"Because" is a conjunction, so it needs an independent clause with its own verb ("is" in this case). "Because of" is a preposition, so it needs a noun or noun phrase, "anatomical structure" in this case. You can't use "is" directly in this case, but you can use another relative clause with it, e.g. "The Indian variety of crocodile is the most widely researched crocodile in the world because of its anatomical structure, which is very unusual".

"because of its" because you are using "its" to refer to the structure of the crocodile, where are the first option, "because its" is trying to say "because it is" which would infer you are about to describe something about the crocodile. I hope this helps, Jack Lindsay

Jack's answer is clearly wrong, and James' answer, although correct, got me a little confused, as he re-wrote the original sentence to fit the 'because of' form. "The Indian variety of crocodile is the most widely researched crocodile in the world because its anatomical structure is so unusual." Is perfectly correct. As James said - 'because' needs a clause with a verb, which that sentence does have.

I'm sorry, It is look like a similar, isn't it?