Heterostraci (

Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic p ...

, ἕτερος+ὄστρακον "those

ith The Ith () is a ridge in Germany's Central Uplands which is up to 439 m high. It lies about 40 km southwest of Hanover and, at 22 kilometres, is the longest line of crags in North Germany. Geography Location The Ith is immediatel ...

a different shell" i is pl. of -us is an extinct subclass of pteraspidomorph jawless

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...

that lived primarily in marine and

estuary An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environm ...

environments. Heterostraci existed from the mid-

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. T ...

to the conclusion of the Devonian.

Description and anatomy

The Heterostracans differed from other Paleozoic agnathan taxa both in the arrangement and

histology Histology, also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vi ...

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 Dentin () (American English) or dentine ( or ) (British English) ( la, substantia eburnea) is a calcified tissue of the body and, along with enamel, cementum, and pulp, is one of the four major components of teeth. It is usually covered by e ...

and aspidine, an acellular bony tissue unique to animals of this class and

Thelodonti Thelodonti (from Greek: "feeble teeth")Maisey, John G., Craig Chesek, and David Miller. Discovering fossil fishes. New York: Holt, 1996. is a class of extinct jawless fishes with distinctive scales instead of large plates of armor. There is much ...

. 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 Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as ...

. 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 extensions.

Taxonomy

Heterostraci are divided into several orders, including the two most diverse orders,

Cyathaspidiformes Cyathaspidiformes is an extinct order of heterostracan vertebrates known from extensive fossil remains primarily from Silurian to Early Devonian strata of Europe, and North America, and from Early Devonian marine strata of Siberia. Anatomy ...

("Cup Shields"), and Pteraspidiformes ("Wing Shields"), as well as the Cardipeltida, 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 ' () or ''problematica'' is a term used for a taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertain ...

'', possibly close to the original basal forms.

Cyathaspidiformes

is divided into two main subgroups: the Amphiaspidida of Early Devonian

Siberia Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive region, geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a ...

(i.e., '' Amphiaspis'', '' Gabreyaspis'', and '' Edaphaspis'', grouped together with their relative '' Ctenaspis'' of

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...

), and the

Cyathaspidida Cyathaspidida is a taxon of extinct cyathaspidiform heterostracan agnathans whose fossils are found in Silurian to Lower Devonian marine strata of Europe and North America.Lundgren, Mette, and Henning Blom. "Phylogenetic relationships of the cy ...

(i.e., '' Cyathaspis'', '' Poraspis'', and '' Anglaspis''), grouped together with their relative '' Nahanbiaspis''.

Pteraspidiformes

The Pteraspidiformes is divided up into five

families Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Ideal ...

, four of which, Anchipteraspididae, Gigantaspididae, 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.

References

External links

tolweb.org

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