Craniata

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Craniata

Craniata (sometimes Craniota) is a proposed clade

Clade

A clade is a term used in modern alpha taxonomy, the scientific classification of living and fossil organisms, to describe a monophyletic group, defined as a group consisting of a single common ancestor and all its descendants.The term "monophyletic group" is used in this article in the conventional sense of "an a...
of chordate animal

Animal

Animals are a major group of multicellular, eukaryotic organisms of the Kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life....
s that contains the vertebrates (subphylum Vertebrata

Vertebrate

Vertebrates are members of the subphylum Vertebrata, chordates with Vertebras or Vertebral columns. The grouping sometimes includes the hagfish, which have no vertebrae, but are genetically quite closely related to lampreys, which do have vertebrae....
) and Myxini (hagfish

Hagfish

Hagfish are marine craniates of the class Myxini, also known as Hyperotreti. Myxini is the only class in the clade Craniata that does not also belong to the phylum Vertebrata....
) as living representatives. Craniata includes all animals with a skull

Skull

The skull is a bone structure found in the head of many animals. The skull supports the structures of the face and protects the head against injury....
, or cranium, as the name suggests.

Craniata as an unranked taxon

Taxon

A taxon or taxonomic unit is a name designating an organism or a group of organisms. In biological nomenclature according to Carl Linnaeus, a taxon is assigned a taxonomic rank and can be placed at a particular level in a systematic hierarchy reflecting evolutionary relationships....
replaces the former use of Vertebrata (Vertebrata sensu lato). The main difference of the old and new (Vertebrata sensu stricto) interpretation of Vertebrata is that Myxini and - usually - the Cephalaspidomorphi

Cephalaspidomorphi

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Encyclopedia

Craniata (sometimes Craniota) is a proposed clade

Clade

Animal

Vertebrate

Hagfish

Skull

Taxon

Cephalaspidomorphi

Cephalaspidomorphs are a taxon of agnatha named for the cephalaspids, a group of Osteostraci. Most of the members of this group are extinct; however, it interests modern biologists because it may include the lampreys....
too are now not included in Vertebrata anymore. These two taxa lack proper vertebrae, which are characteristic for vertebrates according to the new interpretation, whereas traditionally—and confusingly—they were not (Hickman et al., 2007).

Characteristics

In the simplest sense craniates are chordates with heads, thus excluding members of chordate subphyla Urochordata (tunicates) and Cephalochordata (lancelets), but including Myxini, which have cartilaginous skulls and tooth-like structures composed of keratin

Keratin

Keratins are a family of fibrous protein; tough and insoluble, they form the hard but mineral structures found in reptiles, birds, amphibians and mammals....
. Craniata also includes all sharks and rays

Chondrichthyes

Chondrichthyes or cartilaginous fishes are jawed fish with paired Fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone....
, true fish

Osteichthyes

Osteichthyes , also called bony fish, are a taxonomy group of fish that includes the ray-finned fish and lobe finned fish . The split between these two classes occurred around 440 mya ....
, amphibian

Amphibian

Amphibians , such as frogs, toads, salamanders, newts and caecilians, are cold-blooded animals that metamorphose from a juvenile, water-breathing form to an adult, air-breathing form....
s, reptile

Reptile

Reptiles, or members of the class Reptilia, are air-breathing, cold-blooded vertebrates that have skin covered in scale as opposed to hair or feathers....
s and mammal

Mammal

Mammals are a class of vertebrate animals whose name is derived from their distinctive feature, mammary glands, with which they feed their young....
s. The craniate head consists of a brain

Brain

The brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as cnidarian and echinoderm have a decentralized nervous system without a brain, while sponges lack any nervous system at all....
, sense organs including eyes, and a skull.

In addition to distinct crania (sing. cranium), craniates possess many derived characteristics which have allowed for more complexity to follow. Molecular-genetic

Molecular biology

Molecular biology is the study of biology at a molecule level. The field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry....
analysis of craniates reveals that, compared to less complex animals, they developed duplicate sets of many gene

Gene

A gene is the basic unit of heredity in a living organism. All living things depend on genes. Genes hold the information to build and maintain their cell and pass genetic trait to offspring....
families that are involved in cell signaling

Cell signaling

Cell signaling is part of a complex system of communication that governs basic cellular activities and coordinates cell actions. The ability of cells to perceive and correctly respond to their microenvironment is the basis of development, tissue repair, and immunity as well as normal tissue homeostasis....
, transcription

Transcription (genetics)

Transcription is the synthesis of RNA under the direction of DNA. RNA synthesis, or transcription, is the process of transcribing DNA nucleotide sequence information into RNA sequence information....
, and morphogenesis

Morphogenesis

Morphogenesis , is the physical process that gives rise to the shape of an organism. It is one of three fundamental aspects of developmental biology along with the control of cell growth and cellular differentiation....
(see homeobox

Homeobox

A homeobox is a DNA sequence found within genes that are involved in the regulation of patterns of development in animals, fungus and plants. Genes that have a homeobox are called homeobox genes and form the homeobox gene family....
).

In general, craniates are much more active than tunicates and lancelets and as a result have greater metabolic demands, as well as several anatomical adaptations. Aquatic craniates have gill slits which are connected to muscles and nerves which pump water through the slits (as opposed to lancelets, whose pharyngeal slit

Pharyngeal slit

Pharyngeal slits, characteristic of both hemichordata and chordata, are used by organisms in feeding. The wall of the pharynx is perforated by up to 200 vertical slits, which are separated by stiffening rods....
s are used only for suspension feeding), engaging in both feeding and gas exchange. Muscles line the alimentary canal, moving

Peristalsis

Peristalsis is the rhythmic contraction of smooth muscles to propel contents through the digestive tract. The word is derived from New Latin and comes from the Greek language peristaltikos, peristaltic, from peristellein, "to wrap around," and stellein, "to place."...
food through the canal, allowing higher craniates like mammals to develop more complex digestive systems for optimal food processing. Craniates have cardiovascular systems which include a heart with two or more chambers, red blood cells, and O₂ transporting hemoglobin

Hemoglobin

Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of vertebrates, and the tissues of some invertebrates....
, as well as kidney

Kidney

The kidneys are Organ that have numerous biological roles. Their primary role is to maintain the homeostasis balance of bodily fluids by filtering and secreting Metabolomics#Metabolitess and minerals from the blood and excreting them, along with water , as urine....
s.

Systematics and taxonomy

CLADE CRANIATA

Class Myxini (= Hyperotreti)

Myxiniformes

Myxinidae

Unranked taxon Vertebrata
Vertebrate

Vertebrates are members of the subphylum Vertebrata, chordates with Vertebras or Vertebral columns. The grouping sometimes includes the hagfish, which have no vertebrae, but are genetically quite closely related to lampreys, which do have vertebrae....

Class Cephalaspidomorphi
Cephalaspidomorphi

Cephalaspidomorphs are a taxon of agnatha named for the cephalaspids, a group of Osteostraci. Most of the members of this group are extinct; however, it interests modern biologists because it may include the lampreys....
Unranked taxon Gnathostomata
Gnathostomata

Gnathostomata is the group of vertebrates with jaws.The group is traditionally a superclass , broken into two top level groupings; cartilaginous fish, and all other members, including the familiar classes of bony fish, birds, mammals, reptiles, and amphibians....

Class Chondrichthyes
Chondrichthyes

Chondrichthyes or cartilaginous fishes are jawed fish with paired Fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone....
Superclass Osteichthyes
Osteichthyes

Osteichthyes , also called bony fish, are a taxonomy group of fish that includes the ray-finned fish and lobe finned fish . The split between these two classes occurred around 440 mya ....

Validity

The validity of this taxon is in doubt. mtDNA sequence

DNA sequence

A DNA sequence or genetic sequence is a succession of letters representing the primary structure of a real or hypothetical DNA molecule or strand, with the capacity to carry information as described by the central dogma of molecular biology....
analysis (Delarbre et al 2002) suggests that Myxini are as close to the Hyperoartia

Hyperoartia

Hyperoartia is a group of jawless fishes that includes the modern lampreys and their fossil relatives, the jawless fishes of the class Anaspida....
as long believed - i.e., that the Agnatha

Agnatha

Agnatha is a class or superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. Many recent textbooks regard the group as paraphyletic but recent molecular data, both from rRNA and from mtDNA strongly supports living agnathans as monophyletic....
are a valid clade. The Cephalaspidomorphs

Cephalaspidomorphi

Lamprey

A lamprey is a parasitic marine animal with a toothed, funnel-like sucking mouth. While lampreys are well known for those species which bore into the flesh of other fish to hematophagy, these species make up the minority....
s) are traditionally allied with the Hyperoartia, which may or may not be correct. Note however that mtDNA is not very reliable in phylogenetic analysis that reach very deep into time, due to its accelerated rate of mutation

Mutation

In biology, mutations are changes to the nucleotide sequence of the genetic material of an organism. Mutations can be caused by copying errors in the genetic material during cell division, by exposure to ultraviolet or ionizing radiation, chemical mutagens, or virus , or can be induced by the organism, itself, by cellular processes such as s...
, and that the fossil record is quite equivocal.

If the Acrania are indeed not paraphyletic, Vertebrata would return to its old content (Gnathostomata

Gnathostomata

Gnathostomata is the group of vertebrates with jaws.The group is traditionally a superclass , broken into two top level groupings; cartilaginous fish, and all other members, including the familiar classes of bony fish, birds, mammals, reptiles, and amphibians....
+ Agnatha

Agnatha

Craniata

Characteristics

Systematics and taxonomy

Validity

See also