It cannot be an Alocasia as this genus doesn't occur in Brazil.
There are 4 species of Xanthosoma registered in the Atlantic rainforest of the Rio de Janeiro state.
Xanthosma maximilianii
Xanthosoma pentaphyllum
Xanthosoma taioba
Xanthosoma riedelianum
It is none of the first 3, unfortunately I cannot find any photos or even herbarium species of X.riedelianum on the web.
The plant was growing in the shade on a very steep, almost vertical slope with a stream underneath heading to the river. It was difficult to take good photos as I had to stand on the crumbling edge of the slope, the plant was tall, and I found the leaf shape interesting, different from the other species I've met during this trip or my previous trip 4 years ago.
I'm not entirely sure but I think it had the beginning of an inflorescence coming up. A pity I couldn't come closer by and take better and more detailed shots.
In the hope to find a bloom about week later, I returned to the same spot, but alas the poor plant had been destroyed by a heavy monsoon, it had probably been washed down and taken by the stream. What a shame!
