Petrolacosaurus was a primitive diapsid reptile featured in Walking with Monsters.
Petrolacosaurus was one of the first reptiles to evolve. It was small and looked similar to modern lizards. It lived during the Carboniferous era, a time when there was around 50% more oxygen in the atmosphere than today. This allowed for insects to dramatically grow in size, placing Petrolacosaurus fairly low on the food chain. Although it was shown to evolve into Edaphosaurus in Walking with Monsters, this was impossible, whereas Petrolacosaurus was a diapsid, while Edaphosaurus was a synapsid and a pelycosaur.
Petrolacosaurus often hunted on the forest floors of the Carboniferous. Unlike amphibians, Petrolacosaurus, like all reptiles, had tough, scaly skin which trapped moisture inside their bodies, vital for all land dwellers. Because they didn’t dry out in the sun, they could venture away from water. They also had evolved a complex heart, which was much more efficient than their enemies. It’s a powerful pump that pushed blood and oxygen around their bodies to their muscles so they could run at high speeds without stopping for a rest. They didn’t need burrows to avoid danger, they had speed and stamina on their side.
To show how the scaly skin of this reptile IS containing moisture, the camera made a zoom on the animal.
In the series
A nest of Petrolacosaurus eggs were hatching. Some hatched and escaped but most of them were killed by a Mesothelae.
One was seen crawling towards a Mesothelae's burrow and was chased by the Mesothelae. The Petrolacosaurus hid in a tree trunk but the Mesothelae broke in from the top and killed it. The Mesothelae dragged it away but a Meganeura stole it. Another was seen scurrying past the same Mesothelae. After a massive storm and forest fire during the night, another Petrolacosaurus finds a fried Mesothelae and proceeds to eat it. The show's narrator says that 'the era of giant insects and spiders is coming to an end and Petrolacosaurus begins to evolve into Edaphosaurus, while the scene shifts from Carboniferous to early Permian.