Source: The Guardian

I was looking at Romer’s Gap the other day. No this isn’t a dirty confession, although I did come back with sand in my hiking boots. Just before Christmas I found myself digging into the fine shingle of a beach opposite Bass Rock, near Edinburgh, in the pouring rain. Beside me were three burly men with shovels, who were undoubtedly making faster progress than I. We were digging down 350 million years into Scotland’s past, in search of bones from a time when the fossil record falls eerily silent: this is Romer’s Gap.

There are often periods of geological time where conditions were not good for fossil preservation, or the creatures that lived didn’t preserve well (such as the very earliest boneless, squishy lifeforms over 540 million years ago). In some cases we haven’t found the right rocks of that age yet, or we can’t access the rocks, for example if someone has peskily built a city on top of them. Occasionally these fossil gaps represent true low points in diversity – there simply weren’t very many animals in existence to then die and become fossilised – but often the fossils have just not been uncovered yet.

Romer’s Gap is a global missing page, a time when the record of tetrapods, our four-limbed vertebrate ancestors, is scanty at best. It was first recognised by the famous and prolific palaeontologist, Alfred Romer, and so it was given his name. The lack of information is a real snag, as it is also when our earliest terrestrial ancestors evolved the key features that would let them breathe air, lift their bodies out of the water, and, 350 million years later, wrap their cold digits around the handle of a shovel on a rocky beach in Scotland.

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What caused Romer’s Gap? The mystery begins at the end of the Devonian, a time period famous for its fishes. The earliest tetrapods had begun to develop: limbed, fish-like creatures clearly on their way to becoming land-lubbers, but with anatomy that wasn’t quite cut out for the job. Their legs were too weak, they still had gills, and their limbs ended in decidedly fishy fins rather than fingers. At the end of the Devonian, 375–360 million years ago, two mass extinctions decimated a much of the life on earth. For the next 15 million years (the start of the Carboniferous period) the fossil record goes strangely quiet. It is probable that the low concentration of oxygen on earth reduced the amount of fossilisation, but there were also fewer animals around to be preserved. This makes the search to uncover remnants from this time especially tricky, and every new find is an exciting piece in the evolutionary jigsaw puzzle.

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