Instructions
Travel to the lake in South Dust Hills. Walk to the same area that you were first dropped in upon discovering this region, just after defeating Ursala.
Speak with the lone character here, who asks you to identify the bones of a dinosaur in a family photo for him. He gives you the photo to take to the protagonist's lab.
Teleport to the player's lab, the D-Site Branch, and speak with Minmi to identify the fossil in the photo as a Tyrannosaurus Rex.
Return to the character who gave you the family photo and speak with him. He gives you a note detailing where to uncover the fossil.
Go to the entrance of the South Dust Hills region, then follow the step instructions on the note. For example, if it says "South 4, West 5, South 6, East 1," then you would walk down four steps, left five, down six more, and right one step. Dig at the spot indicated by the note to retrieve the Tyrannosaurus Rex fossil.
Speak with Minmi again to re-animate the dinosaur and add it to your collection.