Australasian Science: Australia's authority on science since 1938

The Heart of a Good Fossil

By John Long

Palaeontologists have found their Holy Grail: the fossilied heart of a Cretacean fish.

Most of us think of fossil vertebrate remains as being bones or teeth. In rare cases we can sometimes see skin impressions or the outline of feathers on fossil birds and dinosaurs, but these are among the top 1% for fossil preservation.

Such exquisite fossils come from deposits called konservat laggerstätten (meaning “place of storage”), which are the result of rapid burial and special chemical conditions involving low-oxygen environments. In special cases these deposits can preserve or mineralogically “ghost” a range of soft tissues, even complete internal organs, in the fossilised organism.

Scientists have long known about the famous Burgess Shale fossils in British Columbia, where soft-bodied worms and other invertebrate creatures were buried by rapid mud slides around 508 million years ago. However, well-preserved fishes from the 113–119 million-year-old Santana Formation of Brazil were among the first vertebrate fossils to show evidence of preserved soft tissues. These include parts of the glandular stomachs and bands of muscles, with the original tissue mostly replaced by chemical processes.

To find whole preserved internal organs in a fossil is the Holy Grail of palaeontology. Such discoveries contribute a wealth of new anatomical information that is essential for understanding evolutionary patterns.

Therefore the recent discovery of a complete, well-preserved fossilised heart in an almost 120 million year old fish ( was a major breakthrough for the team of largely Brazilian scientists led by José Xavier-Neto of the Brazilian Biosciences National Laboratory. The heart is perfectly preserved in 3D in a number of specimens, even showing valves that regulate the outflow of blood from the conus arteriosus. The Rhacolepis specimens fill in a gap from what is known from primitive living ray-finned fishes with many heart valves to the teleosts, which have only one valve.

This new finding used synchrotron X-ray tomography to image Rhacolepis fossils that were still entombed within limestone concretions. This technique images the fossil in thin sections; these images can then be processed to render the heart slice by slice and digitally restore the features of the organ. This method has been widely applied in palaeontology for the past decade or so to reveal many intricate soft tissue structures in fossils.

Other examples of superb soft-tissue preservation in fossils include the remarkable preservation of actual fossilised sperm in a 16 million-year-old ostracode (a bivalved crustacean) from the World Heritage Riversleigh sites in Queensland (Fossil File, July 2014), and a preserved shrunken brain inside the braincase of a 300 million-year-old shark-like fish from North America. Our 380 million-year-old Gogo fish fossils from Western Australia have yielded a collection of superbly preserved soft tissues that include the twisted umbilical cords associated with an embryo, nerve axial plate cells and muscle bundles. Recently the complete neural and vascular systems were reported in 520 million-year-old arthropods from Chengjiang in China, preserved as differently coloured chemical traces of the original brain, nerve cords, blood vessels and veins.

However, caution must always be exercised when identifying such structures in fossils. About 16 years ago a fossilised heart made international news headlines when it was reported in a Cretaceous plant-eating dinosaur (Thescelosaurus) nicknamed Willo. A large scale CT scan of the dinosaur’s body cavity showed a heart-like structure preserved in ironstone. In 2011 a new study demonstrated that it is not a heart but the infill of iron-rich sediment washed into the decaying body cavity of the animal.

John Long is Strategic Professor in Palaeontology at Flinders University, and current President of the Society of Vertebrate Palaeontology.