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Are humans Gnathostomates?

Are humans Gnathostomates?



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Gnathostomates are vertebrates having jaws. But when I search on the internet for Gnathostomates, I always find out that they are sharks and fish-like organisms.

But are humans not also Gnathostomates? Don't we have jaws?


Absolutely, humans belong to the superclass Gnathostomata. It's simply the class of jawed vertebrates, and includes 99% of vertebrates. It also includes the class Mammalia, which includes humans.

For basic and fairly light reading, check out Wikipedia on Gnathostomata and Mammalia.


Yes, all jawed vertebrates belong to the infraphylum Gnathostomata, including all mammals (and therefore all humans).

Here are 3 different taxanomic trees showing this:

Source: Wikipedia

Source: UCL

Source: INTECH


Yes of course Humans are Gnathostomes. Under division Gnathostomata you have 2 Superclasses that include Pisces and Tetrapods. Under Tetrapoda are 3 classes- reptiles, Aves, mammals. So Humans are indeed Gnathostomes.


Taxonomically humans have been grouped as mammals placed under superclass Tetrapoda (Includes amphibians, reptiles, Aves and mammals) being different from other superclass Pisces which includes sharks and bony fishes. All the animals placed under superclass Pisces and Tetrapod are have been grouped as gnathostomes characterised by the presence of jaws in their mouth. In contrast all animals which do not posses jaws like lampreys and hagfishes (primitive jawless fishes) have been grouped as agnathans.Both gnathostomes and agnathans have been grouped as vertebrata (sub phylum) and as Chordata (Phylum) groups.

Source : Vertebrates: Comparative anatomy, function & evolution By Kenneth V Kardong


Timeline of human evolution

The timeline of human evolution outlines the major events in the evolutionary lineage of the modern human species, Homo sapiens, throughout the history of life, beginning some 4.2 billion years ago down to recent evolution within H. sapiens during and since the Last Glacial Period.

It includes brief explanations of the various taxonomic ranks in the human lineage. The timeline reflects the mainstream views in modern taxonomy, based on the principle of phylogenetic nomenclature in cases of open questions with no clear consensus, the main competing possibilities are briefly outlined.


Where humans split from sharks: Common ancestor comes into focus

The common ancestor of all jawed vertebrates on Earth resembled a shark, according to a new analysis of the braincase of a 290-million-year-old fossil fish that has long puzzled paleontologists.

New research on Acanthodes bronni, a fish from the Paleozoic era, sheds light on the evolution of the earliest jawed vertebrates and offers a new glimpse of the last common ancestor before the split between the earliest sharks and the first bony fishes -- the lineage that would eventually include human beings.

"Unexpectedly, Acanthodes turns out to be the best view we have of conditions in the last common ancestor of bony fishes and sharks," said Michael Coates, PhD, professor of organismal biology and anatomy at the University of Chicago and senior author of the study published in Nature. "Our work is telling us that the earliest bony fishes looked pretty much like sharks, and not vice versa. What we might think of as shark space is, in fact, general modern jawed vertebrate space."

The group gnathostomes, meaning "jaw-mouths," includes tens of thousands of living vertebrate species, ranging from fish and sharks to birds, reptiles, mammals and humans. Cartilaginous fish, which today include sharks, rays, and ratfish, diverged from the bony fishes more than 420 million years ago. But little is known about what the last common ancestor of humans, manta rays and great white sharks looked like.

Coates and colleagues Samuel Davis and John Finarelli found answers to this mystery in an unexpected place: the acanthodians, extinct fishes that generally left behind only tiny scales and elaborate suites of fin spines. But armed with new data on what the earliest sharks and bony fishes looked like, Coates and colleagues re-examined fossils of Acanthodes bronni, the best-preserved acanthodian species.

Davis created highly detailed latex molds of specimens revealing the inside and outside of the skull, providing a valuable new data set for assessing cranial and jaw anatomy as well as the organizations of sensory, circulatory and respiratory systems in the species.

"We want to explore braincases if possible, because they are exceptionally rich sources of anatomical information," Coates said. "They're much better than scales, teeth or fin spines, which, on their own, tend to deliver a confusing signal of evolutionary relationships."

The analysis of the sample combined with recent CT scans of skulls from early sharks and bony fishes led the researchers to a surprising reassessment of what Acanthodes bronni tells us about the history of jawed vertebrates.

"For the first time, we could look inside the head of Acanthodes, and describe it within this whole new context," Coates said. "The more we looked at it, the more similarities we found with sharks."

However, analysis of the evolutionary relationships of Acanthodes bronni -- even with these new data added -- still connected this species to early bony fishes. Meanwhile, some acanthodian species turned out to be primitive sharks, while others were relatives of the common ancestor of sharks and bony fishes.

This result explains some of the longstanding confusion about the placement of acanthodians in vertebrate history. But additional analyses went a step further. Using more than 100 morphological characters, the researchers quantified the mutual resemblance among the earliest jawed fishes. Acanthodians as a whole, including the earliest members of humans' own deep evolutionary past, appear to cluster with ancient sharks.

"The common ancestors of all jawed vertebrates today organized their heads in a way that resembled sharks," said Finarelli, PhD, Lecturer in Vertebrate Biology at University College Dublin. "Given what we now know about the interrelatedness of early fishes, these results tell us that while sharks retained these features, bony fishes moved away from such conditions."

Furthermore, the analysis demonstrated that all of these early members of the modern gnathostomes are clearly separated from what now appear to be the most primitive vertebrates with jaws: a collection of armored fishes called placoderms.

"There appears to be a fundamental distinction between the placoderms and all other vertebrates with jaws," Finarelli said.

This new revision of the lineage of early jawed vertebrates will allow paleontologists to dig into deeper mysteries, including how the body plan of these ancient species transformed over the transition from jawless to jawed fishes.

"It helps to answer the basic question of what's primitive about a shark." Coates said. "And, at last, we're getting a better handle on primitive conditions for jawed vertebrates as a whole."

"This study is an example of the power of phylogenetics combined with the comparative morphology of living and fossil organisms," said Maureen Kearney, program director in National Science Foundation's Division of Environmental Biology, which co-funded the research. "It shows us important evolutionary transitions in the history of life, providing a new window into the sequence of evolutionary changes during early vertebrate evolution."

The study, "Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes," will be published on June 14 by Nature. The research was also funded by the Natural Environment Research Council.


Lateral line system

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Lateral line system, also called lateralis system, a system of tactile sense organs, unique to aquatic vertebrates from cyclostome fishes (lampreys and hagfish) to amphibians, that serves to detect movements and pressure changes in the surrounding water. It is made up of a series of mechanoreceptors called neuromasts (lateral line organs) arranged in an interconnected network along the head and body. This network is typically arranged in rows however, neuromasts may also be organized singly. At its simplest, rows of neuromasts appear on the surface of the skin however, for most fishes, they lie embedded in the floor of mucus-filled structures called lateral line canals. These canals are placed just underneath the skin, and only the receptor portion of each neuromast extends into the canal. In amphibians the lateral line system occurs only in larval forms and in adult forms that are completely aquatic.

Neuromasts are made up of a cluster of sensory and support cells encapsulated within a jellylike sheath called the cupula. Each sensory cell, or hair cell, bears several small cilia, and each cilium may be stimulated by water movement or pressure from a single direction. The lateral line system allows the fish to determine the direction and rate of water movement. The fish can then gain a sense of its own movement, that of nearby predators or prey, and even the water displacement of stationary objects.

In sharks and rays, some neuromasts have been evolutionarily modified to become electroreceptors called ampullae of Lorenzini. These receptors are concentrated on the heads of sharks and can detect the minute electrical potentials generated by the muscle contractions of prey. Ampullae of Lorenzini can also detect Earth’s electromagnetic field, and sharks apparently use these electroreceptors for homing and migration.


This would work well as a table in human article, but doesn't really deserve it's own article. Fallsend 01:43, 7 November 2005 (UTC)

It's really already there with the taxobox, though this page does add to that by explaining what each of the sections means and what it relates to. It could be educational as an introduction to taxonomy and the evolutionary position of humans relative to other animals. This page could actually be expanded by incorporating a historical perspective on how humans were treated taxonomically in the past. It needs work, but it could be encyclopedic and worthwhile. If we keep it, it should be moved to Human taxonomy (lowercase t). --Aranae 06:29, 8 November 2005 (UTC)

Agree, even worse, this barely deserves to exist at all. There is no content, no value to this at all. So many unreferenced paragraphs, and only one reference in ref section. This should be a delete. Nothing here is not already in the Homo sapiens article. --Tallard (talk) 06:18, 28 June 2016 (UTC)

  • Systema: Naturae
    • Superdomain: Biota
      • Domain: Eucytota
        • Kingdom: Metazoa
          • Subkingdom: Eumetazoa
            • Branch: Bilateria
              • Grade: Deuterostomia
                • Infrakingdom: Chordonia
                  • Phylum: Chordata
                    • Subphylum: Vertebrata
                      • Infraphylum: Gnathostomata
                        • Superclass: Tetrapoda
                          • Clade: Amniota
                            • Subclade: Mammaliaformes
                              • Class: Mammalia
                                • Sublcass: Theriiformes
                                  • Infraclass: Holotheria
                                    • Superlegion: Trechnotheria
                                      • Legion: Cladotheria
                                        • Sublegion: Zatheria
                                          • Infralegion: Tribosphenida
                                            • Supercohort: Theria
                                              • Cohort: Placentalia
                                                • Magnorder: Epitheria
                                                  • Superorder: Preptotheria
                                                    • Grandorder: Archonta
                                                      • Order: Primates
                                                        • Suborder: Euprimates
                                                          • Infraorder: Haplorhini
                                                            • Parvorder: Anthropoidea
                                                              • Superfamily: Cercopithecoidea
                                                                • Family: Hominidae
                                                                  • Subfamily: Homininae
                                                                    • Tribe: Hominini
                                                                      • Subtribe: Hominina
                                                                        • Genus: Homo
                                                                          • Species: Sapiens
                                                                            • Subspecies: Sapiens
                                                                              • Variety: Cro-magnon
                                                                                • Form: (all the human forms (races) descanting from the Cro-magnon "strain" like hispanic, caucasian, semitic, etc. ) —The preceding unsigned comment was added by 87.3.168.101 (talk) 02:34, 10 February 2007 (UTC).

                                                                                Actually, I agree with that. Independently I got: Biota (All life) Magnodomain Cytota


                                                                                Are humans Gnathostomates? - Biology

                                                                                2 - pharynx with pouches or slits in wall (at least in the embryo)

                                                                                3 - dorsal, hollow nervous system

                                                                                Notochord = rod of living cells ventral to central nervous system & dorsal to alimentary canal

                                                                                • Head region - incorporated into floor of skull
                                                                                • Trunk & tail - surrounded by cartilaginous or bony vertebrate (except in Agnathans)
                                                                                • Fishes & amphibians - notochord persists the length of the trunk & tail but is constricted within the centrum of each vertebra
                                                                                • Reptiles, birds, & mammals - notochord almost disappears during development (e.g., remains as a pulpy nucleus in the vertebrae of mammals)
                                                                                • Protochordates - notochord remains as the chief axial skeleton
                                                                                • Agnathans - lateral neural cartilages are located on notochord lateral to the spinal cord
                                                                                  • permanent slits - adults that live in water & breathe via gills
                                                                                  • temporary slits - adults live on land

                                                                                  • fishes that occurred in the late Cambrian period (see The Cambrian Explosion) through the Devonian (about 400 - 525 million years before present)
                                                                                  • had bony plates and scales (&, therefore, were easily fossilized)
                                                                                  • jawless vertebrates called 'armored fishes'
                                                                                  • Myllokunmingia fengjiaoa (pictured below) & Haikouichthys ercaicunensis- primitive fish that have many similarities to living hagfishes and are the oldest vertebrates (530 mybf) ever found.
                                                                                  • Cathaymyrus diadexus(literally the 'Chinese eel of good fortune') is not the fossil of an eel. At just 5 cm long, but 535 m.y. old, it is the earliest known chordate (fossil shown below for a 'reconstruction' check http://www.gs-rc.org/GOODS/GOOD_3e.HTM). Researchers think that Cathaymyrus is a fossil relative of modern lancelets (amphioxus).

                                                                                  Non-vertebrate chordates still alive today include tunicates (or sea squirts urochordates) & amphioxus (or branchiostoma). (cephalochordates)

                                                                                  2 - pharyngeal pouches or slits

                                                                                  3 - dorsal, hollow nervous system

                                                                                    Chordate 'ancestor' of vertebrates:
                                                                                    • sessile (like adult tunicates)
                                                                                    • tail evolved as adaptation in larvae to increase mobility
                                                                                    • 'higher forms' - came about by retention of tail (neoteny)
                                                                                    • notochord is confined to the tail
                                                                                    • notochord is lost during metamorphosis into sessile adult
                                                                                    • possess pharyngeal slits

                                                                                    A 530 million-year-old (although perhaps as old as 560 million years) creature, Cheungkongella ancestralis, probably a tunicate, found in the Chengjiang fauna in China's southwest Yunnan Province, might be the earliest known fossil evidence of primitive chordates (Shu, D.-G., L. Chen, J. Han, X.-L. Zhang. 2001. An early Cambrian tunicate from China. Nature 411:472 - 473.)

                                                                                      Subphylum Cephalochordata= Amphioxus (or Branchiostoma)

                                                                                    • notochord
                                                                                    • dorsal, hollow nervous system
                                                                                    • pharyngeal gill slits
                                                                                    • 'circulatory' system - vertebrate pattern with 'pumping vessels' (but no heart)

                                                                                      1 - a dorsal, hollow nervous system

                                                                                    • semicircular canals
                                                                                      • agnathans have 1 or 2
                                                                                      • gnathostomes have 3
                                                                                      • agnathans have none
                                                                                      • gnathostomes do
                                                                                      • agnathans have none
                                                                                      • gnathostomes do

                                                                                      6 - Petromyzontia (lampreys)

                                                                                        1 - extinct Paleozoic (Cambrian to Devonian) jawless fish with an external skeleton of bone ('bony armor')
                                                                                        2 - oldest known vertebrates
                                                                                        3 - many had flattened appearance (some may have been bottom-dwellers)

                                                                                        Acanthodians:
                                                                                          1 - earliest known gnathostomes (Silurian about 440 mybp)

                                                                                        2 - probably related to modern bony fishes

                                                                                        3 - small (less than 20 cm long) with large eyes

                                                                                        The relationships of acanthodians to other vertebrates has been the subject of considerable debate. Early researchers considered them to be most closely related to the ray-finned fishes, but most scientists during the mid-20th Century considered acanthodians to have a closer affinity to the sharks. Opinion has now generally swung back in favor of a closer relationship with ray-fins, but this is far from universally accepted.

                                                                                          1 - Silurian (about 420 million years before present)

                                                                                        2 - probably off the main line of vertebrate evolution

                                                                                        3 - many had bony dermal shields

                                                                                          1 - ancestors had bony skeletons so cartilaginous skeleton is specialized

                                                                                        2 - pelvic fins of males are modified as claspers


                                                                                          Subclass Elasmobranchii - most common cartilaginous fishes
                                                                                            O. Cladoselachii - primitive sharks (300-400 mybp)

                                                                                          O. Selachii - 'modern' sharks


                                                                                            Elasmobranchs:
                                                                                              1 - 1st pharyngeal slit modified as a spiracle

                                                                                            2 - naked gill slits (no operculum)

                                                                                                Subclass Holocephali
                                                                                                  O. Chimaeriformes (photo & drawing)
                                                                                                  • marine
                                                                                                  • gill slits have a fleshy operculum & the spiracle is closed
                                                                                                  • few scales
                                                                                                  • common ancestor with sharks but an independent line

                                                                                                    1 - skeleton is partly or chiefly bone

                                                                                                  2 - gill slits are covered by a bony operculum

                                                                                                  3 - skin has scales with, typically, little bone

                                                                                                  4 - most have a swim bladder


                                                                                                    Subclass Actinopterygii - ray-fins
                                                                                                      Superorder Chondrostei
                                                                                                      • most primitive ray-fins
                                                                                                      • chiefly Paleozoic (300-400 mybp)
                                                                                                      • include present-day Sturgeons & Paddlefish (below)
                                                                                                            Superorder Neopterygii
                                                                                                              Order Semionotiformes
                                                                                                              • dominant Mesozoic fishes
                                                                                                              • possess ganoid scales
                                                                                                              • two extant genera:
                                                                                                                • Lepidosteus - predatory includes present-day gars
                                                                                                                • Amia - includes present-day bowfins (or dogfish)
                                                                                                                • recent bony fishes
                                                                                                                • 95% of all living fish
                                                                                                                • about 40 living orders
                                                                                                                • well-ossified skeleton
                                                                                                                • cycloid & ctenoid scales (flexible & overlapping)
                                                                                                                • pelvic fins often located far forward
                                                                                                                • no spiracle


                                                                                                                        3 - skull similar to that of early amphibians

                                                                                                                            O. Dipnoi - lungfish (3 living genera Africa, Australia, & South America)
                                                                                                                            • African & South American species have inefficient gills & will drown if held under water
                                                                                                                            • Australian species (Neoceratodus spp.) relies on gills unless oxygen content of water is too low

                                                                                                                              Oldest known = subclass Labyrinthodontia
                                                                                                                                Fish-like features:

                                                                                                                                  1- small bony scales in the skin

                                                                                                                                2- fin-rays in the tail (for swimming)

                                                                                                                                3- a skull similar to that of some Crossopterygians

                                                                                                                                Labyrinthodonts are distinguished by deeply folded structure of enamel and dentine layers in the teeth, that look like an intricate labyrinth in the cross section, hence the name of this group. Labyrinthodonts were probably similar to fishes in their mode of living. Labyrinthodonts, like fishes and most modern amphibians, laid eggs in the water, where their larvae developed into mature animals. All labyrinthodonts had special sense organs in the skin, that formed a system for perception of water fluctuations. Moreover, some of them possessed well developed gills. In contrast, many labyrinthodonts seemingly had primitive lungs. They could breath atmospheric air, that was a great advantage for residents of warm shoals with low oxygen levels in the water. The air was inflated into the lungs by contractions of a special throat sac. Primitive members of all labyrinthodont groups were probably true water predators, and only advanced forms that arose independently in different groups and times, gained an amphibious, semi-aquatic mode of living. Mature individuals of advanced labyrinthodonts could live on land, feeding mainly on insects and other small invertebrates. Well ossified robust skeletons in some Late Carboniferous and Early Permian labyrinthodonts prove their adaptation to the terrestrial mode of life. It suggests that amphibians had successfully 'organized' actual terrestrial assemblages prior to the wide expansion of reptiles.

                                                                                                                                The most diverse group of the labyrinthodonts was the batrachomorphs ('similar to a frog'). Though these animals looked more like crocodiles, they most probably gave rise to the order Anura, the amphibians without tails, which include, in particular, the modern frogs. Batrachomorphs appeared in the Late Devonian, but they had worldwide distribution in the continental shallow basins of the Permian (Platyoposaurus, Melosaurus) and Triassic Periods (Thoosuchus, Benthosuchus, Eryosuchus). Some batrachomorphs existed until the end of the Cretaceous.

                                                                                                                                  Subclass Lepospondyli
                                                                                                                                  • ancestry uncertain due to lack of fossil evidence
                                                                                                                                  • probably on a 'side branch' of vertebrate evolution
                                                                                                                                    Subclass Lissamphibia - modern amphibians
                                                                                                                                      O. Anura - frogs & toads
                                                                                                                                      O. Urodela - tailed amphibians
                                                                                                                                      O. Gymnophiona (apodans) - wormlike, burrowing amphibians
                                                                                                                • 1 - aquatic larval stage with external gills

                                                                                                                2 - middle ear cavity with ear ossicle (columella)

                                                                                                                  Reptile Subclasses:
                                                                                                                    1 - Anapsida
                                                                                                                      O. Cotylosauria - stem reptiles
                                                                                                                      O. Chelonia - turtles & tortoises
                                                                                                                      • unchanged for about 175 million years
                                                                                                                      • identified by bony dermal plates to which ribs & trunk vertebrae are fused
                                                                                                                        O. Rhynchocephalia (Sphenodonta) - only living representative is the Tuatara
                                                                                                                        O. Squamata - lizards, geckos, & snakes
                                                                                      • O. Thecodontia - stem archosaurs
                                                                                        O. Pterosauria (check this short video & this one)
                                                                                        O. Saurischia - 2 major groups: sauropods & theropods (check this short video)
                                                                                        O. Ornithischia (like Iguanodon)
                                                                                        O. Crocodilia
                                                                                  • 5 - Synapsida
                                                                                      O. Pelycosauria - first stage in evolution to mammals
                                                                                      O. Therapsida
                                                                            • Saurischia (sawr-RIS-kee-ah) & Ornithischia are the two orders of dinosaurs, with the division based on the shape of the pelvic bone. The saurischian pubis (left) juts forward, and its ischium points backward. The ornithischian pubis and ischium (right) both point backward. The ornithischians were all herbivorous, and included some of the most interesting-looking dinosaurs. Ornithischian dinosaurs include three suborders: Ornithopoda, Marginocephalia and Thyreophora. The famous carnivorous dinosaurs were from the saurischian order, as were the largest herbivorous dinosaurs. The saurischian dinosaurs include two suborders: Theropoda and Sauropodomorpha.

                                                                              The first vertebrates to evolve true flight were the pterosaurs, flying archosaurian reptiles. After the discovery of pterosaur fossils in the 18th century, it was thought that pterosaurs were a failed experiment in flight a humorous mishap or that they were simply gliders, too weak to fly. More recent studies have revealed that pterosaurs were definitely proficient flyers, and were no evolutionary failure as a group they lasted about 140 million years (about as long as birds have)! Pterosaurs are thought to be derived from a bipedal, cursorial (running) archosaur in the late Triassic period (about 225 million years ago). No other phylogenetic hypothesis has withstood examination however, the early history of pterosaurs is not yet fully understood because of their poor fossil record in the Triassic period. We can infer that the origin of flight in pterosaurs fits the "ground up" evolutionary scenario, supported by the fact that pterosaurs had no evident arboreal adaptations.

                                                                              The pterosaur wing was supported by an elongated fourth digit (imagine having a "pinky finger" several feet long, and using that to fly!). Pterosaurs had other morphological adaptations for flight as a keeled sternum for the attachment of flight muscles, a short and stout humerus (the first arm bone), and hollow but strong limb and skull bones. Pterosaurs also had modified scales that were wing-supporting fibers, and that possibly formed hairlike structures to provide insulation -- bird feathers are analogous to the wing fibers of pterosaurs, and both are thought to possibly have been evolved originally for the primary purpose of thermoregulation (which implies, but does not prove, that both pterosaurs and the earliest birds were endothermic).

                                                                              Early pterosaurs (such as Dimorphodon) had long tails that assisted balance, but later pterosaurs had no tails, and may have been more adept flyers. The most derived pterosaurs, such as Pteranodon and Quetzalcoatlus, were so large that soaring was the only feasible option these were the largest flyers ever to cast a shadow on the Earth's surface.

                                                                              • Synapsid type = mammal-like reptiles
                                                                              • Anapsid type = stem reptiles & turtles
                                                                              • Diapsid type = rhynchocephalians, lizards, & snakes
                                                                              • Euryapsid type = extinct plesiosaurs.

                                                                              Temporal fenestration has long been used to classify amniotes. Taxa such as Anapsida, Diapsida, Euryapsida, and Synapsida were named after their type of temporal fenestration. Temporal fenestra are large holes in the side of the skull. The function of these holes has long been debated. Many believe that they allow muscles to expand and to lengthen. The resulting greater bulk of muscles results in a stronger jaw musculature, and the longer muscle fibers allow an increase in the gape.

                                                                              2 - lost several dinosaur characteristics (e.g., long tail & teeth) but retained others (e.g., claws, scales, diapsid skull, single occipital condyle &, perhaps, feathers) (see AMNH website & ABC News website)

                                                                                Genera: Archaeopteryx & Archaeornis
                                                                                  Characteristics:
                                                                                    1 - solid bones


                                                                                  Myelin sheaths are formed with proteins that originated in vertebrate lineages

                                                                                  All vertebrate nervous systems, except those of agnathans, make extensive use of the myelinated fiber, a structure formed by coordinated interplay between neuronal axons and glial cells. Myelinated fibers, by enhancing the speed and efficiency of nerve cell communication allowed gnathostomes to evolve extensively, forming a broad range of diverse lifestyles in most habitable environments. The axon-covering myelin sheaths are structurally and biochemically novel as they contain high portions of lipid and a few prominent low molecular weight proteins often considered unique to myelin. Here we searched genome and EST databases to identify orthologs and paralogs of the following myelin-related proteins: (1) myelin basic protein (MBP), (2) myelin protein zero (MPZ, formerly P0), (3) proteolipid protein (PLP1, formerly PLP), (4) peripheral myelin protein-2 (PMP2, formerly P2), (5) peripheral myelin protein-22 (PMP22) and (6) stathmin-1 (STMN1). Although widely distributed in gnathostome/vertebrate genomes, neither MBP nor MPZ are present in any of nine invertebrate genomes examined. PLP1, which replaced MPZ in tetrapod CNS myelin sheaths, includes a novel 'tetrapod-specific' exon (see also Möbius et al., 2009). Like PLP1, PMP2 first appears in tetrapods and like PLP1 its origins can be traced to invertebrate paralogs. PMP22, with origins in agnathans, and STMN1 with origins in protostomes, existed well before the evolution of gnathostomes. The coordinated appearance of MBP and MPZ with myelin sheaths and of PLP1 with tetrapod CNS myelin suggests interdependence - new proteins giving rise to novel vertebrate structures.

                                                                                  Figures

                                                                                  Fig. 1. Electron micrographs of rodent (A)…

                                                                                  Fig. 1. Electron micrographs of rodent (A) PNS and (B) CNS myelin sheaths (originals kindly…

                                                                                  Compact myelin (bordered by red bands in PNS and by blue bands in CNS) is surrounded by glial cell cytoplasm (*), containing organelles where myelin-destined lipids and proteins are synthesized. For peripheral nerves, a supportive basement membrane (arrowhead) is also present and functions to stabilize the fibers and signal myelination (Jessen and Mirsky, 2005). Regions of compact myelin, where MBP, MPZ and PLP1 reside, are separated from regions of non-compacted myelin where MBP, MPZ and PLP1 are made, by tight junctions that form inner and outer mesaxons (arrows). Of many proteins synthesized in glial cell cytoplasm, few accumulate in and help structure myelin. A Schmidt–Lanterman incisure, with cytoplasm included between layers is common in MPZ-based though not PLP1-based myelin ( ).

                                                                                  Phylogenetic tree constructed with the…

                                                                                  Phylogenetic tree constructed with the UniTree program (Methods) with murine MBP (NP_001020426.1) as…

                                                                                  Pair-wise amino acid identity comparisons…

                                                                                  Pair-wise amino acid identity comparisons of all full-length sequences (see supplementary Tables 1A…

                                                                                  Fig. 4. Alignments of MBP sequences (isoform…

                                                                                  Fig. 4. Alignments of MBP sequences (isoform 3, contains all but exon II) from species…

                                                                                  Accession numbers are listed in supplementary Tables 1A and 1B online. Alignments are made, exon-by-exon with Clustal W (Methods), merged into a single alignment, and finally modified by eye. Comparisons among all listed species, excepting human use white letters on a black background ( ) for complete identity, white letters on gray background for blocks of identity (>2) and conserved substitutions ( ) the background shading is set to the number of sequences with the same amino acid – darker background are for greater number of sequences. For simplicity, conserved substitutions are considered the same as identical amino acids. Exon boundaries are marked (θ) with exon numbers above the human sequence. Positions of all basic (K, R) amino acids are shown with coloring indicating complete identity ( ) and ( ) conservation and presence in sequences of one or two species (*). Sixty-nine HMM logo (PFAM) amino acids were selected based on individual or conserved substitution contributions >1.0 and shown with white lettering on orange backgrounds. As above, darker background shading indicates greater sequence identity. Letters and basic residue designations are placed between human and murine sequences. A single arginine methylation site is shown below the human sequence (M). A portion of exon III, absent in the common zebrafish 88 amino acid isoform is indicated [ ] and assigned IIIA (see supplementary Table 3 online).

                                                                                  Fig. 5. Phylogenetic tree constructed with the…

                                                                                  Fig. 5. Phylogenetic tree constructed with the Unitree program, with murine MPZ (NP_032649) as ‘BAIT’…

                                                                                  Fig. 6. Alignment of MPZ sequences from…

                                                                                  Fig. 6. Alignment of MPZ sequences from species representing different vertebrate taxa


                                                                                  Oral apparatus and the mandibular arch: heterotopy and loss of homology

                                                                                  According to Wagner & Müller (2002), an 𠆎volutionary novelty’ can be defined as a new structure that arises by overriding ancestral developmental constraints, so that morphological homology is lost between the novel and ancestral structures. Thus, the gnathostome jaw could be counted as an evolutionary novelty, as discussed below. In the following discussion, we have to bear in mind that the term ‘mandibular arch’ universally refers to an identical developmental unit among vertebrates (morphologically homologous throughout vertebrates), whereas the ‘oral apparatus’ or ‘oral region’ may differentiate from different regions of the embryonic head in each animal group.

                                                                                  Although the classical transformation theory of the jaw predicts the initially identical, undifferentiated pharyngeal arches, the cephalic crest cells (ectomesenchyme) never simply form single divided cell streams each filling a single PA. Instead, in all the vertebrate embryos observed, there are three distinct crest cell populations, and the most rostral one not only populates the MA, but also expands rostrally to the entire pharynx (Noden, 1988 Osumi-Yamashita et al. 1994 Kuratani, 1997 Graham, 2001 Kuratani et al. 2001 Graham et al. 2004 reviewed by Horigome et al. 1999, and Kuratani et al. 2001 Figs 4 and ​ and5 5 top). At the early pharyngula stage, when cephalic crest cells cease emigration, the distribution pattern of the crest cells is very similar between gnathostome and the lamprey – the cephalic crest cells form three major streams of migration, and the most rostral cell population (trigeminal crest cells) is distributed in the MA as well as in the premandibular (PM) region ( Fig. 5A Horigome et al. 1999 Shigetani et al. 2000). Here the term ‘premandibular’ does not imply the presence of a ‘premandibular arch’ once assumed by comparative morphologists, but simply indicates the position rostral to the MA. [In this review, the premandibular crest cells are defined as those crest cells that surround the premandibular mesoderm, as opposed to the MA crest cells that surround the mandibular mesoderm. Owing to the absence of the pharyngeal pouch that normally limits the MA rostrally, only the mesodermal component can be used as a landmark. This terminology inevitably relates to the classical concept of head segmentation in vertebrates, which used to be based primarily on the segments in the head mesoderm (see de Beer, 1937 Jarvik, 1980 Jefferies, 1986). However, as has been discussed previously (Kuratani et al. 1999), neither lamprey nor gnathostome embryos show any sign of segmentation in the cephalic mesoderm, except for the late-forming premandibular mesoderm that is more or less separated from the rest of the cell population, and the lateral part of the head mesoderm, which is partially and secondarily divided by the growth of pharyngeal pouches. Thus, primary mesodermal segmentation is not assumed in the vertebrate head in this review. For more discussion on head segmentation and metamerism, see Kuratani (2003). The term ‘trigeminal crest cells’ stems from the fact that the distribution of these cells corresponds to the peripheral distribution patterns of the trigeminal nerve in the later embryo (see Kuratani, 1997 Kuratani et al. 2001 Fig. 6 ). The crest cells in the PM region are further subdivided into preoptic and postoptic cell populations ( Figs 4 , ​ ,5A 5A and ​ and6 6 ).

                                                                                  Comparison of mesenchymal developmental patterning. In the lamprey and gnathostome embryonic heads, shared patterns of mesodermal (yellow) and ectomesenchymal (green) distribution can be detected. The endodermal derivatives are coloured pink. The crest cells rostral to the first pharyngeal pouch (pp1) are collectively called the trigeminal crest cells that can be divided into those in the mandibular arch (mandibular crest cells MC) and the premandibular crest cells (PMC). The posterior region comprising the mandibular crest cells surrounds the premandibular mesoderm (pmm) that arises rostral to the tip of the notochord (n). Different subsets of the trigeminal crest cells are patterned on this shared pattern on mesenchymal distribution, as the oral apparatus in each animal groups arises through different distribution of growth factors such as FGF8 and BMP4 that define the mouth (shown by blue bars). In the lamprey, the upper lip arises from PMC and the lower lip from MC, whereas in gnathostomes both the upper and the lower jaw arise from MC, and the PMC differentiate into the prechordal cranial elements. Blue arrows indicate the position of mouth openings. Abbreviations: Ba1𠄲, branchial arches di, diencephalon e, eye hm, hyoid mesoderm ph, pharynx pp1𠄳, pharyngeal pouches pog, preoral gut.

                                                                                  Comparisons of equivalent ectomesenchymal regions between the lamprey and gnathostomes. Terminology for the crest cell populations and subpopulations is based on the topographical distribution of the crest cells with respect to the other embryonic structures such as mesoderm and pharyngeal pouches, not in terms of their developmental fates. This division defines the morphological homology of ectomesenchymal portions, which can be applied throughout vertebrates, laid down at the early pharyngula stages (see Figs 4 and ​ and5). 5 ). Note, however, that different portions of ectomesenchyme are utilized to differentiate into the ‘oral apparatus’ in the lamprey and gnathostomes (shaded). Because of this heterotopic shift in tissue interactions, the ectomesenchymal part with the same name in the lamprey and gnathostomes do not always differentiate into the same skeletal elements (see Kuratani et al. 2001 Shigetani et al. 2002). For the homology of the cartilages called ‘trabeculae’ between the two animal groups, see Kuratani et al. (2004).

                                                                                  Importantly, at an early stage of pharyngular development, there is no clear boundary between the PM and MA regions (Kuratani et al. 1999 Shigetani et al. 2000, 2002), and the oral apparatus is formed in different ways between gnathostomes and the lamprey (it is composed of upper and lower jaws in the gnathostomes of upper and lower lips in the ammocoete larva of the lamprey). Each domain of the trigeminal ectomesenchyme can be identified by its topographical relationships with the mesodermal components (reviewed by Kuratani et al. 2001 Figs 4 – 6 ). The MA crest cells surround the mandibular mesoderm and the postoptic part of the PM crest cells surrounds the premandibular mesoderm (primordia of the extrinsic eye muscles in gnathostomes Koltzoff, 1901 see Boorman & Shimeld, 2002, for a case of specific gene expression in the premandibular mesoderm of lamprey embryos and see Kuratani et al. 1999, for comparative embryology of this mesoderm). Interestingly, both the upper and the lower jaws develop from the MA crest cells, whereas in the lamprey the MA differentiates only into the velum and the lower lip, and the upper lip is derived from PM crest cells (Kuratani et al. 2001 Shigetani et al. 2002). Based on the basic architecture of the embryo therefore the term ‘mandibular arch’ should be reserved for the specific PA found in the same relative position in the embryo, as a developmental unit that is specifically and topographically associated with the mandibular mesoderm, irrespective of its fate in later developmental stages, which can differ in each animal lineage. Thus, in gnathostomes, the oral apparatus called the ‘jaw’ develops from the MA, whereas the lamprey oral region also requires the PM region as an embryonic material ( Figs 4 – 6 ). The term ‘oral apparatus’, on the other hand, implies only a functional resemblance of structures, which can be derived from varied sets of embryonic tissues. It is important to note that a gene expression pattern is not always associated with a homologous set of cell populations, as is discussed below.

                                                                                  In the amniote MA, BMP4 in the distal ectoderm induces the expression of the target gene, Msx1, in the underlying ectomesenchyme, and the proximally distributed FGF8 induces Dlx1 expression in the proximal ectomesenchyme (Barlow & Francis-West, 1997 Neubüser et al. 1997 Tucker et al. 1998 Shigetani et al. 2000). Shigetani et al. (2002) have also found that molecules involved in the proximodistal patterning of the gnathostome jaw are apparently used in similar patterning of the upper and lower lips of the lamprey larva, as if lips and jaws were homologous to each other ( Fig. 5B ). Namely, LjFgf8/17 (the lamprey cognate of Fgf8) and LjDlx1/6 (the lamprey cognate of Dlx1) are expressed widely in the oral ectoderm and mesenchyme, respectively. Moreover, LjBmp2/4a (the lamprey cognate of Bmp4) and LjMsxA (the lamprey cognate of Msx1) are expressed in the tips of lips, respectively ( Fig. 5B also see Shigetani et al. 2002). As the functions of ectomesenchymally expressed homeobox genes are most prominent in the MA-derivatives in gnathostomes (Satokata & Maas, 1994 Martin et al. 1995 Qiu et al. 1995, 1997 Yamada et al. 1995 reviewed by Hall, 1998), and apparently masked by the Hox gene expression in HA and posterior PAs (Mallo & Gridley, 1996), the Hox-free default state of the lamprey MA is again consistent with the apparently similar functions of these gene cognates in the lamprey (proximodistal specification in oral patterning). In the above comparison, homologous sets of genes are not expressed in homologous embryonic materials, but rather are associated with functionally similar structures, namely the oral apparatus (see below for a similar discussion on adenohypophysial differentiation). To resume the agnathan mode of Dlx1 gene expression, which denotes the oral and pharyngeal region in the ectomesenchyme, the domain of the upstream factor, FGF8, has to be expanded in the gnathostome embryo. Actually, implantation of an FGF8-soaked bead into the PM region mimics the lamprey-type Dlx1 expression in the chick embryo (Shigetani et al. 2000, 2002).

                                                                                  These experimental results imply that epithelial–mesenchymal interactions have been topographically shifted in the transition from the lamprey-like agnathan to the gnathostome states, based on a shared pattern of embryonic tissues. As a result, morphological homology was apparently lost between the lamprey and gnathostome oral apparatus expression patterns of orthologous genes are not associated with morphologically equivalent cell populations. This evolutionary implication is curious: even if these genes always functioned in defining the oral apparatus, their regulation does not seem to be restricted to the same (homologous) embryonic component during this transition (see Manzanares & Nieto, 2003, for a similar discussion on gene usage). Such a situation simultaneously implies that the invention of the jaw deserves to be understood in the context of evolutionary novelty as defined by Wagner & Müller (2002): because the newly acquired pattern is not homologous with the ancestral pattern, the former was brought about by overriding ancestral developmental constraints, not simply modifying it for adaptation.

                                                                                  Alternatively, it is also possible that lips and jaws are homologous, derived from homologous cell populations with homologous gene expressions. In this case, however, morphological identities of crest cell populations cannot rely on the mesodermal components that are shared in vertebrates (Kuratani et al. 1999). Furthermore, the developmental nature of the lamprey trabecula, the premandibular cartilage, does present a conundrum and cannot be explained using this consideration, as will be discussed below.


                                                                                  Class Chondrichthyes: Cartilaginous Fishes

                                                                                  The class Chondrichthyes (about 1,000 species) is a morphologically diverse clade, consisting of subclass Elasmobranchii (sharks – Figure 2), rays, and skates, together with the obscure and critically endangered sawfishes), and a few dozen species of fishes called chimaeras, or “ghost sharks” in the subclass Holocephali. Chondrichthyes are jawed fishes that possess paired fins and a skeleton made of cartilage. This clade arose approximately 370 million years ago in the early or middle Devonian. They are thought to be descended from the placoderms, which had endoskeletons made of bone thus, the lighter cartilaginous skeleton of Chondrichthyes is a secondarily derived evolutionary development. Parts of shark skeleton are strengthened by granules of calcium carbonate, but this is not the same as bone.

                                                                                  Figure 2. Hammerhead sharks tend to school during the day and hunt prey at night. (credit: Masashi Sugawara)

                                                                                  Most cartilaginous fishes live in marine habitats, with a few species living in fresh water for a part or all of their lives. Most sharks are carnivores that feed on live prey, either swallowing it whole or using their jaws and teeth to tear it into smaller pieces. Sharks have abrasive skin covered with tooth-like scales called placoid scales. Shark teeth probably evolved from rows of these scales lining the mouth.

                                                                                  A few species of sharks and rays, like the enormous whale shark (Figure 3), are suspension feeders that feed on plankton. The sawfishes have an extended rostrum that looks like a double-edged saw. The rostrum is covered with electrosensitive pores that allow the sawfish to detect slight movements of prey hiding in the muddy sea floor. The teeth in the rostrum are actually modified tooth-like structures called denticles, similar to scales.

                                                                                  Figure 3. Whale shark in the Georgia Aquarium. Whale sharks are filter-feeders and can grow to be over 10 meters long. Whale sharks, like most other sharks, are ovoviviparous. (credit: modified from Zac Wolf [Own work] [CC BY-SA 2.5 (http://creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons)

                                                                                  Sharks have well-developed sense organs that aid them in locating prey, including a keen sense of smell and the ability to detect electromagnetic fields. Electroreceptors called ampullae of Lorenzini allow sharks to detect the electromagnetic fields that are produced by all living things, including their prey. (Electroreception has only been observed in aquatic or amphibious animals and sharks have perhaps the most sensitive electroreceptors of any animal.) Sharks, together with most fishes and aquatic and larval amphibians, also have a row of sensory structures called the lateral line , which is used to detect movement and vibration in the surrounding water, and is often considered to be functionally similar to the sense of “hearing” in terrestrial vertebrates. The lateral line is visible as a darker stripe that runs along the length of a fish’s body. Sharks have no mechanism for maintaining neutral buoyancy and must swim continuously to stay suspended in the water. Some must also swim in order to ventilate their gills but others have muscular pumps in their mouths to keep water flowing over the gills.

                                                                                  Sharks reproduce sexually, and eggs are fertilized internally. Most species are ovoviviparous: The fertilized egg is retained in the oviduct of the mother’s body and the embryo is nourished by the egg yolk. The eggs hatch in the uterus, and young are born alive and fully functional.

                                                                                  Figure 4. Shark egg cases. Shark embryos are clearly visible through these transparent egg cases. The round structure is the yolk that nourishes the growing embryo. (credit: Jek Bacarisas)

                                                                                  Some species of sharks are oviparous: They lay eggs that hatch outside of the mother’s body. Embryos are protected by a shark egg case or “mermaid’s purse” (Figure 4) that has the consistency of leather. The shark egg case has tentacles that snag in seaweed and give the newborn shark cover. A few species of sharks, e.g., tiger sharks and hammerheads, are viviparous: the yolk sac that initially contains the egg yolk and transfers its nutrients to the growing embryo becomes attached to the oviduct of the female, and nutrients are transferred directly from the mother to the growing embryo. In both viviparous and ovoviviparous sharks, gas exchange uses this yolk sac transport.

                                                                                  In general, the Chondrichthyes have a fusiform or dorsoventrally flattened body, a heterocercal caudal fin or tail (unequally sized fin lobes, with the tail vertebrae extending into the larger upper lobe) paired pectoral and pelvic fins (in males these may be modified as claspers), exposed gill slits (elasmobranch), and an intestine with a spiral valve that condenses the length of the intestine. They also have three pairs of semicircular canals, and excellent senses of smell, vibration, vision, and electroreception. A very large lobed liver produces squalene oil (a lightweight biochemical precursor to steroids) that serves to aid in buoyancy (because with a specific gravity of 0.855, it is lighter than that of water).

                                                                                  Rays and skates comprise more than 500 species. They are closely related to sharks but can be distinguished from sharks by their flattened bodies, pectoral fins that are enlarged and fused to the head, and gill slits on their ventral surface (Figure 5). Like sharks, rays and skates have a cartilaginous skeleton. Most species are marine and live on the sea floor, with nearly a worldwide distribution.

                                                                                  Figure 5. Cartilaginous fish. (a) Stingray. This stingray blends into the sandy bottom of the ocean floor. A spotted ratfish (b) Hydrolagus colliei credit a “Sailn1″/Flickr (credit: a “Sailn1″/Flickr b: Linda Snook / MBNMS [Public domain], via Wikimedia Commons.)

                                                                                  Unlike the stereotypical sharks and rays, the Holocephali (chimaeras or ratfish) have a diphycercal tail (equally sized fin lobes, with the tail vertebrae located between them), lack scales (lost secondarily in evolution), and have teeth modified as grinding plates that are used to feed on mollusks and other invertebrates (Figure 5b). Unlike sharks with elasmobranch or naked gills, chimaeras have four pairs of gills covered by an operculum. Many species have a pearly iridescence and are extremely pretty.


                                                                                  Affiliations

                                                                                  Marine Biology Research Division, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, 92093-0202, USA

                                                                                  Department of Organismal Biology, Uppsala University, 75236, Uppsala, Sweden

                                                                                  School of Natural Sciences, University of California Merced, Merced, CA, 95343, USA

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                                                                                  Contributions

                                                                                  LZH wrote the manuscript. DOD edited the manuscript. Both authors read and approved the final manuscript.

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