 Vertebrate Palaeontology. Vertebrate Palaeontology is the subfield of Palaeontology at CX 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 Mmehaanum, million years ago. The Deconian period 395 to 345 Mmeha brought in the changes that allowed primitive air-breathing fish to remain on land as long as they wish, thus becoming the first terrestrial vertebrates, the Amphidians. Amphidians 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 Mmeha. The reptilian changes and adaptations to diet and geography are chronicled in the fossil record of the varying forms of thoraxida. True mammals showed up in the Triassic period 225 to 190 Mmeha around the same time as the dinosaurs, which also sprouted from the reptilian line. Birds first diverged from dinosaurs between 100 Mmeha and 60 Mmeha.