how did scientist know it was an index fossil?

Answer 1

The reason how scientist can tell whether something is a fossil is by a method call carbon dating. Basically, what it is in short is to measure the amount of C-14 (a radioactive isotope of natural C-12) on the substance, then you can do a back calculation to find out how old something is.

The theory behind the carbon dating is like this. On earth, there is a trace amount of C-14 in the atmosphere, presence in the form of CO2. This substance is radioactive, it release radiation continuously and after a certain period of time (called half life), its radioactive property will be reduced by a factor of 2. After another half life, it will be further reduced by another factor of 2, etc.

For any living human being or plant, there is a (close to) constant level of this trace substance in our bodies because we keep breathing everyday to replenish any C-14 that disappear. For any dead body, since they can no longer breath, the amount of C-14 present in their body at the moment when they are just dead will be the same as that in the surrounding. As time goes on, some of these C-14 will be lost (depending on how many half life it has undergone). As a result, by measuring the amount of C-14 on the fossil and that in the atmosphere, we can estimate how long this fossil is.

Answer 2

You may already know that index fossils are common, widely distributed species that only lived for limited periods of geologic time and so they are used as guides to the age of the rocks in which they are preserved.
So, I think you are asking how paleontologists decide that a particular fossil would be a good index fossil to use in their zonation. That's actually a very good question--because they clearly need to have looked at lots of different rock sections with fossils --crossing similar time periods. Some fossils that have been picked to be "index" markers for time periods have later been found not to be so great, because they lived longer than thought, or they were too rare. But, over time, many different species have been documented as being good index fossils. Here's a website that shows some key shell and coral fossils used for important zones: http://pubs.usgs.gov/gip/geotime/fossils.html . Relatively big and widespread fossils like shells are good markers for many limestone beds. Tiny microfossils that show up in drilling cores are good index fossils for the petroleum industry. Fossil teeth can be good markers in sediments deposited on land.