Environmental Control of Trace Fossil Morphology
Marked differences in the morphology of modem gastropod trails can be related to sediment consistency. In uncompacted sediment of tidal channels, tidal channel banks, and low tidal flats in Barnstable Harbor, Massachusetts, the gastropod Polinices duplicates produces deep V-shaped grooves as the animal moves several centimeters below the sediment surface. Bilobed, transversely ridged trails are formed in firmer substrate of the high tidal flat where P. duplicates moves nearer the surface. Results of laboratory experiments on P. duplicates moving through various sediment mixtures suggest that grain size and substrate consistency are two controlling factors of trail morphology.
Trails, presumably formed by snails, occur in Pennsylvanian sandstones of tidal origin in Tennessee. Substrate characteristics of the rocks were determined qualitatively by examination of sedimentary features. Comparison of trail morphology with syn-depositional sediment mass properties suggests a close relationship between substrate consistency and trail morphology. In sandstones likely deposited on higher parts of a tidal flat, single trails change from (1) V-shaped furrows to (2) bilobed, transversely ridged trails to (3) longitudinal rows of tiny knobs. These morphologies apparently resulted from the snail moving at decreasing depth in a uniformly firm substrate. In sandstones representing looser sand of lower tidal flat origin the snails produced V-shaped and lobed trails that lack prominent transverse ridges.
Recognition that different trace morphologies can be produced by the same organism is useful for paleoecological interpretation for at least three reasons. (1) The variety of trails produced may be a clue to syn-depositional sediment mass properties. (2) Variety (or degree of variation) of gastropod trail morphology may help identify the tidal flat environment of deposition. (3) Recognition of variation will result in a truer picture of trace producer diversity and development of more accurate compilations of ichnogeneric paleoenvironmental distributions.