Chemical fingerprints and microbial biomineralization of fish muscle tissues from the Late Cretaceous Múzquiz Lagerstätte, Mexico

Abstract

Fossil fish specimens from the Múzquiz Lagerstätte (Late Cretaceous) of northern México have been analysed using UV light-induced visible fluorescence microscopy, Particle-induced X-ray Emission (PIXE), X-Ray Diffraction (XRD), and Scanning Electron Microscopy (SEM). Specimens examined with UV light microscopy show tightly packed trunk muscle tissues and digestive tract contents, as well as a color gradient from pink to orange to brown associated with the chemical state of the muscle tissues. PIXE analysis shows a 0.346 P/Ca ratio in muscle tissues, as well as a phosphorus increase by a factor of more than four compared to surrounding sediment. Quantitative XRD analysis shows that cryptocrystalline flourapatite (FAP) is the predominant mineral phase and calcite is complementary in the muscle tissues. Nucleation of FAP and calcite may have occurred simultaneously with organic decay, forming adhesive pellets in the soft watery carbonate mud, and caused immobilization of the carcasses. Electron microscope scans show muscle tissues preserved with cellular and subcellular features as well as digestive tract contents with calcareous nanoplankton. Fossil biofilms with bacteria have also been exceptionally preserved as intact cells, casts and molds. This cell-specific, rapid mineralization can be explained by a crystal seed process, which is discussed here.