The Journey of Fish onto Land

All terrestrial vertebrates belong to the group known as tetrapods, and tetrapods ultimately evolved from fishes that left the oceans and began exploring life on land. Determining when this transition began relies largely on fossil evidence. Until recently, the oldest traces of vertebrate movement on land were known from only a handful of sites, mainly in Europe and Australia. For example, locomotion traces preserved in early Middle Devonian strata in the Holy Cross Mountains of Poland and similarly aged traces on Valentia Island in Ireland indicate that tetrapods were already capable of efficient terrestrial movement by the early Middle Devonian. These discoveries suggested that the origins of quadrupedal locomotion must reach back even earlier, into the Early Devonian.

Photograph of the quarry section where the fossils were discovered; the trace fossils described in the study are visible in G, H, and I（圖片來源：Szrek, P et al. (2025).，採用 CC BY-NC-ND 4.0 授權）

A recent study, however, describes even older trace fossils from the Early Devonian (Emsian) deposits of the Holy Cross Mountains in Poland. These traces are interpreted as having been produced by lungfishes (Dipnoi) with relatively short snouts, such as Dipnorhynchus or Chirodipterus. The fossils include complex locomotion traces as well as resting traces, providing insight into how early vertebrate relatives of tetrapods moved across extremely shallow water or even briefly onto exposed sediment.

The crawling trace described in the study has been named Reptanichnus acutori. It consists of several distinct structural elements. These include sinuous or nearly straight elongated depressions accompanied by narrow longitudinal grooves and bilobed pits. The bilobed pits represent a variant of the trace fossil Osculichnus tarnowskae. The elongated depressions typically measure 20–30 centimeters wide, can reach depths of about 10 centimeters, and may extend for more than one meter in length.

Reptanichnus acutori specimen; 2 indicates the narrow groove, 3 indicates the bilobed depression（圖片來源：Szrek, P et al. (2025).，採用 CC BY-NC-ND 4.0 授權）

Alongside these depressions are grooves that may be either straight or curved. Their cross-sections are generally V-shaped, reaching widths of up to about 11.5 millimeters and depths of around 12 millimeters, with maximum lengths of roughly 70 millimeters. Toward their ends, the grooves gradually or abruptly become narrower and shallower before disappearing entirely. Additional shallow grooves, approximately 3.5 millimeters deep, also occur. Although shorter, these grooves align with the direction and form of the main depressions.

Bilobed pits appear along both sides of the long depressions. These structures may reach depths of about 20 millimeters and typically have elliptical or crescent-shaped outlines. A wavy groove divides each pit into two unequal lobes, forming a larger and a smaller portion.

The resting trace fossils have been named Broomichnium ujazdensis. These traces consist of two pairs of grooves spaced approximately 35–40 millimeters apart. The second pair is wider and deeper than the first, and the grooves within each pair meet at an angle of about 40°. The pairs are arranged in a radial orientation.

Broomichnium ujazdensis specimen, along with reconstructions of the two types of trace fossils（圖片來源：Szrek, P et al. (2025). 復原圖繪師：Jakub Zalewski，採用 CC BY-NC-ND 4.0 授權）

Researchers interpret the locomotion trace Reptanichnus acutori as having been produced by lungfish moving across extremely shallow water, possibly even across sediment partially exposed to air. Under such conditions, buoyancy becomes greatly reduced or disappears entirely, preventing normal swimming. Instead, the fish must move along the bottom with its body pressed against the substrate. Each body part involved in locomotion leaves its own mark in the sediment.

The broad elongated depressions were formed as the fish’s body dragged across the surface. The sinuous portions reflect lateral twisting of the body, while straighter sections indicate sliding movements. The grooves associated with the depressions were produced when the fish lifted and lowered its fins during movement. The bilobed pits record moments when the fish pressed its snout into the sediment. By anchoring its snout in the substrate, the fish could lift and pull its body forward while using its fins to adjust posture.

Similar behavior can be observed today in the West African lungfish Protopterus annectens. Modern individuals sometimes move across land by rotating their bodies and supporting themselves using their heads while the fins assist with stabilization. The resting traces Broomichnium ujazdensis likely formed when lungfish paused and supported themselves with their pectoral and pelvic fins resting on the sediment, a behavior also observed in living lungfishes.

Another intriguing observation concerns the orientation of the traces. Among 36 examined trace fossils, 35 show the fish’s snout and body tilted toward the left. This pattern suggests that early lungfish may have preferred using the left side of their body—essentially exhibiting left-sided dominance or “left-handedness.” Such asymmetry implies that behavioral lateralization may have originated earlier than previously believed, possibly before vertebrates evolved true limbs. Interestingly, this differs from the right-side dominance commonly observed in many modern terrestrial vertebrates, suggesting that the evolution of handedness in vertebrates was more complex than previously assumed.

The evolution of vertebrate locomotion on land was therefore a complicated process that did not occur within a single evolutionary lineage. During the Devonian, some early fishes had already developed the ability to move in extremely shallow water environments. Many possessed elongated bodies and eyes positioned on the top of the head, features that would have helped them navigate environments where parts of the body were exposed above water. This indicates that walking-like movement may have existed even before true tetrapods appeared.

Lungfish are the closest living relatives of tetrapods. As such, they represent an important model for understanding how early vertebrates experimented with moving onto land before the emergence of true four-limbed animals. By venturing onto shallow shorelines, these fishes could avoid predators and competition while gaining access to new feeding areas.

The trace fossils described in this study push back the earliest evidence of vertebrates leaving the water by roughly ten million years. Because lungfish belong to the sister lineage of tetrapods, their mode of movement may closely resemble the behavior of the earliest vertebrates that first explored terrestrial environments. For this reason, the discovery provides valuable insight into one of the most transformative events in vertebrate evolutionary history—the transition from water to land.

Author: Bai Leng

Reference:

Szrek, P., Uchman, A., Grygorczyk, K., Salwa, S., Dworczak, P. G. (2025). Traces of dipnoan fish document the earliest adaptations of vertebrates to move on land. Scientific Reports.