A surprising discovery in the feathers of a fossil vulture from central Italy has revealed that volcanic deposits can preserve delicate tissue structures in unprecedented detail, offering new insights into the fossilisation process.
Authors
- Valentina Rossi
Postdoctoral researcher, Palaeontology, University College Cork
- Dawid Iurino
Professor of Vertebrate Palaeontology, University of Milan
- Maria McNamara
Professor, Palaeobiology, University College Cork
In 1889 in the foothills of Mount Tuscolo, 25km south-east of Rome, farm workers discovered something extraordinary. While digging the ground for a new vineyard, they encountered a layer of bedrock with a strange void. This contained the skeleton of a large bird, including apparent imprints of its plumage on the surrounding rocks.
The bizarre find prompted the landowner to call in the renowned Italian geologist Romolo Meli. By the time of Meli's arrival on site, however, the workers had consigned most of the fossil blocks to the waste pile, and many were broken.
After salvaging most of the rocks, Meli identified the specimen as a fossilised griffon vulture. He also noted that the preservation of the plumage was unusual considering the host rock was volcanic.
Meli produced a report about the discovery later that year, and then the fossil vulture faded into obscurity and most of the rock samples were lost. All that remains today are blocks containing the plumage of one wing and the imprint of the bird's head and neck.
A few years ago, advances in analytical approaches to studying fossils prompted researchers to become more interested in the specimen, which probably dates from around 30,000 years ago. In 2014 one of us (Dawid Iurino) led a new study using CT scanning (computed tomography) of the imprint of the head and neck.
This revealed three-dimensional details of the bird's eyelids, tongue and the texture of its skin and neck (see the video below). Such fine preservation of biological features exceeds even that of the victims of Pompeii .
In our new study, we then examined the feathers and it became clear that we were looking at something out of the ordinary. Our preliminary microscope analyses surprisingly revealed that the feathers, which have an orange colour that contrasts with the host rock, were preserved in three dimensions.
Three-dimensional fossil feathers are more commonly found in amber , whereas those in rocks are normally two-dimensional thin layers of dark-coloured organic matter.
Yet there were still important unanswered questions around how the feathers were preserved in a volcanic deposit, so we carried out some further investigations.
A new way to fossilise
A more detailed microscopic analysis revealed that this three-dimensional preservation extended to the delicate branches of the feathers. We could even see feather structures that were less than one micron (0.001mm) wide, specifically tiny cell organelles (part of a cell) called melanosomes whose pigments contribute to the colouration of feathers.
Even stranger was the fact that the fossil feather was made of a mineral called zeolite. This mineral is not associated with any other fossil tissues, revealing a means of fossilisation that has never been recorded before. It came about because zeolite forms via the dissolution of volcanic ash and glass.
The fossil's level of tissue detail, plus the chemical composition of the feathers, indicates some important differences between the pyroclastic flows that entombed the vulture and the flow that buried Pompeii.
The ancient residents of Pompeii were buried alive by hot fast-moving, turbulent flows of gas and ashes known as pyroclastic flows, at temperatures exceeding 500°C. At these temperatures, their soft tissues were vaporised, leaving only skeletons and charcoal.
On the other hand, we do not know exactly how the vulture died. It may have been asphyxiated by toxic clouds of volcanic gas, or may have been killed directly by the pyroclastic flow. What we do know is that the flow was relatively cool because it was diluted with water or far from the volcanic source.
The processes by which the volcanic sediment hardened into rock and formed zeolite happened relatively quickly (within days), which may explain why delicate structures such as feathers can preserve well in three dimensions. This opens up the possibility that many other ash-rich volcanic rocks may contain remarkable fossils, and are therefore exciting new targets for palaeontological research.
Valentina Rossi received funding from The Palaeontological Association (UK) and The Paleontological Society (USA). She is currently receiving funding from the European Research Council (H2020-ERC-CoG-1010003293-PALAEOCHEM awarded to Prof. Maria Mcnamara).
Maria McNamara receives funding from the European Research Council and Research Ireland.
Dawid Iurino does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
