Soft-tissue preservation in an ichthyosaur from the lower Cretaceous (Barremian-Aptian) of Colombia

In recent years, global research on soft-tissue preservation in fossils has grown significantly, offering insights into the biology, taphonomy, and diagenesis of extinct organisms. However, studies from low-latitude regions, particularly northern South America, remain limited, and this gap, together with the Lower Cretaceous Gap, restricts our understanding of the processes that enable soft-tissue preservation in these regions. The Paja Formation is a Lower Cretaceous (Hauterivian to Aptian) marine sequence located in central Colombia, notable for its abundant and well-preserved fossils, including vertebrates, invertebrates, and plants. In this study, we analyzed a Barremian-Aptian ichthyosaur from the Paja Formation to investigate soft-tissue preservation. We applied various analytical techniques, including EDTA demineralization, thin-sectioning, transmitted and cross-polarized light microscopy, SEM-EDS, ATR-FTIR spectroscopy, and Raman microspectroscopy. Our findings reveal net-like structures interpreted as fragments of the stratum spinosum layer of the epidermis, composed of N-heterocyclic polymers. We identified laminar and flexible structures defined as the skin-like layer of preserved integumentary tissue, with original proteins transformed into geochemically stable components, including N-heterocyclic polymers and kerogen-like materials. This study reports the first known case of preserved integument in a marine reptile from northern South America. We propose a taphonomic and preservational model based on rapid burial in soft sediment, persistent dysoxic-anoxic regional conditions, microbial biofilm activity, and the formation of early diagenetic carbonate concretions that influenced soft-tissue preservation in this specimen.