Geological Time and Mass Extinctions: From the Ordovician to the Cretaceous

Geological Time

Geological time (time units used to study Earth's history) allows us to place biological and geological events on Earth's chronological scale, which spans approximately 4.57 billion years. These markers are essential for understanding the evolution of life and episodes of mass extinction.

Mass Extinctions Through History

A mass extinction (rapid and significant disappearance of species on a geological time scale) is characterized by the disappearance of at least 75% of species within a geologically short time frame, often ranging from a few hundred thousand to a few million years. These events represent major disruptions in the evolution of life, leading to profound ecological upheavals and the reorganization of ecosystems.

The five major extinctions recognized by the scientific community are:

• Ordovician-Silurian (≈ 445 Ma): climatic cooling and significant glaciation, with a major drop in sea levels primarily affecting marine fauna.
• Devonian (≈ 375 Ma): prolonged episodes of oceanic anoxia, sea level variations, and possible impacts, leading to the disappearance of many marine species and some terrestrial vertebrates.
• Permian-Triassic (≈ 252 Ma): the most severe known extinction, with ≈ 96% of marine species and 70% of terrestrial species disappearing, linked to massive volcanic eruptions in Siberia and global climatic disruptions.
• Triassic-Jurassic (≈ 201 Ma): associated with intense volcanism and rapid climate change, this extinction facilitated the diversification of dinosaurs in the Jurassic.
• Cretaceous-Paleogene (≈ 66 Ma): combining the impact of the Chicxulub meteorite and significant volcanic activity, causing the extinction of non-avian dinosaurs and many marine species.

Mechanisms and Causes

The causes of mass extinctions are generally multiple and often combined. They include: meteorite impacts, major volcanic eruptions, rapid climate changes, and disruptions in the chemical composition of the oceans. These factors can interact, combine, and amplify catastrophic effects, leading to a rapid collapse of ecosystems.

For example, the Chicxulub impact (impact of a large asteroid on Earth) in the Cretaceous is directly associated with the disappearance of non-avian dinosaurs, while avian dinosaurs survived and gave rise to modern birds. This impact, combined with volcanic eruptions and temporary global cooling, caused drastic changes in the atmosphere and oceans, simultaneously affecting fauna and flora.

N.B.:
Radiometric dating relies on the decay of radioactive isotopes, such as carbon 14 or uranium 238, to estimate the age of rocks and fossils. This method allows precise positioning of extinction events on the geological time scale.

Comparative Table of Major Extinctions