Although fossilised remains of long dead animals are normally impressive (in size at least, as small fossils are much more likely to shatter under natural stress) what is really amazing is to find preservation of an animal’s soft tissue. Fossils are really just rocks that have taken on the shape of the bone as if poured into a mould, any soft tissue that can be recovered is the actual flesh that existed on the animal.
The Santana Formation in Brazil is a place of sedimentary deposit formed in the early Cretaceous period. The remains found here are located in deposits called Konservat-Lagerstätten which developed in an anoxic environment so the potential for rotting is minimised. This region is particularly known for its fish of which some 25 species have been found with the contents of their stomachs preserved. Once, the entire heart and connected valves of one of these fish was recovered, an incredibly rare find indeed.
All of the organic remains found have fallen into two categories, either they’re black and carbonaceous or orange and tinged with iron oxyhydroxide (Fe(OH)3). Previous studies have concluded that these differences in preservation are due to different rock weathering; but these studies were done on insects and the current data on fish remains seems to refute this.
Soft tissue from the fish called Dastilbe crandalli was collected from beige limestone, where iron oxyhydroxide permineralisation is found, and grey limestone where carbonaceous development is found. Through careful examination of the samples conclusions could be drawn about the preservation pathways and how they differed. The fish from the beige limestone were found to have a high concentration of iron present within them along with sulphur that should not have been there naturally any more. This leads to the conclusion that these fish have been pyritised (the compound iron sulphide is called pyrite) where the iron sulphide replaced carbon parts of the fish. The other fish, from the grey limestone, were instead kerogenised. The massive carbon content, along with the clear black colour, highly implied the presence of bitumen and kerogen in general. This also can explain some of the excellent muscle fibre preservation as the organic molecules in the kerogen could have formed a macromolecular chain with it which would have sufficiently resisted degradation.
Understanding both these methods is important as they operate to preserve different aspects of the animals. While pyritsation can save eyes, tendons and skeletal muscles; kerogenisation saves skin, connective tissue and muscle fibres. Put together and we get some of the best preserved specimens that can ever be hoped for in palaeontology.