Abstract
Some primarily aquatic vertebrates make brief forays onto land, creating traces as they do. A lack of studies on aquatic trackmakers raises the possibility that such traces may be ignored or misidentified in the fossil record. Several terrestrial Actinopterygian and Sarcopterygian species have previously been proposed as possible models for ancestral tetrapod locomotion, despite extant fishes being quite distinct from Devonian fishes, both morphologically and phylogenetically. Although locomotion has been well-studied in some of these taxa, trackway production has not. We recorded terrestrial locomotion of a 35 cm African lungfish (Protopterus annectens; Dipnoi: Sarcopterygii) on compliant sediment. Terrestrial movement in the lungfish is accomplished by planting the head and then pivoting the trunk. Impressions are formed where the head impacts the substrate, while the body and fins produce few traces. The head leaves a series of alternating left-right impressions, where each impact can appear as two separate semi-circular impressions created by the upper and lower jaws, bearing some similarity to fossil traces interpreted as footprints. Further studies of trackways of extant terrestrial fishes are necessary to understand the behavioural repertoire that may be represented in the fossil track record.
Highlights
Some primarily aquatic vertebrates make brief forays onto land, creating traces as they do
Several terrestrial Actinopterygian and Sarcopterygian species have previously been proposed as possible models for ancestral tetrapod locomotion, despite extant fishes being quite distinct from Devonian fishes, both morphologically and phylogenetically
The head leaves a series of alternating left-right impressions, where each impact can appear as two separate semi-circular impressions created by the upper and lower jaws, bearing some similarity to fossil traces interpreted as footprints
Summary
Some primarily aquatic vertebrates make brief forays onto land, creating traces as they do. Actinopterygian fishes were uncommon in the Devonian, several modern taxa have evolved ‘fin-driven’ locomotion on land, and can be useful models for investigating intermediate stages between fully aquatic and terrestrial locomotion (e.g., ‘walking’ catfish1, ‘crutching’ mudskipper[9]). These taxa tend to exhibit specialisations of the pectoral appendages rather than the pelvic fins—the latter of which is considered by some to be a major tetrapod innovation[10,11,12]. It is impossible to know for certain how many Devonian taxa made terrestrial forays, the diversity of locomotory behaviours exhibited by extant, morphologically diverse semi-aquatic fishes[1] suggests there was a potential for Devonian fish to produce enigmatic sub-aerial traces
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