The Cretaceous Period: A Pivotal Era in Earth’s Geological and Biological History

The Cretaceous Period, spanning approximately 79 million years from 145.5 to 66 million years ago, represents the final and longest segment of the Mesozoic Era, widely recognized as the “Age of Dinosaurs”.¹ This geological interval was characterized by profound transformations in Earth’s geography, climate, and biodiversity, culminating in a cataclysmic mass extinction event that fundamentally reshaped the trajectory of life on Earth.¹ During this time, supercontinents continued to fragment, sea levels rose dramatically, and a warm, humid “greenhouse” climate prevailed globally.¹ Biologically, the Cretaceous witnessed the peak diversity of dinosaurs, the revolutionary rise and diversification of flowering plants (angiosperms), and the emergence of modern groups of mammals, birds, and insects.¹

The Cretaceous served as a dynamic crucible where the foundations of modern life were forged, even as the previous era concluded. While it is often noted as the last portion of the “Age of Dinosaurs,” implying an end, this period simultaneously saw the appearance of new dinosaur lineages and the initial fossils of many modern insect groups, mammals, and birds, alongside the groundbreaking evolution of flowering plants.² This period was not merely a terminal phase but a time of intense biological innovation. The subsequent mass extinction event at its close then acted as a powerful catalyst, clearing ecological niches and allowing these newly diversified groups, which had previously played secondary roles, to proliferate and become dominant in the ensuing Cenozoic Era.¹ The rich fossil record left behind from this period, alongside significant geological formations such as vast chalk deposits and organic-rich black shales, provides invaluable insights into Earth’s dynamic past and holds critical implications for understanding present and future environmental changes.¹

Defining the Cretaceous Period: Timeframe and Characteristics

The Cretaceous Period, deriving its name from the Latin word “creta” for chalk, stands as the longest geological period of the Mesozoic Era, encompassing approximately 79 million years.¹ Its commencement is marked at roughly 145.5 million years ago, succeeding the Jurassic Period, and its abrupt conclusion occurred 66 million years ago with the momentous Cretaceous-Paleogene (K-Pg) extinction event.² For chronological clarity, this extensive period is further subdivided into two primary epochs: the Early Cretaceous, spanning from 145 to 100 million years ago, and the Late Cretaceous, from 100 to 66 million years ago.³

The very name of the Cretaceous Period encapsulates a defining geological and biological characteristic of the time. The vast deposits of chalk, which are primarily composed of the skeletal remains of marine organisms, are a hallmark of this period.⁵ These extensive formations are indicative of the warm, shallow marine environments that prevailed across much of the globe, demonstrating how the period’s etymology directly reflects a dominant biogenic and environmental phenomenon.³

Globally, the Cretaceous was characterized by a warm and humid climate, notably lacking polar ice caps and experiencing significantly higher sea levels, which led to the inundation of continental margins and the formation of extensive inland seas.¹ Geographically, it was a time of intense tectonic activity marked by the continued fragmentation of the supercontinent Pangaea, a process that had initiated in the Jurassic.¹ This continental breakup played a crucial role in shaping global geography, creating new oceanic basins and extensive coastlines. The resulting large-scale geographic isolation of landmasses promoted the evolutionary divergence among terrestrial life forms, illustrating a profound and multi-faceted interplay between Earth’s physical processes and biological evolution.² The period is also notable for its rich fossil records and the formation of significant natural resources, including the aforementioned chalk and limestone deposits, and organic-rich black shales, which are vital source rocks for petroleum and natural gas.¹

Global Geography and Climate: A Greenhouse World

The Cretaceous Period was fundamentally a “greenhouse world,” distinguished by a significantly warmer climate than the present day and the complete absence of polar ice caps.³ Global average temperatures were elevated, and the climate was generally humid, a condition largely supported by high levels of atmospheric carbon dioxide (CO2).³ Evidence, such as the presence of palm trees and reptiles in continental interiors north of the Arctic Circle, suggests remarkably warm conditions even at high latitudes.⁶ However, despite these elevated CO2 levels, it remains a subject of ongoing scientific inquiry how the polar continental interiors maintained such warmth throughout the year, indicating that the Cretaceous greenhouse was not a static, uniformly hot state and that its mechanisms are still being explored.⁶ Furthermore, while generally warm, a cooling trend became discernible towards the Late Cretaceous, with tropical regions becoming more restricted and northern latitudes experiencing more pronounced seasonal climatic conditions.² This shift influenced the evolution of forests, which began to resemble modern forests with the increasing commonality of oaks, hickories, and magnolias in North America by the period’s end.²

A defining geological feature of the Cretaceous was the continued fragmentation of the supercontinent Pangaea, a process that had commenced its breakup approximately 200 million years ago.² By the mid-Cretaceous, Pangaea had split into several smaller continents, gradually moving towards the present-day configuration of Earth’s landmasses.² This extensive tectonic activity led to the expansion of the Atlantic Ocean as the Americas drifted westward, and the formation of significant features like the Western Interior Seaway, which bisected North America into eastern (Appalachia) and western (Laramidia) halves.⁹ The rifting also generated extensive new coastlines, increasing available near-shore habitats and creating large sedimentary basins such as the Gulf of Mexico and the North Sea.² This demonstrates that the tectonic processes of the Cretaceous were not merely about the rearrangement of landmasses but were a fundamental, interconnected driver of habitat creation, biogeographic isolation, and consequently, the diversification and evolution of life on a global scale.

Accompanying the warm climate were exceptionally high eustatic sea levels, reaching up to 200 meters higher than today, which flooded large portions of continents and formed vast inland lakes and seas.³ These conditions fostered the widespread deposition of marine sediments, including the massive chalk and limestone deposits that lend the period its name.³ The interplay of these geological events significantly shaped the Earth’s surface and its biological inhabitants.

Table 1: Key Geological Events and Their Impacts on Landscape and Life Forms

Event	Impact on Landscape	Impact on Life Forms
Break-up of Pangaea	Creation of new oceans and seas	Isolation of species, driving evolution
Formation of sedimentary basins	Creation of new habitats	Support for diverse marine life
Creation of new mountain ranges	Changes to climate and geography	Creation of new habitats and isolation of species
Source: 3

Flora: The Rise and Diversification of Angiosperms

The Cretaceous Period marks a profound turning point in terrestrial plant evolution, primarily due to the emergence and explosive diversification of flowering plants, or angiosperms.¹ This fundamental change in the dominant group of terrestrial autotrophs began in the Early Cretaceous, around 125 to 130 million years ago, with their initial appearance.² Their rapid radiation followed in the middle Cretaceous, approximately 90 to 100 million years ago.²

The rise of angiosperms had profound consequences for the other organisms inhabiting terrestrial ecosystems and for the planet as a whole.¹¹ This vegetational change, and the accompanying shifts in ecological dominance, impacted the evolution of other organisms and ecological processes across all scales, from interactions between individual species to key processes in the global carbon and hydrological cycles.¹¹ The diversification of angiosperms served as a co-evolutionary catalyst, not merely an isolated botanical phenomenon. For instance, the evolution of eusocial bees was integral to the proliferation of flowering plants, and many modern groups of insects, including the oldest known ants and butterflies, diversified around the same time as angiosperms.² This highlights the “Cretaceous terrestrial revolution” as a system-wide ecological transformation, where the development of flowers and fruits provided new food sources and niches, leading to reciprocal evolutionary advancements among insects (e.g., pollinators, herbivores) and other organisms.¹¹

Early angiosperms did not initially develop shrub- or tree-like morphologies, but by the close of the Cretaceous, many recognizable modern forms had evolved, with oaks, hickories, and magnolias becoming common in North America.² Angiosperms thrived in diverse environments, such as damper climates, habitats previously favored by cycads and cycadeoids, and riparian zones, though their invasion of high southern latitudes was delayed until the end of the period.²

While angiosperms were diversifying, other plant groups also played significant roles. Conifer diversity, initially low in higher latitudes of the Northern Hemisphere, increased exponentially by the middle of the period.² Swamps were dominated by both conifers and angiosperm dicots.² Ferns continued to dominate open, dry, and/or low-nutrient lands.²

Understanding the origin and early diversification of angiosperms was historically hindered by what was perceived as an uninformative fossil record, uncertain relationships, and apparent morphological gaps.¹² However, significant progress has been made over the past 50 years. The plant fossil record, including studies of pollen, leaves, and exquisitely preserved mesofossils (flowers, stamens, fruits, and seeds) from sites like the Potomac Group in eastern North America and the Catefica mesofossil flora in Portugal, has provided previously unanticipated detail on floral form and increasing complexity.¹¹ These advancements, combined with phylogenetic analyses of living plants, have led to a broadly consistent and coherent picture of angiosperm evolution through more than half of their entire evolutionary history, illustrating how technological innovations in analysis and imaging can resolve long-standing scientific questions.¹¹

Fauna: Dominance, Diversification, and Evolution

The Cretaceous Period, often synonymous with the “Age of Dinosaurs,” saw these terrestrial giants reach their peak diversity and dominance across all continents, including cold polar latitudes.¹ New types of dinosaurs, such as the first ceratopsian and pachycephalosaurid dinosaurs, appeared during this time.² Iconic Late Cretaceous dinosaurs included the large land predator

Tyrannosaurus rex, the horned Triceratops, and “duck-billed” hadrosaurs like Shantungosaurus.⁴ Fossil finds from sites like the Liaoning lagerstätte (Yixian Formation) in China have provided remarkable insights into Early Cretaceous dinosaur life, revealing small coelurosaur dinosaurs, including Maniraptora (a group encompassing modern birds and their closest non-avian relatives), with notable hair-like feathers.⁴

In the skies, pterosaurs were common in the early and middle Cretaceous, but their diversity declined towards the end of the period for reasons not fully understood, with only three highly specialized families (Pteranodontidae, Nyctosauridae, and Azhdarchidae) remaining by the close of the era.⁴ The giant

Quetzalcoatlus, an azhdarchid, was one of the largest flying animals known.⁴ Concurrently, avian dinosaurs, the ancestors of modern birds, diversified significantly throughout the period, with the earliest crown group birds appearing towards the end.⁴ Examples include the crow-sized

Confuciusornis from the Early Cretaceous and the toothed, seabird-like Ichthyornis from the Late Cretaceous.⁴

The Cretaceous was an “Age of Dinosaurs” but also an “Age of Emerging Modernity.” While dinosaurs were at their zenith, the foundational groups of modern terrestrial and aerial life were simultaneously undergoing significant diversification and evolution. The period saw the first fossils of many insect groups, modern mammal and bird groups, and the first flowering plants.² New groups of mammals and birds appeared, including the earliest relatives of placentals and marsupials, and the earliest crown group birds.⁴ This highlights a critical duality: a period of intense biological innovation and co-existence of old and new dominant forms, rather than just a static “dinosaur age.”

Marine environments were dominated by formidable marine reptiles such as ichthyosaurs (present in early to mid-Cretaceous, extinct by the Cenomanian-Turonian anoxic event), plesiosaurs (present throughout), and mosasaurs (appearing in the Late Cretaceous, with Tylosaurus being a large example).⁴ Ammonites, extinct shelled relatives of squids, flourished and were a principal food source for mosasaurs.⁴ Other marine life included diverse fish (rays, modern sharks, diversifying teleosts like Acanthomorpha), reef-building rudist clams, inoceramids, and various plankton and echinoderms.⁴

Recent research has significantly re-evaluated marine ecosystem dominance during the Cretaceous. A study published in Science in 2025, utilizing advanced 3D digital fossil-mining, revealed that ancient squids were far more prevalent and dominant in oceans 100 million years ago than previously thought.¹⁵ This research, based on the identification of thousands of fossilized cephalopod beaks in Late Cretaceous rocks from Japan, demonstrated that squids (including both near-shore Myopsida and open-sea Oegopsida) had already originated and diversified explosively long before the K-Pg extinction event, challenging the long-held belief that their flourishing began only after the demise of dinosaurs.¹⁵ This suggests squids were “pioneers of fast and intelligent swimmers” that dominate modern oceans, overturning a long-held view and indicating a more complex and dynamic marine food web than previously assumed.¹⁵

Early mammals, though generally small-sized, were a relevant component of the fauna.⁴ While true marsupials and placentals only appeared at the very end of the period, a variety of non-marsupial metatherians and non-placental eutherians diversified, occupying roles as carnivores, aquatic foragers, and herbivores.⁴ Multituberculates, particularly cimolodonts, even outnumbered dinosaurs in some northern sites by the Late Cretaceous.⁴ Insects also underwent significant diversification, with the appearance of the oldest known ants, termites, butterflies, aphids, grasshoppers, and gall wasps.² Rhynchocephalians (today represented only by the tuatara) declined in North America and Europe but remained diverse in high-latitude southern South America.⁴

Table 2: Major Faunal Groups of the Cretaceous Period

Faunal Group	Key Examples/Types	Significance/Characteristics
Dinosaurs (Non-avian)	Tyrannosaurus rex, Triceratops, Hadrosaurs	Peak diversity, dominant land animals
Marine Reptiles	Mosasaurs, Plesiosaurs, Ichthyosaurs	Dominant marine predators
Pterosaurs	Quetzalcoatlus	Common early/mid-Cretaceous, specialized decline
Birds (Avian Dinosaurs)	Confuciusornis, Ichthyornis	Diversified, ancestors of modern birds
Mammals	Multituberculates, Metatherians, Eutherians	Generally small-sized, diversified into various niches
Insects	Ants, Butterflies, Aphids, Termites	Diversified, appearance of modern groups
Source: 1

Significant Geological Events and Formations

Beyond the pervasive continental drift, the Cretaceous Period was marked by several other significant geological events and the formation of distinctive rock types. The continued breakup of Pangaea led to the creation of new mountain ranges, such as the Rocky Mountains and the Himalayas, and the formation of large sedimentary basins like the Gulf of Mexico and the North Sea.³ These events profoundly impacted Earth’s landscape, climate, and the isolation and evolution of species.³

A hallmark of the Cretaceous is the widespread deposition of vast quantities of chalk, a white limestone formed from the skeletal remains of marine organisms, which gives the period its name.⁵ These deposits are indicative of the warm, shallow marine environments that characterized much of the period.³

Another critical geological formation of the Cretaceous is the episodic deposition of organic-rich black shales.⁸ These dark-colored sedimentary rocks are geologically and economically important as primary source rocks for petroleum and natural gas worldwide.⁸ Cretaceous black shales are particularly well-studied due to their widespread, contemporaneous deposition in various oceanic settings, notably during the early Aptian (around 120 million years ago, OAE 1a) and the Cenomanian-Turonian boundary (around 94 million years ago, OAE 2).⁸ These Oceanic Anoxic Events (OAEs) are characterized by global distribution and exceptionally high organic carbon content, sometimes exceeding 50%.⁸

The formation of these black shales is strongly correlated with massive volcanic events and the emplacement of Large Igneous Provinces (LIPs), such as the Ontong Java Plateau.⁸ These LIPs, massive crustal emplacements of mafic rock, formed over short geological intervals due to mantle plume upwellings.⁸ The proposed mechanism linking volcanism to black shale deposition involves volcanic outgassing of greenhouse gases like CO2, leading to global warming, enhanced continental weathering, and increased riverine nutrient input to the oceans.⁸ This nutrient influx could have stimulated N2-fixer blooms, leading to increased primary productivity and subsequent deep ocean anoxia, which enhanced the preservation of organic matter.⁸ Hydrothermal activity from submarine magmatism might also have reduced dissolved oxygen, further contributing to ocean anoxia.⁸ This reveals that black shales are not merely geological formations but direct indicators of past global environmental perturbations—specifically, periods of intense volcanism, greenhouse warming, excessive nutrient loading, and widespread ocean deoxygenation. Their economic value as fossil fuel sources is a direct, long-term consequence of these ancient, large-scale environmental stresses.

The interplay of tectonics and climate during the Cretaceous was also instrumental in shaping the distribution of natural resources. The breakup of Pangaea led to the formation of large sedimentary basins, such as the Gulf of Mexico and the North Sea, which are well-known modern regions for petroleum and natural gas extraction.³ The black shales, which serve as source rocks for these fossil fuels, were deposited within these basins during the Cretaceous.⁸ This establishes a direct causal link between the large-scale tectonic movements of the Cretaceous, which created the geological structures for sediment accumulation, and the subsequent formation and global distribution of significant fossil fuel reserves, demonstrating a profound, long-term impact of Earth’s dynamics on human energy resources.

The Cretaceous-Paleogene (K-Pg) Extinction Event

The Cretaceous Period concluded with one of Earth’s most famous and devastating mass extinction events: the Cretaceous-Paleogene (K-Pg) extinction (formerly K-T extinction), approximately 66 million years ago.¹ This catastrophic event led to the demise of three-quarters of plant and animal species on Earth, most notably all non-avian dinosaurs, pterosaurs, and large marine reptiles like mosasaurs and plesiosaurs, as well as ammonites.¹

The prevailing scientific consensus attributes the K-Pg extinction primarily to the impact of a massive asteroid, estimated to be 10 to 15 km wide, which struck the Yucatán Peninsula in Mexico, forming the 180 km Chicxulub crater.⁵ The impact released immense quantities of dust, ash, sulfur, and other aerosols into the atmosphere by striking carbonate and sulfate-rich sediments.¹⁰ This atmospheric contamination led to prolonged sunlight screening and global cooling, often termed an “impact winter,” which severely inhibited photosynthesis and caused widespread ecological collapse.¹⁰ Estimates suggest subfreezing temperatures for years and a recovery time of decades, with potential cooling for centuries.¹⁰ Ocean acidification due to sulfur aerosols also killed organisms with calcium carbonate shells.¹⁰ The global distribution of an iridium-rich ejecta layer at the K-Pg boundary provides strong evidence for the asteroid impact.¹⁰

The role of massive volcanic activity, particularly the Deccan Traps in present-day India, as an alternative or contributing cause has been a subject of intense debate.⁹ The Deccan Traps erupted in pulses, releasing vast amounts of magma and atmospheric gases like CO2 and SO2, which could have caused global climate change.¹⁶ While some argue for volcanism as a primary kill mechanism for earlier extinctions ¹⁶, recent research suggests a more nuanced role. The asteroid impact was the main driver for the non-avian dinosaur extinction due to the prolonged cold winter it induced, which suppressed potential global dinosaur habitats.¹⁶ Interestingly, some studies propose that the long-term CO2-induced warming from Deccan volcanism might have acted as an “ameliorating agent,” buffering some of the asteroid’s negative effects and accelerating climate recovery by approximately 10 years, thus potentially reducing the overall extinction severity for some groups.¹⁶ This illustrates a complex interaction where one major perturbation (the impact) caused the primary extinction mechanism, while another (volcanism) potentially mitigated some of the long-term climatic effects, influencing the recovery trajectory.

The K-Pg extinction was globally synchronous and highly selective.¹⁰ While 75% of species vanished, there was significant variability in extinction rates between and within different groups.¹⁰ Species dependent on photosynthesis were severely affected, leading to a major reshuffling of dominant plant groups.¹⁰ Conversely, omnivores, insectivores, and carrion-eaters, which could subsist on detritus (dead organic matter), fared better than strict herbivores or carnivores, likely due to the increased availability of their food sources.¹⁰ Surviving mammals and birds, for instance, fed on insects, worms, and snails, which in turn consumed detritus.¹⁰ Aquatic communities in streams and lakes, also relying on detritus, experienced fewer extinctions.¹⁰ This highlights that the extinction was not random but highly selective based on ecological niche and food source. The collapse of primary productivity due to sunlight blockage fundamentally restructured food webs, favoring detritivores and generalists. While non-avian dinosaurs perished, many groups, including flowering plants, gastropods, pelecypods (snails and clams), amphibians, lizards, snakes, crocodilians, and small mammals, survived with fewer apparent losses.² This differential survival laid the groundwork for the remarkable adaptive radiation and diversification of mammals and modern birds in the subsequent Cenozoic Era, fundamentally shaping life as it exists on Earth today.²

Table 3: Comparison of K-Pg Extinction Causes and Their Hypothesized Effects

Cause	Mechanism	Primary Effect on Life	Evidence
Chicxulub Asteroid Impact	Global dispersal of dust, ash, sulfur, aerosols; prolonged sunlight screening; global cooling (“impact winter”); ocean acidification	Main driver of non-avian dinosaur extinction; severe ecological cascade effects; extinction of photosynthesis-dependent species	Iridium anomaly, Chicxulub crater, global ejecta layer 10
Deccan Traps Volcanism	Release of CO2 and SO2; global temperature excursions	Increased habitat suitability for dinosaurs (long-term CO2 warming); potential mitigation of asteroid effects; accelerated climate recovery	Large Igneous Province, magma volume, gas release 16
Source: 10

Notable Fossil Discoveries and Ecosystem Insights

The rich fossil record of the Cretaceous Period provides invaluable glimpses into ancient ecosystems and paleoclimates, with several sites offering exceptional preservation and unique insights. The Liaoning lagerstätte (Yixian Formation) in China is a particularly important site for the Early Cretaceous, renowned for its exquisitely preserved remains of numerous types of small dinosaurs (including feathered coelurosaurs like Maniraptora), early birds, and mammals.⁴ These discoveries have significantly advanced our understanding of dinosaur-bird evolutionary links and the early diversification of mammalian groups. Sites like Liaoning function as integrated “time capsules,” allowing paleontologists to reconstruct entire ancient ecosystems, including the interactions between flora, fauna, and the prevailing environmental conditions, providing a much richer and more accurate understanding of past life than isolated discoveries.⁴

In North America, Denali National Park in Alaska offers a rare and comprehensive window into a Late Cretaceous (~70 million years ago) high-latitude ecosystem.¹⁷ The sedimentary rocks of the Cantwell Formation preserve an ancient forest where dinosaurs roamed, providing integrated plant, vertebrate, invertebrate, and environmental records.¹⁷ Researchers have documented over 12 different leaf types, woody and herbaceous stems, and at least 25 pollen and spore types, thriving in freshwater pond and stream environments.¹⁷ Vertebrate specialists have also discovered new dinosaur fossils in the region, with variations in fossil preservation reflecting different depositional environments.¹⁷ Studies in Denali help scientists understand what high-latitude ecosystems were like in a globally warmer world and how they responded to climate change and extinction events.¹⁷

Recent advancements in fossil discovery techniques continue to reshape our understanding of Cretaceous life. A new study published in Science in 2025, utilizing advanced 3D digital fossil-mining, revealed that ancient squids were far more prevalent and dominant in oceans 100 million years ago than previously thought.¹⁵ This research, based on the identification of thousands of fossilized cephalopod beaks hidden inside Late Cretaceous rocks from Japan, demonstrated that squids (including both near-shore Myopsida and open-sea Oegopsida) had already originated and diversified explosively long before the K-Pg extinction event, challenging the long-held belief that their flourishing began only after the demise of dinosaurs.¹⁵ This suggests squids were “pioneers of fast and intelligent swimmers” that dominate modern oceans.¹⁵ This discovery exemplifies how technological innovations in analysis, imaging, and data processing can unlock previously hidden information from existing samples, leading to significant revisions of long-standing scientific paradigms and highlighting the dynamic nature of scientific understanding.

Legacy and Importance in Shaping Subsequent Evolution

The Cretaceous Period, despite ending in a cataclysmic extinction, holds immense legacy and importance in shaping the subsequent geological and biological evolution of Earth, fundamentally setting the stage for the Cenozoic Era, often referred to as the “Age of Mammals”.²

Biologically, the Cretaceous was a pivotal time for the emergence and diversification of many life forms that would play key roles in the Cenozoic world.² The most significant of these was the rise of flowering plants (angiosperms), which began in the Early Cretaceous and radiated extensively by the middle of the period.² Their proliferation transformed terrestrial ecosystems, providing new food sources and habitats that drove the co-evolution and diversification of modern insect groups, including ants, butterflies, and eusocial bees.² By the end of the Cretaceous, forests began to resemble modern ones, with familiar species like oaks, hickories, and magnolias becoming common.² The period also saw the first appearance of modern mammal and bird groups, which, though playing secondary roles during the dinosaurian dominance, were poised for explosive diversification post-extinction.² In the oceans, the first radiation of diatoms occurred, a group that would become crucial primary producers in modern marine food webs.²

Geologically, the continued breakup of Pangaea during the Cretaceous created the continental configuration that gradually evolved into the present-day arrangement.² This rifting generated extensive new coastlines, new mountain ranges, and large sedimentary basins, which continue to influence global geography and climate.² The vast chalk deposits formed during this period continue to be prominent geological features globally.⁵ Furthermore, the episodic deposition of organic-rich black shales during the Cretaceous laid the groundwork for significant petroleum and natural gas reserves, making the geological processes of this era directly relevant to modern energy resources.⁸

The K-Pg extinction event, while catastrophic, served as a crucial ecological reset. By eliminating dominant groups like non-avian dinosaurs, it cleared vast ecological niches, allowing the surviving groups—particularly mammals, modern birds, and flowering plants—to undergo remarkable adaptive radiations and diversify into the myriad forms observed today.² This event fundamentally reshaped the trajectory of life, leading to the ecosystems that characterize the Cenozoic Era. Moreover, paleoclimatological studies of the Cretaceous greenhouse conditions, with their high CO2 levels and warm temperatures, provide valuable past analogs for understanding the potential impacts of current atmospheric CO2 emissions and future climate change.