TAPHONOMIC ROLE OF PERIOSTRACUM ON THE PRESERVATIONAL POTENTIAL OF THE FRESHWATER BIVALVE MOLLUSK SHELLS

Dissolution is one of the most pervasive taphonomic processes in tropical environments. Modern and Subfossil freshwater Unioidea assemblages from fluvial channels of the Touro Passo Stream, southern Brazil, yielded severely corroded shells, which integrity was only kept by the periostracum. This indicates that a diagenetic-bias driven by the presence/absence of periostracum may occur in freshwater bivalve mollusk assemblages. In order to understand the role of periostracum in the preservational potential of Unioidea shells, periostracum-bearing shells were submitted to a series of dissolution experiments. Fresh shells of Anodontites (n= 12), and Diplodon (n=20) were split apart, and the periostracum of one shell (of a given matching valve) were mechanically removed. Subsequently, all shells were immersed in 4% acetic acid solution for 24hours to determine relative rates of dissolution. Examinations were made at 0.5h intervals for the first 3h, at hourly intervals for the subsequent 6h, and from then on 3h until 24h. Dissolution rate (DR, %/h); dissolution half-life (DHL, h); and the percent of weight lost after 30 minutes (W30) were calculated. For periostracum-bearing shells, DRs are 26.1 (Anodontites), 31.3 (Diplodon); DHLs are 15 (Anodontites), 21 (Diplodon); and W30s are 2.5 (Anodontites), 2.8 (Diplodon). Peristracum-removed shells showed DRs of 45.5 (Anodontites), 61.3 (Diplodon); DHLs of 7 (Anodontites), 7 (Diplodon); and W30 of 5.1 (Anodontites), 5.8 (Diplodon). Hence, periostracum-removed shells dissolved twice as faster. Dissolved periostracum-bearing valves showed a complete loss of the inner shell layer. Due to this, dissolved freshwater bivalve shells quickly loose the muscle scars. This may explain the common poor preservation of these anatomical features in many freshwater bivalve fossils preserved as internal molds. Other typical dissolution shell signatures found are: (a) loss of luster and internal valve color, (b) presence of holes in the internal surface of shell (externally hidden by the periostracum), (c) shell bleaching, and peeling, (d) thinning of edges, (e) cracks along growth lines (chemically-facilitated fragmentation). Hence, the presence of periostracum plays a role in the post-mortem history of freshwater bivalve mollusk shells.