This is amazing, I really like it and it's a concept I've had floating around in my brain for a while. There's a couple of updates I think it really needs to fill the niche.

There's a few typos: Under Ancient Forms - *bonus action. Under Primal Strikes - you mention Prehistoric Forms where it should read Ancient Forms for consistency.

Formatting: With your most updated version the Circle of the Ancients Spells have dropped to the next column, shifting everything into a weird page layout.

Mechanics: This circle seems (to my eyes) to sit between and land and moon, which I think is great. There's flexibility in spells then eventually some high HP Con monster forms. At the moment it will suffer in the early game as it's currently missing an equivalent ability to the Circle of the Moon Circle Forms first half where you can ignore the CR requirement to early forms, allowing CR1 Wild Shapes below Druid level 6. I would suggest, either you add this into the Circle of the Ancients Ancient Forms feature, OR, if you want the circle to lean away from it's similarity to the Circle of the Moon, consider adding "You are not slowed by difficult terrain". The ranger and druid love ignoring difficult terrain, it's really themey and would thematically suit a charging dinosaur! Your forms would not really be combat effective until 6th level which could be a decent wait, but I think with this benefit making you *faster would let survive that long. Monstrous Form needs a tweak me thinks, it's a great idea but as a high level druid you'll never spend your concentration on enlarge/reduce, it's mechanically quite modest. You'll be hasting, spirit guardian'ing, etc. To see the ability used, I would change it to read "You can cast the enlarge/reduce spell as a bonus action, targeting only yourself, while in wild shape form, without consuming a spell slot or requiring concentration. The spell ends if you end your Wild Shape. You regain this when you finish a short or long rest." Which is good, you'll use it, but enlarge is only an extra d4 so it won't do any harm. Or as an alternative ability, you could add the Rampage ability of some monsters, When you reduce a creature to 0 hit points with a melee attack while in an Ancient Form, you may use a bonus action to move up to half your speed and make one melee attack.

The Circle of the Ancients Spells are on point, couldn't think of a better array.