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| Heterostraci | |
|---|---|
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| Panamintaspis | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Infraphylum: | Agnatha |
| Class: | †Pteraspidomorpha |
| Subclass: | †Heterostraci Lankester 1868 |
| Orders | |
Heterostraci (Ancient Greek, ἕτερος+ὄστρακον "those [with] a different shell" [-i is pl. of -us]) is anextinctsubclass ofpteraspidomorph,ostracoderm, jawlessvertebrate that lived primarily inmarine andestuary environments. Heterostraci existed from the mid-Ordovician to the conclusion of theDevonian.
The heterostracans differed from other Paleozoicagnathan taxa both in the arrangement andhistology 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 asLepidaspis, 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 ofdentine and aspidine, an acellular bony tissue unique to animals of this class andThelodonti. The middle layer was honeycombed with tiny spaces called "cancella."
As with many agnathan groups, heterostracans had no fins besides the tail orcaudal fin. In somepteraspids, especially in thepsammosteids, the ends of the branchial plates (the plates that covered the gills) is drawn out to form wing-like extensions.
Heterostraci are divided into severalorders, including the two most diverse orders,Cyathaspidiformes ("Cup Shields"), andPteraspidiformes ("Wing Shields"), as well as theCardipeltida,Corvaspidida,Tolypelepidida,Tesseraspidida, and theTraquairaspidiformes. The predominantlySilurian Tolypelepidida (the best known member beingAthenaegis), is regarded as being a sister group of the Cyathaspids and Pteraspidids, while the Early DevonianLepidaspis is regarded as beingincertae sedis, possibly close to the original basal forms.
Cyathaspidiformes is divided into two main subgroups: theAmphiaspidida of Early DevonianSiberia (i.e.,Amphiaspis,Gabreyaspis, andEdaphaspis, grouped together with their relativeCtenaspis ofCanada), and theCyathaspidida (i.e.,Cyathaspis,Poraspis, andAnglaspis), grouped together with their relativeNahanbiaspis.
ThePteraspidiformes is divided up into fivefamilies, four of which,Anchipteraspididae,Gigantaspididae,Pteraspididae, andProtaspididae are monophyletic: Protopteraspididae has recently been determined to be paraphyletic within Pteraspidiformes, and thus, no longer valid, whilePsammosteidae is now regarded as a separate subtaxon within Pteraspidiformes, "Psammosteida," as the sister taxon of the most basal family, Anchipteraspidae. Pteraspididae, Gigantaspididae and Protaspididae, in turn form the suborderPteraspidoidei, with various genera of Protopteraspididae as sister taxa.[3]
Based on the work of Mikko Haaramo.[4]
| Heterostraci | |