Historical Scope of the Tertiary Period
The “Tertiary Period” historically referred to a significant span of Earth’s history, encompassing the early part of the Cenozoic Era. This period began approximately 66 million years ago (Ma), immediately following the mass extinction event that marked the end of the Cretaceous Period and the Mesozoic Era. It concluded around 2.58 Ma, leading into the Quaternary Period.
During the Tertiary, Earth experienced considerable geological activity, including the continued breakup of continents and the uplift of major mountain ranges such as the Himalayas and the Alps. Global climates shifted from the warmer conditions of the Mesozoic, with a general cooling trend observed over millions of years. This period saw the diversification of life forms, particularly mammals and flowering plants, which began to dominate terrestrial ecosystems.
Subdivisions and Defining Features
The Tertiary Period was traditionally divided into five distinct epochs: the Paleocene, Eocene, Oligocene, Miocene, and Pliocene. These subdivisions provided a framework for understanding the progressive climatic, geological, and biological changes that occurred over tens of millions of years.
Paleocene Epoch
The Paleocene Epoch, spanning from approximately 66 to 56 Ma, marked the initial recovery of life following the Cretaceous-Paleogene extinction event. Mammals, previously overshadowed by dinosaurs, began to diversify and grow in size, filling ecological niches. The climate was generally warm, with tropical conditions extending to higher latitudes.
Eocene Epoch
The Eocene Epoch (56 to 33.9 Ma) followed the Paleocene. It is recognized for its exceptionally warm global temperatures, including a period known as the Paleocene-Eocene Thermal Maximum. This epoch saw the emergence of many modern mammalian orders, including early primates, horses, and whales. Extensive forests covered much of the land, supporting a rich biodiversity.
Oligocene Epoch
The Oligocene Epoch, from approximately 33.9 to 23.03 Ma, represented a significant shift towards cooler and drier global climates. This cooling led to the expansion of grasslands and the retreat of dense forests, influencing the evolution of grazing mammals. Antarctica became more glaciated during this time, marking a step towards modern ice ages.
Miocene Epoch
The Miocene Epoch (23.03 to 5.33 Ma) was characterized by continued global cooling and drying trends. The widespread development of grasslands further facilitated the evolution and diversification of large grazing mammals. Major mountain-building events, such as the uplift of the Himalayas, significantly altered global weather patterns.
Pliocene Epoch
The Pliocene Epoch (5.33 to 2.58 Ma) was the final epoch of the traditional Tertiary Period. During this time, global temperatures continued to cool, leading to the formation of the Arctic ice cap and a general increase in glacial cycles. This epoch is notable for the continued evolution of hominins in Africa, marking important steps in human ancestry.
The Reclassification of the Tertiary
The term “Tertiary Period” is no longer formally recognized by the International Commission on Stratigraphy (ICS), the body responsible for standardizing Earth’s geological timescale. This decision was part of a broader effort to refine and modernize geological nomenclature based on more precise global stratigraphic boundaries and consistent principles. The original classification system, which included “Tertiary,” stemmed from an older, less scientifically rigorous understanding of Earth’s history.
Historically, the Earth’s rock layers were broadly categorized into four “orders”: Primary, Secondary, Tertiary, and Quaternary. This system, developed in the 18th and 19th centuries, was based more on relative position and general characteristics of rock formations rather than defined global events. As geological science advanced, particularly with the development of radiometric dating and detailed biostratigraphy, the imprecision of these older terms became apparent.
The term “Tertiary” became problematic because it represented a long, geologically diverse interval lacking a single, unifying characteristic. Modern stratigraphic principles emphasize defining geological periods by specific, globally recognizable boundary stratotypes. The “Tertiary” did not align with these updated criteria, making its continued use inconsistent with the rigorous framework of the International Chronostratigraphic Chart.
Current Geological Nomenclature
To address the inconsistencies and imprecision of the “Tertiary Period,” modern geology has replaced it with two distinct periods: the Paleogene Period and the Neogene Period. This revised classification provides a more scientifically robust and globally applicable framework for understanding the Cenozoic Era. These new divisions are precisely defined by global boundary stratotype sections and points (GSSPs).
Paleogene Period
The Paleogene Period encompasses the earlier part of what was formerly the Tertiary, spanning from approximately 66 to 23.03 Ma. It includes the Paleocene, Eocene, and Oligocene epochs. This period captures the critical time immediately following the Mesozoic Era, characterized by the initial diversification of mammals and a generally warm climate that transitioned to cooler conditions.
Neogene Period
The Neogene Period follows the Paleogene, extending from about 23.03 to 2.58 Ma. It comprises the Miocene and Pliocene epochs. This period is marked by significant global cooling trends, the expansion of grasslands, and the ongoing evolution of many modern animal groups, including the development of hominins.