Speculation had arisen awhile ago when someone spotted some dino-related concept art in a Pixar interview photo and with this announcement last year:
Discovery Channel and Pixar Team Up For New Dino Show | Dinosaur Tracking
And finally an official announcement:
Disney Announces Two New Pixar Projects - The Hollywood ReporterIn addition to Docter's project, the Untitled Pixar Movie about Dinosaurs will be directed by Pixar veteran Bob Peterson and produced by John Walker.
The movie is aiming for a holiday 2013 release. The story is set in a world where an asteroid didn't hit the Earth, thus allowing the creatures to keep living and evolving.
An image of concept art shown to the hall was of a small humanoid silhouette on top of the head of a brachiosaurus.
Peterson said he was inspired by a World's Fair he attended as a child where he saw animatronic dinos created by Disney.