The new carnivore fossils have been described by a researcher from the University of Bristol working with palaeontologists from the US.
One of the dinosaurs probably scavenged its prey like a hyena, the other probably hunted live animals.
Details appear in the journal Acta Palaeontologica Polonica.
The fossilised remains of two 110-million-year-old carnivorous dinosaurs were found along the western edge of the Tenere Desert in Niger by Dr Paul Sereno, from the University of Chicago, eight years ago.
"They are the earliest records of both major carnivore groups that would go on to dominate Africa, South America, and India during the next 50 million years, in the Cretaceous Period," said co-author Steve Brusatte, from the University of Bristol.
Hidden face
One of the creatures was about 8m (25ft) in length and sported a short snout with a horny covering. It has been named Kryptops palaios or "old hidden face".
Kryptops may have scavenged food in a manner similar to a hyena.
Like later members of its dinosaur group - known as the abelisaurids - in South America and India, it had short, armoured jaws and small teeth, well designed for gobbling guts and gnawing carcasses.
The other discovery is of a similar-sized contemporary called Eocarcharia dinops or "fierce-eyed dawn shark".
It possessed blade-shaped teeth and a prominent bony eyebrow ridge. Unlike Kryptops, its teeth were more suited to attacking live prey and severing body parts.
The Carcharodontosaurids, the group to which Eocarcharia belongs, included predators as big, if not bigger, than Tyrannosaurus rex.
A swollen bony brow over Eocarcharia's eye gave it a menacing appearance and may have been used as a battering ram against rivals for mating rights, say the researchers.
These two meat eaters were contemporaries of another carnivorous dinosaur which is known from the same area: Suchomimus, a large fish-eating theropod.
"It is clear from their anatomy that they were eating different things: Suchomimus ate fish, Kryptops ate smaller animals and Eocarcharia was the top predator of its day," said Mr Brusatte.
"Just like in the African savannah today, lions, cheetahs and hyenas must eat different food to survive side by side. It is fascinating to see this in a 110-million-year-old ecosystem."
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