Glossary

The Cretaceous Period is the third and final period of the Mesozoic Era, spanning from approximately 145.0 million years ago (Ma) to 66.0 Ma. At roughly 79 million years in duration, it is the longest period of the entire Phanerozoic Eon. It follows the Jurassic Period and precedes the Paleogene Period of the Cenozoic Era. During the Cretaceous, the breakup of Pangaea accelerated: the Atlantic Ocean widened, India rifted away from Gondwana and began its northward migration, and most modern continents approached their present positions. Vigorous seafloor spreading caused sea levels to rise 100–250 metres above present-day levels, flooding continental interiors with vast epicontinental seas such as the Western Interior Seaway of North America. Under a warm greenhouse climate with no polar ice sheets and elevated atmospheric CO₂ (estimated at times exceeding 1,000 ppm), forests grew at high latitudes including Antarctica. Flowering plants (angiosperms) diversified explosively, fundamentally reshaping terrestrial ecosystems. The period witnessed peak dinosaur diversity, with iconic taxa such as Tyrannosaurus, Triceratops, and hadrosaurs dominating on land, while mosasaurs, plesiosaurs, and pterosaurs ruled the seas and skies. The Cretaceous ended with the K-Pg mass extinction event approximately 66 Ma, triggered primarily by the Chicxulub asteroid impact on the Yucatán Peninsula (Mexico), compounded by massive Deccan Traps volcanism in India. About 76% of all species were lost, including all non-avian dinosaurs, pterosaurs, and large marine reptiles, clearing ecological space for the subsequent radiation of birds and mammals in the Cenozoic.

The **Jurassic Period** is the second of three periods constituting the Mesozoic Era. According to the International Commission on Stratigraphy's (ICS) 2024 International Chronostratigraphic Chart, it spans from approximately **201.4 Ma** (±0.2) to **143.1 Ma** (±0.6), a duration of roughly **58 million years**. The period commenced immediately after the end-Triassic mass extinction — one of Earth's five largest extinction events, which eliminated approximately half of all marine invertebrate genera — and dinosaurs rapidly filled the vacated ecological niches to become the dominant terrestrial vertebrates. Pangaea continued to rift into the northern landmass Laurasia and the southern landmass Gondwana, the proto-Atlantic Ocean began to open, and globally warm greenhouse conditions prevailed, with atmospheric CO₂ concentrations estimated at four or more times present levels. Giant sauropods such as *Brachiosaurus* and *Diplodocus*, large theropod predators including *Allosaurus*, and armoured dinosaurs like *Stegosaurus* flourished on land, while the Late Jurassic yielded the earliest known bird fossil, *Archaeopteryx*, providing pivotal evidence for the dinosaur-to-bird evolutionary transition.

The **Mesozoic Era** is the second of the three major geologic eras of the Phanerozoic Eon, spanning from approximately 251.9 million years ago (Ma) to 66.0 Ma — a duration of roughly 186 million years. It is bounded by two of the most catastrophic mass extinction events in Earth's history: the Permian–Triassic extinction at its base and the Cretaceous–Paleogene (K–Pg) extinction at its top. The era is subdivided into three periods — the Triassic (251.9–201.4 Ma), the Jurassic (201.4–145.0 Ma), and the Cretaceous (145.0–66.0 Ma). During the Mesozoic, the supercontinent Pangaea progressively fragmented into the modern continental configuration, and a predominantly warm, ice-free greenhouse climate prevailed, with sea levels at times exceeding present levels by as much as 170–200 meters. Archosaurs — particularly dinosaurs — dominated terrestrial ecosystems, while pterosaurs ruled the skies and marine reptiles such as ichthyosaurs and plesiosaurs occupied the oceans. The era witnessed the origination of key modern lineages including mammals, birds, and angiosperms (flowering plants), as well as a fundamental restructuring of marine ecosystems through escalating predation pressures termed the Mesozoic Marine Revolution. The extinction of all non-avian dinosaurs at 66 Ma brought the Mesozoic to a close and opened the way for the mammal-dominated Cenozoic Era.

The **Triassic Period** is the first of three geological periods of the Mesozoic Era, spanning from approximately 251.902 ± 0.024 Ma to 201.4 ± 0.2 Ma, a duration of roughly 50.5 million years according to the ICS International Chronostratigraphic Chart (v2024/12). It is preceded by the Permian Period and followed by the Jurassic Period. The Triassic opened in the immediate aftermath of the Permian–Triassic mass extinction ("the Great Dying"), the most catastrophic extinction event in Earth's history, which eliminated approximately 81% of marine species and 70% of terrestrial vertebrate species. During the Triassic, all major landmasses were joined in the supercontinent Pangaea, straddling the equator. This configuration produced a predominantly hot and arid global climate, with no polar ice caps and extreme continentality in the interior, while monsoonal circulation dominated coastal zones. Pangaea began rifting apart in the Middle to Late Triassic, initiating the opening of the Tethys Ocean and proto-Atlantic basins. The Triassic is of profound evolutionary significance as the period during which many of the dominant modern terrestrial vertebrate lineages first appeared, including dinosaurs (earliest undisputed fossils ~231 Ma), pterosaurs (~228 Ma), mammaliaforms (~225 Ma), crocodylomorphs, turtles, and lepidosauromorphs. Throughout most of the period, however, ecosystems were dominated not by dinosaurs but by non-dinosaurian archosaurs, particularly pseudosuchians (the crocodile-line archosaurs). The end-Triassic extinction event (~201.4 Ma), associated with massive volcanism of the Central Atlantic Magmatic Province (CAMP), eliminated approximately 76% of all species and removed many of the dinosaurs' competitors, thereby setting the stage for dinosaurian dominance during the Jurassic and Cretaceous.