Pteraspidiformes is an extinct

order 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 ...

of heterostracan

agnatha 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, cyclos ...

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

s known from extensive fossil remains primarily from Early

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, w ...

strata of

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a subcontinent of Eurasia and it is located enti ...

and North America, and from Upper

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 Paleozoi ...

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 tota ...

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, in snakes and scaled reptiles Other uses * Rostral col ...

pineal The pineal gland, conarium, or epiphysis cerebri, is a small endocrine gland in the brain of most vertebrates. The pineal gland produces melatonin, a serotonin-derived hormone which modulates sleep patterns in both circadian and seasonal cyc ...

plates, 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 ) refers to the actively swimming aquatic organisms in a body of water. The term was proposed by German biologist Ernst Haeckel to differentiate between the active swimmers in a body of water, and the passive organisms t ...

.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 ''Pycnosteus'' is an extinct genus of agnatha, jawless fish from the Devonian. It is thought to have cruised through vegetation, eating small invertebrates which it knocked loose . Sources

* ''Fishes of the World'' by Joseph S. Nels ...

'', 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 extinction consisted of several extinction events in the Late Devonian Epoch, which collectively represent one of the five largest mass extinction events in the history of life on Earth. The term primarily refers to a major ext ...

). 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 In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

, 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 (Agna ...

'') closer to the Pteraspidoidei.

Pteraspidoidei

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

Doryaspis ''Doryaspis'' (from el, δόρῠ 'spear' and el, ἀσπίς 'shield') (also known by its synonym, "''Lyktaspis''") is an extinct genus of primitive jawless fish that lived in the Devonian period. Fossils have been discovered in Spitsbergen ...

'' 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, and with an explosive warhead designed to detonate either on contact with or in proximity to the target. Historically, s ...

-shaped pteraspidids, such as the

type genus In biological taxonomy, the type genus 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-bearing type of a nomina ...

, ''

Pteraspis ''Pteraspis'' (from el, πτερόν 'wing' or 'fin' and el, ἀσπίς 'shield') is an extinct genus of pteraspidid heterostracan agnathan vertebrate that lived from the Lochkovian to Eifelian epochs of the Devonian period in what is now ...

'', 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 Placode ...

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 family that was erected for '' Gigantaspis'', a genus of pteraspidids restricted to the Early Devonian marine strata of Svalbard. 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 straight-line distance from the northernmost part of western Canada, to New Mexico in ...

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