Ternary taphograms and the comparative taphonomy of recent mollusks from the north sea and the Gulf of California

We examined the taphonomic condition of bivalve shells from the East Frisian Islands of the German North Sea coast (Cerastoderma edule), and two localities in the northern Gulf of California: Bahia la Choya (Chione spp.) and Colorado Delta (Mulinia coloradoensis). We classified each shell according to its taphonomic condition with respect to seven features: abrasion, edge preservation, bioerosion, encrustation, internal and external color, and fragmentation. The northern Gulf of California has a macrotidal regime and the German North Sea coast has a mesotidal regime. The Gulf of California has a hotter and drier climate than the German North sea coast. The Bahia la Choya locality has coarser sediments and a lower rate of sedimentation than the other two localities. We used ternary diagrams (taphograms) to characterize the frequency of preservational grade within a sample and to compare the taphonomic signatures of the three regions. Comparison of North Sea, Bahia la Choya, and Colorado Delta samples shows: (a) high wear among North Sea shells; (b) high fragmentation and encrustation among Bahia la Choya shells; and (c) high variation in surface condition and limited bioerosion and encrustation among Colorado Delta shells. The variation in shell condition among these three localities reflects (1) the relatively high energy regime of the German North Sea localities, as compared to the northern Gulf of California; (2) the low sedimentation rates of Bahia la Choya, as compared with the North Sea and Colorado Delta localities; and (3) the soft substrates and limited subaqueous exposure of Colorado Delta shells, as compared with the Bahia la Choya and North Sea shells. This study demonstrates that regional variation in the preservational quality of fossil shells can reflect many of the differences in the original environments of deposition.