Oxygen requirements, biology and phylogenetic significance of the late Precambrian worm<i>Dickinsonia</i>, and the evolution of the burrowing habit

Abstract

Dickinsonia was a very thin (<3 mm) subcircular to elliptical worm that grew to about a metre in length. It appears to have been a common, conspicuous and widespread Late Precambrian animal. Dickinsonia probably had a hydrostatic skeleton (true coelom); metamerically arranged dorsoventral muscles; longitudinal, transverse and possibly diagonal muscles; an inextensible collagenous basement membrane; a well-developed circulatory system; and possibly a gut with lateral lobes. It was probably an annelid, but may have been neither a polychaete nor an oligochaete, and its geometry and ontogeny suggest that its similarity to the living discoidal polychaete Spinther is due to convergence. Calculations indicate that Dickinsonia could have respired in sea water containing about a tenth of the present amount of oxygen. Since it lived in well-aerated sublittoral environments, its peculiar shape may reflect the low level of oxygen in the Late Precambrian atmosphere. The geometry of coeval animals reinforces this view. Few, if any, Precambrian trace fossils can be attributed to the activities of sizeable animals burrowing by peristaltic movements of the body wall. The existence of Dickinsonia prior to the first clear evidence for peristaltic burrowing by macroscopic animals casts doubt on Clark's hypothesis that both body cavities and segmentation developed in response to the need for an efficient hydrostatic skeleton which was used for burrowing in soft substrates. It now seems possible that both structures first appeared in surface dwellers as a means for increasing body size. Only subsequently were both used as devices for invading soft substrates.