This was done on mobile.

Finding and catching:

Diplocaulus is found in the swamps I'd say they're uncommon but predators will kill them if they're nearby making them hard to find. I strongly recommend using a flying Dino when looking for one I used a pteranodon for it but you can use whatever can pick it up. They're pretty simple to find but DO NOT TRY TO TAME IN THE SWAMP. Unless your really prepared I wouldn't recommend risking it. When I was catching it I picked it up with my ptera and put it in a taming pen at my base. When picking it up wait for it to be on land, because if it's in the water it's much harder to pick up or maybe it's not possible I don't know(I spent hours trying to pick one up in the water and died many times). It may take a while to find one on land it'll save you a lot of trouble.

Note: don't try to go for one that's already being attacked by another Dino especially kaprosuchus

Knocking out and taming:

In the pen knocking it out is simple just use tranqs or darts, also keep in mind the health you don't want it to die. When it's knocked out taming is like any other Dino just put the kibble or meat and wait. I starved mine to make the taming a bit faster and found that the hunger decreases relatively quick and you can speed that up by sleeping while it's torpor is around 3/4 or more. Not really much to say it's torpor drains slow so you won't need to many narcotics unless your starving it and by the end of the tame you should have yourself a newly tamed diplocaulus.

-AHS