Interpretation of the Reptanichnus acutori igen. et isp. nov
The newly discovered trackway is a crucial trace fossil for the analysis presented in this paper. It was produced by a fish moving in extremely shallow water or even across exposed sediment, with at least part of its body emerging. In this state, buoyancy is either reduced or absent, preventing the fish from swimming. Under such conditions, all parts of the body involved in locomotion left different traces (Figs. 3, 4 and 5). The elongated depression was formed by the trunk being dragged across the sediment, partly due to lateral body twists, as reflected in the sinuous stretches, and partly by sliding in straighter stretches. Swimming is excluded because the sinuous pattern characteristic of swimming is absent, unlike in the well documented fish swimming trace Undichna. The various ichnospecies of Undichna are preserved on bedding surfaces of very fine-grained deposits and display single, paired, or tangled sinusoidal waves. The shape, amplitude, regularity and mutual relationship of these waves reflect the type of fins and manner of swimming of the fish that left them7.
The longitudinal furrows (Fig. 6A, B), previously interpreted as the invertebrate locomotion trace fossil Protovirgularia4 are here reinterpreted as traces of fins that were raised and lowered, touching the bottom and leaving isolated impressions during the locomotion of the fish. The ribs in some of the furrows may reflect the structure of the fins. The bilobate depressions (low-angle Osculichnus) are traces of the snout, which was pushed into the sediment to anchor and create leverage for lifting the body. The fins were used as an auxiliary tool to adjust the movement.
This interpretation of the trackway is supported by observations of the modern West African lungfish Protopterus annectens, which leaves characteristic traces when moving on land by rotating its body and supporting itself using its head8. The body and fins of primitive fishes rarely leave clear traces on the ground. The exception is when the fish slips during a rotational movement, in which case shallow, sinusoidal traces of the body dragging may appear on soft ground. On sand, due to the lack of cohesion of the substrate, the traces are more irregular and less distinct than on mud. The traces left during pauses in movement do not differ morphologically from those left during continuous movement. As a result, it produces circular headprints that form a left-right alternating series. These imprints are variable and can take the form of single semi-circular indentations or double, more elongated imprints when the fish has its mouth open, with the upper and lower jaws forming separate imprints. The spacing between the imprints is irregular, ranging from 1 to 15 cm, although it is most often around 10 cm8.
Interpretation of the Broomichnium ujazdensis isp. nov
The pairs of furrows were produced by ventral fins of a fish resting on the sediment with its body was raised at low angle enough that only one pair of fins left an impression (Fig. 6C). This corresponds to the behavior of modern lungfish, where the rear part rests on the sediment9. A few traces of this type were also found in double pairs. They are imprints of the pectoral and pelvic fins, formed when the entire fish rested on the bottom. It is likely that there are more resting traces, but the recognition of convincing pairs is difficult due to the high density and overlapping of the furrows.
Similar trace fossils are known as Broomichnium flirii10 from late Pleistocene glacial lacustrine sediments. These are characterized by two bilaterally symmetrical two pairs of thin, linear or curved imprints corresponding to the pectoral and pelvic fins, with additional straight imprints in the middle, attributed to the anal fin. They occur either in a series or as solitary traces and have been interpreted as locomotion traces of a bony fish from the family Cottidae, using their pectoral and pelvic fins to “hop” or “crawl10.
The trace maker
The identification of the trace maker was discussed already at the stage of the description of the Osculichnus4. Characteristic features of the imprints that help to identify the trace maker are the trapezoidal outline of the snout in dorsal view, the deep, curved profile of the lower jaw in lateral view, and the presence of a pair of arches in the ventral margin of the upper lip. All of these characteristics match in short-snouted Devonian lungfishes such as Dipnorhynchus or ‘Chirodipterus’ australis4. Since Osculichnus is now one of the elements of the described trackway, their production by the same group seems obvious.
Dipnoan fishes have a fascinating evolutionary history that began about 415 million years ago in an aquatic environment. During this time, they made the transition from marine environments to inland waters, where they continue to thrive today11,12. At the same time, their anatomical structure has undergone only minor changes, which is why they are sometimes referred to as “living fossils“11,13. Some representatives of this taxonomically small group (Protopterus and Lepidosiren) are capable of surviving in periodically drying water. reservoir. In particular, Protopterus8 can crawl in search of water and, as a last resort, dig a burrow where they enter torpor, awaiting favourable conditions. This ability has been known since the early Triassic, when they first appeared in the fossil record14. The fossil record of dipnoan fishes is sparse, particularly from the early stages of their appearance in the Early Devonian. Only a few forms are known from this time: Melanognathus15 Uranolophus16,17 Jessenia18 Sorbitorhynchus19 Tarachomylax20 and Dipnorhynchus21 preserved as more or less complete skull remains. Until the description of the hunting trace fossil Osculichnus tarnowskae4 details of the dipnoans’ way of life were inferred primarily from their skeletal remains.
Adaptaions to invasion on land
Adaptations of vertebrates for moving on land are complex and not limited to a single evolutionary lineage, having evolved independently in at least two groups of Devonian vertebrates. The ability to move in very shallow water, where part of the body is emerged and swimming is not efficient, may have been present in early/lower vertebrates22. Many fossil fish groups, such as the Late Devonian elpistostegids Tiktaalik, Panderichthys, and Elpistostege adapted to semi-terrestrial environments. Their elongated bodies and eyes positioned on top of their heads helped them to move with a semi-submerged body. Such adaptations suggesting a transition to land were observed in Tiktaalik23,24,25. The process of vertebrate terrestrialisation was therefore multi-staged, suggesting that the ability to walk and/or near-walking in very shallow water may have existed even before the appearance of tetrapods. In this context, morphological studies and molecular data on lungfishes show that they are a modern-day sister group to tetrapods26,27. These assumptions are strongly supported by the described trackway, which represents the oldest evidence of pre-tetrapod preadaptations and attempts to move in terrestrial or near-terrestrial conditions. By moving towards land, the fishes avoided competition and predation, gaining access to new feeding areas. Feeding was possible during low tide, while non-adapted taxa have had to retreat to the subtidal zone.
Handedness of early dipnoans
The presence of fish snout traces pushed into the sediment, coordinated with the body twisting to the left, was already reported4 as evidence of an almost exclusive left-handedness phenomenon based on ten trace fossils from the Ujazd section. These observations were later supplemented by an additional 26 traces from the Ujazd and Kopiec sections. The merged dataset of 35 left-turning traces appears to be statistically significant and may represent the earliest known instance of handedness among vertebrates.
Asymmetries in whole-body actions, particularly those with a right-sided predominance, are common across various vertebrate groups, including amphibians, reptiles, birds, and mammals. Existing literature28 on the evolution of lateralization in vertebrates—the preference of one side of the body over the other—provides information on humans and marine mammals but lacks detailed data on fishes. This right-sided preference28 primarily controlled by the left hemisphere of the brain, may have originated in early vertebrates, possibly including fishes, even before the development of limbs. However, data from the studied sections suggest that the process of handedness in early vertebrates was more complex and that later right-handedness was not present from the outset.
Overall significance
The surfaces containing the described trace fossils are exceptionally well-preserved because they were covered by tuffite. This coverage prevented further bioturbation, which might have obliterated the traces left by fishes. As a result, the traces have been conserved, and the surfaces represent a form of “true substrate“29 capturing the original sediment-water or sediment-air interface and recording a period of stasis before subsequent deposition.
The significance of the presented trackway Reptanichnus acutori igen. et isp. nov. lies in its documentation of both the locomotory abilities of fishes as well as their preadaptation to terrestrial conditions. It also presents the oldest known evidence of vertebrate movement in the transition between sea and land. Furthermore, it is relevant to the ongoing discussion regarding the interpretation of early tetrapod traces, such as those from Zachełmie2. The suggested possibility that these traces originated from fishes8,30 appears less likely in light of the evidence presented in this paper, as the morphology and development of these traces differ from the Middle Devonian material2,31.
A neoichnological experiment showing that the tracks left by crawling Protopterus resemble those of tetrapods8 in light of our observations, demonstrated the identical behaviour of this creature on land to that of its Devonian ancestors.
The pattern of tetrapod footprints is different from those of moving fishes primarily through the presence (someplace only subtle) of fingers imprints and regular alignment. However, the additional structures left by the fins and thorax are absent.
An important characteristic of the traces left by the crawling dipnoans is the absence of a clear body midline, which is often present in quadruped traces. Additionally, the trackways of early fishes can display irregular distances between head imprints, although they may become more regular when the animal moved continuously in a single direction. In a palaeontological context, such traces—featuring alternating, evenly spaced, rounded impressions—can be mistaken for those of early tetrapods, particularly in the absence of distinct digit imprints.
Some trace fossils record various behaviours of the trace-maker, with a classic example being the tellinacean bivalve trace fossil Hillichnus lobosensis32 whose morphological elements reflect locomotion, feeding, and ventilation. The described trackway exhibits different morphological elements that are genetically related, although only locomotion is inferred. Nevertheless, different parts of fish’s body played distinct roles during locomotion. Therefore, it can also be considered as compound trace fossil, which should be classified under a single ichnotaxonomic name. It further represents an example of structural and developmental complexity33.
The described trackway Reptanichnus acutori igen. et isp. nov. suggests that Osculichnus is not necessarily a hunting trace when it forms a part of the trackway. Such trackways can be difficult to recognize if their remaining elements are less distinct. However, Osculichnus occurring outside of trackways can still be considered a hunting trace, as demonstrated by examples of younger, Late Devonian trace fossils from China4,7.
