TIME AVERAGING ON A SHALLOW SUBTROPICAL SHELF: THE SEARCH FOR A TAPHONOMIC CLOCK

Studies of rates of time averaging in present-day settings are important for understanding the temporal resolution of the fossil record. The taphonomy of shells has been used in paleontological research as a proxy for time averaging and the duration of pre-burial taphonomic history (age-since-death) with varying results. Here, we use rates of time averaging estimated from directly dated specimens of the tellinid bivalve Semele casali and the terebratulid brachiopod Bouchardia rosea to analyze the informative value of taphonomic signatures on pre-burial history.

The two studied organisms were collected along a transect from two shallow (10 and 30 m) nearshore localities from Ubatuba Bay located on the inner part of the Southeast Brazilian Bight (SW Atlantic), a mixed carbonate-siliciclastic shelf. The two targeted species are approximately the same in terms of size, but differ in many respects. M. cleryana has an aragonitic shell and a shallow infaunal life habit, whereas B. rosea is calcitic and an epifaunal, free-lying suspension feeder. To quantify specimens in terms of preservation, additive taphonomic scores for the presence or absence of disarticulation, fragmentation, edge completeness, glossiness, interior and exterior surface alteration, visibility of muscle scars and shell color were computed. A higher taphonomic score indicates that a shell is more degraded. Ages of shells were estimated using amino acid (D/L aspartic acid) racemization, calibrated against paired AMS-radiocarbon analyses. Taphonomic scores were compared against shell age both between taxa and between collection sites.

Previous studies have indicated that due to shell microstructure, brachiopods may disarticulate and disintegrate faster than bivalves. Data suggest however, that brachiopods in this assemblage (median age 695.7 years) are slightly older than bivalves (median age 250.9 years). Taphonomic scores were also generally greater for brachiopods than for bivalves, and greater at the 30 m site than at the 10 m site. Furthermore, the data agree with previous studies that taphonomic grade of individual shells can not be used alone as a proxy for the duration of its pre-burial history. The use of taphonomy is more promising at the assemblage level, and the level of taphonomic degradation, or taphonomic score of the assemblage does appear to increase with age since death.