It happened about 390 million years ago, “really, really quickly” according to a Harvard team:
Despite the rapid evolution, the team found there wasn’t a rapid accumulation of new species, which suggests the anatomy required to get on land evolved several million years before species diversification of tetrapods. Put simply: just a few early fish grew all the right anatomy before ‘leggy bois’ became abundant and diverse.
“What we’ve been finding in the last couple of years is that you have lots of anatomical changes during the construction of new animal body plans at short periods of geological time, generating high rates of anatomical evolution, like we’re seeing with the first tetrapods,” explains Simões.
“But in terms of number of species, they remained constrained and at really low numbers for a really long time, and only after tens of millions of years do they actually diversify and become higher in number of species.”
Deborah Devis, “‘Never skip leg day’ – Tetrapods evolved superfast” at Cosmos Magazine August 24, 2021
The way they describe it, it sounds like design. The unfolding of a plan.
The paper is closed access.