If humans evolved from the sea where did the dinosaurs evolve from?

Humans evolved from mammals like dinosaurs such as dimetrodon.

All Vertebrate's evolved from the sea, Dinosaurs and Humans alike (the first were neither Reptiles nor Mammals but Tetrapods). These Tetrapods were the first to make the move from ocean to land:

At the time of the Dinosaurs the only Mammals were tiny mouse like creatures unable to increase in size and complexity due to the domination of the Dinosaurs - that small size helped the early mammals to survive underground whilst the Dinos perished during the extinction event. And the rest, is history!