Gordia marina Emmons, 1844

Diagnosis.—Thin, arcuate to winding burrows or trails with self-overcrossing patterns (De Gibert & others, 2000).

Discussion.-Tunnels, which are systematically observed at the top of rippled beds, are clearly dug very superficially. Locally, their upper wall may collapse. Some areas show simple gently sinuous forms, probably illustrating locomotion. Other areas show dense looped forms, probably indicating grazing. These traces are clearly more abundant and more complex along the crests of ripplemarks (Fig.5.6). On these crests, the sediment is thinner and more oxygenated. This last point is attested to by a concentration of probable air bubbles along the ripple crests (Fig. 8.8), possibly because of the presence of a thin microbial film. Gordia probably corresponds to a surficial burrow made soon after the deposit that shows an active search for food by a shallow infaunal animal. These traces generally are interpretated as grazing traces (Buatois et al.,1997).

The smooth, unbranched shallow horizontal burrows have a constant diameter of 1mm and wind irregularly over a length of several centimetres, where they cross themselves and other burrows. Some examples exhibit a distinct median groove that is persistent over a distance of a few millimetres. Burrow fill is similar to the host rock and may exhibit delicate transverse striae.