Dinosaurs and Mesozoic Reptiles

Dinosaurs and their Mesozoic reptile relatives dominated terrestrial, marine, and aerial ecosystems for over 150 million years and gave rise to birds.

Definition

Dinosaurs are a clade of archosaurian reptiles, including living birds, that arose in the Triassic; Mesozoic reptiles also encompass the flying pterosaurs and various secondarily marine reptile groups.

Scope

This topic covers the origin, diversity, and biology of dinosaurs, the origin of birds from theropods, and the contemporaneous pterosaurs and marine reptiles such as ichthyosaurs, plesiosaurs, and mosasaurs, along with the end-Cretaceous extinction of non-avian dinosaurs.

Core questions

• How did dinosaurs originate and come to dominate land ecosystems?
• What evidence links birds to theropod dinosaurs?
• What physiology and behavior can be inferred from dinosaur fossils?
• What caused the end-Cretaceous extinction of non-avian dinosaurs?

Key concepts

• Saurischia and Ornithischia
• Feathered theropods and bird origins
• Bone histology and growth
• End-Cretaceous mass extinction

Key theories

Birds as living dinosaurs

Abundant feathered theropod fossils and shared skeletal features establish that birds are nested within theropod dinosaurs, making the dinosaur lineage extant.

Archosaur dominance and physiology

Bone histology, posture, and growth rates suggest many dinosaurs had elevated metabolisms and active lifestyles, reshaping views of reptilian physiology.

Clinical relevance

Dinosaur fossils are key archives of Mesozoic terrestrial ecosystems and evolutionary innovation, and the end-Cretaceous extinction that ended their reign is a central case study in catastrophic environmental change.

History

The name Dinosauria was coined by Richard Owen in 1842, and the great bone rushes of the late nineteenth century filled museums. John Ostrom's work on Deinonychus in the 1960s revived the dinosaur-bird link and launched the modern, dynamic view of dinosaur biology.

Debates

Dinosaur metabolism and thermoregulation

Whether dinosaurs were ectothermic, endothermic, or intermediate remains debated, with evidence from bone histology, posture, and isotopes.

Key figures

• John Ostrom
• Robert Bakker
• Stephen Brusatte

Seminal works

• weishampel2004
• brusatte2018

Frequently asked questions

Are birds really dinosaurs?

Yes. Birds evolved from small feathered theropod dinosaurs and are classified within the dinosaur clade, so the dinosaur lineage survives today as birds.

What killed the non-avian dinosaurs?

The leading explanation is the end-Cretaceous mass extinction triggered by a large asteroid impact, compounded by massive volcanism and environmental disruption.