Why Did Some Dinosaurs Go Extinct Before the Asteroid?

The Cretaceous Era—roughly 145 to 66 million years ago—was the last hurrah of the dinosaurs. A massive asteroid impact brought them to a violent end, but there’s more to the story. The Cretaceous period was shaped by a series of smaller extinction events throughout the era. Though less famous, these events were powerful enough to influence evolutionary changes in dinosaurs and countless other organisms.

Background Extinction

Extinction is part of the natural rhythm of life on Earth. Even when no dramatic events are occurring, species go extinct all the time, a phenomenon called the background extinction rate. This baseline reflects the gradual disappearance of species due to competition, habitat change, and evolutionary turnover, often without any single catastrophic cause.

Episodic Extinctions

Episodic extinctions differ from the major mass extinction events that often receive the most attention. These smaller events are more targeted, typically driven by specific environmental changes that affect certain groups more severely than others. They tend to be regional rather than global and often reshape ecosystems by clearing the way for other species to rise.

Possible Causes of Episodic Extinction Events

What caused episodic extinction events? Here are some of the possibilities:

• Volcanic eruptions – Massive eruptions can release ash and gases that alter climate and ocean chemistry, triggering die-offs.
• Ocean anoxia – Sudden drops in ocean oxygen levels can collapse marine food webs. These can occur due to changes in geology, climate, sea level, and biological processes that deplete marine oxygen. These can happen because of changes in geology, climate, sea level, and biological changes that soak up marine oxygen.
• Climate shifts – Rapid cooling or warming periods can stress species that cannot adapt or migrate.
• Sea level changes – Rising or falling sea levels destroyed coastal and shallow marine habitats, making them untenable for the creatures that lived there.
• Meteor impacts – Smaller than mass-extinction-causing asteroids, regional impacts can still devastate ecosystems.
• Emergence of new predators or competitors – Invasive species or evolving predators can overtake existing niches, driving older groups extinct.
• Vegetation turnover – Rapid changes in plant life, like the angiosperm radiation during the Cretaceous, can alter ecosystems and starve out herbivores adapted to older food sources.

Cretaceous Extinction Events

Volcanic eruptions in India, along with a massive asteroid impact in Mexico, helped bring about the end of the dinosaurs.

During the Cretaceous, there were periods of more extensive extinction that hit specific groups hard. Two especially significant extinction pulses occurred around 94 million years ago, during the Cenomanian–Turonian transition. Toward the end of the Cretaceous, the Chicxulub asteroid impact, combined with environmental changes from the Deccan Traps volcanic eruptions, led to the extinction of dinosaurs.

How Scientists Identify Smaller Extinction Events

Large extinction events leave such extensive fossil evidence that they’re hard to miss. Smaller ones require more sophisticated tools. Researchers use advanced data analysis, molecular clock modeling, and Bayesian birth–death models drawing from comprehensive fossil databases. This allows researchers to reconstruct speciation and extinction rates with great accuracy. Extinction trends can be determined for some species by using molecular dating techniques paired with well-dated fossil finds. Researchers also track extinction pulses by examining fossil shapes, locations, and evolutionary relationships. These methods reveal patterns that might be easy to miss with traditional fossil counting alone, and they’ve revolutionized our understanding of extinction as a dynamic process.

What Happened to the Ichthyosaurs?

Ichthyosaurs are a prime example of a successful group of marine reptiles that died out due to episodic extinction events. They dominated the oceans for much of the Mesozoic, from about 250 to 90 million years ago. They were not dinosaurs, but they shared the seas with them and resembled modern dolphins in body shape. Over time, ichthyosaurs evolved into a wide range of sizes, from a few feet long to giants over 60 feet in length.

By the middle of the Cretaceous, ichthyosaurs began to decline. At the same time, other major marine predators, such as pliosaurids, also experienced sharp declines. By around 90 million years ago, ichthyosaurs had become completely extinct. Their extinction had a ripple effect, opening the door for other marine predators to take over. Paleontologists now understand that ichthyosaur extinction was not a single event but occurred in two phases. The first phase, during the early Cenomanian, saw a significant drop in diversity, eliminating many ecological niches. The second phase, around the Cenomanian-Turonian boundary, resulted in the final extinction of the group around 90 million years ago.

What Drove These Mid-Cretaceous Extinctions?

Researchers point to instability in the climate and the oceans as the culprit. There were several factors at play. First, ocean anoxia played a huge role—when oceans lose oxygen, marine food chains fall apart. On top of that, atmospheric CO₂ levels were high, which led to warmer ocean temperatures that stressed marine organisms even further. This all combined into a perfect storm for marine extinction. Predators like mosasaurs evolved over time to fill the geological niche left behind.

Extinction on Land

Land creatures were also affected, including insects, which we might normally think are more resilient than many other species. Some studies suggest that in the mid-Cretaceous period, there were several spikes in extinction among different species of darkling beetles (Tenebrionidae). This decline happened during what scientists call the Cretaceous Terrestrial Revolution, a time when flowering plants began to explode in diversity and climates began to shift. Many darkling beetles were well adapted to arid environments, but as angiosperms spread and ecosystems transformed, those dry habitats began to shrink. The combination of climate change and the ecological upheaval triggered by rapidly evolving plant life reduced the niches these beetles depended on, contributing to the decline and extinction of some of their species.

The Deccan Traps Extinction

The Deccan Traps were massive volcanic eruptions in present-day west-central India that began around 66 million years ago, near the end of the Cretaceous. These eruptions released huge volumes of lava, carbon dioxide, and sulfur gases over thousands of years, leading to global climate change, including cooling from aerosols and warming from CO₂. The environmental stress from these eruptions likely contributed to the mass extinction event that also involved the Chicxulub asteroid, impacting ecosystems worldwide and weakening species already under pressure.

What Dinosaurs Went Extinct as a Result of the Deccan Traps?

So many factors were at play at the end of the Cretaceous that scientists cannot attribute the extinction of any specific dinosaur species solely to the Deccan Traps eruptions. However, the eruptions likely contributed to the disappearance of some of these species:

• Hadrosaurs (duck-billed dinosaurs) – especially in Asia and North America
• Ceratopsians (like Triceratops) may have suffered ecological pressure before the asteroid
• Theropods (including raptors and large carnivores like Tyrannosaurus) – many lineages disappeared except for the ancestors of birds

Why These Smaller Extinctions Matter

Studying these episodic extinction events offers valuable insights into how evolution works. While the giant asteroid strike at the end of the Cretaceous is an obvious turning point, these smaller events help fill in the details of how ecosystems evolve. They show that extinction is often selective: species that are highly specialized or have limited ability to migrate tend to be most vulnerable. These smaller-scale extinctions also play a role in shaping future biodiversity. As some lineages disappear, new ones emerge to fill the ecological gaps they leave behind. Recognizing these lesser-known events gives us a deeper, more complete understanding of evolution—not as a smooth, continuous process, but as a jagged path shaped by pressures, disruptions, and cycles of loss and renewal.