One set of bones belonged to a quick-moving meat-eater and the other a giant plant-eater.
The carnivore's remains have rested for millenniums at the bottom of an Antarctic sea, while remains of the 30m long herbivore were found on the top of a mountain.
They would have lived in a different Antarctica - one that was warm and wet, according to two teams of researchers - both funded by the National Science Foundation.
The little carnivore - about 1.8 meters tall - was found on James Ross Island, off the coast of the Antarctic Peninsula.
Not yet named, the animal probably floated out to sea after it died and settled to the bottom of what was then a shallow area of the Weddell Sea, said Judd Case of St. Mary's College of California, who helped find the fossils.
Its bones and teeth suggest it may represent a population of two-legged carnivores that survived in the Antarctic long after other predators took over elsewhere on the globe.
"For whatever reason, they were still hanging out on the Antarctic continent," Case said in a statement.
A second team led by William Hammer of Augustana College in Rock Island, Illinois found the 200 million-year-old plant-eater's fossils on a mountaintop 3900m high near the Beardmore Glacier.
"We have so few dinosaur specimens from the whole continent, compared to any other place, that almost anything we find down there is new to science"
Augustana College paleontologist
The animal would have been a primitive sauropod - a long-necked, four-legged grazer similar to the better known brachiosaurs.
Now known as Mount Kirkpatrick, the area where the fossils were found was once a soft riverbed.
Potentially more finds
Hammer and colleagues were scouring the area for fossils after having found other new species there in the 1990s.
The team included Peter Braddock, a mountain safety guide. "I jokingly said to him, 'Keep your eyes down. Look for weird things in the rock'," Hammer said in a statement.
"He had marked four or five things he thought were odd, including some fossilized roots. But I realized that one of these things was bone: part of a huge pelvis and ilium."
"This site is so far removed geographically from any site near its age, it's clearly a new dinosaur to Antarctica.
"We have so few dinosaur specimens from the whole continent, compared to any other place, that almost anything we find down there is new to science."