Exploring the restricted Lontras Shale fossil record: Data biases or preservational casualties?

Abstract

Throughout the Phanerozoic, numerous well-preserved fossiliferous sites have provided valuable insights into the evolution of ecosystems and organisms. Among these sites, the Lontras Shale Lagerstätte (LSL) within the Lontras Member in southern Brazil stands out as a rare glimpse into Carboniferous ecosystems of western Gondwana. The LSL is renowned for its diverse array of well-preserved fossils like insects, wood, fishes, larval cases, and poriferans, with different types of preservational pathways. Moreover, the Lontras Member is considered a maximum flooding surface that marks an interglacial warming peak of the final Late Paleozoic Ice Age, highlighting its paleoclimatological and paleogeographical relevance. However, the occurrence of fossils within the extensive Lontras Member is restricted to a few sites, raising questions about the factors influencing this constrained lateral and vertical distribution. Through meticulous literature review, this study aims not only to solidify the understanding of the Lontras Member's lithostratigraphy and paleontology but also to explore its broader implications and paleoenvironmental significance. We propose a comprehensive stratigraphic section for the Lontras Member, integrating local and regional sections from Santa Catarina and Paraná States, surveying paleontological sites and fossils exhaustively. By examining two additional fossil-rich sites in addition to the Lagerstätte outcrop, and analyzing vertical and horizontal distributions of the Member, we thoroughly explore the methodological, paleoenvironmental, and taphonomic factors that restricts this fossil record. This analysis extends beyond comparisons with paleontological insights from other units, locations, and Lagerstätten, encompassing modern analogous environments. The methodological biases are tied to stratigraphic focus, fieldwork scale, fossil size, and weathering degree of outcrops, constraining new fossil discoveries. Paleobiological and paleoclimatological factors, like transitional climatic conditions, limited organism diversity, inhospitable deep environments distant from the coast, and anoxic bottoms impact organism distribution and fossil recovery. The fossil transportability and the compositional resilience under widespread anoxia contributed as preservational constraints. While the LSL offers unique conditions for the proliferation and preservation of organisms, limiting the taphonomic window, facies of more energetic depositional conditions in other Lontras Member sites is more likely to harbor fossils, particularly transportable fragments and disarticulated pieces of chitinophosphatic or phosphatic composition. Addressing these biases unveils insights into flourishing ecosystems and organisms during this specific time period.