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
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
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
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.
2 Systematics and taxonomy
5 See also
In the simplest sense, craniates are chordates with well-defined
heads, thus excluding members of the chordate subphyla Tunicata
Cephalochordata (lancelets), 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 (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 that pump water through the
slits, engaging in both feeding and gas exchange (as opposed to
lancelets, whose pharyngeal slits are used only for suspension
feeding). 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
Systematics and taxonomy
Linnaeus (1758) used the terms Craniata and Vertebrata
interchangeably to include lampreys, jawed fishes, and terrestrial
vertebrates (or tetrapods). Hagfishes were classified as Vermes,
possibly representing a transitional form between 'worms' and fishes.
Dumeril (1806) 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
(Marspibranchii). Cyclostomes were regarded as either degenerate
cartilaginous fishes or primitive vertebrates. Cope (1889) coined
Agnatha ("jawless") 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:
Agnatha and the
Gnathostomata (jawed vertebrates). Stensiö
(1927) suggested that the two groups of living agnathans (i.e. the
cyclostomes) arose independently from different groups of fossil
Løvtrup (1977) argued that lampreys are more closely related to
gnathostomes based on a number of uniquely derived characters,
Arcualia (serially arranged paired cartilages above the notochord)
Extrinsic eyeball muscles
Radial muscles in the fins
A closely set atrium and ventricle of the heart
Nervous regulation of the heart by the vagus nerve
A typhlosole (a spirally coiled valve of the intestinal wall)
A differentiated anterior lobe of the pituitary gland
Three inner ear maculae (patches of acceleration sensitive 'hair
cells' used in balance) organized into two or three vertical
Neuromast organs (composed of vibration sensitive hair cells) in the
An electroreceptive lateral line (with voltage sensitive hair cells)
Electrosensory lateral line nerves
A cerebellum, i.e. the multi-layered roof of the hindbrain with unique
structure (characteristic neural architecture including direct inputs
from the lateral line and large output Purkinje cells) and function
(integrating sensory perception and coordinating motor control)
In other words, the cyclostome characteristics (e.g. horny teeth on a
"tongue", gill pouches) 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 (1978)
proposed to use the names Vertebrata and Craniata as two distinct and
The validity of the taxon "Craniata" was recently examined by Delarbre
et al. (2002) using mt
DNA sequence data, concluding that
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 (
Gnathostomata + Cyclostomata) 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 (from among tunicate-like chordates)
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:
Although I was among the early supporters of vertebrate paraphyly, I
am impressed by the evidence provided by Heimberg et al. and prepared
to admit that cyclostomes are, in fact, monophyletic. The consequence
is that they may tell us little, if anything, about the dawn of
vertebrate evolution, except that the intuitions of 19th century
zoologists were correct in assuming that these odd vertebrates
(notably, hagfishes) are strongly degenerate and have lost many
characters over time.
Phylogenetic tree of the
Chordate phylum. Lines show probable
evolutionary relationships, including extinct taxa, which are denoted
with a dagger, †. Some are invertebrates. The positions
(relationships) of the Lancelet, Tunicate, and Craniata clades are as
reported in the scientific journal Nature. Note that this
cladogram, in showing the extant
Cyclostomata (hagfish and lamprey) as
paraphyletic, is contradicted by nearly all recent molecular data,
which support the monophyly of the extant cyclostomes (see Ota and
Kurakani 2007 and references therein for a review of evidence).
Haikouella, extinct genus
Haikouichthys, extinct genus
^ Nielsen, C. (July 2012). "The authorship of higher chordate taxa".
Zoologica Scripta. 41 (4): 435–436.
^ a b c Campbell & Reece 2005 p. 676
^ Cracraft & Donoghue 2004 p. 390
^ Janvier, Philippe (2011). "Comparative Anatomy: All Vertebrates Do
Have Vertebrae". Current Biology. 21 (17): R661–R663.
doi:10.1016/j.cub.2011.07.014. ISSN 0960-9822.
^ Campbell & Reece 2005 pp. 675-7
^ Parker & Haswell 1921
^ a b c d e f Janvier, Philippe. "Craniata - Animals with skulls".
Tree of Life Web Project (ToL). Tree of Life Web Project.
^ "MicroRNAs revive old views about jawless vertebrate divergence and
evolution." Proceedings of the National Academy of Sciences (USA)
^ Putnam, N. H.; Butts, T.; Ferrier, D. E. K.; Furlong, R. F.;
Hellsten, U.; Kawashima, T.; Robinson-Rechavi, M.; Shoguchi, E.;
Terry, A.; Yu, J. K.; Benito-Gutiérrez, E. L.; Dubchak, I.;
Garcia-Fernàndez, J.; Gibson-Brown, J. J.; Grigoriev, I. V.; Horton,
A. C.; De Jong, P. J.; Jurka, J.; Kapitonov, V. V.; Kohara, Y.;
Kuroki, Y.; Lindquist, E.; Lucas, S.; Osoegawa, K.; Pennacchio, L. A.;
Salamov, A. A.; Satou, Y.; Sauka-Spengler, T.; Schmutz, J.; Shin-i, T.
(19 June 2008). "The amphioxus genome and the evolution of the
chordate karyotype". Nature. 453 (7198): 1064–1071.
^ Ota, K. G.; Kuratani, S. (September 2007). "Cyclostome embryology
and early evolutionary history of vertebrates". Integrative and
Comparative Biology. 47 (3): 329–337. doi:10.1093/icb/icm022.
Wikispecies has information related to Chordata Craniata
Campbell, Neil A.; Reece, Jane B. (2005). Biology (Seventh ed.). San
Francisco CA: Benjamin Cummings.
Cleveland P. Hickman, J., Roberts, L. S., Keen, S. L., Larson, A.
& Eisenhour, D. J. (2007).
Animal Diversity (Fourth ed.). New
York: McGraw Hill. CS1 maint: Uses authors parameter (link)
Cracraft, Joel; Donoghue, Michael J. (2004). Assembling the Tree of
Life. New York: Oxford University Press US.
Delarbre, Christiane; Gallut, C; Barriel, V; Janvier, P; Gachelin, G;
et al. (2002). "Complete Mitochondrial DNA of the Hagfish, Eptatretus
burgeri: The Comparative Analysis of Mitochondrial DNA Sequences
Strongly Supports the Cyclostome Monophyly". Molecular Phylogenetics
and Evolution. 22 (2): 184–192. doi:10.1006/mpev.2001.1045.
Parker, T. J.; Haswell, W. A. (1921). A Text-book of Zoology.
Macmillan & Co.
Ctenophora (comb jellies)
Cnidaria (jellyfish and relatives)
craniates / vertebrates
Echinodermata (starfish and relatives)
Kinorhyncha (mud dragons)
Priapulida (penis worms)
Nematomorpha (horsehair worms)
Onychophora (velvet worms)
Chaetognatha (arrow worms)
Gnathostomulida (jaw worms)
Dicyemida or Rhombozoa
Annelida (ringed worms)
Nemertea (ribbon worms)
Entoprocta or Kamptozoa
Ectoprocta (moss animals)
Brachiopoda (lamp shells)
Phoronida (horseshoe worms)
Anthozoa inc. corals
Medusozoa inc. jellyfish
Asterozoa inc. starfish
Phyla with ≥5000 extant species bolded
Monoblastozoa (nomen dubium)
Extant chordate classes
Ascidiacea (sea squirts)
Thaliacea (pyrosomes, salps, doliolids)
(Vertebrates + Myxini)
(fish + Tetrapods)
Agnatha (jawless fish)
Chondrichthyes (cartilaginous fish: sharks, rays, chimaeras)
Actinopterygii (ray-finned fish)
Squamata (scaled reptiles)²
¹subclasses of Sarcopterygii
²orders of class Reptilia (reptiles)
³traditionally placed in Anapsida
italic are paraphyletic groups