Tiny dinosaur left “perfect footprint” in the rain

Workers were excavating an area for construction. Then paleontologists found dinosaur tracks, raindrop impressions, and a window into a 110-million-year-old ecosystem.

A small dinosaur—probably about the size of a blackbird—stalks a lakeshore. That animal left footprints in the mud that hardened and lasted for over 110 million years, until a team of scientists uncovered them in South Korea. (Artist’s interpretation by Zifeng Wang.)

His colleagues call him “eagle eye.” In the middle of excavations for development near the South Korean city of Jinju, Kyung Soo Kim — a paleontologist at Chinju National University of Education — spotted a small footprint on a broken slab of stone.

It wasn’t the only one. Four more tracks followed, and the researchers really had something: the first fossil tracks ever unearthed with traces of dinosaur skin covering entire footprints.

The find — tiny theropod tracks amidst a spattering of raindrop impressions — gives insight into the appearance and surrounding ecosystem of these ancient animals, which lived over 100 million years ago.

Kyung Soo Kim and his team found the fossils south of Jinju, South Korea. (Image: Satellite view of area around Jinju, from Google Maps.)

Revealed earlier this month in Scientific Reports, the fossils come from Minisauripus, a distant relative of Tyrannosaurus rex about the size of a blackbird. The small animal’s tracks have skin imprints showing scales no bigger than half a millimeter wide arranged in perfect arrays.

What made this level of detail possible was the environment: an Early Cretaceous lakeshore with a thin, sticky film of mud atop a firm layer of sand. “It was just the right texture so that the animal would leave a perfect footprint,” says Martin Lockley, a paleontologist with the University of Colorado Denver who worked on the study.

You can see the footprints in the journal Scientific Reports, where the researchers published their findings earlier this month.

Small Tracks, Big Deal: High-Definition Skin Traces

“It’s a fantastic find, because it gives us a glimpse of what these little dinosaurs were doing a hundred million years ago,” says Anthony Martin, a paleontologist at Emory University who was not involved in the new study. “But it also gives us a snapshot of what their teeny weeny little feet look like. That’s something that you just don’t get every day out of the fossil record.”

Why not? Well, it takes a lot to make a fossil: Most things that die never fossilize. They’re just gone, lost to deep time. Of the remains that do get preserved, paleontologists most often find remnants of bone or animal tracks etched into ancient rock.

Bones from tiny dinosaurs like Minisauripus are too delicate to preserve well. That’s why artists’ reconstructions of these dinos rely on related species, and the thing these animals more often left behind: tracks. (Illustration by Zifeng Wang.)

What’s much more rare are traces of soft tissue like muscle or skin. Dinosaur tracks discovered previously have only preserved skin impressions covering parts of footprints. This new Minisauripus trackway changes that.

Prehistoric bird species found in China around the same time sported similar scale patterns, which Lockley says highlights the evolutionary relationship between birds, the last surviving dinosaurs, and their extinct dino relatives. Modern birds still have scales on their feet.

The resemblance, however, ends there: the fossilized Minisauripus steps show fleshy feet more suited to living on the ground than taking to the skies.

Rain on the Lakeshore: the Ecosystem Context

These fossils capture a brief moment in an ecosystem that flourished about 110 million years ago.

That includes weather. Surrounding the Minisauripus tracks are raindrop imprints, which the researchers say is evidence of a light shower right before or during the small dinosaur’s jaunt along the shore.

Dinosaurs had to deal with the weather, too! The same sandstone slabs that hold the new fossil footprints also show evidence of rain. (Photo: Olia Gozha.)

Martin, the paleontologist not involved in the study, is normally skeptical when researchers claim they’ve found raindrop impressions in fossils. “I always have an alternative explanation for them, that they could be gas bubble escape structures—that you get gas bubbling up from underneath,” he says. “I was surprised. These, I do think they’re raindrop impressions.”

Also present on these sandstone slabs? Two pterosaur tracks. (Pterosaurs, close cousins of dinosaurs, were the first vertebrates to fly.)

In only the past six years or so, scientists working in South Korea’s Jinju Formation — where the new footprints were unearthed — have discovered Early Cretaceous trackways revealing a diverse ecosystem of animals: non-avian dinos, birds, pterosaurs, frogs, crocodile relatives, and even mammals. These new trackways are a key piece in that puzzle.

“When you’re dealing with tracks, you are dealing with living animals,” Lockley says. “I have these visions of this little dinosaur walking along on the lakeshore, and there are birds and there are lizards and there are frogs.”

The scene Lockley describes is a reminder: Extinct dinosaurs weren’t all big and, despite their role in Hollywood films, they weren’t characters or monsters. They were real animals — animals that spent time living, breathing, and walking alongside other animals, as the weather changed above, millions of years before humans took our first step.