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''Hemicyclaspis'' ( or 'semicircle plate') is an extinct genus of primitive jawless 
fish
Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...
, closely related to ''Cephalaspis
''Cephalaspis'' (from el, κεφαλή , 'head' and el, ἀσπίς , 'shield') is a possibly monotypic genus of extinct osteostracan agnathan vertebrate. It was a trout-sized detritivorous fish that lived in the early Devonian.
Description
L ...
'', that lived in the Late Silurian
The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...
( Pridoli) to Devonian
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
period in what is now Europe and North America.
A typical cephalaspid, ''Hemicyclaspis'' had a heavily armored
Armour (British English) or armor (American English; see spelling differences) is a covering used to protect an object, individual, or vehicle from physical injury or damage, especially direct contact weapons or projectiles during combat
Co ...
, shovel-shaped headshield. It is thought to have been a better swimmer than most of its relatives because of its powerful tail, stabilizing dorsal fin
A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through conv ...
and the keel-shaped hydrodynamic
In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including ''aerodynamics'' (the study of air and other gases in motion) and ...
edges of its head shield. ''Hemicyclaspis'' probably foraged the ocean floor for food.
''Hemicyclaspis'' grew to a length of 5 inches (13 centimeters) and would most likely have fed on particles from the muddy sea bed.
References
* Parker, Steve. Dinosaurus: the complete guide to dinosaurs. Firefly Books Inc, 2003. Pg. 58 Osteostraci genera Early Devonian fish of Europe Devonian fish of North America Paleozoic life of Nunavut {{Devonian-jawless-fish-stub