Obviously, something strange is going on, because biomes don't just up and change like that. This is a relatively short area, but there is a new enemy type to be on the lookout for: the Spider Hopper. Again, though, it's really nothing to worry too much about. Continue on and after the dialogue, save and enter the Forest Ruins. There will be yet another scene, and then a battle with a tougher new opponent known as a Skull Guts. This can be challenging, seeing as how it can hit for a good 20 points per attack. You're best bet is liberal use of the Defense Corps co-op attack, as it does between 55-60 points of damage if you're at a decent level, and can finish the battle in three turns or so.
After the battle, two more Skull Guts appear and block the exit. You're given the option to fight or flee; I opted to look for another exit, so I don't know what happens if you stay and fight. Anyway, proceed through the ruins, and eventually you will come to an inner sanctum of sorts. Here, the hero finds a book. When he, Marica and Jale touch it, they witness a vision of a great battle and are filled with a mysterious new power they refer to as "The Marks of the Stars". After the scene, the party is challenged by a trio of Skull Guts. Using the party's newfound powers--the hero's Breath of Flame, Marcia's Healing Gleam and Jale's Bind Flash--will make short work of the enemies this time around. More story sequences follow, and ultimately your goal will be to make your way back to Citro Village.