@misc{indiciae8f28011f1f46,
  title = {Graptolite identification chart for field determination of geologic age},
  author = {Michael Churkin and Claire Carter},
  year = {1972},
  doi = {10.3133/oc66},
  url = {https://pubs.usgs.gov/publication/oc66},
  note = {Source identifier: oc66}
}