The Pterosaur was a mysterious female phoenix-like entity that guided the D-Team and the Alpha Gang through time in Mesozoic Meltdown. Her body (not including the wings and tail) appears slightly larger than the Backlander. She seems to somewhat resemble a large, glowing version of the anime's Microraptor.
Throughout the series, the Pterosaur guides the D-Team into the different time periods, following the Spectral Space Pirates. She also warns them about the danger of fusing the 7 Cosmos Stones together.
Although it isn't stated, the Pterosaur may come from the fusing of the 7 Element Stones, as these stones and the Pterosaur herself are "positive" opposites of the "negative" Cosmos Stones and Dark Pterosaur.
The Dark Pterosaur is the nemesis of the Pterosaur. The two are physically identical except for the opposite light/dark colors and the fact that the Dark Pterosaur can expand its outer shell to be as large as an entire city, while still having its "main" self as a separate entity inside.
It was created when all 7 Cosmos Stones were fused together by Spectre, and has the ability to rip time and space apart. The Dark Pterosaur was destroyed by Seth, who, with the help of the 7 Element Stones and the Pterosaur, rammed into it with the Backlander and destroyed it, the Element Stones' and Cosmos Stones' opposite positive/negative "charges" canceling each other out.