What If The Dinosaurs Never Went Extinct?

The Actual History

The extinction of non-avian dinosaurs 66 million years ago represents one of the most significant turning points in Earth's biological history, an event that fundamentally reshaped the trajectory of evolution and ultimately allowed for the rise of mammals and eventually humans.

The Age of Dinosaurs

Dinosaurs first appeared during the Triassic Period, approximately 245 million years ago. Following the Triassic-Jurassic extinction event around 201 million years ago, dinosaurs became the dominant terrestrial vertebrates for the next 135 million years. The dinosaur era encompasses three major geological periods:

Late Triassic Period (237-201 million years ago): Early dinosaurs emerged and began to diversify, though they shared the landscape with many other reptile groups.
Jurassic Period (201-145 million years ago): Dinosaurs achieved worldwide dominance. This era saw the rise of massive sauropods like Brachiosaurus and Diplodocus, predators like Allosaurus, and the first birds evolved from small theropod dinosaurs.
Cretaceous Period (145-66 million years ago): The final and longest dinosaur period witnessed tremendous diversity. Famous dinosaurs from this time include Tyrannosaurus rex, Triceratops, and Velociraptor. Flowering plants (angiosperms) also evolved and spread during this period, changing terrestrial ecosystems.

By the Late Cretaceous, dinosaurs had evolved into an extraordinary range of forms occupying virtually every terrestrial ecological niche:

Theropods: Bipedal carnivores ranging from chicken-sized Compsognathus to the massive Tyrannosaurus and Giganotosaurus
Sauropods: Long-necked herbivores, the largest land animals ever, including Argentinosaurus and Patagotitan
Ceratopsians: Horned, frilled herbivores like Triceratops and Styracosaurus
Hadrosaurs: Duck-billed herbivores, often with elaborate head crests
Ankylosaurs: Armored herbivores with club-like tails
Stegosaurs: Plated herbivores with distinctive back plates and tail spikes
Ornithopods: Diverse bipedal herbivores including Iguanodon and Parasaurolophus
Pachycephalosaurs: Thick-skulled herbivores that may have engaged in head-butting contests

Dinosaurs had also taken to the air in the form of pterosaurs (flying reptiles, not technically dinosaurs but close relatives) and birds (which are theropod dinosaurs). The oceans were dominated not by dinosaurs but by marine reptiles like plesiosaurs, mosasaurs, and ichthyosaurs.

During this time, mammals existed but remained small, mostly nocturnal creatures rarely exceeding the size of a modern badger. They lived in the shadows of the dinosaur-dominated world, unable to compete directly with the ruling reptiles for ecological niches.

The Cretaceous-Paleogene Extinction Event

The reign of non-avian dinosaurs came to an abrupt end 66 million years ago in what is known as the Cretaceous-Paleogene (K-Pg) extinction event, formerly called the Cretaceous-Tertiary (K-T) extinction. This mass extinction eliminated approximately 75% of all species on Earth.

The primary cause of this extinction is now widely accepted to be the impact of a massive asteroid or comet:

The Impact: An object approximately 10-15 kilometers (6-9 miles) in diameter struck Earth in what is now the Yucatán Peninsula in Mexico, creating the Chicxulub crater, which spans about 180 kilometers (110 miles) in diameter.
Immediate Effects: The impact released energy equivalent to billions of atomic bombs, causing:
- Massive tsunamis in surrounding oceans
- Global wildfires ignited by ejected superheated material
- A shockwave that triggered earthquakes and volcanic eruptions worldwide
- Vaporization of rock that released sulfur compounds into the atmosphere
Long-term Consequences: The most devastating effects came from atmospheric changes:
- Dust and aerosols blocked sunlight for months or years, creating an "impact winter"
- Photosynthesis was severely reduced or halted globally
- Food chains collapsed, beginning with plant life and affecting herbivores and then carnivores
- Acid rain from sulfur compounds further damaged ecosystems
- Global temperatures dropped dramatically before later rising due to greenhouse gases

Some scientists believe the Deccan Traps—massive volcanic eruptions in what is now India—may have contributed to environmental stress before the asteroid impact, potentially making ecosystems more vulnerable to collapse.

The extinction was highly selective:

All non-avian dinosaurs disappeared
Marine reptiles like mosasaurs and plesiosaurs went extinct
Pterosaurs vanished from the skies
Ammonites and many other marine invertebrates disappeared
Many plant species, particularly in North America, went extinct

However, certain groups showed higher survival rates:

Birds (avian dinosaurs) survived, though many lineages were lost
Small mammals endured, possibly due to their burrowing habits, omnivorous diets, and small size
Crocodilians and turtles survived, perhaps due to their semi-aquatic lifestyles and ability to remain dormant
Many marine organisms, especially those in deep ocean environments, were less affected

The Rise of Mammals

In the aftermath of the extinction, the Earth entered the Paleogene Period, and the absence of large dinosaurs created ecological opportunities that mammals rapidly exploited:

Immediate Aftermath (First Million Years): Small, generalist mammals began to increase in size and diversity. The early Paleocene saw mammals evolve to fill niches previously occupied by dinosaurs.
Adaptive Radiation: Over the next several million years, mammals underwent an extraordinary diversification:
- Herbivores evolved to exploit plant resources no longer consumed by dinosaurs
- New carnivore lineages emerged to prey on these herbivores
- Mammals adapted to arboreal (tree-dwelling), fossorial (burrowing), and eventually aquatic lifestyles
- Primates began to evolve, eventually leading to humans
Modern Mammal Groups Emerge: By 55-50 million years ago, most modern mammalian orders had appeared, including:
- Rodents and lagomorphs (rabbits and hares)
- Carnivores (ancestors of cats, dogs, bears)
- Cetaceans (whales and dolphins evolved from land-dwelling ancestors)
- Perissodactyls (odd-toed ungulates like horses and rhinos)
- Artiodactyls (even-toed ungulates like cattle and deer)
- Early primates

Birds also diversified significantly, evolving into many of the major groups we recognize today. As the only surviving dinosaurs, they continued their evolutionary journey, developing new adaptations and filling aerial niches.

The K-Pg extinction event thus represents a pivotal moment in Earth's history—a cosmic accident that ended the 165-million-year reign of dinosaurs and set the stage for the Age of Mammals. Without this chance event, the world as we know it, including human existence, would likely never have come to be. The dinosaurs' misfortune became mammals' opportunity, leading eventually to the evolution of our own species and the development of a creature capable of understanding this profound history.

The Point of Divergence

In this alternate timeline, the catastrophic asteroid that struck Earth 66 million years ago misses our planet entirely, fundamentally altering the course of evolutionary history. The point of divergence occurs through a series of astronomical circumstances:

Altered Orbital Trajectory (Primary Divergence): In this timeline, the Chicxulub impactor—the asteroid or comet approximately 10-15 kilometers in diameter—follows a slightly different orbital path through the solar system. This minor variation, perhaps caused by a subtly different gravitational interaction with Jupiter millions of years earlier, places the object on a trajectory that passes close to Earth but does not intersect with our planet.
Near Miss Event: Instead of the devastating impact that occurred in our timeline, the massive space rock passes within tens of thousands of kilometers of Earth—close enough to be visible as an extraordinarily bright object in the sky for several hours, but far enough to prevent any direct effects on the planet's ecosystems.
Continued Dinosaur Evolution: Without the sudden, catastrophic extinction event, the diverse ecosystems of the Late Cretaceous continue to flourish and evolve. The approximately 700 known species of non-avian dinosaurs—and likely many more yet undiscovered by paleontologists—continue their evolutionary trajectories alongside the early mammals, birds, and other life forms of the period.
Persistent Cretaceous Conditions: The warm, generally equable climate of the Late Cretaceous continues without the dramatic cooling and darkening caused by impact debris. The lush forests dominated by conifers, ginkgoes, cycads, and the relatively recently evolved flowering plants continue to provide habitat and food for the diverse fauna.
Gradual Environmental Change: Rather than a sudden extinction, environmental changes occur more gradually. The ongoing volcanic activity of the Deccan Traps in what is now India continues to influence global climate, but without the compounding effects of the asteroid impact, these changes unfold over millions of years, allowing species time to adapt or migrate.

The absence of this extinction event means that the dominant terrestrial vertebrates of the past 165 million years—the non-avian dinosaurs—never relinquish their ecological supremacy. Mammals remain largely small, nocturnal creatures, continuing in the ecological roles they had occupied since the Jurassic Period. The evolutionary history of Earth takes a dramatically different path, one where the Age of Mammals never begins and the Age of Dinosaurs continues uninterrupted to the present day.

This divergence represents one of the most profound possible alterations to Earth's history, affecting not just a single species or ecosystem but the entire trajectory of life on our planet. The world that develops over the subsequent 66 million years would be utterly alien to us—a world where dinosaurs, not mammals, continue to be the dominant form of terrestrial life, and where humans almost certainly never evolve.

Immediate Aftermath

The First Million Years: Continued Cretaceous Ecosystems

Without the sudden extinction event, the rich ecosystems of the Late Cretaceous would continue to flourish and gradually evolve:

Terrestrial Ecosystems Stability: The diverse dinosaur-dominated ecosystems would persist with relatively minor changes initially:
- Large herbivorous dinosaurs like ceratopsians, hadrosaurs, and sauropods would continue browsing on conifers and early flowering plants
- Predatory theropods would maintain their position as apex predators
- Small mammals would remain in their established niches as nocturnal insectivores and omnivores
- Birds would continue diversifying, particularly in forest and coastal environments
Marine Ecosystem Continuation: The oceans would maintain their Late Cretaceous character:
- Mosasaurs would remain the dominant marine predators
- Plesiosaurs would continue in their specialized niches
- Ammonites and other cephalopods would persist as important components of marine food webs
- Sharks and ray-finned fishes would continue their evolutionary trajectories without the opportunity to expand into niches vacated by extinct marine reptiles
Gradual Climate Change: The ongoing eruptions of the Deccan Traps would continue to influence global climate:
- Periods of warming due to volcanic greenhouse gas emissions
- Some regional extinctions and range shifts as species adapt to changing conditions
- Potential sea level fluctuations affecting coastal habitats
- Unlike the asteroid impact, these changes would occur gradually enough for many species to adapt
Continued Floral Evolution: Plant communities would continue their ongoing transitions:
- Flowering plants (angiosperms) would continue their expansion, gradually replacing conifers in many environments
- Co-evolutionary relationships between plants and their pollinators would continue developing
- Grasslands, which began to appear in the Late Cretaceous, would continue their slow expansion, though likely at a different pace than in our timeline

1-10 Million Years: Evolutionary Adaptations

Over the next several million years, evolution would continue shaping Earth's biota, but along very different lines than in our timeline:

Dinosaur Adaptations to Changing Flora: As flowering plants became increasingly dominant, dinosaur herbivores would evolve new adaptations:
- New dental and digestive specializations to process different plant materials
- Potential co-evolutionary relationships with specific plant groups
- Behavioral adaptations to exploit new food resources
Predator Evolution: Carnivorous dinosaurs would continue evolving in response to their prey:
- Continued arms race between predator and prey adaptations
- Potential for new hunting strategies and social behaviors
- Specialized predators adapted to particular prey types or hunting environments
Mammalian Constraints: Mammals would remain largely constrained to small body sizes and nocturnal niches:
- Continued specialization within these constraints
- Possible diversification in arboreal (tree-dwelling) environments
- Limited opportunity to evolve larger body sizes or diurnal (day-active) lifestyles while dinosaurs remained dominant
Avian Dinosaur Diversification: Birds would continue their evolutionary journey as a successful dinosaur lineage:
- Further specialization for different flight styles and ecological niches
- Continued competition with pterosaurs in some aerial niches
- Potential development of flightless forms in environments with few terrestrial predators
Pterosaur Persistence: Unlike in our timeline, pterosaurs would not face sudden extinction:
- Continued competition with birds in some niches
- Potential for new adaptive radiations as environments changed
- Maintenance of their dominance in the large flying animal niche

10-20 Million Years: New Ecological Patterns

As time progressed, new patterns would emerge in this dinosaur-dominated world:

Response to Paleogene Climate Shifts: The warming event at the Paleocene-Eocene boundary (around 56 million years ago in our timeline) would still occur due to natural carbon cycle processes:
- Range shifts as species moved to maintain their preferred climate conditions
- Potential for new adaptive radiations in tropical environments
- Some extinctions of species unable to adapt to warmer conditions
Continental Drift Effects: The continuing breakup of the supercontinent Gondwana would create new biogeographical patterns:
- Isolation of dinosaur populations on different continents
- Divergent evolution creating distinct dinosaur faunas in different regions
- New endemic species evolving on island continents like Australia and Madagascar
Ecosystem Engineering: Large dinosaurs would continue shaping their environments:
- Megaherbivores maintaining open woodlands through their feeding activities
- Creation of specific microhabitats through behaviors like nest building
- Nutrient cycling affected by the presence of very large animals
Specialized Niches: As ecosystems matured without disruption, more specialized niches would develop:
- Highly specialized feeding adaptations
- Complex symbiotic relationships
- Intricate community structures with multiple trophic levels

20-40 Million Years: Alternative Cenozoic

The period corresponding to the early-to-mid Cenozoic in our timeline would feature a world still dominated by dinosaurs, but with significant evolutionary developments:

New Dinosaur Groups: Novel dinosaur lineages would likely evolve to exploit new ecological opportunities:
- Potential for entirely new major groups beyond those known from the fossil record
- Further specialization of existing lineages
- Possible convergent evolution with some mammalian forms from our timeline
Mammalian Diversity Within Constraints: Mammals would continue to diversify but within the ecological constraints imposed by dinosaur dominance:
- Increased diversity of small-bodied forms
- Specialization for niches unavailable to or ignored by dinosaurs
- Possible development of some unique adaptations not seen in our timeline
Response to Oligocene Cooling: The global cooling trend that began in the Oligocene in our timeline would affect dinosaur distributions:
- Potential for the evolution of cold-adapted dinosaur species
- Range contractions for heat-loving species
- New selection pressures favoring endothermy (warm-bloodedness) or enhanced thermoregulation
Novel Ecosystems: The combination of continued dinosaur evolution and changing global conditions would create ecosystems with no parallel in our timeline:
- Unique community structures
- Food webs organized differently than in mammal-dominated ecosystems
- Different patterns of energy flow through ecosystems

By approximately 25 million years after the point of divergence (roughly equivalent to the mid-Oligocene in our timeline), Earth would host a biosphere that would be increasingly unrecognizable to paleontologists from our reality. The continued dominance of dinosaurs would have prevented mammals from undergoing their dramatic Cenozoic radiation, while the dinosaurs themselves would have evolved into forms beyond what we can confidently predict based on the fossil record.

This alternative world, some 40 million years post-divergence, would feature dinosaurs that had continued to adapt and evolve for a span of time equal to that between Tyrannosaurus rex and the present day in our timeline—sufficient time for significant evolutionary innovation and the emergence of entirely new adaptations and behaviors in these remarkable animals.

Long-term Impact

40-66 Million Years: The Modern Dinosaur World

As we approach the present day in this alternate timeline, the world would be dominated by dinosaur lineages that have continued evolving for the full 66 million years that separate the K-Pg boundary from modern times—a span of time equal to the entire Cenozoic Era in our timeline:

Advanced Dinosaur Evolution: Non-avian dinosaurs would have undergone extensive evolutionary change:
- Forms unimaginably different from the dinosaurs known from our fossil record
- Potential for significantly increased encephalization (brain size relative to body size) in some lineages
- Novel sensory adaptations, potentially including enhanced color vision, infrared sensing, or electroreception in specialized groups
- Sophisticated social behaviors and communication systems
Ecological Specialization: After 66 million years of uninterrupted evolution, dinosaur specialization would reach extraordinary levels:
- Highly specific adaptations to particular food sources, habitats, or climatic conditions
- Complex co-evolutionary relationships with plants, insects, and other organisms
- Specialized dinosaur species filling niches occupied by mammals in our timeline
- Potential for tool use or environmental manipulation in the most cognitively advanced theropods
Mammalian Adaptation: Mammals would have continued their own evolutionary journey, though constrained by dinosaur dominance:
- Highly specialized small-bodied forms exploiting niches unavailable to dinosaurs
- Possible underground or nocturnal civilizations of intelligent mammals (though less likely than continued dinosaur dominance)
- Unique adaptations with no parallel in our timeline
- Potential commensalism or mutualism with certain dinosaur species
Modern Biomes: The major biomes of this alternate Earth would feature dinosaurs in the dominant roles:
- Forests: Diverse arboreal dinosaurs exploiting different levels of the canopy
- Grasslands: Herds of grazing dinosaurs analogous to but distinct from mammalian ungulates
- Deserts: Specialized dinosaurs with water conservation adaptations
- Tundra: Cold-adapted dinosaur species with insulation and thermoregulatory adaptations
- Oceans: Marine reptiles like mosasaurs potentially evolving adaptations similar to cetaceans

Potential for Dinosaur Intelligence

One of the most intriguing possibilities in this alternate timeline is the potential evolution of advanced intelligence in certain dinosaur lineages:

Candidates for Intelligence: The most likely candidates for evolving advanced intelligence would be:
- Troodontids: Small, carnivorous dinosaurs already possessing the highest brain-to-body ratios among non-avian dinosaurs
- Dromaeosaurids (raptor dinosaurs): Shown evidence of complex behaviors and relatively large brains
- Advanced Avialans: Bird-like dinosaurs that might evolve intelligence similar to corvids (crows and ravens) or parrots in our timeline, but potentially taken further
Selection Pressures for Intelligence: Several factors might drive the evolution of greater intelligence:
- Social complexity requiring advanced communication and problem-solving
- Predator-prey arms races favoring greater cognitive abilities
- Environmental challenges requiring behavioral flexibility
- Competition within and between species
Potential Cognitive Abilities: Advanced dinosaurian intelligence might manifest as:
- Complex social structures with cultural transmission of knowledge
- Sophisticated communication systems, potentially including primitive language
- Tool use, initially simple but potentially becoming more complex over time
- Environmental manipulation, such as constructing complex nests or shelters
Limitations on Dinosaur Technology: Even intelligent dinosaurs would face certain constraints:
- Manual dexterity limitations compared to primates (though some small theropods had relatively dexterous hands)
- Different relationship with fire due to their evolutionary history and potentially different environmental needs
- Alternative technological pathways based on their specific sensory and physical capabilities

The Question of Dinosaur Civilization

While highly speculative, we can consider whether a technological civilization might have evolved in this alternate timeline:

Civilization Probability: The emergence of a technological civilization would be far from inevitable:
- Intelligence evolves in response to specific selection pressures, not as an inevitable endpoint
- The 66 million years of potential dinosaur evolution is less than the time it took for primates to evolve into humans
- Many contingent factors would need to align for a technological civilization to emerge
Potential Form: If a dinosaur civilization did emerge, it would differ fundamentally from human civilization:
- Based on different cognitive architectures and sensory experiences
- Utilizing different resources and technologies suited to dinosaur physiology
- Potentially more integrated with natural ecosystems rather than dominating them
- Different social structures reflecting dinosaurian rather than primate psychology
Technological Differences: Dinosaur technology would follow different pathways:
- Potentially biological or organic technology rather than metallurgy
- Different energy sources based on their environmental relationships
- Alternative forms of record-keeping and knowledge transmission
- Technologies designed for bodies and sensory systems very different from our own
Multiple Intelligences: The dinosaur world might feature multiple intelligent species:
- Different dinosaur lineages evolving intelligence independently
- Potential for competition or cooperation between intelligent species
- Complex interspecies relationships unlike anything in human experience

Environmental and Climatic Considerations

The continued presence of dinosaurs would have significant implications for Earth's climate and environments:

Megafauna Effects on Ecosystems:
- Large dinosaurs would continue to act as ecosystem engineers
- Different patterns of nutrient cycling due to dinosaur metabolism and behavior
- Potential maintenance of more open woodland environments rather than closed forests
- Unique patterns of seed dispersal and plant evolution
Response to Cenozoic Climate Changes:
- Dinosaurs would have adapted to the major cooling trends of the Cenozoic
- Different evolutionary responses to glacial-interglacial cycles
- Potential migrations and range shifts during climate fluctuations
- Specialized adaptations to new biomes as they emerged
Human-Era Climate Change:
- Without human industrial activity, the recent anthropogenic climate change would not occur
- Natural climate cycles would continue without human interference
- Different carbon cycle dynamics due to dinosaur-dominated ecosystems
- Alternative patterns of biomass distribution across the planet

The Absence of Humans

Perhaps the most profound aspect of this alternate timeline is the absence of humans and human-like intelligence:

Evolutionary Pathway Blocked:
- Primates as we know them likely would not evolve in a dinosaur-dominated world
- The specific conditions that led to hominin evolution would not occur
- The ecological niches that allowed ape evolution would likely be occupied by dinosaurs
Alternative Observers:
- If intelligence did evolve, the beings contemplating their world's evolutionary history would be fundamentally different from humans
- They would perceive and conceptualize their world through entirely different sensory and cognitive systems
- Their understanding of their world's history might focus on different aspects than human paleontology
Different Planetary Impact:
- Without human civilization, the planet would not experience the Anthropocene
- No mass extinction caused by human activity
- No agriculture, urbanization, or industrialization as we know it
- Different patterns of species distribution without human-mediated introductions

The world 66 million years after our point of divergence would be utterly alien to human observers—a planet where the Age of Mammals never began, where dinosaurs continued their evolutionary journey uninterrupted, and where the particular combination of circumstances that led to human evolution never occurred. It would be a world both familiar in its basic physical geography yet profoundly strange in its biota, a parallel Earth that followed a completely different evolutionary path due to the absence of a single catastrophic event.

Expert Opinions

Dr. Emily Saurian, Paleontologist at the Museum of Evolutionary Biology, suggests:

"If non-avian dinosaurs had survived the K-Pg boundary, I believe we would see extraordinary evolutionary developments far beyond what we can confidently predict. The 66 million years since the extinction is longer than the time from the first dinosaurs to T. rex—more than enough time for entirely new body plans and adaptations to evolve.

One fascinating possibility is the evolution of dinosaurs to fill niches occupied by mammals in our timeline. We might see dinosaur analogues of grazing ungulates, burrowing insectivores, and perhaps even marine forms similar to cetaceans. The theropod lineage, which already produced birds, might have generated additional highly successful groups with novel adaptations.

Regarding intelligence, while it's not inevitable, I find it plausible that certain dinosaur lineages—particularly among the maniraptoran theropods like troodontids—might have evolved significantly enhanced cognitive abilities. These dinosaurs already possessed relatively large brains and complex behaviors. Given tens of millions of years of additional evolution, advanced intelligence might have emerged, though it would likely manifest very differently from primate intelligence due to different neurological architecture and sensory systems."

Dr. Marcus Chen, Evolutionary Biologist specializing in contingency theory, observes:

"The absence of the K-Pg extinction event would represent one of the most profound possible alterations to Earth's evolutionary history. This scenario highlights the role of contingency in evolution—how unpredictable events shape the course of life on Earth.

Mammals had coexisted with dinosaurs for over 150 million years without displacing them from dominant ecological roles. Without the extinction event creating ecological opportunity, it's highly unlikely that mammals would have undergone their dramatic Cenozoic radiation. Our own existence as humans is the product of a long chain of contingent events that began with that asteroid impact.

What's particularly interesting is how different selection pressures might operate in a world where large dinosaurs remained the dominant terrestrial vertebrates. Would the arms race between predators and prey drive the evolution of intelligence as it may have done in primates? Would the continued competition between different dinosaur lineages lead to novel adaptations we can't even imagine based on the fossil record? These questions highlight how evolutionary history is not predetermined but rather contingent on specific historical circumstances."

Dr. Sophia Avialae, Paleoecologist focusing on ancient ecosystems, notes:

"The ecological implications of dinosaur survival would be profound. Modern ecosystems would be structured fundamentally differently, with dinosaurs occupying the roles of dominant herbivores and carnivores across most terrestrial biomes.

Plant evolution would also follow a different trajectory. The coevolution between flowering plants and mammals, particularly primates and ungulates, has shaped much of our modern flora. In a dinosaur-dominated world, different coevolutionary relationships would develop. We might see plant defenses specifically adapted to counter dinosaur herbivory, and seed dispersal mechanisms evolved to utilize dinosaur movement patterns.

Climate would be affected as well. Large dinosaurs, like modern elephants but potentially on a greater scale, would act as ecosystem engineers, maintaining more open woodland environments in many regions. The cycling of nutrients and carbon would follow different patterns based on dinosaur metabolism and behavior.

Perhaps most fascinating is the question of how dinosaurs would have adapted to the major climate shifts of the Cenozoic, particularly the cooling trends of the Oligocene and the glacial-interglacial cycles of the Quaternary. Would we see dinosaurs with fur-like integument expanding into northern latitudes? Would migration patterns develop to cope with seasonal changes? These adaptations would reveal new dimensions of dinosaur evolutionary potential that we can only glimpse through the limited window of the fossil record."