Ciderius cooperi van der Brugghen 2015
Ciderius cooperi is a highly unusual fossil fish belonging to the Euphaneropidae. This is a poorly-understood group of fishes because its fossils have only been discovered in a few locations, and the fossils themselves are composed almost entirely of soft tissues. This means that well-preserved specimens are few and far between. Two striking features make euphaneropids unique among vertebrates; around 60% of the length of the body is flanked by a greatly elongated gill apparatus. In effect, this fish can best be described as a gill apparatus with fins attached. Its other unique feature is the presence of paired anal fins, which it shares with no other vertebrate (other than highly mutated goldfishes).
Ciderius is from the Fish Bed Formation, which is a fossil formation known for more than a century. This fish has only been described recently because its fossils have went unnoticed for a long time. The fossils are composed of faint impressions which are unremarkable to the untrained eye, and visibly quite unlike the other fishes occurring at the same fauna. Wherever it occurs, Ciderius is not particularly rare. However, a well-preserved articulated example is highly uncommon, and it is usually found as fragmentary remains.