Does This Fossil Reveal a Jurassic Tropical Freeze?
On view for over a century, a fossil slab may display evidence of tropical freezing during the Jurassic, but scientists never noticed it—until one finally did. Some colleagues are not convinced.
“Hiding in plain sight for over 123 years is physical evidence that the tropics froze for brief periods of time 200 million years ago, at the beginning of the age of dinosaurs,” according to Paul Olsen, a paleontologist at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, N.Y.
Addressing reporters at AGU’s Fall Meeting last December, Olsen described his utter surprise when he looked at a fossil that he had seen at least 40 times since the 1970s and realized it revealed more than he had ever previously noticed. In fact, the fossil in question has been on public display since 1896 and has been viewed by hundreds of thousands of visitors, including hundreds of scientists, first at Wesleyan University and more recently at Dinosaur State Park, both in Connecticut.
The fossil is a sandstone slab about 3.4 meters in length, dates to the early Jurassic period, and was recovered at a quarry in Portland, Conn., in the 19th century. It is famous for clearly displaying five footprints of Otozoum moodii, an early ancestor of Brontosaurus.
Equally conspicuous in the slab, but somehow unremarked before Olsen’s epiphany in 2017, is a groove that he and his coauthors interpret as the track left by a “sailing rock” and, with it, evidence of Jurassic freezing.
Sailing rocks are stones that occasionally move across flat landscapes, gouging indented trails like the one seen in the fossil slab. Present-day sailing rocks are famously visible at Racetrack Playa, a dry lake bed in Death Valley National Park, Calif.
The cause of the rocks’ movement was the subject of much speculation prior to experiments conducted in 2014 by researchers at the Scripps Institution of Oceanography (SIO). With the aid of GPS trackers placed on stones they left on the playa floor, researchers determined that the rocks moved following rainy periods when the lake bed was covered by a few centimeters of water that froze overnight. When the Sun warmed the ice, thin sheets broke away and were pushed by the wind, taking the stones with them. When the water evaporated, trails left by the moving stones became visible.
That is what Olsen thinks occurred 200 million years ago in what is now Connecticut. However, at that time, Connecticut was part of the Pangea supercontinent and was located at around 20°N latitude—in the tropics.
Two Hypotheses for the Track
Why was there ice in the tropics 200 million years ago? Olsen suggests a period of global cooling occurred, created by a series of powerful volcanic eruptions in the Central Atlantic Magmatic Province that spewed sulfurous emissions into the atmosphere. Those emissions blocked sunlight from reaching the surface, allowing groundwater to freeze and resulting in a mass reptilian extinction. He thinks that dinosaurs’ protofeathers insulated them from the cold, whereas stem mammals’ fur protected them, as did their underground burrows.
There is, however, another known cause of sailing rocks on flat landscapes, aside from ice slabs: thick and slippery microbial mats that allow the wind to push rocks across their surface, like what occurs now on a dry lake near Toledo, Spain. Olsen does not think that is the explanation for the Connecticut track because the dinosaur’s footprints would not have been so well defined in a soft microbial mat, but he does not completely rule it out.
Some scientists are not convinced. Konrad Hughen of the Woods Hole Oceanographic Institution in Massachusetts thinks the microbial mat explanation is more plausible because of the unlikelihood, in his view, of a brief freeze during a massive volcanic winter in the middle of a carbon dioxide–fueled warm period. Hughen added that for him to be more sure of this hypothesis, there would have to be microfossil or geochemical evidence of a microbial mat, which is so far lacking.
Richard Norris of SIO noted that Chilean researchers reported “something similar” to Olsen’s discovery in Jurassic rocks east of Santiago. Regarding the Connecticut fossil, he says, “I don’t know what the track maker was, but it was probably not a moving rock.”
Norris noted that the track is “a negative impression, and the presumed levees on the trail are actually grooves along the edge of the trail. Hence, the feature cannot be made by a rock that was pushing sediment out of the way when the rock was in motion.”
Seeing and Believing
Whatever hypothesis eventually prevails, why did Olsen see and comprehend in 2017 what had eluded him and other scientists dozens of times before?
Olsen said that all of us tend not to recognize things that don’t fit into our existing ideas. In 2017, he was in China, studying evidence of freezing in the Jurassic and Triassic periods. He was actually looking for traces of sailing rocks, which would be evidence of ice. At the time, he didn’t remember that he had seen exactly that in Connecticut, he said, but the next time he visited the exhibit at Dinosaur State Park, he slapped his forehead and exclaimed, “Oh—that looks like a sailing rock trace!”
Meanwhile, Patrick Getty of Collin College in Spring Creek, Texas, attributes a set of small tracks on the fossil, equally unnoticed for more than a century, to a tiny stem mammal, bounding along near the dinosaur track.
“Both of those things [traces of a sailing rock and a stem mammal] are examples of things which nobody observed because they weren’t, in a way, prepared to see them,” Olsen said. “I wouldn’t have seen it, if I hadn’t believed it,” he concluded.
Citation: Leifert, H. (2020), Does this fossil reveal a jurassic tropical freeze?, Eos, 101, https://doi.org/10.1029/2020EO140315. Published on 19 February 2020.