Santanichthys diasii is a species of extinct fish that existed around 115 million years ago during the Albian age.

Santanichthys is a small, characiform fish that would have superficially resembled more advanced tetras or the unrelated herrings.

The formation, situated in the hills of the Araripe Basin, is highly renowned for its rich Early Cretaceous fauna.

The specific rock strata where fossils of Santanichthys were gathered from date back to the Aptian stage of the Cretaceous period.

A large part of this faunal assemblage are masses of fossilized fish found at semi-regular intervals, including shoals of Santanichthys in various states of preservation.

The Cretaceous deposits of the Brazilian Riachuelo Formation, specifically the Taquari Member have yielded specimens of Santanichthys.

This supports a marine-to-freshwater model of the evolution and dispersal of characiform species from the breakup of the continent Gondwana (Africa and South America) to the present day.

[9][10] Subsequent analyses of available Santanichthys fossil material have determined some structures to be akin to a primitive Weberian Apparatus, prompting reclassification of the taxon as a basal otophysan and within the Characiformes.