Rocks continually break down and are continuously formed. Any volcanic tableau will show you that. So will tectonic plate action in the formation of mountains and oceanic erosion. That said they have ways to date fossils and could give you a range for many. But these could be eroded and resettled as sedimentary rock. Consider that limestone is predominately crushed shell. It cements itself together and gets eroded becoming sediment again. The process repeats itself. Fossils are harder than limestone. They could wash out and be recemented in place in a new rock. Sharks are 300,000,000 years old, mostly unchanged. Best advice I can give is take it to a university or a museum of natural history and ask. I have never been refused if there was someone to help. They are usually very cordial and enthusiastic. I took two twenty pound rocks to a geology center in my state. A professor was there having a class. After his class I asked him to identify the rocks. He identified one as a two billion year old rock which he also said was mudstone. The other he said was a composite stone of which the softer minerals were wearing away leaving the harder rock behind. The hard rock looked like wavy swirls. He asked me where I found them. I told him my yard and said I had other anomolies. He thinks glaciers dragged the rocks to my property. This of course has fueled my interest for years.