Our understanding of when fishes evolved into tetrapods — animals with a backbone and four limbs — and began to walk on land needs radical revision. A study published recently in the journal Nature provides convincing evidence that the first vertebrates started walking nearly 385 million years ago — about 18 million years earlier than previously thought. There is already sufficient evidence that land vertebrates evolved from fish when the fins first became lobe-shaped without digits. Fishes with lobe-fins are considered transitional forms that gradually developed into vertebrates with limbs. The latest discovery — footprints of unknown creatures that were as long as 2.5 metres — from rocks in a disused quarry in southeast Poland confirms the fish-tetrapod transition theory. The footprint tracks resemble the early tetrapod fossils.
The discovery has resulted in two major scientific reassessments of the transition. First, the age of the footprint tracks is ten million years earlier than the oldest known transitional fishes called elpistostegalians (such as tiktaalik roseae). This would mean that transitional fish forms and those with limbs coexisted for a certain period of time. The discovery highlights the fact that elpistostegalians were neither the early transitional forms of fish-vertebrates nor a short-lived ‘transitional grade’ between fish and tetrapods. They were at best “late-surviving relics rather than direct transitional forms,” to quote the editorial summary in Nature. Secondly, the discovery builds a strong case for reassessing the environmental setting of the transition. The impression that the transition took place in seasonally flooded environments of rivers is no longer valid. The footmark tracks studied in Poland are from rocks formed in a marine environment — a tidal flat environment and/or a lagoon. According to the authors of the study published in Nature, an intertidal environment would have provided a ready food source twice a day. Although no body fossils of the tetrapods are seen in the quarry rocks, the discovery suggests that any further search for body fossils should be in such marine environments; footprint tracks in the absence of body fossils are difficult to interpret with full confidence. The footprint tracks highlight the fact that fossils recording the early stages of vertebrate evolution (predating both the elpistostegids and those seen in Poland) are missing. The study cautions that “the timing of the fish-tetrapod transition is best regarded as uncertain.” The evolutionary tree of early tetrapods will be sketchy as long as this gap remains unfilled.