Final answer:
Amphibians were the first terrestrial tetrapods but remained water-dependent for reproduction. Amniotes evolved amniotic eggs, allowing full terrestrial reproduction and leading to the divergence into mammals and reptiles/birds. This adaptation spurred their success and dominance on land.
Step-by-step explanation:
Evolution of Tetrapods and Amniotes
Amphibians were the first tetrapod vertebrates to colonize land. Nonetheless, they maintained a dependency on water for reproduction due to their non-amniotic eggs, which required a moist environment to avoid desiccation. In contrast, the evolution of amniotes—which include reptiles, birds, and mammals—represented a pivotal point in vertebrate evolution.
The earliest amniotes appeared around 350 million years ago and were key to the colonization of terrestrial environments due to their amniotic eggs. These eggs are equipped with special membranes that allow for gas exchange while retaining moisture, enabling them to be laid on land without drying out. This adaptation allowed amniotes to become fully terrestrial, no longer requiring a water body for their reproductive cycle.
Synapsids and sauropsids are two significant clades of early amniotes that diverged. Synapsids led to the evolution of mammals, while sauropsids gave rise to reptiles, birds, and eventually dinosaurs. This division was primarily based on differences in their skull structures.
While the Paleozoic era saw a proliferation of large amphibian tetrapods, the first amniotes were comparatively smaller but had the significant advantage of the amniotic egg, which led to greater reproductive success on land. This eventually resulted in the dominance of amniotes in most terrestrial ecosystems.