Fossilized tracks trace ancient animal behaviors in Oregon

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25-04-2025

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About 50 million years ago, a small bird waded along a lakeshore in what today is central Oregon. A worm wriggled at its feet. The bird appeared to probe the silty earth with its beak, once, twice, three times, looking for food. On the fourth try, the bird may have found something. Or perhaps it missed again and moved on.
This glimpse into prehistory is possible thanks to two tiny fossil footprints and the dogged work of an undergraduate intern at John Day Fossil Beds National Monument with a particular interest in ancient tracks, known as trace fossils.
'Trace fossils definitely do tell stories,' said Conner Bennett, lead author of a study describing the find and three others that was published in February in the journal Palaeontologia Electronica. 'We don't have the body of this bird. It's more just like we're left with its actions, its behaviors. It's like trying to study ghosts.'
Trace fossils can fill in gaps in the fossil record, said Dr. Anthony Martin, professor of practice in the department of environmental sciences at Emory University in Atlanta. 'This paper has tracks that are definitely from a bird of some sort, and then tracks that are definitely from a lizard,' said Martin, who researches modern and fossil traces and was not involved in the research. 'So those are showing that those animals actually were there, even though there's not a single bone or feather or any other bodily evidence of those two types of animals being there.'
The science of trace fossils isn't as 'sexy' as studying bones, teeth and other physical remains, Bennett said. Fossilized tracks don't get as much attention. That's part of the reason he got the chance to undertake this research in the first place. Bennett, now a graduate student in the department of Earth, environmental and planetary sciences at the University of Tennessee, Knoxville, applied for a summer internship at John Day in 2022. The national monument holds a rich paleontological record from the Eocene Epoch — part of the age of mammals — which lasted from about 56 million to 34 million years ago.
Bennett combed through the park's online catalog of finds and came across several animal track impressions that had been found decades ago but never studied. He'd previously worked with 3D modeling software to describe dinosaur tracks and endeavored to give the unidentified animal tracks the same treatment.
The analysis could help researchers form a better understanding of the prehistoric ecosystems of Oregon. Bennett said he hopes his findings will inspire other researchers to look for trace clues they may have previously overlooked. 'I'm sure it's going to happen soon where people are like, 'Oh, it's really interesting to learn about fossil tracks. Let's pull this stuff out of our archives and start looking at it.'
While interning at John Day, Bennett took hundreds of overlapping photos of four sets of fossilized tracks, each only centimeters in size. He fed the pictures into the modeling software, which created 3D representations that could be blown up, zoomed in on and scrutinized in greater detail than the fossils.
In one, there were tracks that reminded him of a small shorebird, such as a plover. There aren't bird fossils in John Day, he explained — the bones are fragile and hollow and don't hold up well. But it would make sense that there would be such birds near the lake that once covered the area where the tracks were excavated.
There were also small, round indentations near the tracks. At first, Bennett and study coauthor Dr. Nicholas A. Famoso — the head paleontologist and museum curator at John Day — thought they could be caused by raindrops, which can leave impressions in the fine grains of shale and clay the tracks were found in. But there are usually many raindrop impressions, and here there were only a few, and only near the footprints. The researchers wondered whether the bird had made them with its beak. Bennett went online and quickly turned up a video of modern plovers pecking into the ground, hunting for food. The connection seemed clear to the research team.
Not only did the trace fossil confirm the previously unproved existence of birds in the area tens of millions of years ago, but it also could paint a picture of how the birds foraged in the shallow water — much the way they do today. And to complete the picture, the fossil contained evidence of what the bird was feeding on or trying to feed on: the squiggly trail of a worm on the move. 'We can track a feeding behavior over 50 million years. That's pretty cool,' Bennett said.
Dr. Danielle Fraser, head of paleobiology at the Canadian Museum of Nature in Ottawa, who was not involved in this study, expressed a similar sentiment. 'This is a really cool example of a behavior that would otherwise not be apparent,' she said.
Martin suggested an alternative interpretation: that the divots could have been made by water droplets falling off the bird's body as it came out of the water. 'I've seen that a lot on the Georgia coast,' he noted.
A separate specimen showed three footprints made up of five thin, splayed digits and clawlike impressions, suggesting some small lizard once darted along the lakeshore. There were even marks showing where the lizard dragged its feet. The researchers found this exciting because there are few examples of tracks from small lizards in North America in this period, and no body fossils have been unearthed at John Day.
'Too few paleontologists are trained in the recognition and interpretation of trace fossils and so a lot of this information is overlooked,' said study coauthor Dr. Daniel I. Hembree, professor and director of undergraduate geology studies in the University of Tennessee department where Bennett is studying.
The traces are the only evidence of small lizards' existence at John Day during the Eocene.
'The fossils of the bird and lizard are the first evidence of those groups of animals from that period of time in our fossil record,' Famoso said. They 'help to paint a more complete picture of what life was like' during the Eocene.
Bennett also analyzed two other trace fossils — from mammals from a more recent time period. One showed prints of a three-toed ungulate, possibly a rhinoceros or ancient tapir. The other: the 29 million-year-old tracks from what the researchers believe to be some sort of saber-toothed cat.
The absence of claw marks suggested that the animal's claws were retractable, like those of modern cats. And the gait appears similar, too, Martin said. The way the rear paw print only partially overlaps the front means the animal may have been walking at a normal pace. 'This is the kind of understep gait that I see in my cats at home,' he said. 'I get kind of excited when I see something that looks so familiar.'
Famoso noted that both sets of prints were found in an ash layer, which means the animals walked across an ash-laden landscape after a volcanic eruption.
'Sometimes fossils are stored in collections until new technologies or methods are developed that can better study them,' he added. 'These fossils were collected and added to collections between 1979 and 1987, but it took until 2022 for us to get a good study out to describe the specimens.'
Amanda Schupak is a science and health journalist in New York City.