VERTEBRATA

Origins: 518 million years ago (early Cambrian, Stage 3)

Extinction: Still extant

Vertebrates are deuterostome animals characterized for a bony or cartilaginous axial endoskeleton, known as the vertebral column, or backbone, along the spinal cord, and englobes all species of fish, including amphibians, mammals and reptiles. There are many groups of vertebrates, with the two most basic groups being the agnathans (jawless fish) and the gnathostomes (jawed fish), the latter including the living chondrichthyans (cartilaginous fish) and the osteichthyans (bony fish). Despite their abundance and diversity, vertebrates only comprise less than 5% of all known animal species, with the rest being invertebrates (a non-taxonomic term that englobes all animals that are not vertebrates). Hagfish are the only vertebrates that lack proper vertebrae, but they do retain a cranium.

The endoskeleton of vertebrates is basally composed of a long vertebral column that starts behind the cranium and continues, often beyond the anus, to form a tail. During development, vertebrates have a notochord as embryos, which is replaced by the vertebral column when they grow older, though hagfish are the only vertebrates that have a notochord and lack a vertebral column at the same time. The earliest vertebrates had gills, which are carried right behind the head, bordering the posterior margins of a series of openings from the pharynx to the exterior. More derived terrestrial vertebrates lack gills as adults, but the gill arches are present in fetal development, forming the basis of structures such as jaws, the thyroid gland, the larynx, the columella and the malleus and incus (the latter two found in mammals). Vertebrates are unique for having neural crest cells, these migrate through the body from the dorsal nerve cord during development, and initiate the formation of neuronal ganglia and structures such as the jaws and skull. Vertebrates also have cephalization, that is, the tendency to form a distinct head, something not observed in other chordates. The forebrain is well developed in advanced vertebrates, such as tetrapods, being subdivided into the telencephalon and diencephalon, while the midbrain dominates the most in fish and some amphibians. Myeline insulation in nerves can be found in annelids and arthropods, but vertebrates differ for their myelination being annular and non-fenestrated and the combination of myelination and encephalization have given vertebrates a unique advantage in developing higher neural functions such as complex motor coordination and cognition.

Vertebrates have conserved signature indels (CSIs) in the following proteins: protein synthesis elongation factor-2 (EF-2), eukaryotic translation initiation factor 3 (eIF3), 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; the CSIs in these protein sequences are predicted to have important functionality in vertebrates.

Vertebrates first evolved in the Cambrian period, and over the course of the early Paleozoic evolved several characteristics that would have benefited them over the course of their following evolutionary history, culminating in the Silurian and the later Devonian periods which saw the inception of the Actinopterygii and Sarcopterygii branches of bony fish, which became increasingly common and diverse since. The Devonian saw several droughts, anoxic events and oceanic competition, forcing a lineage of sarcopterygians to leave the water and evolve into the terrestrial tetrapods by the Carboniferous period. Nowadays, tetrapods are the dominant megafauna of most terrestrial environments, with many groups that have secondarily returned to the sea.

Early attempts at classifying modern vertebrates initially focused on the creation of a handful of classes, based on gross anatomical and physiological traits. This classification mode still persists today in many school books, separating vertebrates in the classes Agnatha, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Mammalia and Aves. Of course, phylogeny is more complex than that, and several different groups are formed as exact relationships become clearer to understand, with fish being a paraphyletic assemblage leading to tetrapods, and birds being included within the reptile group. Conventions in phylogeny may be perpetuated or ignored depending on the applications of each scientist. Many intermediate vertebrate groups have been extinct in Earth's evolutionary past, which help paint a picture on the relationships between modern groups. For example, tetrapods are well inserted within the sarcopterygian fish group, with coelacanths proved to be closer to tetrapods than ray-finned fish, and lungfish closer to tetrapods than to coelacanths.

Among extant species there's a nearly equal number of tetrapod and non-tetrapod vertebrate species. There are continuously updated species databases for each main grouping of vertebrates alive today.

Vertebrate reproduction involves merging two haploid gametes (sperm and egg), and upon fertilisation, it forms a diploid zygote, which then develops into new individuals. Inbreeding can occur in many vertebrate species and appears to have a negative effect in juvenile mortality in a few species. For example, selective breeding in dogs often involved some level of inbreeding, which caused negative effects in the juveniles survival. Because of this, many vertebrates have developed methods to avoid inbreeding. For example, toads are capable of recognizing that the pond that they were born in has the likeliest chances of having one of their own siblings, so mating is avoided in places recognized as places where close of kin could be inhabiting. Advertisement vocalizations created by males can serve as cues by which females can recognize their kin. Female lizards can actively select the sperm cells of various males that she has mated, enhancing her fitness and reducing the chance of inbreeding. Efforts to breed with unrelated individuals are also observed in many reproductive strategies employed by vertebrates. Some birds avoid inbreeding by either dispersing away from their natal grounds or simply avoiding familiar group members as mates. Both sexes will disperse locally, moving outside the range where genetically related individuals may lie. When in a group, individuals only acquire breeding positions when the breeder of the opposite sex is unrelated. Male birds may delay their dispersal to remain with the family and help raise their younger kin, while females will be driven to move away earlier to breed independently and find unrelated groups. Some vertebrates only breed through parthenogensis, such as the mole salamanders, which have been living in a unisexual lifestyle for a few million years, by now. In the polyploid unisexual mole salamander females, a premeiotic endomitotic event doubles the number of chromosomes, and 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. This causes few or no genetic variation in the offspring, however. A specific species of killifish apparently has remained for the course of hundreds of thousands of years in a state of perpetual self-fertilization. Each individual hermaphroditic fish normally fertilizes itself through uniting inside the fish's body of an egg and a sperm that it has produced by an internal organ.

main source: Wikipedia

PHYLOGENY

Centroneuralia

- Protostomia

- Vertebrata

- Chondrichthyes

- Metopacanthus bollensis

- Hybodontiformes

- Crassodus reifi

- Hybodus hauffianus

- Osteichthyes

- Actinopterygii