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Craniata (sometimes Craniota) is a proposed clade of chordate animals that contains the Myxini (hagfish), Petromyzontida (including lampreys), and Gnathostomata (jawed vertebrates)[1][2] 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 sometimes the Petromyzontida (see Systematics and taxonomy, below) are no longer included in Vertebrata. The Myxini lack proper vertebrae, which are characteristic for vertebrates according to the new interpretation, whereas traditionally, and confusingly, they were not (Hickman et al., 2007). When Vertebrata is defined as excluding the lampreys as well, it becomes another name for Gnathostomata.
[edit] CharacteristicsIn 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 lampreys and armored jawless fishes, sharks and rays, bony fish, amphibians, reptiles and mammals. The craniate head consists of a brain, sense organs including eyes, and a skull[3][4]. 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)[1]. 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[1]. [edit] Systematics and taxonomyLinnaeus (1758) used the terms 'Craniata' and 'Vertebrata' interchangeably to include lampreys, jawed fishes, and terrestrial vertebrates (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 the name '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 Gnathostomata (jawed vertebrates). Stensiö (1927) suggested that the two groups of living agnathans (i.e., cyclostomes) arose interpedently from different groups of fossil agnathans. Løvtrup (1977) argued that lampreys are more closely related to gnathostomes based on a number of uniquely derived characters, including: 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 (spirally coiled valve of the intestinal wall), true lymphocytes, a differentiated anterior lobe of the pituitary gland (adenohypophysis), three inner ear maculae (patches of acceleration sensitive 'hair cells' used in balance) organized into two or three vertical semicircular canals, neuromast organs (composed of vibration sensitive hair cells) in the laterosensory canals, an electroreceptive lateral line (with voltage sensitive hair cells) and electrosensory lateral line nerves, and 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, "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. Janvier (1978) proposed to use the names 'Vertebrata' and 'Craniata' as two distinct and nested taxa. [edit] ValidityThe validity of the taxon "Craniata" was recently examined by Delarbre et al. (2002) 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 Acrania is indeed monophyletic, Vertebrata would return to its old content (Gnathostomata + Cyclostomata) and the name Craniata, being superfluous, would become a junior synonym. However, mtDNA may not be a reliable marker for phylogenetic analysis of such ancient divergence times, due to its rapid rate of mutation. Further, the root of the molecular phylogeny for Craniata may be difficult to resolve due to the very poor representation of deep branches among living taxa; in other words that the taxon sampling afforded by extant taxa alone results in very long branches, within the basal craniate lineages, and among their closest relatives (i.e., Branchiostoma). Because extinction has eliminated so much of the important transitional states needed to reconstruct the early branching order of craniate lineages, the molecular tree of these taxa may effectively be an unrooted network. [edit] See also
[edit] References
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