The biggest fossil mammals found in the Badlands are brontotheres —giant, rhino-like “thunder beasts.” The National Park Service says they are the largest fossil mammals found there.

Quick Scoop

If you mean the largest single animal fossil people usually talk about from the Badlands, a juvenile T. rex found in the North Dakota Badlands is one of the standout dinosaur discoveries, but it is not the largest fossil overall.

What makes them big

Brontotheres were huge prehistoric mammals that lived millions of years ago, and the Badlands preserve a lot of their remains. The park’s geology and paleontology page specifically highlights them as the largest fossil mammals found in the Badlands.

Other famous finds

Recent news has also highlighted rare dinosaur fossils from the Badlands, including a juvenile T. rex skeleton and exceptionally preserved duck-billed dinosaurs from Wyoming badlands. Those are famous because they are rare and well preserved, not because they are the biggest fossil ever found there.

Bottom line

So, the best short answer is: brontotheres are the biggest fossils found in the Badlands in terms of fossil mammals. If you meant the biggest dinosaur fossil or the most famous discovery, that answer shifts toward the rare T. rex and other dinosaur finds.