I would use the period instead of the comma.
I could understand the following sentence as saying that Mary laughed while saying what quoted.
But Mary suddenly laughed, "How could that be?"
In that case, the comma before the quote would be considered optional from somebody. (See Should I always use a comma before a quote?)
Since the quote is followed by "she said," that is an example of comma splice.
She smiled, she told me "I will see you later."
In the sentence shown in the question, replacing the comma with "and said" would be acceptable.
But Mary suddenly laughed and said "How could that be?"
Replacing the comma with the period is also another solution.
But Mary suddenly laughed. "How could that be?" she said, "It's only 2008 this year."