonian Stage of Siberia (Cambrian Stage 4). Perissopyge triangulata, from the Harkless Formation (Cambrian Stage 4) of Nevada, is also discussed and tectonic deformation of the type suite is recognized. An indeterminate species of Perissopyge is identified in the Ella Island Formation of North-East Greenland. The Ella Island Formation yields Olenellus hanseni, which is possibly synonymous with O. cf. truemani from the Henson Gletscher Formation. Together, these occurrences enhance stratigraphical correlation between central North Greenland and North-East Greenland and suggest some utility for Perissopyge in correlation of the lower half of the Laurentian Cambrian.