Tetrapods evolved from sarcopterygian fish, and subsequently adapted to terrestrial land, what caused many anatomical changes and subtler morphological and behavioural adaptations, but it is not precisely known how and when.
It is thought that vertebrate terrestrialization took place during the early Carboniferous, but tetrapod fossils are extremely scarce between 359-329 Ma. The oldest known undoubtedly terrestrial tetrapods are small, 331-327 Ma animals from Scotland.
Ossinodus pueri, a >2 m-long stem tetrapod from Queensland (Australia), dated 333 Ma, is remarkable among these group (along with Tulerpeton) for his degree of ossification. A high-resolution finite element analysis of a bone fracture, and new osteological observations, by Bishop & al. 2015, suggests that it spent significant amounts of time on land, what pushes the date of first land tetrapod at least two million years earlier and raises the possibility that terrestriality evolved from big Gondwanan tetrapods rather than small European ones, as previously thought.
- (2015) Oldest Pathology in a Tetrapod Bone Illuminates the Origin of Terrestrial Vertebrates. PLoS ONE 10(5): e0125723. doi: 10.1371/journal.pone.0125723