Everything about Heterostraci totally explained
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.
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 most other known 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, an acellular bony tissue unique to this class. The middle layer was honeycombed with tiny spaces called "cancella."
As with many agnathan groups, heterostracans had no fins besides the tail or
caudal fin. In some pteraspids, especially in the psammosteids, the ends of the branchial plates (the plates that covered the gills) is drawn out to form wing-like extentions.
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 a sister group of both orders, while the Early Devonian
Lepidaspis is regarded as being
incertae sedis, possibly close to the original basal forms.
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.
Further Information
Get more info on 'Heterostraci'.
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