Tafonomia da paleoictiofauna da Formação Tremembé (Oligoceno da Bacia de Taubaté), Sudeste do Brasil

Abstract

The Tremembé Formation represents a lacustrine system and fish are the most abundant vertebrates in its different shale and argillite levels. The taphonomic analysis of these fossil fishes is necessary for the refinement of knowledge about the environmental, depositional and preservation aspects of Paleolago Tremembé. The present study aims to infer the taphonomic history of fossil fish from the Tremembé Formation, through the identification and interpretation of macroscopic taphonomic signatures in skeletal elements of representatives of the ichthyofauna of the Tremembé Formation. A total of 491 specimens of fossil fish deposited in the paleontological collections of the State University of Rio de Janeiro (Laboratory of Paleontology of the Faculty of Geology and the Department of Zoology of the Institute of Biology Roberto Alcântara Gomes), of the Federal University of Rio de Janeiro (Laboratory of Macrofossil Collection) and the Museum of Earth Sciences (CPRM-RJ). For material analysis, the following taphonomic signatures were evaluated: (1) general articulation of the remains; (2) degree of scale preservation; (3) degree of articulation of the cranial bones; (4) degree of pectoral, pelvic, dorsal and anal fin articulation; (5) degree of articulation of the caudal fin; (6) degree of articulation of the pleural ribs; (7) soft tissue decomposition; (8) layout patterns in space; (9) association with other fossil remains and (10) sedimentary facies. The results indicated that this paleoichthyofauna exhibits three distinct taphonomic classes: (1) complete specimens, (2) disarticulated head and/or caudal fin, and (3) body fragments. The best preservation of fossil fish occurs in the black shale facies due to the low occurrences of post-mortem buoyancy, caused by the lower salinity and high hydrostatic pressures. The second best preservation is found in the gray shale facies, due to the higher occurrences of postmortem buoyancy, caused by the higher salinity and low hydrostatic pressures. The lowest quality of preservation is found in the green argilite facies, due to their association with higher energy environments, governed by mud currents. Mass mortality in the black and gray shale facies is reinforced by the high density and diversity of fossil fish. Arrangement patterns in space consistent with sedimentary stratification and lithology indicate that fish from the black shale and gray shale facies were deposited along with the sediments by decantation processes, at the place of death (in situ), with no reworking.