Acronichthys was a tiny fish measuring just 4 cm (1.6 inches) long which lived in freshwater environments within western North America during the late Cretaceous Period 90-66 million years ago.
Fossils of this fish, or fossils which might belong to it, have been found in multiple geologic formations including the Scollard Formation, the Milk River Formation, and Dinosaur Park Formation of Alberta, Canada, the Hell Creek Formation in Montana, the Lance Formation of Wyoming, the Iron Springs Formation of Utah, the Williams Fork Formation of Colorado, and the Aguja Formation of Texas. In 2025, Juan Liu and colleagues named the species found in Alberta, Canada Acronichthys maccagnoi. The fossils found in other formations might belong to this same genus or even the same species.
Its very small size meant that it likely served as prey for a multitude of animals. However, what’s more noteworthy than its small size is its evolutionary relationship to other fish. Acronichthys was a primitive member of the fish group Otophysi, a group which today encompasses around two-thirds of all freshwater fish species. It’s been proposed that it might be a primitive catfish, but that remains to be proven.
Acronichthys maccognoi. © Jason R. Abdale (July 4, 2026).
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