These are the large fishes that would have been living in the inland sea that covered much of Western North America roughly 275 to 265 million years ago. It's specifically based off formations dated to the leonardian to roadian stages.

This is based off the kaibab formation which is correlated temporally to the phosphoria formation of Idaho and the word formation of Texas both of which contain helicoprion. It therefore would have likely been a contemporary of these animals