In the late 1930s, our understanding of the transition of fish to tetrapods — and the eventual jump to modern vertebrates — took an unexpected leap forward. The evolutionary a'ha came from a single partial fossil skull found on the shores of a riverbank in Eastern Canada. 

Meet the Stegocephalian, Elpistostege watsoni, an extinct genus of finned tetrapodomorphs that lived during the Late Givetian to Early Frasnian of the Late Devonian — 382 million years ago. 

Elpistostege watsoni — perhaps the sister taxon of all other tetrapods — was first described in 1938 by British palaeontologist and elected Fellow of the Royal Society of London, Thomas Stanley Westoll. Westoll's research interests were wide-ranging. He was a vertebrate palaeontologist and geologist best known for his innovative work on Palaeozoic fishes and their relationships with tetrapods. 

As a specialist in early fish, Westoll was asked to interpret that single partial skull roof discovered at the Escuminac Formation in Quebec, Canada. His findings and subsequent publication named Elpistostege watsoni and helped us to better understand the evolution of fishes to tetrapods — four-limbed vertebrates — one of the most important transformations in vertebrate evolution. 

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