Why Some Dinosaurs Left Swim Marks Instead of Footprints; What “Almost Floating” Really Looked Like

When we picture dinosaur tracks, we expect deep, three-toed prints in ancient mud. Yet, at some fossil sites, scientists find faint scratches and narrow claw marks instead.

At first glance, these marks look incomplete, as if the dinosaur barely stepped down. But research shows they capture something far more interesting: moments when dinosaurs were moving through water deep enough to lift much of their weight, yet shallow enough for their toes to still scrape the bottom.

They weren’t fully swimming. They weren’t fully walking. They were almost floating.


How Water Changed the Way Dinosaurs Moved

The science behind this is surprisingly simple. Water creates buoyancy. The deeper an animal moves into water, the more of its body weight it supports. That reduces the pressure its feet apply to the ground below.

A study published in the journal Geology examined unusual trackways at the La Virgen del Campo site in Spain’s Cameros Basin. Instead of full footprints, researchers found sets of elongated claw scratches — some reaching about 50 centimeters in length. The pattern suggested a theropod dinosaur moving through water roughly 3 meters deep. Its body was supported by water, and only the tips of its claws touched the muddy bottom.

The marks form alternating sets, consistent with a swimming motion using the hind limbs. Researchers have compared this to a paddling movement, similar in rhythm to how certain modern birds propel themselves.

The dinosaur was moving forward — but not by taking firm steps. It was gliding, lightly brushing the substrate beneath it.

From Deep Footprints to Light Scratches

One of the clearest examples of this shift comes from Bolivia’s Carreras Pampa site in the El Molino Formation. A 2023 academic study documented nearly 1,400 fossil trackways, including both traditional walking tracks and distinctive swimming marks.

On firmer sediment, the dinosaurs left clear, three-toed footprints with well-defined digit impressions. But in areas interpreted as shallow water environments, the impressions changed. The deep prints transitioned into parallel or subparallel scratch marks, with only the claw tips contacting the mud.

This gradual change supports the idea that increasing water depth reduced the effective body weight pressing down on the surface. As buoyancy increased, the footprints became shallower — until they were no longer footprints at all, grooves.

It’s a physical record of changing conditions captured in real time.

Reading Ancient Mud Like Evidence



Understanding these marks requires more than simply looking at shape. Paleontologists carefully study the surrounding sediment. Ripple marks in the rock, formed by moving water, often appear alongside swim tracks. These ripples help confirm that the environment was aquatic when the traces were made.

Researchers also distinguish between true tracks and “undertracks.” Undertracks form when weight from above distorts lower sediment layers. But swim marks show clear directional drag patterns that match claw movement, not just downward pressure. Studies in peer-reviewed journals, including Scientific Reports, emphasize the importance of this distinction when interpreting unusual trackways.

At Carreras Pampa, scientists documented hundreds of swim traces alongside firm walking tracks in the same general area. That combination suggests dinosaurs repeatedly moved through environments where water levels shifted over time.



What These Marks Tell Us About Dinosaur Behavior

These scratch marks reveal that dinosaurs were not confined to dry ground. They interacted with rivers, lakes, and shallow basins. They crossed water bodies, navigated muddy margins, and adapted to changing landscapes.

The Bolivian tracksite, in particular, preserves thousands of traces that show multiple behaviors — walking on firm sediment, wading in shallow water, and swimming where buoyancy reduced contact with the ground.

This expands our understanding of dinosaur ecology. Rather than imagining them as striding only across dry plains, evidence shows they could navigate complex, water-rich environments.

And it’s not their bones that reveal this — it’s the marks they left behind.



Small Grooves, Big Insights



What appears at first to be a faint or incomplete footprint turns out to be a snapshot of movement in water. A dinosaur enters deeper terrain. Its body grows lighter. Its claws scrape gently along the muddy bottom as it paddles forward.

The mud records that moment. Over millions of years, it turns to stone.

Today, scientists read those scratches as evidence of partial buoyancy — proof that some dinosaurs were moving through water in ways that left only the lightest touch behind.

Sometimes, the smallest marks carry the clearest story.