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Why did reptiles become more diverse during the Triassic recovery?

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Line graph showing the number of marine animal families from the Precambrian era to the present, with illustrations of key groups such as trilobites, ammonoids, marine reptiles, and diatoms. The graph highlights major extinction events and a rise to 1,900 living families today. — Permian extinction The diversity of marine animal families since late Precambrian time. The data for the curve comprise only those families that are reliably preserved in the fossil record; the 1,900 value for living families also includes those families rarely preserved as fossils. The several pronounced dips in the curve correspond to major mass-extinction events. The most catastrophic extinction took place at the end of the Permian Period.

Following the Permian-Triassic mass extinction—the most severe in Earth's history—the Triassic Period served as a recovery phase where the relative importance of land animals increased significantly. Reptiles became more diverse and numerous as they filled ecological niches left vacant by the extinction of many other groups.1

This diversification was driven by several key evolutionary developments:

Adaptive Radiation: The Triassic saw the emergence of major new lineages, including the first dinosaurs and archosaurs, which began a great radiation that would define the Mesozoic Era.12
Marine Colonization: As sea levels rose, continental shelves were colonized for the first time by large marine reptiles, such as ichthyosaurs and sauropterygians, which evolved specialized body forms for aquatic life.13
Locomotion and Specialization: Archosaurs developed distinct limb and pelvic modifications, including upright gaits in some groups, which allowed for greater agility and diverse modes of life, such as the development of pterosaurs for flight.452
Niche Expansion: Reptiles adapted to various environments, ranging from terrestrial predators to specialized marine forms and even early gliders.352

This period of recovery established the foundation for the dominance of reptiles throughout the remainder of the Mesozoic Era.14