(top right). Most heterostracans are pteraspidiforms, such as the Pteraspidiformes (bottom right), doryaspis (bottom left) and the huge Psammosteida (top left), which are the youngest known members of the group.[Philippe Janvier (1997) Heterostraci The Tree of Life Web Project.]
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Heterostraci (Ancient Greek, ἕτερος+ὄστρακον "those with a different shell" -i) is an extinct subclass of Pteraspidomorphi, ostracoderm, jawless vertebrate that lived primarily in marine and estuary environments. Heterostraci existed from the mid-Ordovician to the conclusion of the Devonian.
Description and anatomy
The heterostracans differed from other Paleozoic
agnathan taxa both in the arrangement and
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 animals of this class and Thelodonti. 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 Pteraspidiformes, especially in the Psammosteida, the ends of the branchial plates (the plates that covered the gills) is drawn out to form wing-like extensions.
Taxonomy
Heterostraci are divided into several orders, including the two most diverse orders, Cyathaspidiformes ("Cup Shields"), and
Pteraspidiformes ("Wing Shields"), as well as the
Cardipeltis,
Corvaspidida,
Tolypelepidida,
Tesseraspidida, and the Traquairaspidiformes. The predominantly
Silurian Tolypelepidida (the best known member being
Athenaegis), is regarded as being a sister group of the Cyathaspids and Pteraspidids, while the Early Devonian
Lepidaspis is regarded as being
incertae sedis, possibly close to the original basal forms.
Cyathaspidiformes
Cyathaspidiformes is divided into two main subgroups: the
Amphiaspidida of Early Devonian
Siberia (i.e.,
Amphiaspis,
Gabreyaspis, and
Edaphaspis, grouped together with their relative
Ctenaspis of
Canada), and the
Cyathaspidida (i.e.,
Cyathaspis,
Poraspis, and
Anglaspis), grouped together with their relative
Nahanbiaspis.
Pteraspidiformes
The
Pteraspidiformes is divided up into five families, four of which, Anchipteraspididae,
Gigantaspis,
Pteraspididae, and
Protaspididae are monophyletic: Protopteraspididae has recently been determined to be paraphyletic within Pteraspidiformes, and thus, no longer valid, while
Psammosteidae 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 suborder
Pteraspidoidei, with various genera of Protopteraspididae as sister taxa.
[Pernegre, Vincent N., and David K. Elliott. "Phylogeny of the Pteraspidiformes (Heterostraci), Silurian–Devonian jawless vertebrates." Zoologica Scripta 37.4 (2008): 391-403.]
Phylogeny
Based on the work of Mikko Haaramo.
The most comprehensive study on pteraspidomorph relationships is by Randle et al. (2025). The result of their maximum parsimony analysis is shown below:
See also
External links
