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Fossil footprints force reassessment of prehistory

Polish and Swedish researchers have discovered fossil footprints from early backboned land animals which could push back the date that animals left their watery homes for land by at least 18 million years.

The study, published in the latest edition of the journal Nature, details discoveries made in the Holy Cross Mountains in south-eastern Poland.

Until now, claims of tetrapod tracks predating verified body fossils have remained disputed as to their age and origin. Tetrapods are vertebrate animals with four feet, legs or leg-like appendages, and while traced to the Devonian Period (408 million to 360 million years ago), their origin remains a mystery.

'These results force us to reconsider our whole picture of the transition from fish to land animals,' said Professor Per Ahlberg of Uppsala University in Sweden, one of the two leaders of the study.

For decades, palaeontologists have been scouring the planet for fossil bones and skeletons of the earliest land vertebrates - the ultimate progenitors of all later amphibians, reptiles, birds and mammals. Discoveries have suggested that the first tetrapods evolved relatively rapidly from lobe-finned fishes, through a short-lived intermediate stage represented by 'elpistostegids', about 380 million years ago. But there is another potential source of information about the earliest tetrapods: the fossilised footprints they left behind.

The Polish and Swedish researchers describe several trackways of different sizes and characteristics, as well as a number of isolated prints of up to 26 centimetres wide, indicative of animals of around 2.5 metres in length. The tracks have distinctive 'hand' and 'foot' prints and no evidence of a dragging body. The finds can be securely dated to the early Middle Devonian period, around 395 million years ago.

'[The study results mean] that not only tetrapods but also elpistostegids originated much earlier than we thought, because the position of elpistostegids as evolutionary precursors of tetrapods is not in doubt, and so they must have existed at least as long,' continued Professor Ahlberg. 'Instead, our distant ancestors may first have left the water in order to feed on stranded marine life left behind by the receding tide.'

The fact that the tracks were left on the shores of an ancient sea is also a major surprise; almost all previous scenarios for the origin of tetrapods have placed this event in a freshwater setting and have associated it with the development of land vegetation and a terrestrial ecosystem.

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