From Ohio History Central
Agnathan Cephalaspid Fish Fossils Death Assemblage. Copyright 2007-2008, fossilmall.com
The earliest known fish-like animals appeared in the Cambrian Period. By the Ordovician Period, a diverse fauna of jawless, bottom-feeding, armored fishes were present in nearshore marine environments. These fishes belonged to a group known as Agnatha (jawless). Although remains of agnathans have been found in many areas of the world in Ordovician rocks, none have been in Ohio rocks representing this time period. Perhaps Ohio was too far offshore for these fishes at that time. Agnathan remains are unknown from Ohio Silurian rocks as well.
However, remains of Agnathan fishes are known from a unique deposit of Early Devonian age that was discovered in a quarry in northwestern Ohio in the 1920's. Numerous specimens of agnathan fishes were found in a thin, laterally discontinuous bed of shale that represented a small, brackish-water embayment of the sea. No similar deposits have been discovered in Ohio. Microscopic turbercles that may have been from agnathan fishes have been found in the Middle Devonian Columbus Limestone. The group became extinct in the Devonian.
- Hansen, M. C., 1996. "Phylum Chordata--Vertebrate Fossils," in Fossils of Ohio, edited by R. M. Feldmann and Merrianne Hackathorn. Ohio Division of Geological Survey Bulletin 70, p. 288-369.