FEEDING MECHANISMS OF MODERN CADDIS-FLY LARVAE: A CAUTION FOR PALEOCURRENT ANALYSES BASED ON ALIGNED TENTACULITES FOSSILS

Alignment of conical Tentaculites fossils has been used for more than a century in paleocurrent analyses of Early Paleozoic shallow marine strata. Although neither the taxonomy nor life style of the Tentaculites animal are known, the hydrodynamic properties of its fossils as clasts invite its use as a paleocurrent indicator. The usual interpretation is that the fossils are aligned parallel to the ambient current, with the apex of the cone pointing upstream.

Observations of modern caddis fly larvae in streams challenge this interpretation, questioning the assumption that the aligned fossils were pas-ive clasts arranged by the current. Larvae of several caddis-fly species build protective structures similar in size and aspect ratio to Tentaculites fossils. At this stage in their life cycle, the larvae are sessile, attaching these structures to stream beds with special appendages and with strongly preferred orientation diametrically opposite that inferred for Tentaculites. The broad end of the cone faces upstream so that the current brings food to the animal.

These observations do not resolve the issue of Tentaculites taxonomy but do serve as a caution for paleocurrent analysts in situations where corroborating evidence for current direction is not available.