Greenhorn Plains is the first place you can only go when you get a level 1 Fossil License, as a dig site. There are few dinosaurs there that will be new to you. V-raptor, S-raptor, and Goyle are the main ones there. Greenhorn Plains has rocks to break for 40 gold. Greenhorn Plains is a grassy area with trees and rocks (of course Fossils too). When you are on chapter 1, "The Missing Medals" you have to give your Dino Medals to the person guarding the place he says has rare fossils. You give your medals to him and he lets you in. After digging for a while you see a girl who's mad (named Rosie you will learn later). She's been getting bad stuff same with others. So you meet her there for your first time. Also there is Snivels the BB Bandit there on Chapter 4 when your there. You will also see there on Chapter 1 the Shady Guy. You will see on side missions there is Blambeau your first encounter with him. The last thing there is one of 5 Sub-idolcomp fragments in Greenhorn Plains (you will get to dig it up on Chapter 8). There is lots of other things like jobs to go there and do stuff, but these are main stuff. Greenhorn Plains is a cool and mysterious place!
In Chapter 1 when you know who took Rosie's Dino Medals, you go to Greenhorn Plains and fight him for Rosie's medals back. The person you fight is Medal-Dealer Joe. When you win police come and take him to jail and Rosie gets her Dino Medals back just in time for level up battles! (You also now can do a level up fight to be level 2)