The 372-million-year-old fossils of tetrapod – creatures that evolved from fish living in water to creatures that walked on land – have been discovered by researchers in Russia. And given how well they were preserved, we can tell this was one weird-looking creature.
Had humans been around at the time, they might have seen large, oval-shaped eyes hovering just above the water line looking back at them.
This tetrapod, dubbed Parmastega aelidae, looked similar to a crocodile, except its protruding eyes were on top of its head. Its nose and jaws, concealing a combination of sharp fangs on the upper jaw and needle-like teeth on the bottom jaw, would have been below the surface of the water, the researchers said. And the way they curved together looked a bit like a menacing grin.
Tetrapods began evolving from fish during the Devonian period. They would eventually give rise to all creatures who walked on land, including reptiles, birds, amphibians and mammals.
Until the discovery of these fossils, everything researchers knew about Devonian tetrapods relied on nearly complete fossils from Ichthyostega and Acanthostega that lived at the end of the Devonian period 360 million years ago, as well as pieces of other species.
But scientists know that tetrapods lived as long as 390 million years ago due to fossilized footprints.
The newly discovered example was an early tetrapod, which maintained characteristics similar to fish in some of its bones. It was discovered in the Sosnogorsk Formation where limestone formed from a once-tropical lagoon. Now, it’s an exposed area on the banks of the Izhma River in Ukhta. The study published Wednesday in the journal Nature.
Researchers believe its eyes were above water while the rest of its body lurked below because there are lines in the skull where sensory organs once helped the tetrapod sense vibrations in the water. It used gills to breath beneath the water and was likely more than three feet long.
Crocodiles also lurk in the water this way, usually to keep an eye on prey that walks on land. But the fossil of this tetrapod held another surprise: it likely never left the water. Its shoulder girdle was partially comprised of cartilage. The vertebra and limbs haven’t been preserved, so they may have been as well. Cartilage is softer than bone, so it’s unlikely that this creature was walking around.
Perhaps it grabbed prey on the shoreline.
“Far from presenting a progressive cavalcade of ever more land-adapted animals, the origin of tetrapods is looking more and more like a tangled bush of ecological experimentation,” the researchers said in a release.
Given their eye shape and position, these tetrapods would have been most comparable to modern mudskippers, according to Nadia Fröbish and Florian Witzman at the Natural History Museum in Berln, authors of a News and Views article that accompanied the study. They were not involved in the study.
But mudskippers use their eyes to watch for predators on land or in the sky. When this tetrapod was alive, neither threat existed yet.
Understanding how vertebrates transitioned from water to land is like following the plot of a crime novel, Fröbish and Witzman said.
“The P. aelidae fossils offer a treasure trove of information that could help to disentangle some of the complex evolutionary changes that took place when vertebrates made the transition from aquatic to terrestrial life,” they said. “This discovery also reminds us that much still remains to be learnt in the next gripping chapter of this detective story.”