Megachelon is a powerful surface hybrid in stats. In appearance, it seems to have scales from Archelon and the shark like body that is inherited from Megaladon.
Megarechelon is incredibly powerful in stats. It is one of the strongest hybrids available in the game currently in aquatics. The stats rival Dunkelosarus very closely apart. If you wish to unlock this hybrid, you must first max out both Archelon and Megaladon. The easiest way to get Archelon is to unlock him by winning the Top 1% prize in the Archelon tournament. Good luck maxing out the two as it will take some time.
(I would recommend using it in the late game.)