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Fossil Swim Tracks

27 February 2015

At www.livescience.com/49937-ancient-reptile-tracks-mass-extinction.html … we are told the end of Permian mass extinction event may even have wiped out marine worms and other burrowing creatures that make holes in the mud on the bottom of the sea. This appears to be an extinction too far – on the face of it. The researchers came to this conclusion as in the following era, the early part of the Triassic, massive reptiles had evolved, and their tracks are preserved in rocks in modern day Utah. They did not so much as leave footprints in the sand (or mud) but lines caused by their claws digging in and trailing through.

The same story is at http://phys.org/print344279715.html …. which describes them as 'trace fossils' made by four footed land living vertebrates as they travelled through water under buoyant conditions. They occur in high numbers in Early Triassic rocks. It is unknown why these tracks are so abundant and well preserved.

Palaeontologists from the University of California (Riverside) have tried to show how swim tracks came about and how they were kept in mint condition over the last 100 million years and more. They cite a lack of sediment mixing (as a result of a lack of burrowing animals such as worms), depositional environment (sediment laid on firm ground) and animal behaviour (swimming and bottom walking, the long claws creating lines rather than footsteps).

The idea swim tracks were preserved instantly, in the blink of an eye so to speak, hence the good preservation quality of them, does not seem to have been part of the agenda. I may be mistaken as there are probably good reasons why that should be so but one can't help but wonder if an actual shortage of Early Triassic rocks would require stretching what they have over a very long period of time. In general, the lack of mention of a catastrophic event may speak louder than words. The swim tracks might represent another graphic example that rocks preserve catastrophic events, including localised natural disasters as well as the big global ones, and they preserve nothing at all on what was going on between such events. A major extinction event occurred at the beginning of the Triassic (the end of Permian) and at the end of the Triassic (at the Jurassic boundary) and between the Permian event and the swim tracks in Utah massive reptiles had developed – on land and in the sea. Where is the record of what was going on between the two?

The lack of interest in immediate burial of the swim tracks may have something to with the publisher, Geology journal – but again, it might not. It might just be a research assumption that such events take place gradually, rather than quickly. Go to http://geology.gsapubs.org/content/43/3/215

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