Vertebrate paleontology

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Vertebrate paleontology is the subfield of paleontology that seeks to discover, through the study of fossilized remains, the behavior, reproduction and appearance of extinct vertebrates (animals with vertebrae and their descendants). 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 modern fish as well as mammals, birds, reptiles and amphibians, 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 485 to 444 Ma (megaannum, million years ago), with jawed vertebrates emerging in the following Silurian period (444 to 419 Ma) with the placoderms and acanthodians. The Devonian period (419 to 359 Ma) saw primitive air-breathing fish to develop limbs allowing them to walk on land, thus becoming the first terrestrial vertebrates, the stegocephalians.

Romer's gap in the early Carboniferous period (359 to 299 Ma) left little of the early stegocephalians, but allowed vertebrates more adapted to life on land to flourish in their wake. Crown-group tetrapods appeared in the early Carboniferous, with temnospondyls dominating the ecosystem and becoming the first land vertebrate megafauna. A lineage of reptiliomorphs developed a metabolism better suited for life exclusively on land, as well as a novel form of reproduction freeing them from the water: the amniotic egg, with full-fledged amniotes appearing in the mid-Carboniferous. Sharks and their holocephalian relatives flourished in the seas, while rivers were dominated by lobe-finned fish like rhizodonts.

During the Permian period (299 to 252 Ma), one of the two major branches of amniotes, the synapsids, flourished, with derived therapsids taking over in the middle of the period. The Great Dying wiped out most of the synapsid diversity, with archosaurs, emerging from the other sauropsid branch, replacing many of them in the Triassic period (252 to 201 Ma). Lissamphibians, modern amphibians, likely arose around that time from temnospondyls. True mammals, derived from cynodont therapsids, showed up in the Middle Triassic around the same time as the dinosaurs, who emerged from a clade of archosaurs. At the same time, ray-finned fish diversified, leading to teleost fish dominating the seas.

Ancestral birds (Avialae) like Archaeopteryx^[1] first evolved from dinosaurs during the Jurassic, with crown-group birds (Neornithes) emerging in the Cretaceous between 100 Ma and 60 Ma.^[2]

The K-Pg mass extinction wiped out many vertebrate clades, including the pterosaurs, plesiosaurs, mosasaurs and nearly all dinosaurs, leaving many ecological niches open. While therian mammals had already evolved in the Late Jurassic, they would rise to prominence in the Paleogene following the mass extinction and remain to this day, although squamates and birds still lead in diversity.