A University of Chicago paleontologist announced the discovery of two new species of dinosaurs from the Cretaceous period in a paper appearing this month in the journal Acta Palaeontologica Polonica.

The fossils of the two species were dug up on a 2000 expedition led by the paleontologist, Paul Sereno, in a part of the Sahara Desert in Niger.

“It was a bonanza,” Mr. Sereno said in an interview yesterday. “We found 20 tons of dinosaur fossils. There’s dozens of new species.”

Mr. Sereno and his co-author, Stephen Brusatte, of the University of Bristol, named the two species Kryptops palaios, or “old hidden face,” and Eocarcharia dinops, or “fierce-eyed dawn shark.” Their significance, Mr. Sereno said, is in illustrating how the theropods, or meat-eating dinosaurs, evolved after the continents broke apart in the Jurassic and Cretaceous periods. In North America, the line evolved into the Tyrannosaurus rex, which as a predator held sole dominion over its environment. In the southern landmass, Gondwana, a more complex ecology developed, in which different types of meat-eaters cohabited. Eocarcharia was a predator — a “slicer and dicer,” Mr. Sereno said — while Kryptops was most likely a scavenger.