Pteraspidiformes is an extinct

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

of heterostracan

agnatha Agnatha (; ) or jawless fish is a paraphyletic infraphylum of animals in the subphylum Vertebrata of the phylum Chordata, characterized by the lack of jaws. The group consists of both extant taxon, living (Cyclostomi, cyclostomes such as hagfish ...

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

s known from extensive fossil remains primarily from Early

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

strata of

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

and

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

, and from Upper

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 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 third and shortest period of t ...

Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...

Anatomy

A pteraspidiform heterostracan has the cephalothorax enclosed in armor, formed from several plates, including dorsal, ventral,

rostral Rostral may refer to: Anatomy * Rostral (anatomical term), situated toward the oral or nasal region * Rostral bone, in ceratopsian dinosaurs * Rostral organ, of certain fish * Rostral scale The rostral scale, or rostral, in snakes and other sca ...

pineal The pineal gland (also known as the pineal body or epiphysis cerebri) is a small endocrine gland in the brain of most vertebrates. It produces melatonin, a serotonin-derived hormone, which modulates sleep patterns following the diurnal cycles. ...

plates Plate may refer to: Cooking * Plate (dishware), broad, mainly flat vessel commonly used to serve food * Plates, tableware, dishes or dishware used for setting a table, serving food and dining * Plate, the content of such a plate (for example: ...

, a dorsal spine derived from a scale, and a large, scale-covered tail. Many genera were benthic, others were apparently active swimming

nekton Nekton or necton (from the ) is any aquatic organism that can actively and persistently propel itself through a water column (i.e. swimming) without touching the bottom. Nektons generally have powerful tails and appendages (e.g. fins, pleopods, ...

.Botella, Hector, and Richard A. Farina. "Flow pattern around the rigid cephalic shield of the Devonian agnathan Errivaspis waynensis (Pteraspidiformes: Heterostraci)." Palaeontology 51.5 (2008): 1141-1150

/ref> Delicate, finger-like components of the anterior end of the ventral plate forming the edges of the mouth suggest that pteraspidiform heterostracans were filter-feeding, filter-feeders that selectively filtered specific sized plankton from the water column.Purnell, Mark A. "Feeding in extinct jawless heterostracan fishes and testing scenarios of early vertebrate evolution." Proceedings of the Royal Society of London B: Biological Sciences 269.1486 (2002): 83-8

/ref>

Taxa

Anchipteraspididae

If '' Weigeltaspis'' is not a psammosteid pteraspidid, then the Anchipteraspidids are the oldest-most known pteraspidid heterostracans, with three genera from Pridolian-aged strata from Upper Silurian Canada. The armor of anchipteraspidids superficially resemble those of cyathaspidids, though, anchipteraspidids already have the typical series of plates diagnostic of pteraspidids, and the growth habits of anchipteraspidids are diagnostic of pteraspidids, and not of cyathaspidids.

Psammosteoidei

The psammosteids are a group of pteraspidiforms that forms the

sister taxon In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

of Protopteraspididae and Pteraspidoidei, and are sometimes treated as a suborder, "Psammosteoidei", or as an order-level taxon as "Psammosteida", or as a family, "Psammosteidae". Anatomically, a typical psammosteid resembles a typical pteraspidid that has had the forebody greatly dorsoventrally depressed, with a long, compressed tail. Some psammosteids, like '' Pycnosteus'', have the ventral shield modified into a pedestal-like structure that some researchers compare to a sled's runner. The psammosteids are the only heterostracans to survive beyond the Middle Devonian, as the last genera became extinct during the Late Devonian

Kellwasser event The Late Devonian mass extinction, also known as the Kellwasser event, was a mass extinction event which occurred around 372 million years ago, at the boundary between the Frasnian and Famennian ages of the Late Devonian period.Racki, 2005McGhee, ...

). The Late Silurian-Early Devonian genus '' Weigeltaspis'' may or may not belong to this group; species of this genus superficially resemble a small, elongated psammosteid.

"Protopteraspididae"

Protopteraspididae was once thought to be a family of primitive or basal pteraspidids. A recent analysis of Pteraspidiformes shows that Protopteraspididae is

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

, as its various taxa show a gradual transition from primitive taxa similar to the Anchipteraspidids towards more derived taxa (such as ''

Panamintaspis ''Panamintaspis snowi'' is an extinct species of pteraspidid heterostracan agnathan which existed during the early Middle Devonian period of Death Valley, California.Elliott, David K., and Robert R. Ilyes. "New Early Devonian pteraspidids (Agn ...

'') closer to the Pteraspidoidei.

Pteraspidoidei

Pteraspidoidei contains the more derived taxa within Pteraspidiformes, though, some protopteraspidids, such as '' Doryaspis'' and ''

'', share various features with these derived pteraspidids.

Pteraspididae

Pteraspididae is an Early Devonian-aged family that contains the stereotypical,

torpedo A modern torpedo is an underwater ranged weapon launched above or below the water surface, self-propelled towards a target, with an explosive warhead designed to detonate either on contact with or in proximity to the target. Historically, such ...

-shaped pteraspidids, such as the

type genus In biological taxonomy, the type genus (''genus typica'') is the genus which defines a biological family and the root of the family name. Zoological nomenclature According to the International Code of Zoological Nomenclature, "The name-bearin ...

, ''

Pteraspis ''Pteraspis'' (from 'wing' or 'fin' and 'shield') is an extinct genus of pteraspidid heterostracan jawless fish. It lived from the Lochkovian to Eifelian epochs of the Devonian period in what is now Brazil (Eifelian Maecuru Formation), Br ...

'', and its close relatives '' Errivaspis'' and '' Rhinopteraspis'', that are popularly thought to have actively swam about in the water column. The elongated

rostral plate A plate in animal anatomy may refer to several things: Flat bones (examples: bony plates, dermal plates) of vertebrates * an appendage of the Stegosauria group of dinosaurs * articulated armoured plates covering the head of thorax of Placodermi ...

is thought to help with their hydrodynamic ability. Various genera are found in various marine and estuarine strata of Early Devonian Europe and North America.

Gigantaspididae

Gigantaspididae is a

monogeneric In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispe ...

family that was erected for '' Gigantaspis'', a genus of pteraspidids restricted to the Early Devonian marine strata of

Svalbard Svalbard ( , ), previously known as Spitsbergen or Spitzbergen, is a Norway, Norwegian archipelago that lies at the convergence of the Arctic Ocean with the Atlantic Ocean. North of continental Europe, mainland Europe, it lies about midway be ...

. Species of ''Gigantaspis'' superficially resemble some genera of its sister taxon, Protaspididae, i.e., that they had armor shaped like a hot bun or a round loaf of bread.

Protaspididae

Protaspididae is a family of bun or bread loaf-shaped pteraspidids found in Early Devonian marine strata from the

Rocky Mountains The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in great-circle distance, straight-line distance from the northernmost part of Western Can ...

of the United States, and of

. Some genera, such as '' Xylaspis'' and '' Woodfjordaspis'', are similar in form to ''Gigantaspis'', while others, such as '' Protaspis'' and '' Tuberculaspis'', have armor that is more mound-shaped, or '' Psephaspis'', a very compressed form that was originally described as a psammosteid closely related to ''Drepanaspis''.

References

{{Taxonbar, from=Q14545384 Devonian jawless fish Silurian jawless fish Heterostraci Prehistoric jawless fish orders Devonian fish of North America Devonian fish of Europe Silurian fish of North America Silurian fish of Europe Silurian first appearances Pridoli taxonomic orders Early Devonian taxonomic orders Devonian extinctions