(half round shield).
Classification: Chordata, Osteostraci, Atelaspidiformes.
Species: H. murchisoni.
Size: Roughly about 15 centimetres long.
Known locations: Canada, Norway, United Kingdom.
Time period: Wenlock to Pridoli of the Silurian.
Fossil representation: Few individuals.
When compared to other atelaspidiformes, Hemicyclaspis is considered to have been a better swimmer thanks to the presence of a more developed tail and two forward flaps that acted like pectoral fins, controlling pitch and allowing for level swimming in open water. Despite thee swimming adaptations, the mouth was still on the bottom which means that Hemicyclaspis was probably still a bottom feeder that searched for small organisms and organic matter in the sediment of Silurian oceans.
- Additional remarks about Hemicyclaspis from Jel°ya, southern Norway. - Norsk Geologisk Tidsskrift 54, pp. 375-384. - A. Heintz - 1974.