Oelandocaris

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Oelandocaris oelandica is an extinct Cambrian arthropod known from seven tiny fossils found in the Orsten deposits on Öland, Sweden. It is the only species in the genus Oelandocaris, which belongs to the family Oelandocarididae. The animal was about 1 millimeter long and had a large head shield that made up about half of its body, with a long rostrum and a smooth top.

In front of the head there were small antennulae and larger antennae near the mouth. A possible median eye appears as two little blisters, and the mouth sits under a rectangular hypostome. The body has six segments; the first five carry limb pairs, while the last segment is cylindrical and lacks limbs. Spines line the sides of the body and head shield.

The limbs are biramous (two-branched). The second through fifth pairs are more complex than the first. The second limb pair has two rows of spines along the inner side of the limb base; its outer branch (exopod) is large and flattened, and the inner branch (endopod) is short. The third, fourth, and fifth limbs are shorter or similar, with various bristle-like hairs. The fourth and fifth limbs have paddle-shaped outer branches with a hinge, and tiny spines are found on some limb edges. The tail is a pointed telson.

Oelandocaris lived in the water as a very small swimmer, likely a plankton feeder. It probably swam in the water column, possibly upside down near the substrate, using a specific swimming stroke with movement of its limbs and bristles to move food toward the mouth. Some younger fossils show the animal before the rostrum fully developed.