Did this ancient creature crawl on land like a seal or swim like a fish?

Nature · Published 3 October 2026 Swipe more like this →

In short: Ichthyostega, a Late Devonian inhabitant of Greenland, was once thought to be a simple salamander-like ancestor of land vertebrates. Modern analysis reveals a far stranger anatomy: a specialized, heavy-bodied creature that likely dragged itself across land using a crutching motion, functioning more like a seal than a lizard.

Discovered in East Greenland, Ichthyostega represents a pivotal moment in vertebrate evolution. Living during the Late Devonian, this genus serves as a transitional fossil between fish and true tetrapods. While it possessed lungs and limbs, it existed long before the modern amphibian group appeared in the Triassic. Early researchers assumed a salamander-like gait, but recent computed tomography and 3D modeling have overturned this, suggesting a unique, seal-like method of terrestrial movement.

The creature’s skeleton was highly specialized for its dual existence. It featured a massive, rigid ribcage with overlapping flanges that likely inhibited the side-to-side movement typical of fish. Its forelimbs were powerful enough to haul its body from the water, but they lacked the range of motion for a standard quadrupedal walk. Instead, Ichthyostega likely employed a synchronous 'crutching' gait, dragging its body forward. Its hindlimbs, which featured seven digits, were smaller and probably could not support its full weight on land.

Despite these terrestrial adaptations, Ichthyostega retained clear aquatic traits. Its tail possessed a low fin supported by bony rays, and its skull included an operculum to cover its gills. The animal also had dorsally placed eyes and a specialized ear structure unlike any fish or later tetrapod. Whether these features evolved independently or represent a direct lineage remains a subject of scientific debate, as researchers continue to refine the evolutionary tree of early stegocephalians.

The classification of Ichthyostega has shifted significantly over time. Once placed in the order 'Ichthyostegalia,' it is now recognized by many as a paraphyletic grade of stem-tetrapods. As new specimens were collected between 1931 and 1955, the understanding of its morphology grew, yet the exact number of digits on its forelimb remains a mystery. By bridging the gap between aquatic tetrapodomorphs like Eusthenopteron and later land-dwelling vertebrates, Ichthyostega provides a window into the immense pressures of transitioning from water to land.

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