First known amphibian

The transition from fish to four-footed animals (tetrapods) occurred at least 393 million years ago (MYA), based on fossil tracks discovered at a quarry in Zachełmie, Poland. As to when the first true amphibian emerged, that is more contested depending upon criteria. A prime contender – unearthed from East Kirkdon Limestone in West Lothian, Scotland, UK, by fossil collector Stan Wood in the 1980s – is the temnospondyl Balanerpeton woodi, dating back to c. 336 MYA in the Early Carboniferous period.

The earliest known fish potentially capable of walking on land (and so considered a transitional species between fish and tetrapods such as amphibians) is Tiktaalik roseae, a species of sarcopterygian that lived c. 375 MYA during the Late Devonian, in what is today the Canadian Arctic. Its pectoral fins possessed primitive wristbones not seen in true fishes but present in tetrapods, and the fins themselves were notably muscular, with massively constructed scapulae and coracoids, as well as functional elbows, all of which would have enabled it to support itself in shallow water and briefly on land.

The next stage on from land-walking fish like Tiktaalik were the earliest tetrapods ("stem tetrapods"), represented by species such as the crocodile-like stegocephalians such as Parmastega aelidae, dating back some 373 million years, based on remains unearthed from limestone in the Sosnogorsk Formation on the Izhma River in Ukhta, Russia; Acanthostega (365 MYA), found in Greenland; and Ichthyostega (c. 364 MYA), also found in Greenland. However, many palaeontologists consider such early tetrapods to be a distinct group that can't at that stage be classified as true amphibians.

Among crown group Amphibia (Lissamphibia), encompassing modern-day frogs, salamanders and caecilians, the earliest-known examples are the proto-frogs Triadobatrachus and Czatkobatrachus from the Early Triassic, both c. 250 million years old. Suggestions of possible ancestors for all living amphibian groups, include Amphibamus grandiceps from the Carboniferous of the USA (~315 MYA) or Eugyrinus wildi from the UK (~318-315 MYA).