Cretaceous

Lore & Background

The twofold division into Early and Late Cretaceous was formalized later, notably by Alcide d'Orbigny in the 1840s, who also divided the French Cretaceous into five stages. The first appearance of Calpionella alpina has been proposed as a definition for the boundary.

Reader's Guide

The Cretaceous was a period of relatively warm climate, high sea levels, and numerous shallow inland seas. It saw the dominance of dinosaurs on land and now-extinct marine reptiles, ammonites, and rudists in the oceans. Flowering plants appeared during the Early Cretaceous and rapidly diversified, becoming dominant by the period's end. Many modern taxonomic groups trace their origins to the Cretaceous, including the earliest relatives of placentals and marsupials, crown group birds, and the most diverse subgroup of teleost fish, Acanthomorpha. This event caused the extinction of non-avian dinosaurs, pterosaurs, and large marine reptiles, while groups like crocodilians and mammals that fed on detritus survived.

Did You Know?

• The name 'Cretaceous' is derived from the Latin 'creta', meaning 'chalk'.
• The Cretaceous is the ninth geological period of the entire Phanerozoic.
• The end of the Cretaceous is defined by the abrupt Cretaceous–Paleogene boundary (K–Pg boundary).

A Name Carved in Chalk

The Cretaceous takes its name from the Latin word creta, meaning chalk, a nod to the vast calcium carbonate deposits laid down by the shells of marine invertebrates—principally coccoliths—during the latter half of the period. Belgian geologist Jean d'Omalius d'Halloy first delineated it as a distinct geological period in 1822, calling it Terrain Crétacé based on strata in the Paris Basin. The period is commonly abbreviated K, drawn from the German word Kreide. Spanning roughly 143.1 to 66 million years ago, the Cretaceous stands as the third and final chapter of the Mesozoic Era. At approximately 77.1 million years in length, it also claims the title of the ninth and longest geological period across the entire Phanerozoic eon. Its internal structure is divided into Early and Late epochs, or Lower and Upper series, with a worldwide framework of twelve stages all rooted in European stratigraphy, though local subdivisions persist in various regions.

A Warm World of Shallow Seas

The Cretaceous unfolded under a notably warm climate that drove eustatic sea levels to heights producing countless shallow inland seas across the continents. These vast marine environments teemed with now-extinct creatures: marine reptiles patrolled the waters alongside ammonites and rudists, while on land the dinosaurs maintained their unchallenged dominance. The planet was largely free of ice, though some evidence points to brief glaciation episodes during the cooler first half of the period. Forests stretched all the way to the poles, a testament to the mild global temperatures. The lower boundary of the Cretaceous remains uniquely problematic in stratigraphy—it is the only system boundary in the entire Phanerozoic to lack a formally defined Global Boundary Stratotype Section and Point. The difficulty stems from the strong regionality of biostratigraphic markers and the absence of clear chemostratigraphic events such as isotope excursions that could anchor a global correlation. Calpionellids, enigmatic planktonic protists with urn-shaped calcitic shells, have been proposed as the most promising candidates for fixing this boundary, with the first appearance of Calpionella alpina serving as a leading candidate marker.

Cradle of Modern Life

Many of the dominant biological groups we recognize today trace their deepest origins to the Cretaceous. New lineages of mammals emerged during this interval, including the earliest relatives of placentals (Eutheria) and marsupials (Metatheria), while the earliest crown-group birds appeared toward the period's close. Among vertebrates, teleost fish—now the most diverse group of modern vertebrates—continued their diversification, with their most varied subgroup, Acanthomorpha, making its appearance during the Cretaceous. Perhaps the most transformative botanical event was the arrival of flowering plants in the Early Cretaceous. These angiosperms diversified at a remarkable pace, ultimately becoming the dominant plant group across the entire Earth by the period's end. This rise coincided with the decline and extinction of previously widespread gymnosperm groups. The Cretaceous thus represents a pivotal window in which the biological architecture of the modern world was assembled, laying the groundwork for the ecosystems that would follow the great extinction at the period's close.

The Day the World Ended

The Cretaceous, and with it the entire Mesozoic Era, was brought to an abrupt close by the Cretaceous–Paleogene extinction event. The upper boundary of the period is sharply defined by an iridium-rich layer found worldwide, dated at 66.043 million years ago, and widely attributed to the impact of a large asteroid that formed the Chicxulub crater, whose boundaries circumscribe parts of the Yucatán Peninsula and extend into the Gulf of Mexico. The catastrophe eliminated three-quarters of Earth's plant and animal species, wiping out non-avian dinosaurs, pterosaurs, and large marine reptiles. Species dependent on photosynthesis suffered the most, as atmospheric particles blocked solar energy and collapsed the food chains built upon phytoplankton and land plants. Herbivores that relied on those primary producers perished alongside them. Despite the severity of the event, extinction rates varied significantly between and within different clades. Earth's biodiversity required substantial time to recover, even though an abundance of vacant ecological niches presumably existed in the aftermath.

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