Heterostraci ( Ancient Greek, ἕτερος+ὄστρακον "those itha different shell"

i is pl. of -us I, or i, is the ninth letter and the third vowel letter of the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''i'' (pronounced ), plural ...

is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

subclass of pteraspidomorph jawless vertebrate that lived primarily in

marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military * ...

and estuary environments. Heterostraci existed from the mid- Ordovician to the conclusion of the

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 ...

Description and anatomy

The Heterostracans differed from other Paleozoic

agnathan Agnatha (, Ancient Greek 'without jaws') is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present (cyclostomes) and extinct (conodonts and ostracoderms) species. Among recent animals, cyclostomes ...

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 ''Lepidaspis serrata'' ("serrated scaley shield") is an extinct heterostracan jawless fish from Early Devonian Canada. Its scientific name refers to the fact that the armor is composed of hundreds of tiny scales with serrated edges. Although it ...

'', 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

pteraspid Pteraspidiformes is an extinct order of heterostracan agnathan vertebrates known from extensive fossil remains primarily from Early Devonian strata of Europe and North America, and from Upper Silurian Canada. Anatomy A pteraspidiform heteros ...

s, especially in the

psammosteid Psammosteida also called as Psammosteoidei is a suborder of pteraspidid heterostracan agnathans. The psammosteids had broad, flattened bodies, suggesting a predominantly benthic habit. The earliest unequivocal psammosteid is '' Drepanaspis'' of ...

s, 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 Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...

, including the two most diverse orders, Cyathaspidiformes ("Cup Shields"), and

Pteraspidiformes Pteraspidiformes is an extinct order of heterostracan agnathan vertebrates known from extensive fossil remains primarily from Early Devonian strata of Europe and North America, and from Upper Silurian Canada. Anatomy A pteraspidiform heter ...

("Wing Shields"), as well as the Cardipeltida, Corvaspidida,

Tolypelepidida Tolypelepidida is an extinct order (biology), order of heterostraci, heterostracan vertebrates. These armored jawless fish superficially resemble their relatives, the Cyathaspidiformes, cyathaspids, though, researchers place the tolypelepids as ...

, Tesseraspidida, and the

Traquairaspidiformes Traquairaspidiformes is an order of extinct heterostracan agnathan fish known from the Silurian and Early Devonian periods. Fossils are predominantly known from Late Silurian fluvial deposits from Wales and England: some species were also fo ...

. The predominantly

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 ...

Tolypelepidida (the best known member being '' Athenaegis''), is regarded as being a sister group of the Cyathaspids and Pteraspidids, while the Early Devonian ''

'' is regarded as being '' incertae sedis'', possibly close to the original basal forms.

Cyathaspidiformes

Cyathaspidiformes is divided into two main subgroups: the

Amphiaspidida Amphiaspidida is a taxon of extinct cyathaspidid heterostracan agnathans whose fossils are restricted to Lower Devonian marine strata of Siberia near the Taimyr Peninsula. Some authorities treat it as a suborder of Cyathaspidiformes,Lundgre ...

of Early Devonian Siberia (i.e., ''

Amphiaspis ''Amphiaspis argo'' is the type species of the cyathaspidid taxon Amphiaspidida, and of the family Amphiaspididae.Novitskaya, Larisse. Les amphiaspides (Heterostraci) du Devonien de la Siberie. Éditions du Centre national de la recherche scien ...

'', '' Gabreyaspis'', and ''

Edaphaspis ''Edaphaspis bystrowi'' is an extinct amphiaspidid cyathaspidid heterostracan.Novitskaya, Larisse. Les amphiaspides (Heterostraci) du Devonien de la Siberie. Éditions du Centre national de la recherche scientifique, 1971. Its fossils are res ...

'', grouped together with their relative ''

Ctenaspis ''Ctenaspis'' (''Ctenaspis kaieri'') is an extinct genus of heterostracan cyathaspid agnathans from the early Devonian of Canada, Norway and Russia 416.0 to 412.3 million years ago. Species of ''Ctenaspis'' are typically 10 to 18 cm long with t ...

'' of Canada), and the Cyathaspidida (i.e., ''

Cyathaspis ''Cyathaspis'' is the type genus of the heterostracan order Cyathaspidiformes. Fossils are found in late Silurian strata in the Cunningham Creek Formation, New Brunswick, Canada and Europe, especially in the Downton Castle Sandstone of Great B ...

'', ''

Poraspis ''Poraspis'' is an extinct genus of heterostracan. Fossils are found in Late Silurian and Early Devonian marine strata of Norway, Canada and the United States The United States of America (U.S.A. or USA), commonly known as the United ...

'', and ''

Anglaspis ''Anglaspis'' is an extinct genus of cyathaspidiform heterostracan agnathan. Fossils are found in marine strata of Europe, from the late Silurian period until the genus' extinction during the Early Devonian. As with other cyathaspidiforms, i ...

''), grouped together with their relative '' Nahanbiaspis''.

Pteraspidiformes

The

is divided up into five families, four of which,

Anchipteraspididae Anchipteraspididae is an extinct family of heterostracan vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Ver ...

, Gigantaspididae,

Pteraspididae Pteraspididae is an extinct family of heterostracan vertebrates. It is an Early Devonian-aged family that contains the stereotypical, torpedo-shaped pteraspidids, such as the type genus, ''Pteraspis'', and its close relatives ''Errivaspis'' and ' ...

, and

Protaspididae Protaspididae is an extinct family of pteraspidid heterostracan agnathans. Fossils of the various genera are found in early Devonian-aged marine strata. Protaspidids were once thought to represent a transitional form between the Pteraspididae an ...

are monophyletic: Protopteraspididae has recently been determined to be paraphyletic within Pteraspidiformes, and thus, no longer valid, while

Psammosteidae Psammosteidae is an extinct family of flattened, benthic heterostracan vertebrates that lived in marine and estuary environments in Europe, Russia & North America. They arose during the Early Devonian, with the first (and best) known genus, '' Dr ...

is now regarded as a separate subtaxon within Pteraspidiformes, "

Psammosteida Psammosteida also called as Psammosteoidei is a suborder of pteraspidid heterostracan agnathans. The psammosteids had broad, flattened bodies, suggesting a predominantly benthic habit. The earliest unequivocal psammosteid is '' Drepanaspis'' of ...

," as the sister taxon of the most basal family, Anchipteraspidae. Pteraspididae, Gigantaspididae and Protaspididae, in turn form the suborder

Pteraspidoidei Pteraspidoidei is an extinct suborder of heterostracan vertebrates. It contains the more derived taxa within Pteraspidiformes, such as ''Pteraspis'' and '' Errivaspis'', though, some protopteraspidids, such as '' Doryaspis'' and ''Panamintaspis ...

, 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.

References

External links

tolweb.org

{{Taxonbar, from=Q2309785 Vertebrate subclasses de:Pteraspidomorphi#Heterostraci