Macromolecular resolution of fossilized muscle tissue from an elopomorph fish

Abstract

THE Santana Formation (Lower Cretaceous, Aptian/Lower Albian) of Ceara, Brazil, contains many exceptionally preserved fish fossils within carbonate concretions^1,2. These concretions formed before compaction of the sediment and nucleated around carcasses of fish killed in mass-mortality events. Some concretions may contain several fish, preserved in three dimensions with fully articulated skeletons¹. A few specimens contain calcium phosphate in the form of cryptocrystalline hydroxyapatite. This occurs as coatings on bones and as a replacement of tissues. Samples of mineralized soft tissues can easily be liberated from the concretions by immersion in 10% acetic acid³. Of several mineral phases often preserving soft tissues, calcium phosphate and silica probably offer the greatest resolution of detail⁴. Fossilized soft tissues are most frequently found in fishes, although in the Santana Formation pterosaur wing membrane⁵ and the cuticles of arthropods^6,7 have been reported. I report here the study of striated muscle tissue from a fossil elopomorph fish⁸ from this formation, in which subcellular ultrastructural features are distinguishable. The exceptional degree of preservation of the specimen raises interesting questions about the mechanism of fossilization.