Although the turtle has been toothless for more than 150 million years, in some contemporary turtle species the moderately sharp and jagged edges of their horny jaws function for teeth.

1) their horny jaws function for teeth
2) its horny jaws function for teeth
3) its horny jaws function as do teeth
4) the horny jaws function as teeth do
5) the horny jaws function as teeth

GMAC : 5) totally does away with the possessive pronoun and places an article(?)
Me: 1)

It's 2. We're talking about turtle which is singular.

We answered this question on this post.