In the center map they spawn around the pond that's on the floating island. Pick up with a quetz and bring them to a taming pen. I'd only use oviraptor kibble and starve tame it a couple times. Knock it out during the day..let it wake up at night and knock it out again then feed it
Fun fact: megalosaurus (name translates to “big lizard”) was the first true dinosaur found. Discovered by Richard Owen, he thought it was quadrupedal (walked on all fours) and resembled a crocodile-lion cross. Later he would correct his data and soon classify this group of extinct line of ancient lizards as “Dinosaurs”.
To the mobile person who doesn’t know where to find them, they are mostly found in the middle chamber, I only found one while going for the artifact, so it might be near the onyc, araneo or the other creepies, just be careful before you get off your mount to get the artefact.