Cast Sandsteppe Mastodon by paying {5}{G}{G}.
Sandsteppe Mastodon enters the battlefield, putting five +1/+1 counters on Sage of Hours.
Activate Sage of Hours by removing all +1/+1 counters from it, causing you to take at least one extra turn after this one.
Move to your next turn.
At the begging of your upkeep, Roaring Primadox triggers, causing you to return Sandsteppe Mastodon from the battlefield to your hand.
Repeat.

Infinite turns

Sandsteppe Mastodon

Roaring Primadox

Sage of Hours

At the beginning of your upkeep, Roaring Primadox triggers, returning Wormfang Manta from the battlefield to your hand.
Wormfang Manta's LTB ability triggers, creating an extra turn after this one.
Activate Scroll of Fate by tapping it, manifesting Wormfang Manta from your hand.
Pay {5}{U}{U} to turn Wormfang Manta face up.
Repeat each turn.

Combos with "Roaring Primadox"