| Birkeniidae | |
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| Fossil and model of Birkenia elegans | |
Scientific classification  | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | † Anaspida |
| Order: | † Birkeniiformes |
| Family: | † Birkeniidae Traquair, 1899 [1] |
| Genera | |
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| Synonyms | |
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Birkeniidae is an extinct family of jawless fish belonging to the order Birkeniiformes. They were defined in 1899 by Ramsay H. Traquair, and were a successful family among the many jawless fish in the Silurian seas.
Birkeniidae had a large body which was ovular in shape, and a strong, mobile tail. Their eyes were positioned on either side of the head. Their mouth was large for the jawless fish, but contained no teeth. Birkeniidae are distinguished by the crown shaped projection on their back, and the other smaller projections which run down its tail. They also possessed fins which faced downwards, an uncommon trait among the jawless fish. They had typical scaly skin.
Gnathostomata are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. Most gnathostomes have retained ancestral traits like true teeth, a stomach, and paired appendages. Other traits are elastin, a horizontal semicircular canal of the inner ear, myelin sheaths of neurons, and an adaptive immune system which has discrete lymphoid organs, and uses V(D)J recombination to create antigen recognition sites, rather than using genetic recombination in the variable lymphocyte receptor gene.
Cephalaspidomorphs are a group of jawless fishes named for Cephalaspis of the osteostracans. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys, because lampreys were once thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the Silurian and Devonian periods to the present day. They are the closest relatives of jawed fishes, who may have emerged from within them; if this is true, they would survive if the jawed fish are included.
Acanthodii or acanthodians is an extinct class of gnathostomes. They are currently considered to represent a paraphyletic grade of various fish lineages basal to extant Chondrichthyes, which includes living sharks, rays, and chimaeras. Acanthodians possess a mosaic of features shared with both osteichthyans and chondrichthyans. In general body shape, they were similar to modern sharks, but their epidermis was covered with tiny rhomboid platelets like the scales of holosteians.
Heterostraci is an extinct subclass of pteraspidomorph, Ostracoderm, jawless vertebrate that lived primarily in marine and estuary environments. Heterostraci existed from the mid-Ordovician to the conclusion of the Devonian.
Thelodonti is a class of extinct Palaeozoic jawless fishes with distinctive scales instead of large plates of armor.
Anaspida is an extinct group of jawless fish that existed from the early Silurian period to the late Devonian period. They were classically regarded as the ancestors of lampreys, but it is denied in recent phylogenetic analysis, although some analysis show these group would be at least related. Anaspids were small marine fish that lacked a heavy bony shield and paired fins, but were distinctively hypocercal.
The class Osteostraci is an extinct taxon of bony-armored jawless fish, termed "ostracoderms", that lived in what is now North America, Europe and Russia from the Middle Silurian to Late Devonian.
Drepanaspis is an extinct genus of heterostracan armoured jawless fish from the Early Devonian. Drepanaspis are assumed to have lived primarily in marine environments and is most commonly characterized by their ray-like, heavily armoured bodies, along with their lack of paired fins and jaws.
Jamoytius kerwoodi is an extinct species of primitive, eel-like jawless fish known from the Patrick Burn Formation in Scotland, dating to the Llandovery epoch of the Early Silurian period.
Astraspis is an extinct, monotypic 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. It is related to other Ordovician fishes, such as the South American Sacabambaspis, and the Australian Arandaspis.
Slimonia is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of Slimonia have been discovered in deposits of Silurian age in South America and Europe. Classified as part of the family Slimonidae alongside the related Salteropterus, the genus contains three valid species, S. acuminata from Lesmahagow, Scotland, S. boliviana from Cochabamba, Bolivia and S. dubia from the Pentland Hills of Scotland and one dubious species, S. stylops, from Herefordshire, England. The generic name is derived from and honors Robert Slimon, a fossil collector and surgeon from Lesmahagow.
Lasanius is a genus of basal jawless fish from the Early Silurian, around 443.8 million years ago, known from fossils found near Lesmahagow, Scotland. Specimens range from 13.3 to 74.5 mm in length.
Furcacauda is a genus of thelodontid agnathan from the Lower Devonian of Canada, and is the type genus of the order Furcacaudiformes. Furcacaudiform thelodontids were deep water jawless vertebrates with symmetrical fork and lobed-finned tails and scales smaller than typical loganellid and nikoliviid thelodonti scales. Furcacaudiform thelodonts are noted as having a laterally compressed body, large anterior eyes, slightly posterior, lateral, and vertical to a small mouth, and a condensed curved row of branchial openings (gills) directly posterior to the eyes. Many but not all had laterally paired fins. Wilson and Caldwell also note the presence of a caudal peduncle and a long caudal fin made of two large lobes, one dorsal and one ventral separated by 8 to 14 smaller intermediate lobes, giving the appearance of a striated half-moon shaped tail resembling the tail of a heterostracan. A large square cavity within the gut connecting a small intestine to an anal opening lead many to believe that it is this genus that exhibits the first vertebrate stomach. According to Wilson and Caldwell their discovery, based on sediment infillings of fossils of the Furcacauda heintze, gives credence to the evolutionary development of stomach before jaws.
Drepanolepis is an extinct genus of thelodont which lived in Canada during the Early Devonian period.
The evolution of fish began about 530 million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. The first fish lineages belong to the Agnatha, or jawless fish. Early examples include Haikouichthys. During the late Cambrian, eel-like jawless fish called the conodonts, and small mostly armoured fish known as ostracoderms, first appeared. Most jawless fish are now extinct; but the extant lampreys may approximate ancient pre-jawed fish. Lampreys belong to the Cyclostomata, which includes the extant hagfish, and this group may have split early on from other agnathans.
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.
Loganellia is a genus of jawless fish which lived between 430 and 370 million years ago, during the Silurian and Devonian periods of the Paleozoic. Loganellia belonged to the Thelodonti class and like other Thelodonts possessed scales instead of plate armor.
Anaspidomorphi (anaspidomorphs) is an extinct superclass of jawless fish.
Euphaneropidae is an extinct family of prehistoric jawless fishes in the extinct order Euphanerida. These fishes are characterised by a greatly elongated branchial apparatus which covers most of the length of the body. Fossils are known from the Lower Silurian and Middle Devonian of Scotland, and the Upper Devonian of Canada. In particular, Euphanerops is unique in that it has two anal fins.
Panderodus Is an extinct genus of jawless fish belonging to the order Conodonta. This genus had a long temporal range, surviving from the middle Ordovician to late Devonian. In 2021, extremely rare body fossils of Panderodus from the Waukesha Biota were described, and it revealed that Panderodus had a more thick body compared to the more slender bodies of more advanced conodonts. It also revealed that this conodont was a macrophagous predator, meaning it went after large prey.
Data related to Birkeniidae at Wikispecies
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