Cephalaspidomorphi

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.

Cephalaspidomorphs
Temporal range: Silurian–Devonian
Osteostraci
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Clade: Craniata
Clade: Cephalaspidomorphi
Subgroups

Osteostraci
Galeaspida
Pituriaspida

Biology and Morphology

Cephalaspis Lyellii
Reconstruction of Cephalaspis lyellii

Cephalaspidomorphi were, like most contemporary fishes, very well armoured. The head shield was particularly well developed, protecting the head, gills and the anterior section of the viscera. The body was in most forms well armoured as well. The head shield had a series of grooves over the whole surface, forming an extensive lateral line organ. The eyes were rather small and placed on the top of the head. There was no jaw proper. The mouth opening was surrounded by small plates, making the lips flexible, but without any ability to bite.[1]

No internal skeleton is known, outside of the head shield. If they had a vertebral column at all, it would have been cartilage rather than bone. Likely, the axial skeleton consisted of an unsegmented notochord. A fleshy appendage emerged laterally on each side, behind the head shield, functioning as pectoral fins. The tail had a single, wrap-around tail-fin. Modern fishes with such a tail are rarely quick swimmers, and the cephalaspidomorphi were not likely very active animals. They probably spent much of their time semi-submerged in the mud. They also lacked a swim bladder, and would not have been able to keep afloat without actively swimming. The head shield provided some lift though, and would have made the cephalaspidomorphi better swimmers than most of their contemporaries.[1] The whole group were likely algae- or filter-feeders, combing the bottom for small animals, much like the modern armoured bottom feeders, such as Loricariidae or Hoplosternum catfish.[2]

Classification

In the 1920s, the biologists Johan Kiær and Erik Stensiö first recognized the Cephalaspidomorphi as including the osteostracans, anaspids, and lampreys, because all three groups share a single dorsal "nostril", now known as a nasohypophysial opening.[3]

Since then, opinions on the relations among jawless vertebrates have varied. Most workers have come to regard the agnatha as paraphyletic, having given rise to the jawed fishes. Because of shared features such as paired fins, the origins of the jawed vertebrates may lie close to the Cephalaspidomorphi. Many biologists no longer use the name Cephalaspidomorphi because relations among Osteostraci and Anaspida are unclear, and the affinities of the lampreys are also contested. Others have restricted the cephalaspidomorphs to include only groups more clearly related to the Osteostraci, such as Galeaspida and Pituriaspida, that were largely unknown in the 1920s.[4]

Lampreys

Some reference works and databases have regarded Cephalaspidomorphi as a Linnean class whose sole living representatives are the lampreys.[5] Evidence now suggests that lampreys acquired the characters they share with cephalaspids by convergent evolution.[6] [7] As such, many newer works about fishes classify lampreys in a separate group called Petromyzontida or Hyperoartia.[8]

External links

References

  1. ^ a b Morales, Edwin H. Colbert, Michael (1991). Evolution of the vertebrates : a history of the backboned animals through time (4th ed.). New York: Wiley-Liss. ISBN 978-0-471-85074-8.
  2. ^ Lucas, F.A. (1922). Animals of the past : an account of some of the creatures of the ancient world. New York: American Museum of Natural History.
  3. ^ Stensiö, E.A. (1927): The Devonian and Downtonian vertebrates of Spitsbergen. 1. Family Cephalaspidae. Skrifter om Svalbard og Ishavet, no 12, pp 1-391.
  4. ^ White, Toby. "Thelodonti: Cephalaspidomorphi". Palaeos. Retrieved 2009-10-27.
  5. ^ Nelson, Joseph S. (1994). Fishes of the World (Third ed.). John Wiley and Sons. ISBN 0-471-54713-1.
  6. ^ Forey, Peter & Janvier, Philippe (2012). "Agnathans and the origin of jawed vertebrates". In Gee, Henry. Shaking the tree: readings from Nature in the history of life. USA: University of Chicago Press; Nature/Macmillan Magazines. pp. 251–266. ISBN 978-0-226-28497-2
  7. ^ Janvier, Philippe (2008). "Early Jawless Vertebrates and Cyclostome Origins". Zoological Science. 25 (10): 1045–1056. doi:10.2108/zsj.25.1045. PMID 19267641.
  8. ^ Nelson, J. S. (2006). Fishes of the World (4th ed.). New York: John Wiley and Sons, Inc. pp. 601 pp. ISBN 0-471-25031-7.

Janvier, Philippe. Early Vertebrates Oxford, New York: Oxford University Press, 1998. ISBN 0-19-854047-7

Agnatha

Agnatha (Greek, ἀ-γνάθος "no jaws") is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present (cyclostomes) and extinct (conodonts and ostracoderms) species. The group is sister to all vertebrates with jaws, known as gnathostomes.Recent molecular data, both from rRNA and from mtDNA as well as embryological data strongly supports the hypothesis that living agnathans, the cyclostomes, are monophyletic.The oldest fossil agnathans appeared in the Cambrian, and two groups still survive today: the lampreys and the hagfish, comprising about 120 species in total. Hagfish are considered members of the subphylum Vertebrata, because they secondarily lost vertebrae; before this event was inferred from molecular and developmental data, the group Craniata was created by Linnaeus (and is still sometimes used as a strictly morphological descriptor) to reference hagfish plus vertebrates. In addition to the absence of jaws, modern agnathans are characterised by absence of paired fins; the presence of a notochord both in larvae and adults; and seven or more paired gill pouches. Lampreys have a light sensitive pineal eye (homologous to the pineal gland in mammals). All living and most extinct Agnatha do not have an identifiable stomach or any appendages. Fertilization and development are both external. There is no parental care in the Agnatha class. The Agnatha are ectothermic or cold blooded, with a cartilaginous skeleton, and the heart contains 2 chambers.

While a few scientists still regard the living agnathans as only superficially similar, and argue that many of these similarities are probably shared basal characteristics of ancient vertebrates, recent classification clearly place hagfish (the Myxini or Hyperotreti), with the lampreys (Hyperoartii) as being more closely related to each other than either is to the jawed fishes.

Chordate

A chordate () is an animal constituting the phylum Chordata. During some period of their life cycle, chordates possess a notochord, a dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail: these five anatomical features define this phylum. Chordates are also bilaterally symmetric; and have a coelom, metameric segmentation, and a circulatory system.

The Chordata and Ambulacraria together form the superphylum Deuterostomia. Chordates are divided into three subphyla: Vertebrata (fish, amphibians, reptiles, birds, and mammals); Tunicata (salps and sea squirts); and Cephalochordata (which includes lancelets). There are also extinct taxa such as the Vetulicolia. Hemichordata (which includes the acorn worms) has been presented as a fourth chordate subphylum, but now is treated as a separate phylum: hemichordates and Echinodermata form the Ambulacraria, the sister phylum of the Chordates. Of the more than 65,000 living species of chordates, about half are bony fish that are members of the superclass Osteichthyes.

Chordate fossils have been found from as early as the Cambrian explosion, 541 million years ago. Cladistically (phylogenetically), vertebrates - chordates with the notochord replaced by a vertebral column during development - are considered to be a subgroup of the clade Craniata, which consists of chordates with a skull. The Craniata and Tunicata compose the clade Olfactores. (See diagram under Phylogeny.)

Craniate

A craniate is a member of the Craniata (sometimes called the Craniota), a proposed clade of chordate animals with a skull of hard bone or cartilage. Living representatives are the Myxini (hagfishes), Hyperoartia (including lampreys), and the much more numerous Gnathostomata (jawed vertebrates).The clade was conceived largely on the basis of the Hyperoartia (lampreys and kin) being more closely related to the Gnathostomata (jawed vertebrates) than the Myxini (hagfishes). This, combined with an apparent lack of vertebral elements within the Myxini, suggested that the Myxini were descended from a more ancient lineage than the vertebrates, and that the skull developed before the vertebral column. The clade was thus composed of the Myxini and the vertebrates, and any extinct chordates with skulls.

However recent studies using molecular phylogenetics has contradicted this view, with evidence that the Cyclostomata (Hyperoartia and Myxini) is monophyletic; this suggests that the Myxini are degenerate vertebrates, and therefore the vertebrates and craniates are cladistically equivalent, at least for the living representatives. The placement of the Myxini within the vertebrates has been further strengthened by recent anatomical analysis, with vestiges of a vertebral column being discovered in the Myxini.

Eudontomyzon

Eudontomyzon is a genus of lamprey in the Petromyzontidae family.

Eudontomyzon stankokaramani

The Drin brook lamprey (Eudontomyzon stankokaramani) is a non-predatory, freshwater resident species of lamprey found in the Drin river system of Albania and Kosovo and the basins of Lakes Ohrid and Shkodra.

Far Eastern brook lamprey

The Far Eastern brook lamprey (Lethenteron reissneri) is a species of non-parasitic lamprey. It is found in lakes and rivers in Japan and the Russian Far East. It may identical to the Siberian brook lamprey, Lethenteron kessleri.

Hyperoartia

Hyperoartia or Petromyzontida is a disputed group of vertebrates that includes the modern lampreys and their fossil relatives. Examples of hyperoartians from early in their fossil record are Endeiolepis and Euphanerops (which possessed a calcified branchial basket), fish-like animals with hypocercal tails that lived during the Late Devonian Period. Some paleontologists still place these forms among the "ostracoderms" (jawless armored fishes) of the class Anaspida, but this is increasingly considered an artificial arrangement based on ancestral traits.

Placement of this group among the jawless vertebrates is a matter of dispute. While today enough fossil diversity is known to make a close relationship among the "ostracoderms" unlikely, this has muddied the issue of the Hyperoartia's closest relatives. Traditionally the group was placed in a superclass Cyclostomata together with the Myxini (hagfishes). More recently, it has been proposed that the Myxini are more basal among the skull-bearing chordates, while the Hyperoartia are retained among vertebrates. But even though this may be correct, the lampreys represent one of the oldest divergences of the vertebrate lineage, and whether they are better united with some "ostracoderms" in the Cephalaspidomorphi, or not closer to these than to e.g. to other "ostracoderms" of the Pteraspidomorphi, or even the long-extinct conodonts, is still to be resolved. Even the very existence of the class Hyperoartia is disputed, with some analyses favoring a treatment of the "basal Hyperoartia" as a monophyletic lineage Jamoytiiformes that may in fact be very close to the ancestral jawed vertebrates.

The only hyperoartians surviving today are lampreys, classified in the Petromyzontiformes. The discovery of the fossil Priscomyzon pushed back the oldest known occurrence of true lampreys to the Late Devonian. The evidence of phylogeny, however, suggests the lamprey lineage diverged much earlier from other vertebrates, rather than arising from among the "ostracoderms". The origin of Hyperoartia may therefore extend back to the early Paleozoic, if not earlier.

IUCN Red List of extinct species

On 29 January 2010, the IUCN Red List of Threatened Species identified 842 (746 animals, 96 plants) extinct species, subspecies and varieties, stocks and sub-populations.

Ichthyomyzon

Ichthyomyzon is a genus of northern lampreys in the sub-family Petromyzontinae, native to North America.

Lampetra

Lampetra is a genus of lampreys in the subfamily Petromyzontinae.

Lampetra ayresii

Lampetra ayresii is a species of lamprey in the family Petromyzontidae. It is also called the river lamprey or western river lamprey. Found in the eastern pacific from Tee Harbor, Juneau in Alaska to the Sacramento-San Joaquin drainage in California, USA.

Lethenteron

Lethenteron is a genus of Lamprey in the Petromyzontidae family.

Lombardy lamprey

The Lombardy lamprey (Lethenteron zanandreai) is a species of lamprey in the Petromyzontidae family. It is found in Croatia, Italy, and Slovenia. Its natural habitats are rivers and freshwater springs. It is threatened by habitat loss.

Mordacia

Mordacia is the sole genus of the family Mordaciidae, also known as the southern topeyed lampreys.

Paraconodontida

Paraconodontida is an extinct order of conodonts. It contains two superfamilies, Amphigeisinoidea and Furnishinoidea.

Prioniodontida

Prioniodontida, also known as the "complex conodonts", is a large clade of conodonts that includes two major evolutionary grades; the Prioniodinina and the Ozarkodinina. It includes many of the more famous conodonts, such as the giant ordovician Promissum (Prioniodinina) from the Soom Shale and the Carboniferous specimens from the Granton Shrimp bed (Ozarkodinina). They are euconodonts, in that their elements are composed of two layers; the crown and the basal body, and are assumed to be a clade.

Procephalaspis

Procephalaspis is an extinct genus of jawless fish.

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 primitive for all vertebrates.

Tetrapleurodon

Tetrapleurodon is a genus of lampreys that are endemic to the Lerma–Chapala basin in west–central Mexico. Both species are threatened.

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