craniate n : animals having a bony or cartilaginous skeleton with a segmented spinal column and a large brain enclosed in a skull or cranium [syn: vertebrate]

For the class of brachiopods, see Craniforma.

Craniata (sometimes Craniota) is a proposed clade of chordate animals that contains the vertebrates (subphylum Vertebrata) and Myxini (hagfish) as living representatives. Craniata includes all animals with a skull, or cranium, as the name suggests.

Craniata as an unranked taxon 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 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. Craniata also includes all sharks and rays, true fish, amphibians, reptiles and mammals. The craniate head consists of a brain, 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 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 (see homeobox).

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 slits are used only for suspension feeding), engaging in both feeding and gas exchange. Muscles line the alimentary canal, moving 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, as well as kidneys.

Systematics and taxonomy

CLADE CRANIATA

• Class Myxini (= Hyperotreti)
  • Myxiniformes
    • Myxinidae
• Unraked taxon Vertebrata
  • Class Cephalaspidomorphi
  • Unranked taxon Gnathostomata
    • Class Chondrichthyes
    • Superclass Osteichthyes

Validity

The validity of this taxon is in doubt. mtDNA sequence analysis (Delarbre et al 2002) suggests that Myxini are as close to the Hyperoartia as long believed - i.e., that the Agnatha are a valid clade. The Cephalaspidomorphs (lampreys) 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, and that the fossil record is quite equivocal.

If the Acrania are indeed not paraphyletic, Vertebrata would return to its old content (Gnathostomata + Agnatha) and Craniata, being superfluous, would become its synonym.

See also

• Extinct genera Haikouella and Haikouichthys

References