First off fry don't fossilize, lol. If the female would abort the pregnancy, the fry would be re-absorbed into the females body, or she would drop the dead fry, depending on the length of term they are. They could of aborted pregnancy though just due to the stress of a new tank. If the tank is populated heavily, they could have also had some fry, but the fry may have been eaten, or hiding. Any number of things is possible.
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Mike
MS Livebearers

All the info you need on mollies:
http://slacker1202.tripod.com/id14.html