Sentence Correction (SC) | Re: Although the turtle has been toothless for more than 150

I understand why its E with regards to the idiom 'function as' - makes sense
What about the 'their' vs 'its' split - tortoises usually have the pronoun 'its' - which is correct in B. Would like some clarity on the doubts below:
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.

a) their horny jaws function for teeth
b) its horny jaws function for teeth -correct in terms of pronoun