A METAMERIC BRACHIOPOD FROM THE LOWER CAMBRIAN

The coelomic subdivision of eucoelomate animal phyla are generally thought to be so fundamental to the respective body plans that similarities between different phyla must reflect a common deep evolutionary root. In all lineages of extant brachiopods the major body cavity is formed by a single coelomic cavity and it is reasonable to assume that this was the case in the last common ancestor of the crown group, which most likely appeared in the Lower Cambrian. However, internal markings in newly collected specimens of the cosmopolitan brachiopod Eoobolus sp. from the Lower Cambrian Mural Formation (Jasper National Park, Canadian Rocky Mountains) indicate a subdivision of the body cavity into an anterior compartment containing two pairs of muscles and a posterior compartment containing up to five pairs of muscles. In the dorsal valve, the boundary between these two compartments can be traced from the left to the right of the proximal ends of the vascula lateralis. The median area of the boundary bends prominently forward, which indicates that this boundary represents the impression of the gastroparietal band, a transverse mesentery that stretches from the dorsal body wall to the gut in extant brachiopods. In the ventral valve of Eoobolus sp., the boundary between the two compartments occurs at a position approximately opposite the impression of the gastroparietal band in the dorsal valve. This arrangement is best explained by a mesentery that stretched continuously from the dorsal to the ventral body wall, thus subdividing the coelom of the main body cavity into two distinct compartments. Such an arrangement is supported by the development in modern linguliform brachiopods of the gastroparietal band from the coelomic lining of the dorsal body wall.

This metameric aspect of an otherwise uncontroversial brachiopod body plan has important implications for resolving the phylogenetic position of brachiopods. The presence of a subdivision in the main body cavity of Eoobolus sp. suggests that the putative deuterostome-like coelom of extant members may be a secondary convergence. If brachiopods are accepted as lophotrochozoans, a segmented Lower Cambrian brachiopod supports the hypothesis that segmentation is a plesiomorphy of the superphylum.