Vertebrata - AccessScience from McGraw-Hill Education https://www.accessscience.com/content/vertebrata/730900 (https://www.accessscience.com:443/) Article by: Wiley, Edward O., III Natural History Museum, University of Kansas, Lawrence, Kansas. Publication year: 2014 DOI: http://dx.doi.org/10.1036/1097-8542.730900 (http://dx.doi.org/10.1036/1097-8542.730900) Content Lampreys and fossil jawless fishes Current classification Bibliography Jawed vertebrates The largest natural group (clade, monophyletic group) of chordate animals. Like all chordates, vertebrates have a notochord, but it is largely replaced by vertebrae (initially neural arches that surround the nerve chord) during development. Together with the living hagfishes (Hyperotreti; Myxiniiformes) and many fossil groups, vertebrates are members of the Craniata. One major theme of craniate evolution is the increasing elaboration of the brain, its cranial nerves, and many sensory organs, such as the organs of smell (olfactory), hearing (inner ear), and feel (sensory organs of the lateral line system). A unique innovation is the evolution of neural crest cells, cells that begin development along the neural crest of the embryo and which then migrate to many parts of the body, giving rise to or contributing to the formation of such structures as the cranium and the branchial arches (a remnant of which remains in humans as parts of the larynx). Vertebrates have a long fossil history. Earliest fossils of jawless vertebrates are known from the late Cambrian (490 million years before present, MYBP). Jawed vertebrate fossils are known from the Ordovician (450 MYBP) and this group began to predominate in the Devonian (about 360 MYBP). See also: Chordata (/content/chordata/133700); Nervous system (vertebrate) (/content/nervous-system-vertebrate/449300); Vertebra (/content/vertebra/730800); Vertebrate brain (evolution) (/content/vertebrate-brain-evolution/757532) Formal vertebrate classifications of the past emphasized overall distinctiveness and grade of organization. Modern (evolutionary phylogenetic) classifications are based on common ancestry and genealogy. Thus, traditional classifications have been highly modified and classifications in many textbooks are out of date. Older classifications recognized a group, Agnatha, which contained lampreys and the more primitive hagfishes. Modern classifications recognize the evidence that lampreys, although jawless, are nevertheless more closely related to jawed vertebrates than to hagfishes. Older classifications placed the lungfishes and coelacanths with the bony fishes. Modern classifications place them with the legged vertebrates, because they share a common ancestor with legged vertebrates not shared with bony fishes. See also: Sarcopterygii (/content/sarcopterygii/601800); Systematics (/content/systematics/036000) Lampreys and fossil jawless fishes The most basal living members of Vertebrata are the lampreys (Hyperoatia; about 35 species). Lampreys are jawless fishes whose eel-like body resembles hagfishes, but whose predatory, round, and toothed mouth is unique among vertebrates. Lampreys and many fossil jawless vertebrates that flourished in the Paleozoic have a number of unique evolutionary innovations. These include neural arches (vertebrae) that enclose the dorsal nerve chord, muscles that work to move the fin rays, a vertebrate heart with atrium and ventricle closely situated, nervous regulation of the heart, and a number of important sensory innovations including the evolution of extrinsic eye muscles, the elaboration of the inner ear to two vertical semicircular canals (later to be three in jawed vertebrates), and the development of true neuromasts (sensory hair organs) in 1 of 4 8/4/2016 12:31 PM Vertebrata - AccessScience from McGraw-Hill Education https://www.accessscience.com/content/vertebrata/730900 the sensory canal system. Of course, some of the soft anatomy cannot be observed in fossils. See also: Jawless vertebrates (/content/jawless-vertebrates/014900) Jawed vertebrates Major evolutionary innovations characterized the jawed vertebrates, Gnathostomata (“jawed mouth”). In addition to jaws and characters mentioned above, gnathostomes have paired fins or the homologue of paired fins, legs. They also have a mineralized exoskeleton that surrounds certain cartilages of the head and fins and consists of true bone (as in human leg bones) or calcified cartilage (as in many sharks). As is usual in evolution, such characters evolve stepwise; there are jawless relatives of gnathostomes with paired fins, and more primitive relatives that have mineralized bone but which lack either paired fins or jaws. One of the remarkable innovations of the gnathostome ancestor was the evolution of the adaptive immune system, the basis of our ability to fight off infection and to develop vaccines to fight many infectious diseases. See also: Gnathostomata (/content/gnathostomata/293800) Placodermi and Chondrichthyes There are two major living clades of gnathostomes and one major extinct clade. The extinct clade is Placodermi, a group of Paleozoic fishes characterized by heavy body armor and which includes some of the largest predatory fishes of Paleozoic waters. The smaller of the living clades is Chondrichthyes. It includes the chimeras (30 species) and sharks and rays (about 830 species). Chondrichthyes has a rich fossil history; the earliest remains are known from the Late Ordovician, and early sharks were common predators in both Paleozoic and Mesozoic seas. They are characterized by several evolutionary innovations including internal fertilization via the unique male clasper organs, and the histology of the calcified cartilage that surrounds the cartilaginous skeleton (micrometric prismatic cartilage). The lack of an obvious bony skeleton is a secondary innovation of sharks and is not primitive. In fact, Chondrichthyes fishes have bone, which form the base of each scale. See also: Chondrichthyes (/content/chondrichthyes/133100); Placodermi (/content/placodermi/520900) Osteichthyes The third major clade of vertebrates is Osteichthyes, a group that in traditional classification contained only the bony fishes, but which now includes the Actinopterygii (bony fishes, about 25,000 species) and the Sarcopterygii (about 24,000 species). Sarcopterygii includes the legged vertebrates (amphibians, reptiles, mammals, and birds), the living lungfishes, coelacanths, and their diverse fossil relatives commonly known as crossopterygians. See also: Osteichthyes (/content/osteichthyes /478500) Current classification A current classification of living vertebrates is shown below. Note that the ranks of this classification serve only to organize and subordinate the groups; ranks do not imply distinctiveness or uniqueness of adaptations. Many of the traditional classes, such as class Mammalia, would have lower rank in modern classifications such as this one. Subphylum Chordata (http://www.accessscience.com/content/chordata/133700) Infraphylum Craniata Superclass Hyperotreti (hagfishes) Superclass Vertebrata (http://www.accessscience.com/content/vertebrata/730900) Class Hyperoartia (lampreys) 2 of 4 8/4/2016 12:31 PM Vertebrata - AccessScience from McGraw-Hill Education https://www.accessscience.com/content/vertebrata/730900 Class Gnathostomata (http://www.accessscience.com/content/gnathostomata/293800) Infraclass Chondrichthyes (http://www.accessscience.com/content/chondrichthyes/133100) (chimeras, sharks, rays) Infraclass Osteichthyes (http://www.accessscience.com/content/osteichthyes/478500) Subclass Actinopterygii (http://www.accessscience.com/content/actinopterygii/009100) (bony fishes, sturgeons, trouts, basses, tunas, etc.) Subclass Sarcopterygii (http://www.accessscience.com/content/sarcopterygii/601800) (lungfishes, coelacanths, amphibians, reptiles, birds, mammals) E. O. Wiley Bibliography M. J. Benton, Vertebrate Palaeontology, 3d ed., Blackwell, Malden, MA, 2005 R. L. Carroll, Vertebrate Paleontology and Evolution, W. H. Freeman, New York, 1988 M. Hildebrand and G. Goslow, Analysis of Vertebrate Structure, 5th ed., Wiley, New York, 1998 3 of 4 8/4/2016 12:31 PM .