After completing this unit, students will be able to:
Vertebrates are members of the subphylum, Vertebrata, chordates with backbones and spinal columns. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land and marine animals. Vertebrates are the animals from the groups of jawless fishes, bony fishes, sharks, stingrays, amphibians, reptiles, mammals, and birds. Extant vertebrates range in size from the carp species, Paedocypris, at as little as 7.9 mm (0.3 of an inch) to the blue whale at up to 33 m (110 feet). Vertebrates make up about 5 percent of all described animal species; the rest are invertebrates, which lack backbones.
The vertebrates traditionally include the hagfish, which do not have proper vertebrae, though their closest living relatives, the lampreys, do have vertebrae. For this reason, the vertebrate subphylum is sometimes referred to as "Craniata," as all members do possess a cranium.
The fossil record shows aspects of the meandering evolutionary path from early aquatic vertebrates to mammals, with a host of transitional fossils, though there are still large blank areas. The earliest known fossil vertebrates were heavily armored fish discovered in rocks from the Ordovician Period, about 500 to 430 (Ma), or million years ago.
The Devonian Period (395 to 345 Ma) brought in the changes that allowed primitive air-breathing fish to remain on land as long as they wished, thus becoming the first terrestrial vertebrates, the amphibians. Amphibians developed forms of reproduction and locomotion and a metabolism better suited for life exclusively on land, becoming more reptilian. Full-fledged reptiles appeared in the Carboniferous Period (345 to 280 Ma). The reptilian changes and adaptations to diet and geography are chronicled in the fossil record of the varying forms of therapsida. True mammals showed up in the Triassic Period (225 to 190 Ma) around the same time as the dinosaurs, which also sprouted from the reptilian line, while birds first diverged from dinosaurs between 100 and 60 Ma.
Figure 1. A series of transverse sections through an embryo of the dog. (After Bonnet.) Section I is the most anterior. In V the neural plate is spread out nearly flat. The series shows the uprising of the neural folds to form the neural canal. a. Aortæ. c. Intermediate cell mass. ect. Ectoderm. ent. Entoderm. h, h. Rudiments of endothelial heart tubes. In III, IV, and V the scattered cells represented between the entoderm and splanchnic layer of mesoderm are the vasoformative cells which give origin in front, according to Bonnet, to the heart tubes, h; l.p. Lateral plate still undivided in I, II, and III; in IV and V split into somatic (sm) and splanchnic (sp) layers of mesoderm. mes. Mesoderm. p. Pericardium. so. Primitive segment.
"Pristella tetra1" by Henry Gray (1827–1861) , Wikimedia Commons is in the Public Domain
The neural crest is unique to craniates. Neural crest cells make up a transient, multipotent, migratory cell population, unique to vertebrates that give rise to a diverse array of cell types.
The emergence of the neural crest was important in vertebrate evolution because many of its structural derivatives are defining features of the vertebrate clade. Several structures that distinguish the vertebrates from other chordates are formed from the derivatives of neural crest cells. Gans and Northcut, in their "New Head" theory, argued that the presence of neural crest was the basis for vertebrate-specific features. The appearance of these features was also pivotal in vertebrate evolution, because it enabled a predatory lifestyle.
Date last modified: August 30, 2016.
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