a) There are missing details here, but generally gastropod fossils date back to the Late Cambrian (497-485 milion years ago).
b) No, it is not possible, because radiocarbon dating method has a limit of approximately 40000-50000 years. This is due to the fact that the carbon-14 half-life is about 5700 years: this is the amount of time after at which we have only 50% left of the initial amount of carbon-14. After 50000 years, the amounf of carbon-14 left is less than 1%, therefore it becomes too hard to measure it accurately and this is why it is impossible to measure a date for the fossil.
