Craniate


A craniate is a member of the Craniata, a proposed clade of chordate animals with a skull of hard bone or cartilage. Living representatives are the Myxini, Hyperoartia, and the much more numerous Gnathostomata. Formerly distinct from vertebrates by excluding hagfish, molecular and anatomical research in the 21st century has led to the reinclusion of hagfish, making living craniates synonymous with living vertebrates.
The clade was conceived largely on the basis of the Hyperoartia being more closely related to the Gnathostomata than the Myxini. 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 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.

Characteristics

In the simplest sense, craniates are chordates with well-defined heads, thus excluding members of the chordate subphyla Tunicata and Cephalochordata, but including Myxini, which have cartilaginous skulls and tooth-like structures composed of keratin. Craniata also includes all lampreys and armoured jawless fishes, armoured fish, sharks, skates, and rays, and teleostomians: spiny sharks, bony fish, lissamphibians, temnospondyls and protoreptiles, sauropsids and mammals. The craniate head consists of a brain, sense organs, including eyes, and a skull.
In addition to distinct crania, craniates possess many derived characteristics, which have allowed for more complexity to follow. Molecular-genetic analysis of craniates reveals that, compared to less complex animals, they developed duplicate sets of many gene families that are involved in cell signaling, transcription, and morphogenesis.
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 that pump water through the slits, engaging in both feeding and gas exchange. Muscles line the alimentary canal, moving food through the canal, allowing higher craniates such as mammals to develop more complex digestive systems for optimal food processing. Craniates have cardiovascular systems that include a heart with two or more chambers, red blood cells, and oxygen transporting hemoglobin, as well as kidneys.

Systematics and taxonomy

used the terms Craniata and Vertebrata interchangeably to include lampreys, jawed fishes, and terrestrial vertebrates. Hagfishes were classified as Vermes, possibly representing a transitional form between 'worms' and fishes.
Dumeril grouped hagfishes and lampreys in the taxon Cyclostomi, characterized by horny teeth borne on a tongue-like apparatus, a large notochord as adults, and pouch-shaped gills. Cyclostomes were regarded as either degenerate cartilaginous fishes or primitive vertebrates. Cope coined the name Agnatha for a group that included the cyclostomes and a number of fossil groups in which jaws could not be observed. Vertebrates were subsequently divided into two major sister-groups: the Agnatha and the Gnathostomata. Stensiö suggested that the two groups of living agnathans arose independently from different groups of fossil agnathans.
Løvtrup argued that lampreys are more closely related to gnathostomes based on a number of uniquely derived characters, including:
In other words, the cyclostome characteristics are either instances of convergent evolution for feeding and gill ventilation in animals with an eel-like body shape, or represent primitive craniate characteristics subsequently lost or modified in gnathostomes. On this basis Janvier proposed to use the names Vertebrata and Craniata as two distinct and nested taxa.

Validity

The validity of the taxon "Craniata" was recently examined by Delarbre et al. using mtDNA sequence data, concluding that Myxini is more closely related to Hyperoartia than to Gnathostomata - i.e., that modern jawless fishes form a clade called Cyclostomata. The argument is that, if Cyclostomata is indeed monophyletic, Vertebrata would return to its old content and the name Craniata, being superfluous, would become a junior synonym.
The new evidence removes support for the hypothesis for the evolutionary sequence by which first the hard cranium arose as it is exhibited by the hagfishes, then the backbone as exhibited by the lampreys, and then finally the hinged jaw that is now ubiquitous. In 2010, Philippe Janvier stated:

Classification

of the Chordate phylum. Lines show probable evolutionary relationships, including extinct taxa, which are denoted with a dagger, †. Some are invertebrates. The positions of the Lancelet, Tunicate, and Craniata clades are as reported.