The Lobopod Animal Aysheaia Pedunculata Walcott, Middle Cambrian, Burgess Shale, British Columbia

Abstract

Fifteen specimens of A. pedunculata have been prepared, photographed, and drawn to show how each specimen is interpreted and how portions preserved only in part or counterpart are related to the whole. The body was elongate, sub-cylindrical, bearing one pair of conical, branched anterior appendages inserted in the lateral wall, and ten pairs of short, uniramous limbs; anteriorly no distinct head region, posteriorly the body merged into the bases of the last pair of limbs. The cuticle was unmineralized, flexible, the body wall, anterior appendage and limbs, annulated. On the trunk the evenly spaced annulations were high, sharp-crested dorsally, changing to low and rounded laterally and faint or absent ventrally; dorsally the annulations appear to have borne a row of seven tubercles, each tubercle sharp and possibly spinose apically. One annulation opposite midline of anterior appendage and limbs 1-0, three in the space intervening between these limbs, five between anterior appendage and first limb; posteriorly annulations of trunk formed a continuous series with those of last pair of limbs. This arrangement implies that the body consisted of at least 12 somites. Annulations of the anterior appendage were sharp-crested, uniform in height; branches of the appendage were long, slim, pointed, three at the tip and three along the anterior side, each branch slightly flexible and movable about its base. About ten annulations on each limb, uniform in height, the cross-section varied from low, rounded, to high, sharp-crested, as the limb was extended, contracted or flexed. The tip of the limb was bluntly rounded, on the posterior wall of limbs 1-8, and the anterior wall of limbs 9, 10, was a group of seven curved claws. On limbs 2-8 a forwardly directed spine on the seventh annulation and a shorter spine on the distal annulation, on limbs 9 and 10 a prominent, backwardly directed spine. In front of the anterior appendage the one or two annulations were faint, the anterior end of the body bluntly rounded, the mouth, surrounded by a ring of six or so slim papillae, situated medially on the anterior wall. The alimentary canal is not preserved as a sediment fill, but as a reflective strip, widest adjacent to the mouth, extending back to end between the bases of the last pair of limbs. Sagittal length 1 to 6 cm, smallest similar to largest specimen. Aysheaia pedunculata is one of the rarer animals in the Burgess Shale, occurring in association with arthropods and worms, and to an exceptional extent with sponge fragments. It was not a burrowing, mud-ingesting animal, and the soft body would seemingly make it vulnerable to predatory arthropods. It may have been protected by living amid sponge colonies, the claws having facilitated clinging to the sponge, the anterior appendage holding the suctorial mouth in position to feed on the soft parts. While it shows resemblances to both Onychophora and Tardigrada, it is not placed in either group, nor in any taxon of higher rank than Family Aysheaiidae. It may be regarded as the sole known example of the types of lobopod animals from which the arthropod phylum Uniramia, and the Tardigrada, may have been derived.