The end-Permian mass extinction around 250 million years ago wiped out over 90 percent of all the species on the planet. Based on the fossil record, "the Great Dying" was followed by a prolonged period of delayed recovery. Now, according to a new Geology study, that delayed ecologic recovery actually created just the right conditions for preserving the fleeting tracks left behind by prehistoric reptiles swimming and wading in the water.
We can learn a lot about past environments and the behavior of dinosaurs and early vertebrates by studying the footprints they leave behind. But a lot of conditions have to be just right for these traces to be preserved over millions and millions of years. That goes especially for underwater swim tracks left behind by buoyant four-legged vertebrates hunting for fish and squid. The production and subsequent preservation of these sorts of footprints require a firm and semi-cohesive substrate in order to maintain details before and after it gets buried by sediment.
Early Triassic deposits contain the highest number of fossil swim tracks in the world compared to other epochs. Some of these have preserved exceptionally detailed features like scale ridges and crescent-shaped claw marks. This spike in swim track occurrences suggests that factors promoting their production and preservation were more common in the Early Triassic—which followed the worst mass extinction event in Earth's history.
Were there simply more reptiles wading around after the Permian, or was it something about the surface they were wading through? To investigate, Tracy Thomson (pictured above) and Mary Droser from the University of California, Riverside, studied swim tracks in the lower Triassic Moenkopi Formation of Utah. This area was once a delta filled with little invertebrates like worms that churn up sediment.
They found that factors unique to the Early Triassic increased the preservation potential of detailed swim tracks. Typically, "there are all sorts of things that keep that sediment mixed," Droser tells Live Science. "But if you take them away, then the mud becomes sticky and hard." The delayed recovery following the mass extinction event resulted in extremely low intensities of bioturbation—or when living organisms disturb the sediment—especially in stressful, brackish areas like this. Without that mixing, the dewatered, “firm-ground” mud substrate was able to better record and maintain trace fossils like swim tracks.