When Rivals of Ixalan was being revealed, I was excited to see what the tribes from the new block could do when both sets could work together. Dinosaurs had seemed like the most pushed tribe for Constructed in Ixalan so it seemed like a natural place to start with the full block. There weren't many exciting cards on the creature side of things, but it had a key component to making the ramp-centered tribe work: a powerful ramp spell.
Thunderherd Migration may not look like much to those of you who remember the days of Rampant Growth. After all, it's a strictly worse version of that seemingly innocuous card. But in reality Rampant Growth is very powerful relative to most Standard-level cards, it's just less flashy. We've seen in recent years a move toward three mana with some minor value, a la Gift of Paradise, be the standard Standard fare for ramp cards, and those cards have been successful. But starting a full turn ahead of schedule in a deck that is built around acceleration is gigantic.
It makes double ramp turns a turn faster, letting you get to six mana on turn 4, which may not seem like a lot in a deck without many six mana plays, but with lands that enter the battlefield tapped and the possibility of missing a land drop, every little bit helps. It also opens up the opportunity to curve two ramp spells into an Hour of Promise, a curve that leads to having nine mana on turn 5 should you make all your land drops.
Nine mana is an important mark for a certain absurdly powerful Dinosaur: Zacama, Primal Calamity. Nine is a high mark to hit in Standard, so this card doesn't leap off the page, but if you put in the work to consistently cast it on time, the payoff is there. I don't think anyone is beating it if they don't have an immediate answer, and even that may not be enough. If you get to activate its abilities that can cause a huge swing in position and if they kill it with the trigger on the stack, you can potentially cast another big threat or a sweeper from your hand.
There aren't a ton of other Dinosaurs here, just enough to enable Thunderherd Migration, but they fill their roles nicely. Carnage Tyrant is a premier threat against midrange and control decks. Ripjaw Raptor provides a nice early threat and roadblock against aggro that synergizes nicely with your red sweepers. And the singleton Ranging Raptors is a downgrade on the other three mana ramp options because it's not consistent, but having one suboptimal slot to make your best card better is a fine sacrifice to make.
With the metagame reacting so harshly to Mono-Red Aggro and moving toward midrange and control decks, going over the top with ramp becomes an attractive option, so the time looks to be ripe for all the big creature lovers out there.