Are these sentences grammatically incorrect?

Hi! I don't think they are grammatically incorrect. You just have to arrange your words.

1) May be you could say..."The scientists illustrated Megalonyx.." instead of ' portrayed the Megalonyx in illustrations..." then you could probably move 'with feline grace' in the end:

The scientists illustrated the Megalonyx as pouncing from boulders to prey with feline grace.

(I'm not sure though. But I think this is better.)

2) I don't get number 2. If I were you, and if i got it right, I'd do it like this:

On the engaging theory that the creature had been aquatic and had used them to anchor itself to trees while dozing, another arranged the tusks so that they curved backwards.

Could you explain no.2? I'm confused, sorry.

Hope I helped. =)

1)could be better said as:

"The scientists portrayed the Megalonyx in illustrations as pouncing from boulders, with feline grace, on prey"

2)is hard to understand but I think the following might be better:

"Another scientist arranged the tusks so that they curved backwards, to support the engaging theory that the creature had been aquatic and had used them to anchor itself to trees while dozing."

The first one might be improved by "...feline grace from boulders onto their prey." It removes a bit of ambiguity and makes the sentence flow better.

Otherwise fine.