DIFFERENCES IN TAPHONOMIC SIGNATURE OF MOLLUSC SHELLS FROM SILICICLASTIC AND CARBONATE SEAGRASS BEDS

Seagrass beds represent an important modern and ancient global marine habitat. Despite their importance, the geologic history of this marine ecosystem is not well understood due to the poor preservation potential of seagrass. Direct and indirect taxonomic proxies have been suggested for use in recognizing seagrass beds, however, they have limited applicability because of preservational or biogeographic limitations. Because the presence of seagrass beds alters the physical, chemical, and biological components of a depositional environment, they may impart a recognizable taphonomic signature to use as a habitat proxy. In an attempt to develop such a proxy we have observed taphonomic differences between mollusc shells from carbonate and siliciclastic seagrass environments.

Based on sediment samples collected from the Western Atlantic and Gulf of Mexico we found that gastropods (especially Cerithium) from tropical to subtropical carbonate environments exhibit a much higher rate of encrustation than do gastropods from similar siliciclastic environments. The apertures of many of the highly encrusted Cerithium are pristine in appearance, suggesting that encrustation occurred during active habitation of the shell. Shells collected live (still containing an operculum) also displayed a high degree of encrustation. These data suggest that postmortem conditions such as slow sedimentation rates or pagurization cannot be cited as the cause of the high rate of encrustation; rather, it must reflect physical or biological conditions acting during the snail’s lifetime, such as low turbidity or high recruitment from adjacent colonized hard substrates (reefs). In addition to differences in encrustation levels, features interpreted as root etchings are found on thin-shelled bivalves, especially Tellina, collected from carbonate environments. Thin-shelled bivalves from siliciclastic seagrass environments lack any clear root etchings. Root etchings are primarily postmortem (occurring on internal valve surfaces) and therefore reflect postdepositional alteration of bioclasts.

The differences between carbonate and siliciclastic seagrass beds illustrates one of the challenges to developing taphonomic proxies for paleoenvironments, and will need to be accounted for in any taphonomic proxy that is developed.