Which was the most likely food source for the terrestrial tetrapods?

Leaves

Explanation:

Tetrapods came to the land surface at around the end of Silurian Period and became widespread in the Devonian Period. Before that at during Ordovician Period land period was covered by non-vascular plants. So these plants became food for the early Tetrapods.

there are so many examples for that in different areas, like biology experiment carried out in our lab recently.

here's one link: