Heterostraci
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 Drepanaspis gemuendenensis, accompanied by Tiaraspis subtilis
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Extinct (fossil)
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Cyathaspidiformes |
Heterostraci ("Different scales") is an extinct class of jawless vertebrate that lived primarily in marine and estuary environments. They arose during the Silurian, and all, save for the Psammosteids, became extinct during the late Devonian. This last group of heterostracans, the Psammosteids, died out in the extinction event at the end of the Devonian.
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[edit] Description and Anatomy
The Heterostracans differed from other Paleozoic agnathan classes both in the arrangement of their scales, as well as the histology of their scales.
Most heterostracans had two plates which form a large dorsal shield and a large ventral shield, and had series of scales arranged in various patterns on the sides of their bodies, the exact pattern differing from one group to another. In a few primitive forms, such as Lepidaspis, the dorsal and ventral shields are composed of a mosaic of tiny scales. In all other forms, though, these tiny scales have fused together to form the shield-plates.
The scales of heterostracans are histologically distinct from other vertebrates, having three layers composed of dentine and aspidine. The middle layer was honeycombed with tiny spaces called "cancella."
[edit] Taxonomy
Heterostraca is divided into two main groups, the orders Cyathaspidiformes ("Cup Shields"), and Pteraspidiformes ("Wing Shields"). The Silurian heterostracan Athenaegis is regarded as being the sister group of both orders, while the Early Devonian Lepidaspis is regarded as being incertae sedis.
Cyathaspidiformes is divided into two main groups: the Amphiaspida of Early Devonian Siberia (grouped together with their relative Ctenaspis), of Canada, and the Cyathaspidida (grouped together with their relative Nahanbiaspis.
The Pteraspidiformes is divided up into five families, Anchipteraspididae, Protopteraspididae, Pteraspididae, Protaspididae, and Psammosteidae.
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