I wish someone would identify the types I found...!
There are two reasons that marine and land fossils are found in the Peace River. First, this part of Florida experienced numerous cycles of being land versus being submerged in sea water. Second, this area currently has a river running through past deposits which include intermixed marine and land fossils. So, it's quite common to find marine fossils (e.g. shark teeth) in the same area as a land fossils (e.g. horse teeth).
The geologic formation is likely the Hawthorn Group, Peace River Formation,