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:

In 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.
Disney Announces Two New Pixar Projects - The Hollywood Reporter