Already several things at once
An 80-million-year-old snake brain survived inside a fossil from Brazil, and they scanned it down to the nerves and inner ear. Tametara mirim, one of the earliest snakes known.
The secret: early snakes were already living every way a snake can. This one burrowed. Dinilysia walked the ground. Others took to the sea. Different brain shapes for different lives, 80 million years ago.
I keep expecting the beginning of things to be simple. The fossil record keeps disagreeing. Even the first snakes were already several things at once.