Taphonomy of the fish fauna of the Tremembé Formation (Oligocene of the Taubaté basin), Southeastern Brazil

Abstract

Tremembé Formation represents an Oligocene lacustrine system from South America in which fishes stand out as the most abundant and well-preserved vertebrates. They occur in different shale (black and gray) and argillite beds. Since a taphonomic analysis of these fish fauna is essential to interpretating both the paleoecology and paleoenvironment of the so-called “Paleolake Tremembé”. This study aims to infer taphonomic modes through the identification and interpretation of macroscopic taphonomic signatures in skeletal elements. We analyzed 491 specimens of fishes from which the following taphonomic signatures were considered: (1) degree of articulation of general skeleton; (2) degree of preservation of the scales; (3) degree of articulation of cranial bones; (4) degree of articulation of pectoral, pelvic, dorsal, and anal fins; (5) degree of articulation and preservation of caudal fin; (6) degree of articulation of pleural ribs; (7) degree of decomposition of soft tissue; (8) tetany; (9) layout pattern in space; (10) association with other fossil remains; and (11) sedimentary facies. The results indicate that this fish fauna exhibits three distinct taphonomic classes: (1) complete and fine skeletal preservation of specimens; (2) disarticulated head and/or caudal fin; and (3) dispersed body fragments. In the black shale facies, the best preservation of fossil fishes occurs due to the low occurrences of post-mortem buoyancy, caused by the lower salinity and high environmental pressures. The second-best preservation is observed in the gray shale facies, due to the higher occurrences of post-mortem buoyancy caused by the higher salinity and low ambient pressures. The lowest quality of preservation occurs in the green argillite facies, due to association with higher-energy environmental conditions governed by muddy currents. The preservation of soft tissues in the black and gray shale facies is probably associated with the partial anoxia and subanoxia of the lake bottom, which guaranteed favorable conditions for authigenic mineralization by microbial mats. The green argillite facies include only siluriform fishes, whose deposition occurred before the tectonic activities on the northeast and southwest edges of the Taubaté Basin, responsible for the diversion of waters that gave rise to a primitive Paraíba do Sul River, allowing the invasion of some Perciformes and Characiformes. The polytypic character of the fish assemblage and cases of tetany indicates mass mortality in the black and gray shale facies. Disposition patterns in space consistent with sedimentary stratification and lithology indicate that fishes from the black shale and gray shale facies were deposited by sediment decantation at the site of death (in situ) and without reworking.