So far, paleontologists have found 76 ichthyosaurs in bedrock adjacent to Tyndall Glacier in the ice field of the Southern Patagonia. Some of the fossils were discovered during an expedition to the site in March and April 2022, when scientists visited 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 from the University of Magallanes.
Just a few decades ago, paleontologists would likely have missed some of these discoveries. Camilo Radaglaciologist at University of Magellanestimated from photographs that Fiona has 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 are being discovered as we speak,” Rada said. .
Fiona would have already been exposed at the time of these images. But according to Dean Lomaxpaleontologist of the University of Manchester: “I’m sure many of the specimens were under the glacier in the 1986 image.” That includes a well-preserved complete skull, previously discovered by Lomax, who was part of the recent expedition to excavate Fiona.
The retreat of the glacier
The exposed bedrock corresponds to an area where, during a typical year, melting snow and ice has outpaced new snow accumulation. A detailed view of this ablation area along the eastern side of the glacier is visible in the natural color image above, acquired by the landsat satellite 8 on April 7, 2022, on one of the few clear days during the recent month-long expedition. The lines indicate the previous locations of the ice edge, including its last maximum extent around the year 1700 during the little ice ageand its receding position as of 1986. In recent decades, parts of the glacier’s edge have receded as much as 2 kilometers.
In 2004, when Pardo-Pérez began exploring this area, scientists estimated that the fossil site covered about 5 square kilometers. But as the ice edge receded and the landscape changed, and as subsequent expeditions located more fossils, the site has expanded to about 15 square kilometers (nearly six square miles), or nearly the entire area of exposed rock 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 periods.
Fossilization occurred millions of years before the glacier appeared, when the area was covered with seawater. Scientists think that some of the ichthyosaurs died of natural causes. Others likely perished in mass mortality events caused by the rapid downslope flow of water, known as a 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 instantly in an anoxic environment that prevented bacterial decomposition and kept their joints articulated. skeletons”.
The glacial ice that eventually covered the fossils has not helped preserve them. Rather, Rada pointed out that before the ice melted, it had been “flowing” for a long time. This ice flow carries rocks and soil at its base, which Rada compares to a heavy sheet of sandpaper, grinding up the bedrock and fossils within it.
In Patagonia, erosion rates vary from 1 to 100 millimeters (0.04 to 4 inches) per year, and are probably at the lower end of that range on the Tyndall Glacier side. “But even with erosion rates of a few milliliters to a centimeter a 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. Fossils have been left vulnerable to fracture by freeze-thaw cycles and erosion by wind and water. “It is important to find ways to protect these valuable records from the past,” Rada said.
Fossils are likely to have been exposed near other glaciers, as the entire Southern Patagonian ice field is melting. But until paleontologists conduct more 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 exceptional fossils are being exposed due to a retreating glacier,” Lomax said.
The site is protected by the National Forestry Corporation of Chile (CONAF), and its fossils are protected by Chilean law that prohibits extraction or excavation without permission. “This place is a fragile ecosystem, located in a periglacial zone within the Torres del Paine National Park,” said CONAF manager Gonzalo Cisternas. The area is closed to tourism and recreational activities and can only be visited by authorized scientists.