About 92 million years ago in what's now central Utah, a terrible storm blew over a huge flowering tree, which was washed down a river and into an ancient delta. Sediments buried part of the tree's trunk, which then mineralized—preserving a fossil that's now breaking records.
In a new study published on Wednesday in Science Advances, researchers present the oldest evidence of large flowering trees in North America. The record-breaking fossil is a petrified log nearly six feet wide and 36 feet long. Researchers say that the tree probably stood about 170 feet tall in life, making it twice as tall as Utah's tallest living tree.
The fossil tree trunk probably belongs to Paraphyllanthoxylon, an ancient genus known from other fossils. But this tree lived and died a full 15 million years before the next oldest North American fossils of large flowering trees. (See the oldest living tree in Europe, and find out why it's having a growth spurt.)
Outside experts say that the tree's existence makes sense. Flowering plants, also called angiosperms, arose about 135 million years ago. By a hundred million years ago, smallish ones dominated some lowlands, and by about 75 million years ago, there's clear evidence of massive flowering trees. So sometime around 90 million years ago, angiosperms must have started reaching for the skies.
“It's not necessarily surprising, because we know that there's a rich angiosperm flora at that time,” says Nan Crystal Arens, a paleobotanist at Hobart and William Smith Colleges who wasn't involved with the study. “But it's the concrete, put-your-hands-on-it proof that those big trees were there.”
To reproduce, moss unite their sperm and egg, forming a cell that grows into a stalk tipped with a capsule (seen here in cross section). Spores mature within this capsule, which then bursts and lets the spores drift away.
Finding fossils from this time period, called the Turonian, has been a challenge, because sea levels ran high at the time, which means that most Turonian sediments capture what was happening in the water, not on land. Fewer than a hundred known fragments of angiosperm wood are more than 84 million years old, and most are from tiny trees less than four inches in diameter.
“This tackles a time period that's really under-sampled ... so it was really nice to see this come out,” adds paleobotanist Dori Lynne Contreras, a Ph.D. student at the University of California, Berkeley.
Finding the massive log came as a stroke of luck. With funding from the National Geographic Society, Adelphi University paleontologist Michael D'Emic visited central Utah in 2014 in search of bones from long-necked dinosaurs. One day, a local U.S. Bureau of Land Management official named John Reay gave D'Emic a tour of the area and showed him some interesting geological features the BLM had long noticed, including the petrified log.
At first, D'Emic didn't know what to make of it. On the one hand, petrified logs aren't uncommon in the western U.S., especially those from conifers. But on the other hand, the log was an impressive sight. (Learn more about Arizona's Petrified Forest National Park.)
“You're standing out there in the desert, and to an untrained eye, you're just looking at a lot of sand and sandstone,” D'Emic says. “But I could see then that there must have been crazy, monsoonal storms to move a tree [such as this one] out onto one of these deltas at that time. I saw the environment come to life.”
D'Emic asked Reay if he could take samples of the log, which he mailed to paleobotanist Nathan Jud, who is now a professor at William Jewell College. Jud first thought that the fragments belonged to a large conifer. But once he examined them under a microscope, he realized that the Utah log wasn't just huge—it was a rare find.
“My jaw dropped,” says Jud, the study's lead author. “I knew immediately it was the oldest angiosperm log over a meter in diameter anywhere in the world.” He soon congratulated D'Emic via text message.
The team has some follow-up studies in mind to figure out where different plants lived on the Turonian landscape near the log's resting place. In the meantime, D'Emic hopes the new study draws more scientific attention to the Ferron Sandstone Member, the Utah rock formation that yielded the fossils. Not only did the team study the petrified log, but on later expeditions to the area they also found pollen, leaves, and vertebrate fossils—including a shark's tooth and some dinosaur bones.
“Previous to our paper, there's like three papers ever from the Ferron Sandstone Member; it's just seen as this barren unit,” D'Emic says. “I'm not saying it's the next big thing or anything, but what it does do is encourage other people to prospect in it for fossils.”