This is a cool game. I think all of the art is really well done and the style is really fitting. I wish there was a little more animation. Some tweening on the art would have been great, but even just some simple squash and stretch on the characters would make everything feel less static I think.
I'm really glad there was an invincible mode in the menu, because I was really stumped by that mammoth fight and I probably would have given up if I couldn't have turned on invincibility to understand what was going on. After that I beat the T-rex after 2 tries without invincibilty. I would say the biggest problem is a lack of player feedback. The first several times I fought the mammoth I couldn't figure out if I wasn't doing damage because I was hitting it in the wrong spot, or missing entirely, or was damaging it and it just took a lot of hits. I think a good thing to add would be some feedback that you're hitting the wrong part of the enemy. Like maybe if you hit the tusks it makes a metal clanking sound or something to show you made contact but didn't deal damage.
I also think a little more telegraphing on the offscreen attacks would be good. Both bosses had attacks where they just flew in from off screen without warning, so some red Xs or an arrow over where they're about to fly in from would be super helpful.
Overall, I think this is a really cool idea, and would definitely love to see it polished and fleshed out into a bigger project. Great job!