Vertebrate

Vertebrate
Temporal range:
Cambrian Stage 3Present,[1]
Vertebrata 002.png
Example of vertebrates: Acipenser oxyrinchus (Actinopterygii), an African bush elephant (Tetrapoda), a Tiger shark (Chondrichthyes) and a River lamprey (Agnatha).
Scientific classification e
Kingdom:Animalia
Superphylum:Deuterostomia
Phylum:Chordata
Clade:Olfactores
Subphylum:Vertebrata
J-B. Lamarck, 1801[3]
Simplified grouping (see text)
Synonyms

Ossea Batsch, 1788[3]

Vertebrates (/ˈvɜːrtɪbrəts/) comprise all species of animals within the subphylum Vertebrata (/vɜːrtɪˈbrtə/) (chordates with backbones). Vertebrates represent the overwhelming majority of the phylum Chordata, with currently about 69,963 species described.[4] Vertebrates comprise such groups as the following:

Extant vertebrates range in size from the frog species Paedophryne amauensis, at as little as 7.7 mm (0.30 in), to the blue whale, at up to 33 m (108 ft). Vertebrates make up less than five percent of all described animal species; the rest are invertebrates, which lack vertebral columns.

The vertebrates traditionally include the hagfish, which do not have proper vertebrae due to their loss in evolution,[5] though their closest living relatives, the lampreys, do.[6] Hagfish do, however, possess a cranium. For this reason, the vertebrate subphylum is sometimes referred to as "Craniata" when discussing morphology. Molecular analysis since 1992 has suggested that hagfish are most closely related to lampreys,[7] and so also are vertebrates in a monophyletic sense. Others consider them a sister group of vertebrates in the common taxon of craniata.[8]

The populations of vertebrates have dropped in the past 50 years.[9]

Etymology

The word vertebrate derives from the Latin word vertebratus (Pliny), meaning joint of the spine.[10]

Vertebrate is derived from the word vertebra, which refers to any of the bones or segments of the spinal column.[11]

Anatomy and morphology

All vertebrates are built along the basic chordate body plan: a stiff rod running through the length of the animal (vertebral column and/or notochord),[12] with a hollow tube of nervous tissue (the spinal cord) above it and the gastrointestinal tract below.

In all vertebrates, the mouth is found at, or right below, the anterior end of the animal, while the anus opens to the exterior before the end of the body. The remaining part of the body continuing after the anus forms a tail with vertebrae and spinal cord, but no gut.[13]

Vertebral column

The defining characteristic of a vertebrate is the vertebral column, in which the notochord (a stiff rod of uniform composition) found in all chordates has been replaced by a segmented series of stiffer elements (vertebrae) separated by mobile joints (intervertebral discs, derived embryonically and evolutionarily from the notochord).

However, a few vertebrates have secondarily lost this anatomy, retaining the notochord into adulthood, such as the sturgeon[14] and coelacanth. Jawed vertebrates are typified by paired appendages (fins or legs, which may be secondarily lost), but this trait is not required in order for an animal to be a vertebrate.

© Raimond Spekking / CC BY-SA 4.0 (via Wikimedia Commons)
Fossilized skeleton (cast) of Diplodocus carnegii, showing an extreme example of the backbone that characterizes the vertebrates.

Gills

Gill arches bearing gills in a pike

All basal vertebrates breathe with gills. The gills are carried right behind the head, bordering the posterior margins of a series of openings from the pharynx to the exterior. Each gill is supported by a cartilagenous or bony gill arch.[15] The bony fish have three pairs of arches, cartilaginous fish have five to seven pairs, while the primitive jawless fish have seven. The vertebrate ancestor no doubt had more arches than this, as some of their chordate relatives have more than 50 pairs of gills.[13]

In amphibians and some primitive bony fishes, the larvae bear external gills, branching off from the gill arches.[16] These are reduced in adulthood, their function taken over by the gills proper in fishes and by lungs in most amphibians. Some amphibians retain the external larval gills in adulthood, the complex internal gill system as seen in fish apparently being irrevocably lost very early in the evolution of tetrapods.[17]

While the more derived vertebrates lack gills, the gill arches form during fetal development, and form the basis of essential structures such as jaws, the thyroid gland, the larynx, the columella (corresponding to the stapes in mammals) and, in mammals, the malleus and incus.[13]

Central nervous system

The central nervous system of vertebrates is based on a hollow nerve cord running along the length of the animal. Of particular importance and unique to vertebrates is the presence of neural crest cells. These are progenitors of stem cells, and critical to coordinating the functions of cellular components.[18] Neural crest cells migrate through the body from the nerve cord during development, and initiate the formation of neural ganglia and structures such as the jaws and skull.[19][20][21]

The vertebrates are the only chordate group with neural cephalisation, the concentration of brain functions in the head. A slight swelling of the anterior end of the nerve cord is found in the lancelet, a chordate, though it lacks the eyes and other complex sense organs comparable to those of vertebrates. Other chordates do not show any trends towards cephalisation.[13]

A peripheral nervous system branches out from the nerve cord to innervate the various systems. The front end of the nerve tube is expanded by a thickening of the walls and expansion of the central canal of spinal cord into three primary brain vesicles: The prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain), further differentiated in the various vertebrate groups.[22] Two laterally placed eyes form around outgrowths from the midbrain, except in hagfish, though this may be a secondary loss.[23][24] The forebrain is well-developed and subdivided in most tetrapods, while the midbrain dominates in many fish and some salamanders. Vesicles of the forebrain are usually paired, giving rise to hemispheres like the cerebral hemispheres in mammals.[22]

The resulting anatomy of the central nervous system, with a single hollow nerve cord topped by a series of (often paired) vesicles, is unique to vertebrates. All invertebrates with well-developed brains, such as insects, spiders and squids, have a ventral rather than dorsal system of ganglions, with a split brain stem running on each side of the mouth or gut.[13]

Molecular Signatures

In addition to the morphological characteristics used to define vertebrates (i.e. the presence of a notochord, the development of a vertebral column from the notochord, a dorsal nerve cord, pharyngeal gills, a post-anal tail, etc.), molecular markers known as conserved signature indels (CSIs) in protein sequences have been identified and provide distinguishing criteria for the subphylum Vertebrata.[25] Specifically, 5 CSIs in the following proteins: protein synthesis elongation factor-2 (EF-2), eukaryotic translational initiation factor 3 (Euk IF-3), adenosine kinase (AdK) and a protein related to ubiquitin carboxyl-terminal hydrolase are exclusively shared by all vertebrates and reliably distinguish them from all other metazoan.[25] The CSIs in these protein sequences are predicted to play important functionally important in vertebrates.

A specific relationship between Vertebrates and Tunicates is also strongly supported by two CSIs found in the proteins predicted exosome complex RRP44 and serine palmitoyltransferase, that are exclusively shared by species from these two subphyla but not Cephalochordates, indicating Vertebrates are more closely related to Tunicates than Cephalochordates.[25]

Evolutionary history

First vertebrates

The early vertebrate Haikouichthys

Vertebrates originated about 525 million years ago during the Cambrian explosion, which saw rise in organism diversity. The earliest known vertebrate is believed to be Myllokunmingia.[1] One of many early vertebrates are Haikouichthys ercaicunensis. Unlike the other fauna that dominated the Cambrian, these groups had the basic vertebrate body plan: a notochord, rudimentary vertebrae, and a well-defined head and tail.[26] All of these early vertebrates lacked jaws in the common sense and relied on filter feeding close to the seabed.[27] A vertebrate group of uncertain phylogeny, small eel-like conodonts, are known from microfossils of their paired tooth segments from the late Cambrian to the end of the Triassic.[28]

From fish to amphibians

Acanthostega, a fish-like early labyrinthodont.

The first jawed vertebrates may have appeared in the late Ordovician (~450 mya) and became common in the Devonian, often known as the "Age of Fishes".[29] The two groups of bony fishes, the actinopterygii and sarcopterygii, evolved and became common.[30] The Devonian also saw the demise of virtually all jawless fishes save for lampreys and hagfish, as well as the Placodermi, a group of armoured fish that dominated the entirety of that period since the late Silurian. The Devonian also saw the rise of the first labyrinthodonts, which was a transitional form between fishes and amphibians.

Mesozoic vertebrates

Amniotes branched from labyrinthodonts in the subsequent Carboniferous period. The Parareptilia and synapsid amniotes were common during the late Paleozoic, while diapsids became dominant during the Mesozoic. In the sea, the bony fishes became dominant. Birds, a derived form of dinosaur, evolved in the Jurassic.[31] The demise of the non-avian dinosaurs at the end of the Cretaceous allowed for the expansion of mammals, which had evolved from the therapsids, a group of synapsid amniotes, during the late Triassic Period.

After the Mesozoic

The Cenozoic world has seen great diversification of bony fishes, amphibians, reptiles, birds and mammals.

Over half of all living vertebrate species (about 32,000 species) are fish (non-tetrapod craniates), a diverse set of lineages that inhabit all the world's aquatic ecosystems, from snow minnows (Cypriniformes) in Himalayan lakes at elevations over 4,600 metres (15,100 feet) to flatfishes (order Pleuronectiformes) in the Challenger Deep, the deepest ocean trench at about 11,000 metres (36,000 feet). Fishes of myriad varieties are the main predators in most of the world's water bodies, both freshwater and marine. The rest of the vertebrate species are tetrapods, a single lineage that includes amphibians (with roughly 7,000 species); mammals (with approximately 5,500 species); and reptiles and birds (with about 20,000 species divided evenly between the two classes). Tetrapods comprise the dominant megafauna of most terrestrial environments and also include many partially or fully aquatic groups (e.g., sea snakes, penguins, cetaceans).

Classification

There are several ways of classifying animals. Evolutionary systematics relies on anatomy, physiology and evolutionary history, which is determined through similarities in anatomy and, if possible, the genetics of organisms. Phylogenetic classification is based solely on phylogeny.[32] Evolutionary systematics gives an overview; phylogenetic systematics gives detail. The two systems are thus complementary rather than opposed.[33]

Traditional classification

Traditional spindle diagram of the evolution of the vertebrates at class level

Conventional classification has living vertebrates grouped into seven classes based on traditional interpretations of gross anatomical and physiological traits. This classification is the one most commonly encountered in school textbooks, overviews, non-specialist, and popular works. The extant vertebrates are:[13]

  • Subphylum Vertebrata

In addition to these, there are two classes of extinct armoured fishes, the Placodermi and the Acanthodii, both of which are considered paraphyletic.

Other ways of classifying the vertebrates have been devised, particularly with emphasis on the phylogeny of early amphibians and reptiles. An example based on Janvier (1981, 1997), Shu et al. (2003), and Benton (2004)[34] is given here († = extinct):

Diversity of various groups of vertebrates through the geologic ages. The width of the bubbles signify the diversity (number of families).
  • Subphylum Vertebrata
    • Palaeospondylus
    • Infraphylum Agnatha or Cephalaspidomorphi (lampreys and other jawless fishes)
      • Superclass Anaspidomorphi (anaspids and relatives)
    • Infraphylum Gnathostomata (vertebrates with jaws)
      • Class Placodermi (extinct armoured fishes)
      • Class Chondrichthyes (cartilaginous fishes)
      • Class Acanthodii (extinct spiny "sharks")
      • Superclass Osteichthyes (bony vertebrates)
      • Superclass Tetrapoda (four-limbed vertebrates)
        • Class Amphibia (amphibians, some ancestral to the amniotes)—now a paraphyletic group
        • Class Synapsida (mammals and the extinct mammal-like reptiles)
        • Class Sauropsida (reptiles and birds)

While this traditional classification is orderly, most of the groups are paraphyletic, i.e. do not contain all descendants of the class's common ancestor.[34] For instance, descendants of the first reptiles include modern reptiles as well as mammals and birds; the agnathans have given rise to the jawed vertebrates; the bony fishes have given rise to the land vertebrates; the traditional "amphibians" have given rise to the reptiles (traditionally including the synapsids or mammal-like "reptiles"), which in turn have given rise to the mammals and birds. Most scientists working with vertebrates use a classification based purely on phylogeny,[35] organized by their known evolutionary history and sometimes disregarding the conventional interpretations of their anatomy and physiology.

Phylogenetic relationships

In phylogenetic taxonomy, the relationships between animals are not typically divided into ranks but illustrated as a nested "family tree" known as a phylogenetic tree. The cladogram below is based on studies compiled by Philippe Janvier and others for the Tree of Life Web Project and Delsuc et al.,[36][37] and complemented (based on[38] and [39]). A dagger (†) denotes an extinct clade, whereas all other clades have living descendants.

The galeaspid Nochelaspis maeandrine from the Devonian period
The placoderm Dunkleosteus terrelli from the Devonian period
The acanthodian fish Diplacanthus acus from the Devonian period
The early ray-fin Cheirolepis canadensis from the Devonian period
The tetrapodomorph Tiktaalik roseae from the Devonian period
The early tetrapod Seymouria from the Permian period
The synapsid "mammal-like reptile" Dimetrodon limbatus from the Permian period
The bird-like dinosaur Archaeopteryx lithographica from the Jurassic period
Vertebrata/
Agnatha/

Hyperoartia (lampreys)Nejonöga, Iduns kokbok.jpg

MyxiniEptatretus polytrema.jpg

Cyclostomes

EuconodontaConodontZICA.png

PteraspidomorphiAstraspis desiderata.png

ThelodontiSphenonectris turnernae white background.jpg

AnaspidaLasanius NT small.jpg

Galeaspida Galeaspids NT.jpg

PituriaspidaPituriaspida.gif

OsteostraciCephalaspis Lyellii.jpg

Gnathostomata

"†Placodermi" (armoured fishes, paraphyletic)[40]Dunkleosteus intermedius.jpg

"†Acanthodii" ("spiny sharks", paraphyletic or polyphyletic)[39] Diplacanthus reconstructed.jpg

Chondrichthyes

"†Acanthodii" ("spiny sharks", paraphyletic or polyphyletic)BrochoadmonesDB15.jpg

Holocephali (ratfish)Chimaera monstrosa1.jpg

Euselachii (sharks, rays)Carcharodon carcharias drawing.jpg

(cartilaginous fishes)
Euteleostomi/

"†Acanthodii" ("spiny sharks", paraphyletic or polyphyletic)Acanthodes BW.jpg

Actinopterygii

Cladistia (bichirs, reedfish) Cuvier-105-Polyptère.jpg

Chondrostei (sturgeons, paddlefish)Atlantic sturgeon flipped.jpg

Neopterygii (includes Teleostei, 96% of fish species today)Cyprinus carpio3.jpg

(ray‑finned fishes)
Sarcopterygii

Onychodontiformes OnychodusDB15 flipped.jpg

Actinistia (coelacanths) Coelacanth flipped.png

Rhipidistia

PorolepiformesReconstruction of Porolepis sp flipped.jpg

Dipnoi (lungfishes) Barramunda coloured.jpg

Tetrapodomorpha/

RhizodontimorphaGooloogongia loomesi reconstruction.jpg

TristichopteridaeEusthenodon DB15 flipped.jpg

TiktaalikTiktaalik BW white background.jpg

IchthyostegaIchthyostega BW (flipped).jpg

Tetrapoda (four-limbed vertebrates)Deutschlands Amphibien und Reptilien (Salamandra salamdra).jpg

(Choanata)
(lobe‑finned fish)
Osteichthyes
(jawed vertebrates)
Craniata

Note that Acanthodii, the "spiny sharks", were shown to be either a paraphyletic or a polyphyletic group, with some taxa being more closely related with cartilaginous fish, others more closely related with bony fish, and again others being more basal on the tree of life.[39] Similarly, the Placodermi and Ostracodermi are not anymore considered monophyletic groups.[40][41]

Also note that Teleostei (Neopterygii) and Tetrapoda (amphibians, mammals, reptiles, birds) each make up about 50% of today's vertebrate diversity, while all other groups are either extinct or rare. The next cladogram shows the extant clades of tetrapods (the four-limbed vertebrates), and a selection of extinct (†) groups:

Tetrapoda

Amphibians (frogs, salamanders, caecilians)Deutschlands Amphibien und Reptilien (Salamandra salamdra).jpg

†"Reptile-like amphibians" (paraphyletic)Diadectes1DB (flipped).jpg

Amniota
Synapsida

†"Mammal-like reptiles" (paraphyletic)Ophiacomorphs2.jpg

Mammals (monotremes, marsupials, placental mammals)Ruskea rotta.png

Sauropsida

Parareptilia Scutosaurus BW flipped.jpg

Diapsida

Scaled reptiles (lizards, snakes) Zoology of Egypt (1898) (Varanus griseus).png

Archosaurs

Crocodilians (crocodiles, alligatorids, gavialids)Description des reptiles nouveaux, ou, Imparfaitement connus de la collection du Muséum d'histoire naturelle et remarques sur la classification et les caractères des reptiles (1852) (Crocodylus moreletii).jpg

Dinosaurs

†"Non-avian dinosaurs" (paraphyletic)Allosaurus Revised.jpg

Birds Meyers grosses Konversations-Lexikon - ein Nachschlagewerk des allgemeinen Wissens (1908) (Antwerpener Breiftaube).jpg

Note that reptile-like amphibians, mammal-like reptiles, and non-avian dinosaurs are all paraphyletic.

The placement of hagfish on the vertebrate tree of life has been controversial. Their lack of proper vertebrae (among with other characteristics found in lampreys and jawed vertebrates) led phylogenetic analyses based on morphology to place them outside Vertebrata. Molecular data, however, indicates they are vertebrates closely related to lampreys. A study by Miyashita et al. (2019), 'reconciliated' the two types of analysis as it supports the Cyclostomata hypothesis using only morphological data.[42]

AnaspidaPharyngolepis2.png

Pipiscius

EuconodontaConodontZICA.png

CyclostomataNejonöga, Iduns kokbok.jpg

PteraspidomorphiAstraspis desiderata.png

ThelodontiSphenonectris turnernae white background.jpg

PituriaspidaPituriaspida.gif

Galeaspida Galeaspids NT.jpg

OsteostraciCephalaspis Lyellii.jpg

jawed vertebratesDunkleosteus intermedius.jpg

Number of extant species

The number of described vertebrate species are split between tetrapods and fish. The following table lists the number of described extant species for each vertebrate class as estimated in the IUCN Red List of Threatened Species, 2014.3.[43]

Vertebrate groupsImageClassEstimated number of
described species[43][44]
Group
totals[43]
Anamniote

lack
amniotic
membrane

so need to
reproduce
in water
JawlessFishEptatretus polytrema.jpgMyxini
(hagfish)
78>32,900
Eudontomyzon danfordi Tiszai ingola.jpgHyperoartia
(lamprey)
40
JawedShark fish chondrichthyes.jpgcartilaginous
fish
>1,100
Carassius wild golden fish 2013 G1.jpgray-finned
fish
>32,000
Coelacanth-bgiu.pnglobe-finned
fish
8
TetrapodsLithobates pipiens.jpgamphibians7,30233,278
Amniote

have
amniotic
membrane

adapted to
reproducing
on land
reptiles10,711
Squirrel (PSF).pngmammals5,513
Secretary bird (Sagittarius serpentarius) 2.jpgbirds10,425
Total described species66,178

The IUCN estimates that 1,305,075 extant invertebrate species have been described,[43] which means that less than 5% of the described animal species in the world are vertebrates.

Vertebrate species databases

The following databases maintain (more or less) up-to-date lists of vertebrate species:

Reproductive systems

Nearly all vertebrates undergo sexual reproduction. They produce haploid gametes by meiosis. The smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova. These fuse by the process of fertilisation to form diploid zygotes, which develop into new individuals.

Inbreeding

During sexual reproduction, mating with a close relative (inbreeding) often leads to inbreeding depression. Inbreeding depression is considered to be largely due to expression of deleterious recessive mutations.[45] The effects of inbreeding have been studied in many vertebrate species.

In several species of fish, inbreeding was found to decrease reproductive success.[46][47][48]

Inbreeding was observed to increase juvenile mortality in 11 small animal species.[49]

A common breeding practice for pet dogs is mating between close relatives (e.g. between half- and full siblings).[50] This practice generally has a negative effect on measures of reproductive success, including decreased litter size and puppy survival.[51][52][53]

Incestuous matings in birds result in severe fitness costs due to inbreeding depression (e.g. reduction in hatchability of eggs and reduced progeny survival).[54][55][56]

Inbreeding avoidance

As a result of the negative fitness consequences of inbreeding, vertebrate species have evolved mechanisms to avoid inbreeding.

Numerous inbreeding avoidance mechanisms operating prior to mating have been described. Toads and many other amphibians display breeding site fidelity. Individuals that return to natal ponds to breed will likely encounter siblings as potential mates. Although incest is possible, Bufo americanus siblings rarely mate.[57] These toads likely recognize and actively avoid close kin as mates. Advertisement vocalizations by males appear to serve as cues by which females recognize their kin.[57]

Inbreeding avoidance mechanisms can also operate subsequent to copulation. In guppies, a post-copulatory mechanism of inbreeding avoidance occurs based on competition between sperm of rival males for achieving fertilization.[58] In competitions between sperm from an unrelated male and from a full sibling male, a significant bias in paternity towards the unrelated male was observed.[58]

When female sand lizards mate with two or more males, sperm competition within the female's reproductive tract may occur. Active selection of sperm by females appears to occur in a manner that enhances female fitness.[59] On the basis of this selective process, the sperm of males that are more distantly related to the female are preferentially used for fertilization, rather than the sperm of close relatives.[59] This preference may enhance the fitness of progeny by reducing inbreeding depression.

Outcrossing

Mating with unrelated or distantly related members of the same species is generally thought to provide the advantage of masking deleterious recessive mutations in progeny[60] (see heterosis). Vertebrates have evolved numerous diverse mechanisms for avoiding close inbreeding and promoting outcrossing[61] (see inbreeding avoidance).

Outcrossing as a way of avoiding inbreeding depression has been especially well studied in birds. For instance, inbreeding depression occurs in the great tit (Parus major) when the offspring are produced as a result of a mating between close relatives. In natural populations of the great tit, inbreeding is avoided by dispersal of individuals from their birthplace, which reduces the chance of mating with a close relative.[62]

Purple-crowned fairywren females paired with related males may undertake extra-pair matings that can reduce the negative effects of inbreeding, despite ecological and demographic constraints.[56]

Southern pied babblers (Turdoides bicolor) appear to avoid inbreeding in two ways: through dispersal and by avoiding familiar group members as mates.[63] Although both males and females disperse locally, they move outside the range where genetically related individuals are likely to be encountered. Within their group, individuals only acquire breeding positions when the opposite-sex breeder is unrelated.

Cooperative breeding in birds typically occurs when offspring, usually males, delay dispersal from their natal group in order to remain with the family to help rear younger kin.[64] Female offspring rarely stay at home, dispersing over distances that allow them to breed independently or to join unrelated groups.

Parthenogenesis

Parthenogenesis is a natural form of reproduction in which growth and development of embryos occur without fertilization.

Reproduction in squamate reptiles is ordinarily sexual, with males having a ZZ pair of sex determining chromosomes, and females a ZW pair. However, various species, including the Colombian Rainbow boa (Epicrates maurus), Agkistrodon contortrix (copperhead snake) and Agkistrodon piscivorus (cotton mouth snake) can also reproduce by facultative parthenogenesis—that is, they are capable of switching from a sexual mode of reproduction to an asexual mode—resulting in production of WW female progeny.[65][66] The WW females are likely produced by terminal automixis.

Mole salamanders are an ancient (2.4–3.8 million year-old) unisexual vertebrate lineage.[67] In the polyploid unisexual mole salamander females, a premeiotic endomitotic event doubles the number of chromosomes. As a result, the mature eggs produced subsequent to the two meiotic divisions have the same ploidy as the somatic cells of the female salamander. Synapsis and recombination during meiotic prophase I in these unisexual females is thought to ordinarily occur between identical sister chromosomes and occasionally between homologous chromosomes. Thus little, if any, genetic variation is produced. Recombination between homeologous chromosomes occurs only rarely, if at all.[68] Since production of genetic variation is weak, at best, it is unlikely to provide a benefit sufficient to account for the long-term maintenance of meiosis in these organisms.

Self-fertilization

Two killifish species, the mangrove killifish (Kryptolebias marmoratus) and Kryptolebias hermaphroditus, are the only known vertebrates to self-fertilize.[69] They produces both eggs and sperm by meiosis and routinely reproduces by self-fertilisation. This capacity has apparently persisted for at least several hundred thousand years.[70] Each individual hermaphrodite normally fertilizes itself through uniting inside the fish's body of an egg and a sperm that it has produced by an internal organ.[71] In nature, this mode of reproduction can yield highly homozygous lines composed of individuals so genetically uniform as to be, in effect, identical to one another.[72][73] Although inbreeding, especially in the extreme form of self-fertilization, is ordinarily regarded as detrimental because it leads to expression of deleterious recessive alleles, self-fertilization does provide the benefit of fertilization assurance (reproductive assurance) at each generation.[72]

Population trends

The Living Planet Index, following 16,704 populations of 4,005 species of vertebrates, shows a decline of 60% between 1970 and 2014.[74] Since 1970, freshwater species declined 83%, and tropical populations in South and Central America declined 89%.[75] The authors note that, "An average trend in population change is not an average of total numbers of animals lost."[75] According to WWF, this could lead to a sixth major extinction event.[76] The five main causes of biodiversity loss are land-use change, overexploitation of natural resources, climate change, pollution and invasive species.[77]

See also

References

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Bibliography

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Media files used on this page

Acanthostega BW.jpg
Author/Creator: Nobu Tamura (http://spinops.blogspot.com), Licence: CC BY 2.5
Acanthostega gunnari, pencil drawing, digital coloring
Spindle diagram.jpg
Evolution of Vertebrate from the Cambrium to the present at a class level as a traditional spindle diagram. The width of the spindle represents the number of families as a rough estimate of diversity. The diagram is based on Benton, M. J. (1998) The quality of the fossil record of vertebrates. P. 269–303, in Donovan, S. K. and Paul, C. R. C. (eds), The adequacy of the fossil record, Fig. 2. Wiley, New York, 312 p.

The figures representing classes are (from left): Agnatha, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves and Mammalia. The two extinct classes are Placodermi and Acanthodii. All classes interpreted traditionally.

Bentons notes to his own tree: Number of families is an imperfect measure of diversity. Reptilia in particular should probably have been shown as far more diverse in the Mesozoic.
Astraspis desiderata.png
Author/Creator: Philippe Janvier, Licence: CC BY 3.0
Astraspis desiderata. Astraspids are still poorly known but recent discoveries of partially complete specimens of Astraspis desiderata, from the Ordovician of Colorado, have considerably increased their knowledge. Their dorsal headshield is made up by large, polygonal bone units and the gill openings are situated more dorsally than in arandaspids.
Cuvier-105-Polyptère.jpg
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Planche N° 105 du livre "Le règne animal distribué d'après son organisation" par Georges Cuvier (Tome 8), seconde édition de 1828, représentant : Polypterus senegalus
Atlantic sturgeon flipped.jpg
Atlantic Sturgeon, Acipenser oxyrhynchus. Scans of artwork commissioned by the Fish and Wildlife Service in the 1970's. Original art is kept at NCTC museum.
Barramunda coloured.jpg
Fig. 89. The Barramunda (Ceratodus) (From Queensland.)
Carassius wild golden fish 2013 G1.jpg
Prussian carp (Carassius gibelio). Mutation of an red color alike golden fish. Was caught in wild near Vinnitsa, Ukraine. Full length with caudal fin is 244 mm.
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Author/Creator: No machine-readable author provided. Bogdan assumed (based on copyright claims)., Licence: CC-BY-SA-3.0
based on PD Image:Coelacanth.png
Caribou from Wagon Trails.jpg
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Appears to be an albino Elk, located at Wagon Trails Animal Park.
Wapiti from Wagon Trails.jpg
Author/Creator: Brian0918, Licence: CC BY-SA 2.5
Appears to be an albino Elk, located at Wagon Trails Animal Park.
Lasanius NT small.jpg
Author/Creator: Nobu Tamura email:nobu.tamura@yahoo.com http://spinops.blogspot.com/ http://paleoexhibit.blogspot.com/, Licence: CC BY-SA 4.0
Life reconstruction of Lasanius problematicus
Lithobates pipiens.jpg
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Northern Leopard Frog (Lithobates pipiens). Photo taken near Ottawa
Eusthenodon DB15 flipped.jpg
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Eusthenodon, гигантская кистеперая рыба из позднего девона Гренландии, Европы и России
Archaeopteryx fossil.jpg
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Archaeopteryx lithographica, found in the Jurassic Solnhofen Limestone of southern Germany.
Haikouichthys cropped.jpg
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The primitive vertebrate Haikouichthys. This file is the cropped version of File:Haikouichthys NT.jpg by Nobu Tamura
Gooloogongia loomesi reconstruction.jpg
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Gooloogongia loomesi - rhizodont fish from Late Devonian of Gogo, Australia
Description des reptiles nouveaux, ou, Imparfaitement connus de la collection du Muséum d'histoire naturelle et remarques sur la classification et les caractères des reptiles (1852) (Crocodylus moreletii).jpg
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Title: Description des reptiles nouveaux ou imparfaitement connus de la collection du Muséum d'histoire naturelle et remarques sur la classification et les caractères des reptiles
Identifier: descriptiondesre00dum (find matches)
Year: 1852 (1850s)
Authors: Duméril, Auguste Henri André, 1812-1870
Subjects: Muséum national d\U+2019\histoire naturelle (France); Reptiles; Reptiles
Publisher: (Paris : Muséum d'histoire naturelle)
Contributing Library: Harvard University, Museum of Comparative Zoology, Ernst Mayr Library
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â¢x > Eâ à =3
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^ ^ 5j 1 I 5 o

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Pharyngolepis2.png
My eighth uploaded image that I made through flash. It depicts the Silurian agnathan Pharyngolepis. EDIT: I changed the size and made it more detailed.
Ichthyostega BW (flipped).jpg
Author/Creator: Nobu Tamura (http://spinops.blogspot.com), Licence: CC BY 2.5
Ichthyostega pencil drawing, digital coloring, based on reconstruction by Ahlberg, 2005
Zoology of Egypt (1898) (Varanus griseus).png
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Identifier: zoologyofegypt01ande

Title: Zoology of Egypt
Year: 1898 (1890s)
Authors: Anderson, John, 1833-1900 Boulenger, George Albert, 1858-1937. Fishes of the Nile De Winton, William Edward
Subjects: Zoology
Publisher: London, B. Quaritch
Contributing Library: Harvard University, Museum of Comparative Zoology, Ernst Mayr Library
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nitor griseus, Peters, Mon. Berl. Ak. 1870, p. 109. Varanus (Psammosaurus) arenarius, Bedr. Bull. Soc. Nat. Mosc. 1879, no. 3, p. 40.Varanus griseus, Blgr. Cat. Liz. B. M. ii. 1885, p. 306; Trans. Linn. Soc. Lond., Zool. v. 1889, p. 99; Fauna Brit. Ind., Rept. 1890, p. 163; Trans. Zool. Soc. xiii. 1891, p. 121; Boettger, Zool. Jahrb. iii. 1888, p. 904; Kat. Rept. Mus. Senck. 1893, p. 49; Anderson, Proc. Zool. Soc. 1895, p. 636 ; Herpet. Arabia & Egypt, 1896, pp. 34, 101; Zander, Zool. Garten, xxxvi. 1895, p. 298; Werner, Verh. zool.-bot. Ges. Wien, xliv. 1894, p. 79; Francaviglia, Boll. Soc. Rom. Zool. v. 1896, p. 46.1 $ . Suez. 1 cJ. Desert N.E. of Cairo.1 ? . Gizeh Desert.1 ? . Tel el Amarna. Prof. W. M. Flinders Petrie, D.C.L. 1 J1 and 1 juv. Suakin. Surgeon-Captain Penton, D.S.O. 2 £ and 2 ? . Suakin.1 J . Tokar. Teeth acute, compressed. Snout slightly depressed at the tip. Canthus rostralismoderately denned. Nostril oblique, rather large and crescentic, close to the eye, its
Text Appearing After Image:
w a Pi Zo ° < - Pi *° VAEANUS GEISBUS. 135 distance from the end of the snout being about four times greater than the intervalbetween it and the eye. Tail rounded at the base, slightly compressed posteriorly; nodorsal ridge. Digits rather short; claws strong and curved. Scales on the uppersurface of the head, including the supraoculars, very small, juxtaposed, smooth,generally hexagonal. Scales on the body and limbs small, rounded or oblong,sometimes feebly keeled, larger than ventrals, those on the sides of the neck generallyconical; ventrals smooth, 110-125 between the fold of the neck to the groin. Caudalscales small, more or less keeled above and below. General colour sandy yellow, with narrow brownish longitudinal lines varying inintensity along the side of the neck, and similarly coloured bands or lines across theback and tail, becoming lost in some adults towards the tip; the upper surface of thebody sometimes with yellow spots. The young is generally pale rufous yellow ab

Note About Images

Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original work.
Scutosaurus BW flipped.jpg
Author/Creator: Nobu Tamura (http://spinops.blogspot.com), Licence: CC BY 2.5
Scutosaurus karpinskii, a pareiasaur from the Late Permian of Russia, pencil drawing (reconstruction after Carroll, 1988)
OnychodusDB15 flipped.jpg
Author/Creator: DiBgd, Licence: CC BY-SA 4.0
Onychodus, примитивная среднедевонская кистеперая рыба
Allosaurus Revised.jpg
Author/Creator: Fred Wierum, Licence: CC BY-SA 4.0
Allosaurus fragilis reconstruction by Fred Wierum. Proportions align with Hartman's reconstruction of UUVP 6000.
Reconstruction of Porolepis sp flipped.jpg
Author/Creator: Dmitry Bogdanov , Licence: GFDL
Reconstruction of Porolepis, Early Devonian Porolepiform sarcoptrygian fish
Tellus Dimetrodon.jpg
Author/Creator: Jonathan Chen, Licence: CC BY-SA 4.0
Skeletal cast mount of Dimetrodon limbatus at the Tellus Science Museum in Cartersville, Georgia.
Tiktaalik roseae.jpg
Author/Creator: Ghedoghedo, Licence: CC BY-SA 3.0
Fossil of Tiktaalin, an extinct fish Took the photo at Musee d'Histoire Naturelle, Brussels
Sphenonectris turnernae white background.jpg
Author/Creator: Apokryltaros, Licence: CC BY-SA 4.0
Reconstruction of the "swimming wedge" thelodont, Sphenonectris turnerae, from the Early Devonian Yukon.
Diplacanthus reconstructed.jpg
Author/Creator: Funyu123, Licence: CC BY-SA 4.0
Reconstructed Diplacanthus acus.
Eptatretus polytrema.jpg
Eptatretus polytrema
Gills (esox).jpg
Author/Creator: User:Uwe Gille, Licence: CC-BY-SA-3.0
Gill arches of a Northern Pike (Esox lucius)
Ophiacomorphs2.jpg
Author/Creator: Creator:Dmitry Bogdanov, Licence: CC BY 3.0
Representative Ophiacomorphs: Cotylorhynchus (background), Ophiacodon and Varanops.
Chimaera monstrosa1.jpg
Chimaera monstrosa
Tiktaalik BW white background.jpg
Author/Creator: Nobu Tamura (http://spinops.blogspot.com), Licence: CC BY 2.5
Tiktaalik rosae, pencil drawing, digital coloring
Vertebrata 002.png
Author/Creator: , Licence: CC BY-SA 4.0
Many examples of vertebrates : Acipenser oxyrinchus, African Elephants, Tiger Shark and a River lamprey
Galeaspids NT.jpg
Author/Creator: Nobu Tamura Email:nobu.tamura@yahoo.com http://spinops.blogspot.com/ http://paleoexhibit.blogspot.com/, Licence: CC BY-SA 4.0
Life restoration of various Galeaspis:
  • Hanyangaspis guodingshanensis
  • Sanchaspis megalorostra
  • Lungmenshanapis kyangyouensis
  • Shuyu zhejiangensis
  • Laxaspis qujingensis
Closed Access logo transparent.svg
Author/Creator: Jakob Voss, based on art designer at PLoS, modified by Wikipedia users Nina and Beao, Licence: CC0
Closed Access logo, derived from PLoS Open Access logo. This version with transparent background.
Squirrel (PSF).png
Line art drawing of a squirrel.
Cheirolepis canadensis.jpg
Author/Creator: Placoderm2, Licence: CC BY-SA 4.0
Cheirolepis canadensis on display at the Miguasha National Park.
Fish evolution.png
Author/Creator: Epipelagic, Licence: CC BY-SA 3.0
Evolution of fishes from the Cambrian to the present as a spindle diagram. The width of the spindles are proportional to the number of families as a rough estimate of diversity. The diagram is based on Benton, M. J. (2005) Vertebrate Palaeontology, Blackwell, 3rd edition, Fig 2.10 on page 35 and Fig 3.25 on page 73, as well as this source, and this Wikimedia Commons file.
Eudontomyzon danfordi Tiszai ingola.jpg
Author/Creator: Zsoldos Márton, Licence: CC BY-SA 3.0
Carpathian brook lamprey (Eudontomyzon danfordi)
Cephalaspis Lyellii.jpg
Cephalaspis lyellii (Old Red Sandstone, Scotland).
BrochoadmonesDB15.jpg
Author/Creator: DiBgd, Licence: CC BY-SA 4.0
Early Devonian acanthodian Brochoadmones milesi, from Man on the Hill (MOTH) locality in Northwest Territories, Canada
Florida Box Turtle Digon3.jpg
(c) I, Jonathan Zander, CC-BY-SA-3.0
Photo of a Florida Box Turtle (Terrapene carolina bauri). Taken in Jacksonville, Florida, USA.
Acanthodes BW.jpg
Author/Creator: Nobu Tamura (http://spinops.blogspot.com), Licence: CC BY 3.0

Acanthodes bronni, an acanthodian from the Early Permian of Germany, pencil drawing

Ref : Janvier, P. (2001) "Ostracoderms and the shaping of gnathostome characters" in Ahlberg, P.E. , ed. Major Events in Early Vertebrate Evolution: Palaeontology, Phylogeny, Genetics and Development, CRC Press, p. 175 Retrieved on 11 January 2009. ISBN: 0415233704.
NochelaspisMaeandrine-PaleozoologicalMuseumOfChina-May23-08.jpg
Author/Creator: User:Captmondo, Licence: Copyrighted free use
A specimen of Nochelaspis maeandrine, on display at the Paleozoological Museum of China.
ConodontZICA.png

From the Englich Wikipedia Mateus Zica draw it with macromedia flash 28 oct 2005 mateus zica

18:25, 28 October 2005 (UTC)
Ruskea rotta.png
A brown rat, Rattus norvegicus?
Deutschlands Amphibien und Reptilien (Salamandra salamdra).jpg

Taf. V.

Feuer-Salamander (Salamandra maculosa)
Diplacanthus acus (acanthodian fish) from Waterloo Farm.jpg
Author/Creator: Funyu123, Licence: CC BY-SA 4.0
Diplacanthus acus, an exceptionally well preserved acanthodian fish (10 cm long) from the Late Devonian Waterloo Farm lagerstätte in the Eastern Cape, South Africa
Dunkleosteus terrelli - Cleveland Museum of Natural History - 2014-12-26 (21137540331).jpg
Author/Creator: Tim Evanson from Cleveland Heights, Ohio, USA, Licence: CC BY-SA 2.0

A fossil Dunkleosteus terrelli skull on display in the Cleveland Museum of Natural History.

Dunkleosteus terrelli was a prehistoric fish that lived about 380 to 360 million years ago. Unlike modern fish, it had massive, heavy bones in its skull. It was a carnivore (look at those teeth!) that was about 33 feet long and it was the second-largest predator in the ocean at the time. It was not very fast, but it was a powerful swimmer -- and its bite was one of the most powerful ever found in nature.

Dunkleosteus terrelli was first discovered in 1873. At the time, scientists thought it was part of the species Dinichthys, but they have since realized it was a separate species. Dunkleosteus was renamed in 1956. The name honors David Dunkle, a paleontologist at the Cleveland Museum of Natural History. There are several sub-species, but Dunkleosteus terrelli is the largest. It has been found throughout the U.S. (Ohio, Pennsylvania, Tennessee, California, and Texas).
Naturkundemuseum Berlin - Dinosaurierhalle.jpg
© Raimond Spekking / CC BY-SA 4.0 (via Wikimedia Commons)
Skelett eines Diplodocus in der Dinosaurierhalle des Museums für Naturkunde, Berlin.
Cyprinus carpio3.jpg
Cyprinus carpio
Pituriaspida.gif
Author/Creator: Philippe Janvier, Licence: CC BY 3.0
Pituriaspis (Pituriaspida). Pituriaspids are mainly known by Pituriaspis, from the Devonian of Australia. As a whole, their headshield is quite similar to that of osteostracans, though devoid of a naso-hypophysial opening. The mouth, gill openings and presumably the nasal aperture were all situated on the ventral side of the head. Well-developed paired fins attached on either sides of the headshield. The only diagnostic feature of pituriaspids is an enigmatic pit adjacent to the eyes.
Seymouria Tambach Lovers.jpg
Author/Creator: James St. John, Licence: CC BY 2.0
The "Tambach lovers", a pair of Seymouria sanjuanensis from the Tambach Formation of Germany. This is a cast of the original fossil displayed at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, USA
Secretary bird (Sagittarius serpentarius) 2.jpg
Author/Creator: Lip Kee Yap, Licence: CC BY-SA 2.0

Secretarybird Sagittarius serpentarius Masai Mara, Kenya August 29, 2008

690V2112
Dunkleosteus intermedius.jpg
Author/Creator: Creator:Dmitry Bogdanov, Licence: CC BY 3.0
Dunkleosteus "intermedius" (synonym of D. terrelli). Based on famous skeletal drawing from B. Dean.
Diadectes1DB (flipped).jpg
Author/Creator: Dmitry Bogdanov , Licence: CC BY-SA 3.0
Diadectes