As glaciers melt in our warming world, they have exposed pieces of the past, from Stone Age artifacts to relics of war. But the retreat of the Tyndall Glacier in Chile has revealed something much older: a prehistoric graveyard for ichthyosaurs.
Ichthyosaurs were marine reptiles, or “lizardfish”, which resembled modern-day porpoises. They swam in the oceans between 250 and 90 million years ago, around the same time dinosaurs walked on the ground and pterosaurs took to the air. These creatures are now extinct, but their fossils continue to inform scientists about the species and its evolution.
So far, paleontologists have found 76 ichthyosaurs in bedrock adjacent to Tyndall Glacier in the Southern Patagonian Ice Field. Some of the fossils were discovered during an expedition to the site in March and April 2022, when scientists traveled to extract “Fiona”, a complete fossilized skeleton of a 4-meter-long female with several embryos. The fossil, which is between 129 and 139 million years old, was discovered in 2009 by Judith Pardo-Pérez of the University of Magallanes. The photo below shows Pardo-Perez and Fiona next to Tyndall Glacier in 2010.
Just a few decades ago, paleontologists would likely have missed some of these discoveries. Camilo Rada, a glaciologist at the University of Magallanes, estimated from photographs that Fiona had been discovered since at least 1965. “But other ichthyosaur fossils in the area were discovered much earlier, others much more recently and in all likelihood some they will be discovered as we speak,” Rada said.
The pair of satellite images at the top of this page show the edge of the glacier on January 14, 1986 and January 17, 2022. The images were acquired by the Thematic Mapper aboard the Landsat 5 satellite, and by the Generator Operational Land Imagers ( OLI) aboard the Landsat 8 satellite, respectively. The added color allows for better color matching between the two images collected by different sensors.
Fiona would have already been exposed at the time of these images. But according to Dean Lomax, a paleontologist at the University of Manchester: “I’m sure a lot of the specimens were under the glacier in the 1986 image.” This includes a well-preserved complete skull, previously discovered by Lomax, who was part of the recent expedition to excavate Fiona.
Exposed bedrock is an area where, in a typical year, snow and ice melt has exceeded fresh snow accumulation. A detailed view of this ablation zone along the eastern side of the glacier can be seen in the natural color image above, acquired by the Landsat 8 satellite on April 7, 2022, one of the few clear days during the recent delivery of a month. The lines show the previous locations of the ice edge, including its last maximum extent around 1700 during the Little Ice Age and its retreating position beginning in 1986. In recent decades, parts of the glacier edge have retreated 2 kilometers away.
In 2004, when Pardo-Pérez began exploring this area, scientists estimated that the fossil site covered around 5 square kilometers. Pero a medida que el borde del hielo retrocedía y el paisaje cambiaba, ya medida que las posterior expediciones localizaban más fósiles, el site ha expandido a unos 15 kilómetros cuadrados (casi seis millas cuadradas), o casi toda el área de roca expuesta visible in this picture. The rock is part of the Zapata Formation, which contains sedimentary rocks and fossils dating from the late Jurassic to early Cretaceous.
The fossilization happened millions of years before the glacier appeared, when the area was covered in seawater. Scientists believe that some of the ichthyosaurs died of natural causes. Others likely perished in mass mortality events caused by the rapid downward flow of water known as the turbidity current. “In these cases,” Pardo-Pérez said, “the ichthyosaurs could have been trapped by the turbidity current and thrown into the abyss, drowned, disoriented, and buried almost instantaneously in an anoxic environment that prevented bacterial decomposition and maintained their articulated joints, skeletons”.
The glacial ice that eventually covered the fossils did not help preserve them. Rather, Rada pointed out that before the ice melted, it had been “flowing” for a long time. This flow of ice carries rocks and dirt to its base, which Rada likens to a heavy sheet of sandpaper, crushing the bedrock and the fossils within.
In Patagonia, erosion rates range from 1 to 100 millimeters (0.04 to 4 in) per year and are likely at the lower end of this range on the Tyndall Glacier side. “But even with erosion rates of a few milliliters to a centimeter per year,” Rada said, “Fiona would have turned to dust if she had remained covered by the glacier for a few more decades.”
However, the loss of the ice poses other problems. The fossils were left vulnerable to fracture by freeze-thaw cycles and erosion by wind and water. “It is important to find ways to protect these precious documents from the past,” Rada said.
Fossils were likely exposed near other glaciers, as the entire southern Patagonian ice field is melting. But until paleontologists conduct further prospecting expeditions, the site near Tyndall Glacier remains a unique paleontological find. “As far as we know, there is no other place in the world where so many outstanding fossils are exposed due to a retreating glacier,” Lomax said.
The site is protected by the National Forestry Society of Chile (CONAF) and its fossils are protected by Chilean law which prohibits extraction or excavation without authorization. “This place is a fragile ecosystem, located in a periglacial area of Torres del Paine National Park,” CONAF director Gonzalo Cisternas said. The area is closed to tourism and recreational activities and can only be visited by authorized scientists.
NASA Earth Observatory images by Joshua Stevens, using US Geological Survey Landsat data. Photograph by Alejandra Zúñiga/National Service for Cultural Heritage. Reporting by Kathryn Hansen.
Read this story in Spanish here.
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