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Ichthyosaur find may challenge notions about prehistoric migrations
Finding a rare fossil is often the result of decades of painstaking, systematic searches by scientists who make careers out of the hunt. Other times it is just a matter of luck.
Two Stanford geologists recently had the good fortune to be in the right place at the right time to make the kind of discovery that fossil hunters only dream of: They uncovered the bones of a large aquatic reptile known as an ichthyosaur near the southernmost tip of Chilean Patagonia a find that may have scientists rethinking theories about ancient migration routes and the demise of a supercontinent. The discovery is reported in the February issue of the journal PALAIOS.
Andrea Fildani and Michael Shultz, graduate students in the Department of Geological and Environmental Sciences, first saw the ichthyosaur fossil three years ago while researching outcrops in North and South America as members of the Stanford Project on Deep-water Depositional Systems (SPODDS) -- a consortium funded by 15 oil companies. They were scouting the wilds of Torres del Paine National Park for rock outcrops that had been sediments on the ocean floor millions of years ago, when local tourist guides told them about something strange they had seen in a remote location of the park.
"They told us they saw a big boulder with something on it. They thought maybe it was a painting," Fildani said. "When they sketched it for us, we thought it might be a fossil. But we didn't know if it was a dinosaur or a plant."
Curiosity got the best of them, and with just a rough map the guides had sketched for them, Fildani and Shultz set out to find the boulder. The map consisted mainly of a river and some black rocks and boulders, Fildani said, "but everything out there is black rocks and boulders." Just when they were about to decide the boulder was a wild goose, they found it. They knew immediately it was a fossil and decided it was probably some type of reptile.
"We were very excited," said Fildani, who thought the bones looked like ichthyosaur fossils he had seen in Berlin-Ichthyosaur State Park, 150 miles southeast of Reno, Nev.
Ichthyosaurs were aquatic reptiles that lived between 250 million and 90 million years ago. They were shaped roughly like dolphins with long, thin noses and disproportionately large eyes. The biggest ichthyosaurs were more than 45 feet long and fed on squid, fish and other marine animals. They had unusual, hockey-puck-shaped vertebrae that help scientists recognize their remains. The Chilean boulder contained 17 vertebrae along with the neural arches that enclose the spinal cord as well as some ribs -- more than enough for a positive identification by a trained paleontologist.
So the pair snapped a slew of photos and drew detailed sketches of the fossil. On their way back to Stanford at the end of their scouting trip, they stopped by Chile's National Geological and Mineralogical Survey (Servicio National de Geología y Minería de Chile) in Santiago. Paleontologists there excitedly agreed with Fildani's suspicion that the bones had belonged to an ichthyosaur that was probably 8 to 9 feet long and had lived around 140 million years ago.
The discovery proved to be even more intriguing because the next nearest confirmed ichthyosaur fossil had been found about 900 miles to the north. A few vague, unconfirmed references to other southern fossils exist, but scientists viewed these old accounts with skepticism. Other researchers reported finding possible ichthyosaur teeth in Antarctica, but they weren't enough to convince most paleontologists. The new fossil extends the known range of the reptiles dramatically and adds substance to the older claims.
The discovery bears directly on a debate about how ichthyosaurs and other marine creatures first found their way to the west coast of South America. "In my opinion, the significance of this discovery lies not simply in recognition of the fossil, but in its implications for the changing global landscape in early Mesozoic time, and how organisms responded to those changes," said geological and environmental sciences Professor Stephan Graham.
Around 250 million years ago, all of Earth's landmass was locked together as a single supercontinent known as Pangea. Then, around 200 million years ago, Pangea began to split apart. Eventually, the southern continents, including South America, Africa, India, Antarctica and Australia, broke away as a single landmass known as the Gondwana supercontinent.
At this time, ichthyosaurs were thriving in the Tethys Sea on the eastern side of Gondwana. But soon, some of these big reptiles, along with other creatures, migrated to the Pacific coast of South America on the western side of Gondwana. Because ichthyosaur fossils are common in the northern parts of South America, most scientists believed that the reptiles had arrived there by traveling around the northern edge of Gondwana through the Caribbean region.
But the new discovery revealed that ichthyosaurs also were living much farther south. "They probably couldn't have gotten all the way around to southernmost Chile," Fildani said. The ancestral Andes mountain range lay between the Pacific and where the ichthyosaur lived, and the southern Pacific was probably too cols for reptiles. Fildani thinks it is more likely that the animals swam through a southern seaway between Africa and the rest of Gondwana that was fed by warmer water from the Tethys. This idea is supported by other fossils found in southern South America, such as ammonites that look just like ones found in Kurdistan and clams like those found in India.
If the ichthyosaur that Fildani and Shultz found did make its way through this southern passage, then there may have been a continuous waterway separating Africa and South America from Antarctica and the rest of Gondwana earlier than previously thought. "It was fun to discover it and then realize that we could make an important tectonic story out of it," said Shultz.
Because the fossil is so rare and potentially critical to reconstructing ancient migration routes, the Chilean National Geological and Mineralogical Survey sent its own geologists to inspect the ichthyosaur. Satisfied with the fossil's authenticity, the Chileans have plans to airlift it out of the park and put it on display in the headquarters of Torres del Paine National Park.
The discovery also grabbed the attention of the local Chilean press. An article about the ichthyosaur appeared in the main newspaper of Southern Chile, Prensa Austral, and Fildani received a request from a local television station for an interview.
Fildani and Shultz plan to work for the Chevron oil company in California after completing their dissertation work at Stanford.
Betsy Mason is a freelance writer and graduate of the Stanford School of Earth Sciences.
By Betsy Mason