Astraspida, or astraspids, are a small group of extinct armored jawless vertebrates, which lived in the

Middle 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. The ...

(about 450 million years ago) in North America. They are placed among the

Pteraspidomorphi Pteraspidomorphi is an extinct class of early jawless fish. They have long been regarded as closely related or even ancestral to jawed vertebrates, but the few characteristics they share with the latter are now considered as basal traits for al ...

because of the large dorsal and ventral shield of their head armor. They are represented by a single genus,

Astraspis ''Astraspis'' ('star shield') is an extinct genus of primitive jawless fish from the Ordovician of Central North America including the Harding Sandstone of Colorado and Bighorn Mountains of Wyoming. It is also known from Bolivia.Sacabambaspis ...

, including possibly two species, ''A. desiderata'' and ''A. splendens'' but their remains are fairly abundant in Ordovician sandstones of the USA (

Colorado Colorado (, other variants) is a state in the Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the western edge of t ...

Arizona Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a state in the Southwestern United States. It is the 6th largest and the 14th most populous of the 50 states. Its capital and largest city is Phoenix. Arizona is part of the Fou ...

Oklahoma Oklahoma (; Choctaw language, Choctaw: ; chr, ᎣᎧᎳᎰᎹ, ''Okalahoma'' ) is a U.S. state, state in the South Central United States, South Central region of the United States, bordered by Texas on the south and west, Kansas on the nor ...

Wyoming Wyoming () is a U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the south ...

) and Canada (

Quebec Quebec ( ; )According to the Canadian government, ''Québec'' (with the acute accent) is the official name in Canadian French and ''Quebec'' (without the accent) is the province's official name in Canadian English is one of the thirtee ...

). The head armor of Astraspis is rather massive, with a series of ten gill openings lining the margin of the dorsal shield, and laterally placed eyes. The dorsal shield is ribbed by strong longitudinal crests, and the tail is covered with large, diamond-shaped scales. They are often grouped together with the

Arandaspidida Arandaspida is a taxon of very early, jawless prehistoric fish which lived during the Ordovician period. Arandaspids represent the oldest known craniates, a proposed group of chordates that contain all chordates with a cartilage-derived skull ( ...

Characteristics

Astraspids are characterized by a dermal ornamentation of large, mushroom shaped tubercles of fine-tubuled dentine ("astraspidine"), covered with a thick, glassy cap of enameloid. Astraspids and eriptychiids were the first Ordovician vertebrates ever discovered in the 19th century, and they have long been the only known Ordovician vertebrates, until the discovery of the arandaspids, in the 1970s. Therefore, their structure, though poorly known, has often been used in evolutionary scenarios to illustrate the primitive condition of the vertebrate dermal skeleton.Smith, M. M. (1991). Putative skeletal neural crest cells in Early Late Ordovician vertebrates from Colorado. Science, 251, 301-303. Because of the acellular structure of their dermal skeleton, they were first regarded as heterostracans. Now, we know that they are widely different from heterostracans, in particular in retaining about ten separate external gill openings. However, they share with heterostracans the relatively dorsal position of these openings. Their dorsal and ventral shield is made up by numerous polygonal platelets of aspidine, ornamented with large dentine tubercles capped with a thick enameloid layer. Nothing is known of their internal anatomy, but they possessed a sensory-line system housed in grooves of the dermal plates.

Taxonomy

Taxonomy based on the work of Mikko's Phylogeny Archive, Nelson, Grande and Wilson 2016 and van der Laan 2018. * Order † Astraspidiformes Berg 1940 ** Family †Astraspididae Eastman 1917 *** Genus †''

'' Walcott 1892 *** Genus †'' Pycnaspis'' Ørvig 1958 * Order † Tesakoviaspidida Karatajūtė-Talimaa & Moya 2004 esakoviaspidiformes Arratia, Wilson & Cloutier 2004** Family Tesakoviaspididae Karatajūtė-Talimaa & Moya 2004 *** Genus †'' Kodinskaspis'' Dzik & Moskalenko 2016 *** Genus †''

Tesakoviaspis ''Tesakoviaspis concentrica'' is an extinct species of primitive jawless fish from the Middle Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million yea ...

'' Karatajūtė-Talimaa 1978 ex Karatajūtė-Talimaa & Meredith-Smith 2004

References

Pteraspidomorphi Ordovician jawless fish Prehistoric jawless fish orders Middle Ordovician animals Ordovician fish of North America Middle Ordovician first appearances Middle Ordovician taxonomic orders Middle Ordovician extinctions {{ordovician-animal-stub de:Pteraspidomorphi#Astraspida