A Fossil Heart In An Ancient FIsh

Organ preservation is now a well documented phenomenon in the fossil record. From the nervous systems of primitive arthropods to the reproductive tracts of a number of exceptionally well preserved dinosaurs, a combination of new technologies and remarkable fossil discoveries, have enabled us to reconstruct the anatomies of long extinct species. In 2000 a fossilised heart was reported from a species of dinosaur known as Thescelosaurus. This was the first time such a structure had been reported from vertebrate remains. The discovery was quickly debunked, however, as an iron rich concretion - a quirk of the geological record. 

The 3D reconstruction of Rhacolepis. The heart is shown
in blue and the position of the valves with white arrows

In 2005, a series of organic traces representative of the heart was reported from fish from the Devonian in Scotland. These were flat films, however, which preserved little detail.

In contrast, the 113 to 119 million year old Santana formation is famous for its beautifully preserved 3D fish fossils which occasionally preserve traces of soft tissues, such as muscle blocks.

A specimen of one well- documented species, Rhacolepis, has been found to contain a heart preserved in 3 dimensions.

A phylogeny showing the placement of Rhacolepis in the actinopterygians

By taking x-ray scans using a synchrotron at 6 micrometre intervals, an international team of researchers were able to make a 3D digital reconstruction of the fossil, including the heart.

The scans clearly showed the conus arteriosus, a bulk-shaped portion of the apex of the heart which contained five valves for directing blood flow. Dissection of the heart of a modern tarpon allowed the researchers to determine the evolutionary complexity of the fossil heart using comparative anatomy.

Actinopterygians, or ray-finned fish, have highly varied heart structures. Primitive forms, such as chondrosteans, have up to nine valves controlling outflow, while the most advanced forms, the teleosts, have just one valve - the bulbus arteriosus.

The five valves of Rhacolepis show that its placement was intermediate within the actinopterygians close to the base of the teleost family itself. Comparative anatomy has been the primary means of identifying a fossil's place within the tree of life, since palaeontology was founded as a science. Comparison, however, has focused almost entirely on a creature's morphology and endoskeleton or exoskeleton. The ability to use elements of physiology within the comparative anatomy of fossils will prove to be an invaluable tool in future examinations of extinct branches of the tree of life.