|Mapusaurus roseae by T-PEKC|
Giganotosaurus and Mapusaurus seems to be two differetnt species if we judge about appearance and the supposed time they lived in. But let's not forget that we only have a few, incomplete fossils to work with. In case of Giganotosaurus we have two specimen: MUCPv-Ch1 (it is the smaller but more complete one) and MUCPv-95 ( we only have a fragment of the lower jaw, based on it it was the bigger one), as for Mapusaurus we have at least five but as you'll see in the picture they're not just fragmentary...
|Mapusaurus fossils from Planet Dinosaur|
What if the growth of the Mapusaurus was like that too? And here comes Giganotosaurus in the picture: we "know"(okay, we can only guess) how a Juvenile Mapusaurus looked like but we don't know how an adult looked like, the opposite applies for Giganotosaurus.
|Giganotosaurus vs tank by Rodrigo Vega|
|Carcharodontosauridae(Carcharodontosaurus, Giganotosaurus, Mapusaurus, Acrocanthosaurus, Eocarcharia and Concavenator) by Vitor-Silva|
So to sum it up: We have two predators, the adults were around if not the same sized, we know(well it's better to say guess) how the adult of one species looked like while we don't how the juveniles looked like and the opposite goes for the other one. They both lived in the same place only with 2-3my time difference. I think it's more than possible that the two were the same. As long as we find more fossils we'll never know for sure. It's your descission to think i'm right or wrong, only time will tell this.