| Achanarella Temporal range: Middle Devonian | |
|---|---|
Scientific classification  | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Order: | † Jamoytiiformes |
| Family: | † Achanarellidae |
| Genus: | † Achanarella |
| Species: | †A. trewini |
| Binomial name | |
| †Achanarella trewini Newman, 2002 | |
Achanarella is a genus of extinct jawless fish from the Achanarras Quarry in Scotland, dating to the Middle Devonian. [1] The only known species in the genus is Achanarella trewini.
Achanarella was first referenced in 1978 by Jack Saxon, but was only briefly described. [2] The name was used again in a paper from 1986, but it was in reference to poorly preserved specimens that could not be used for formal diagnosis. [3] Achanarella trewini was finally described as a new genus and species in 2002 by Michael J. Newman based on new and better preserved fossils. [1]
It was proposed in 1996 that the known Achanarella specimens may actually be heavily decayed acanthodians. [4] This idea was later argued against on the basis that Achanarella fossils do not follow decay patterns that acanthodians, like the contemporary Mesacanthus , displayed in their fossils. This was further supported due to the fact that its branchial openings were small and uniform in size compared to the larger, more irregular holes found in decayed fish. [1]
Achanarella was a small, slender jawless fish. It ranged from 20–90mm long. Overall, its form was similar to a lamprey. It had a caudal fin, anal fin, and possibly a small dorsal fin. Its eyes were large and it had a rimmed, circular mouth on the end of its head. It had a branchial basket, though it is hard to define exactly in the fossil. Thirteen branchial openings, and possibly associated branchial arches, are discernible, but there may be over twenty. [1]
Achanarella is able to be placed under Chordata with reasonable confidence due to its apparent bilateral symmetry and paired gill openings. Though a notochord is not present, epichordal fin supports are, which are also seen in vertebrates Endeiolepis and Cornovicthys . The overall morphology of Achnarella is also comparable to other early vertebrates like Jamoytius, so it is justified to assume that Achanarella was also a vertebrate. [1]
The presence of an unidentifiable tube, leading from the branchial openings to the head, may suggest a piston cartilage as in Mayomyzon . [5] This would suggest Achanarella is a close relative of lampreys, though the true function of the tube structure is not for certain. Instead, Euphaneropidae most closely resembles Achanarella, though the body proportions are different between them. Achanarella was placed within its own family Achanarellidae, which is under Jamoytiiformes. [1]
Agnatha is a paraphyletic infraphylum of non-gnathostome vertebrates, or jawless fish, in the phylum Chordata, subphylum Vertebrata, consisting of both living (cyclostomes) and extinct. Among recent animals, cyclostomes are sister to all vertebrates with jaws, known as gnathostomes.
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.
Vertebrate paleontology is the subfield of paleontology that seeks to discover, through the study of fossilized remains, the behavior, reproduction and appearance of extinct vertebrates. It also tries to connect, by using the evolutionary timeline, the animals of the past and their modern-day relatives.
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.
Placoderms are vertebrate animals of the class Placodermi, an extinct group of prehistoric fish known from Paleozoic fossils during the Silurian and the Devonian periods. While their endoskeletons are mainly cartilaginous, their head and thorax were covered by articulated armoured plates, and the rest of the body was scaled or naked depending on the species.
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.
Climatius is an extinct genus of spiny shark. This genus is known from the Early Devonian (Lochkovian) of Europe, previously considered Silurian remains actually belong to Nostolepis instead.
Cheiracanthus is an extinct genus of a group of fish called Acanthodii. It was a deep-bodied acanthodian about 12 in. (30 cm) in length. It had a blunt head, upturned tail, and fins protected by spines. Unlike many other acanthodians, it had one, solitary dorsal fin. Cheiracanthus swam at mid-depth in lakes and rivers, seizing small prey in its gaping jaws. Whole fossils of this fish occur only in Mid-Devonian rocks in Scotland, but its distinctive small, ornamented scales crop up around the world, as far south as Antarctica.
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.
Brochoadmones is an extinct genus of acanthodian from the Devonian of what is now Canada. It is the only genus in the suborder Brochoadmonoidei, whose relationship to other acanthodian orders remains currently in flux.
Priscomyzon riniensis is an extinct lamprey that lived some 360 million years ago during the Famennian in a marine or estuarine environment in South Africa. This small agnathan is anatomically similar to the Mazon Creek lampreys, but is some 35 million years older. Its key developments included the first known large oral disc, circumoral teeth and a branchial basket.
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.
Diplacanthus is an extinct genus of Mid to Late Devonian fish in the class Acanthodii, known as spiny sharks.
Mayomyzon pieckoensis is an extinct species of lamprey that lived during the Late Carboniferous period, about 300 million years ago. It is the only known species of the genus Mayomyzon, which belongs to the family Mayomyzontidae. It is known from the Mazon Creek fossil beds located in present-day Illinois.