Traces of dipnoan fish document the earliest adaptations of vertebrates to move on land

A new trackway produced by crawling fishes, which includes imprints of the trunk, snout, tail, body drag traces, and pectoral fins, was discovered in the Lower Devonian (middle–upper Emsian) marginal marine deposits in the Holy Cross Mountains, Poland. The snout imprints are represented by a low-angle variant of the already described Osculichnus tarnowskae, which has generally been interpreted as a hunting trace of fishes. However, in this case, it is considered an imprint of a fish’s snout, used for anchoring in the sediment during the locomotion of at least partially emerged fish. This compound trackway provides the first evidence of the previously unknown life behaviour and locomotion abilities of dipnoan fishes in the early stage of their evolution and documents a testing land mobility skills of vertebrates, predating by about 10 million years fully terrestrial tetrapods locomotion traces. Similar trackways are produced by extant lungfish during terrestrial locomotion. The trackway co-occurs with a new resting trace produced by a dipnoan fish supporting itself with one or two pairs of fins on the bottom.