First an alga, then a squid, enigmatic fossil is actually a fish
(PHYSorg.com Archaeology & Fossils)

A fossil slab discovered in Kansas 70 years ago and twice misidentified-first as a green alga and then as a cephalopod-has been reinterpreted as the preserved remains of a large cartilaginous fish, the group that includes sharks and rays. In a study published in the Journal of Paleontology, American Museum of Natural History researchers describe the fishy characteristics of the animal, which lived between 70-85 million years ago.