The best use of turtles in base defense (especially on a primitive server) is a whole group of them. When you get raided they will swarms the gigas and rexes rendering immobile. That allows your plant X and turrets to go to work for longer. And nothing can tank like a well leveled turtle with its shell defense and a (hopefully bumped up) saddle
Create the ultimate turtle breeding group. Call yourself the turtle god. Put your mega tortols in aggressive, pick it up with quetzal, drop these babies on other tribe's turrets.
You now have the ultimate turret soaker. Let all other players feel the wrath of the turtle god. Your power is limitless, your defense is unfathomable, they shall all bow before your might. You are turtle god, bullet's demise, the ultimate waster of resources, who would dare defy you?
The carbonemys is perhaps one of the best creatures to tame in the early game, and provides decent protection against dilos and lone raptors. The carbonemys can easily be knocked out with a slingshot, and even a survivor with base movement speed can outrun one, making it one of the easiest tames for the early game. Not only can it carry a lot, it can be used a bait for taming (or escaping) big carnivores that would love to eat you and you (amazing pink dodo) for a snack.
Amazing for early game
Can carry lots of stuff
An easy source of meat, keratin, hide, and XP for early game players