Paleogene

• 1. Introduction

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  The Paleogene Period begins at one of the most dramatic boundary lines in the fossil record: a thin, rusty-colored layer of clay, just 50 centimeters thick, deposited at a site called Oued Djerfane west of El Kef in Tunisia. That clay marks the end of the Cretaceous and the start of 43 million years of biological and geological transformation. Packed inside it are microtektites, nickel-rich spinel crystals, shocked quartz, and an iridium anomaly tied to the asteroid impact whose crater remnants lie at Chicxulub on Mexico's Yucatan Peninsula. The clay itself, scientists believe, was laid down over only a few days.

  What followed those few days shaped the living world we inhabit today. Non-avian dinosaurs, pterosaurs, marine reptiles, and many fish groups vanished, leaving ecological space that surviving creatures rushed to fill. Mammals, birds, and fish all diversified at a pace rarely matched in Earth's history. Meanwhile the planet's surface was cracking apart and buckling upward, opening new oceans, raising mountain ranges from the Mediterranean to the Himalayas, and swinging global temperatures from greenhouse warmth to the first Antarctic ice sheets.

  Three epochs divide those 43 million years: the Paleocene, the Eocene, and the Oligocene. Each carried a different climate, a different fauna, a different configuration of continents. How survivors became dynasties, how warm seas turned to polar ice, and how the world's plates rearranged themselves into something resembling the modern map are the questions this documentary will answer.

• 2. An Explosion of Life

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  Percomorph fish, the most diverse group of vertebrates alive today, had their first appearance near the end of the Cretaceous, but it was during the Paleogene that they radiated into the enormous range of forms and families seen in modern oceans. Mammals tell a similar story. A handful of small, generalized survivors from the extinction event expanded into orders that colonized land, sea, and air. Cetaceans and sirenians descended from mammals that returned to the ocean. Sciuridae and primates, the group that includes humans, arose among those that took to trees.

  Birds, the only lineage of dinosaurs that survived, inherited skies that pterosaurs no longer occupied. From the very few neognath and paleognath clades that made it through the extinction, birds diversified into multiple orders and achieved what the source describes as an extreme level of morphological diversity. Flightless groups found new roles too. Penguins, ratites, and terror birds colonized niches vacated by hesperornithes and other extinct dinosaurs.

  Tropical taxa diversified faster than those at higher latitudes, producing a pronounced latitudinal diversity gradient. Myctophid fish first appeared in the late Paleocene or early Eocene, and for most of the Eocene and Oligocene they were restricted to shelf seas before expanding into open ocean during the warm interval at the close of the Oligocene. The surviving fauna were not simply filling empty space at random; climate, geography, and the sequence of warming and cooling events channeled diversification in specific directions, a theme that runs through every chapter of the Paleogene.

• 3. Heat, Ice, and the Swinging Climate

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  At the Paleocene-Eocene boundary, global mean surface temperatures climbed to 31.6 degrees Celsius, making the Paleocene-Eocene Thermal Maximum one of the warmest intervals of the entire Phanerozoic eon. A study published in 2018 found that from about 56 to 48 million years ago, annual air temperatures over land at mid-latitudes averaged roughly 23-29 degrees Celsius, some 10-15 degrees higher than those regions experience today. Tropical and subtropical forests pushed into polar regions, and rising humidity left a clear record in the form of increased kaolinite in sediments.

  The trigger for that spike was volcanic. Magmatic intrusions into organic-rich seafloor sediments in the North Atlantic Igneous Province, between roughly 56 and 54 million years ago, rapidly released large quantities of greenhouse gases. That warming then melted frozen methane hydrates on continental slopes, adding more greenhouse gases in a cascade. Higher temperatures also accelerated bacterial decomposition of organic matter, returning additional carbon dioxide to the ocean rather than allowing it to be buried.

  Around 70 percent of deep-sea foraminifera species went extinct as the Arctic Ocean warmed during the PETM. On land, many modern mammals, primates among them, appeared at roughly the same time. The PETM was followed by two lesser warming events: the Eocene Thermal Maximum 2 at about 53.69 million years ago, and the Eocene Thermal Maximum 3 at roughly 53 million years ago.

  The reversal came from an unexpected source. Around 48.5 million years ago, aquatic ferns of the genus Azolla proliferated across the Arctic Ocean and sequestered large quantities of atmospheric carbon dioxide. From that Azolla event until about 34 million years ago, a slow cooling trend known as the Middle-Late Eocene Cooling drew temperatures steadily downward. By about 37 million years ago there is evidence of glaciation in Antarctica, and by the Eocene-Oligocene boundary, ocean sediments indicate a full ice sheet in western Antarctica reaching the sea. The Early Oligocene Glacial Maximum lasted roughly 200,000 years, and by the early Oligocene, North American and Eurasian tropical and subtropical forests had been replaced by dry woodlands and spreading grasslands.

• 4. Continents on the Move

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  Australia and South America rifted away from Antarctica during the Paleogene, opening the Southern Ocean. The passages that formed south of Tasmania and through the Drake Passage established the Antarctic Circumpolar Current, which played a direct role in the cooling that drove Antarctica's glaciation. Dense, cold polar water sank into the deep ocean and moved northward, reducing temperatures globally. That cooling, some evidence suggests, may have occurred over less than 100,000 years.

  In the North Atlantic, seafloor spreading propagated northward from the Central Atlantic during the Paleocene, reaching the Labrador Sea at roughly 62 million years ago and Baffin Bay at roughly 57 million years ago, then pushing into the northeastern Atlantic between Greenland and Eurasia by about 54 million years ago. Greenland briefly became an independent plate from the Eocene into the early Oligocene, drifting northward and rotating anticlockwise. Around 47 million years ago, the Reykjanes Ridge propagated northward and split off the Jan Mayen microcontinent from Greenland's eastern margin.

  The Indian continent had rifted from Madagascar at roughly 83 million years ago and moved northward at about 18 centimeters per year during the Paleocene. A drop in speed to roughly 5 centimeters per year in the early Eocene records the collision of the Tethyan Himalayas with the Lhasa terrane of Tibet. The Pacific Ocean, meanwhile, reorganized dramatically around 50 million years ago when the Izanagi-Pacific spreading ridge was subducted beneath East Asia, changing the forces on the Pacific plate and triggering subduction along the Izu-Bonin-Mariana and Tonga-Kermadec arcs. In the Oligocene, at roughly 28 million years ago, the first segment of the Pacific-Farallon spreading ridge entered the North American subduction zone near Baja California, initiating the major strike-slip movements that formed the San Andreas Fault.

• 5. Mountains Rising

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  The Zagros mountain belt, stretching roughly 2,000 kilometers from the eastern border of Iraq to the Makran coast in southern Iran, took shape as the Arabian and Eurasian plates converged and the Neotethys Ocean closed. Continental collision there began during the Eocene at around 35 million years ago and continued into the Oligocene, until roughly 26 million years ago.

  The Alpine orogeny produced a different geometry. As the African plate moved toward Eurasia and the Adriatic promontory, known as Adria, pushed northward from the African plate, several short subduction zones developed rather than one long system. This configuration produced the arc-shaped mountain chains that run from the Tell-Rif-Betic cordillera in the western Mediterranean through the Alps, Carpathians, Apennines, Dinarides, and Hellenides to the Taurides in the east. Between about 40 and 30 million years ago, roll-back of the subducting slab under the western Mediterranean arcs explains why those ranges curve the way they do.

  In North America, the Laramide orogeny built the Rocky Mountains as the dip of the subducting Farallon plate decreased, creating a flat-slab segment that transmitted friction far inland, producing faults that reached mid-crustal depths and uplifted basement rocks more than 700 kilometers from the trench. By the mid to late Eocene, between 50 and 35 million years ago, plate convergence rates slowed, the Farallon slab steepened again, and much of the uplifted terrain was leveled by erosion. The Red Sea began to open in the Oligocene when flood basalts erupted across Ethiopia, northeast Sudan, and southwest Yemen as the Afar mantle plume began pushing against the base of the African lithosphere.

• 6. Flora Transformed

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  Pronounced cooling in the Oligocene drove one of the largest plant turnover events of the Cenozoic. Grasses and herbs, including Artemisia, proliferated as tropical vegetation retreated. Conifer forests established themselves in mountainous areas. The evidence for this shift comes from the palynological record, meaning pollen preserved in sediments that allows scientists to reconstruct which plants were living where and when.

  Earlier in the Paleogene, during the peak warmth of the Eocene, the opposite had been true. Tropical and subtropical forests had extended into polar regions, and the humidity associated with those forests fed additional water vapor into the atmosphere, itself a greenhouse gas that reinforced the warm conditions. The Azolla event that curtailed those conditions around 48.5 million years ago reshaped not just climate but the competitive landscape for plants across the planet. The cooling trend that began then continued, with fluctuations, through the Pleistocene.

  The early Oligocene shift to grasslands and dry woodlands across North America and Eurasia created the ecological stage on which many later mammal groups would evolve. Among the plants that came to prominence during this Oligocene cooling were the ancestors of the grass-dominated ecosystems that today cover much of Earth's land surface.