The jawless fish Eriptychius americanus already has an articulated skull. It was discovered in Colorado’s Harding Sandstone Formation in 2023 AD and is the oldest 3D fossil evidence of the cranial anatomy of a primitive vertebrate, according to a study in the journal Nature. The skull arrangement of E. americanus was unlike anything seen in living vertebrates or the fish’s extinct relatives, with unfused cartilage sections—some symmetrical, some not—at the front of the head and surrounding the mouth, olfactory organs, and eyes. “This fossil fills a gap in our knowledge of the evolution of the vertebrate head,” said paleobiologist Lauren Sallan, assistant professor in the macroevolution unit at the Okinawa Institute of Science and Technology Graduate University in Japan. “This gap was partly due to the fact that the Ordovician ancestors of jawed fishes were relatively rare and mostly confined to very shallow marine waters. After death, the remains of these early fishes, including Eryptychius, were usually destroyed by waves, and we find mostly pieces of them,” said Sallan, who researches the origins of marine biodiversity and was not involved in the study.
