Vertebrate paleontology is the subfield of paleontology that seeks to discover, through the study of fossilized remains, the behavior, reproduction and appearance of extinct animals with vertebrae or a notochord. It also tries to connect, by using the evolutionary timeline, the animals of the past and their modern-day relatives.
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 (megaannum, 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.
Birds first diverged from dinosaurs between 100 Ma and 60 Ma. One specimen of Archaeopteryx from around 150 Ma subjected to x-ray microtomography revealed significant bird-like alterations.
One of the people who helped figure out the vertebrate progression was French zoologist Georges Cuvier (1769–1832), who realized that fossils found in older rock strata differed greatly from more recent fossils or modern animals. He published his findings in 1812 and, although he steadfastly refuted evolution, his work proved the (at the time) contested theory of extinction of species.
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A grade is a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit. An evolutionary grade is a group of species united by morphological or physiological traits, that has given rise to another group that has major differences from the ancestral condition, and is thus not considered part of the ancestral group, while still having enough similarities that we can group them under the same clade.
The evolution of fish began about 530 million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. The first fish lineages belong to the Agnatha, or jawless fish. Early examples include Haikouichthys. During the late Cambrian, eel-like jawless fish called the conodonts, and small mostly armoured fish known as ostracoderms, first appeared.
The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the Pennsylvanian sub-period of the late Carboniferous period. By the mid-Triassic, there were many synapsid species that looked like mammals. The lineage leading to today's mammals split up in the Jurassic; synapsids from this period include Dryolestes, more closely related to extant placentals and marsupials than to monotremes, as well as Ambondro, more closely related to monotremes.
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A milestone in vertebrate evolution, the transition from water to land, owes its success to the development of a sprawling body plan that enabled an amphibious lifestyle. The body, originally adapted for swimming, evolved to benefit from limbs that enhance ...
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