The Rise and Fall of the Dinosaurs

Steve Brusatte, a paleontologist at the University of Edinburgh, traces the complete evolutionary arc of dinosaurs, from their humble origins after a global catastrophe to their sudden extinction 66 million years ago, weaving his own fieldwork with cutting-edge research from colleagues worldwide. Writing during what he calls a golden age of discovery, when a new species is found roughly once a week, Brusatte argues that dinosaurs were not the dim-witted, lumbering failures of popular stereotype but remarkably successful animals whose story holds urgent lessons for the present.

The narrative begins approximately 252 million years ago, at the end of the Permian Period, when a massive volcanic hot spot beneath Siberia erupted for hundreds of thousands of years, releasing gases that triggered a runaway greenhouse effect. This end-Permian mass extinction killed roughly 90 percent of all species. Brusatte and his Polish colleague Grzegorz Niedźwiedzki study tracks in the Holy Cross Mountains of Poland that document the aftermath: a devastated world in which survivors hid underground and new animals slowly emerged. Among the newcomers were archosaurs, reptiles with an upright posture that split into two lineages: the pseudosuchians, ancestors of crocodiles, and the avemetatarsalians, which gave rise to pterosaurs, dinosaurs, and birds. Around 250 million years ago, small cat-size creatures called dinosauromorphs, near-relatives not yet true dinosaurs themselves, began leaving tracks in the Polish muds, their narrow trackways foreshadowing dinosaur anatomy.

True dinosaurs arose between 240 and 230 million years ago, with Nyasasaurus from Tanzania as a possible oldest candidate. The earliest unquestionable fossils come from Ischigualasto Provincial Park in Argentina, which preserves species across all three major dinosaur lineages: meat-eating theropods, long-necked sauropods, and plant-eating ornithischians. Yet these first dinosaurs comprised only 10 to 20 percent of their ecosystems, overshadowed by larger amphibians, mammal relatives, and crocodile-line archosaurs. Brusatte's statistical analysis of morphological disparity, a measure of anatomical diversity, shows that pseudosuchians consistently outpaced dinosaurs throughout the Triassic Period. Confined to humid regions of the supercontinent Pangea, a single landmass stretching pole to pole, early dinosaurs were underdogs unable to tolerate the extreme deserts and violent megamonsoons closer to the equator.

The turning point came 201 million years ago, when Pangea cracked apart along its center. Massive volcanic eruptions along the rift zone, lasting roughly 600,000 years, released carbon dioxide that melted seafloor clathrates, ice-like structures trapping methane, a greenhouse gas over 35 times more potent than CO₂. Temperatures spiked, food chains collapsed, and over 30 percent of species went extinct, including nearly all pseudosuchians. Dinosaurs somehow survived. Brusatte concedes this may simply be luck. Freed from competition, dinosaurs diversified rapidly in the ensuing Jurassic Period, spreading globally, growing larger, and colonizing all environments.

Sauropods, the iconic long-necked giants, epitomize this rise. Brusatte explains their extraordinary size through five key adaptations: long necks for efficient feeding at height; fast growth rates inherited from early ancestors; ultra-efficient bird-style lungs that extracted oxygen during both inhalation and exhalation; air sacs that hollowed out bones, making skeletons strong yet lightweight; and the same air sacs dissipating excess body heat. By the Late Jurassic, around 150 million years ago, diverse sauropods like Brontosaurus, Brachiosaurus, and Diplodocus coexisted through niche partitioning, each adapted to different foods and feeding heights. Predators like Allosaurus topped the food chain. Because land connections between continents still existed, similar dinosaur communities appeared worldwide.

The Jurassic-Cretaceous transition, around 145 million years ago, brought gradual environmental changes: cooling temperatures, falling sea levels, and continued continental fragmentation. Sauropod diversity crashed, though titanosaurs, a later branch of sauropods, eventually produced supergiants like Argentinosaurus. Smaller plant-eating ornithischians, including ancestors of ceratopsians (horned dinosaurs) and hadrosaurs (duck-billed dinosaurs), rose as dominant herbivores. Among predators, carcharodontosaurs, close relatives of Allosaurus, became apex hunters worldwide, with species rivaling T. rex in size.

Living in the carcharodontosaurs' shadows were tyrannosaurs, which Brusatte reveals to have a surprisingly long history. The oldest known tyrannosaur, Kileskus from Siberia, dates to about 170 million years ago and was barely seven feet long. For roughly 80 million years, tyrannosaurs remained small to midsize hunters. CAT scans of Timurlengia, a horse-size tyrannosaur from Uzbekistan dating to about 90 million years ago, show that tyrannosaurs developed enlarged brains and heightened senses before they evolved large bodies. When carcharodontosaurs vanished from North America and Asia around 84 million years ago during environmental upheaval, tyrannosaurs seized the opportunity and grew to monstrous proportions.

Brusatte devotes extensive attention to T. rex, which lived from about 68 to 66 million years ago in western North America. Drawing on bite-force measurements by Greg Erickson of Florida State University, skull-stress modeling by Emily Rayfield of the University of Bristol, speed analysis by John Hutchinson of the Royal Veterinary College near London, and brain imaging by Amy Balanoff of Johns Hopkins University, Brusatte reconstructs Rex as a forty-two-foot, seven-ton predator with a bone-crushing bite of about 3,000 pounds per tooth, binocular vision, chimp-level intelligence, and a world-class sense of smell. Too heavy to sprint, Rex relied on ambush tactics and probably hunted in packs. Growth-ring analysis shows it lived fast and died young, gaining about five pounds per day during its teenage years and never surviving past thirty.

By the latest Cretaceous, the continents had drifted to near-modern positions, creating distinct ecosystems on each landmass. North America hosted T. rex, Triceratops, and Edmontosaurus; South America and Africa were ruled by carcharodontosaurs and abelisaurids, fierce short-skulled predators, alongside titanosaur sauropods; and the flooded islands of Europe produced dwarfed dinosaurs and the unusual raptor Balaur bondoc, a double-clawed predator that served as top carnivore on islands too small to support a giant tyrannosaur. Dinosaurs were at the peak of their diversity.

Simultaneously, one lineage of small, feathered theropods was evolving into birds. Thousands of fossils from Liaoning, China, preserved by volcanic eruptions, reveal that feathers first evolved as simple insulating filaments in many dinosaur groups, then developed into complex structures forming wings in maniraptorans, the small theropods most closely related to birds. Brusatte argues that wings initially served as display structures, only later assuming aerodynamic function in smaller species. Multiple dinosaur lineages experimented with different wing configurations, but one eventually produced modern birds, which diversified rapidly once fully airborne.

Then, 66 million years ago, an asteroid roughly six miles wide struck the Yucatán Peninsula of Mexico. Brusatte recounts how geologist Walter Alvarez, studying a thin clay layer at the Cretaceous-Paleogene boundary in Gubbio, Italy, discovered anomalous concentrations of iridium, a metal rare on Earth but common in space, leading to the 1980 hypothesis that an asteroid impact caused the extinction. The subsequent identification of the 110-mile-wide Chicxulub crater confirmed the theory. The impact generated catastrophic earthquakes, rains of molten glass, continent-wide wildfires, and a years-long nuclear winter, followed by intense global warming from trapped carbon dioxide.

Brusatte and an international team synthesize the evidence and conclude that the asteroid was the primary cause of the extinction. Most dinosaur groups were thriving right up to the impact, though declining diversity among large herbivores had made food webs more vulnerable. Non-bird dinosaurs were doomed by their large size, dependence on plant-based food chains disrupted by the collapse of photosynthesis, high metabolic demands, and slow-hatching eggs. Modern-style birds survived, possibly through flight, fast reproduction, and seed-eating.

In the epilogue, Brusatte describes collecting mammal fossils in New Mexico's Paleogene rocks, deposited just after the extinction. Within 500,000 years, mammals had diversified to fill the ecological vacuum, including early primates like Torrejonia, a puppy-size creature whose lineage would eventually produce humans. Brusatte closes with a warning: Dinosaurs ruled for over 150 million years and were destroyed in a geological instant, and the species now wearing the crown should consider whether a similar fate awaits.