Produced by the Population Genetics and Evolution class, Furman University

The Silurian: Heterostracans
Hesterostracans were one of the jawless, ostracoderm fish lineages distinguished by a single pair of external gill openings (Janvier 1997). They possessed a large, three layered, dermal armor of dentine and acellular bone that covers the head region and is pierced laterally by external brachial openings. The armor was made up of a number of plates that were specialized in different species. Some fused the plates to form a box-like armor, while others developed spines or other amplifications. Fossilized portions of the head armor have allowed us to learn more about their internal anatomy, by revealing impressions of the brain, pineal organ, semicircular canals, and gill chambers. The brachial opening minimized the surface area exposed to parasites. Most were marine, living in lagoons and deltas, although some species have been classified as fresh-water. They fed by scraping their fan shaped oral plates against the ocean or riverbed. Their bodies had no internal skeleton, merely reinforced by lines of scales (Kazlev 1999). They are not believed to have been great swimmers. There are two major clades, the Cyathaspidiformes (parallel and finely crenulated dentine ridges) and the Pteraspidiformes (concentric dentine ridges with serrated margins; Janvier 1997).

Page by Megan Aprill
Heterostracans. Photo from: Tree of Life Project

Janvier P. 1997. Hesterostraci. Tree of Life Web Project. Accessed Feb. 17, 2010.

Kazlev, MA. 1999. Pteraspidomorphi: Heterostraci Cyathaspidiformes. www.paleos.com. Accessed Feb. 17, 2010

Wilson MVH. 2010. Heterostraci. Access Science@ McGraw Hill. Accessed Feb. 17, 2010.