On May 6, 2013, a volcano in Ecuador, South America erupted, spewing ash into the air. The Cretaceous Era, spanning from roughly 145 to 66 million years ago, marked the final era of the dinosaurs. Despite the catastrophic end brought about by a massive asteroid impact, there were several smaller extinction events throughout this period that played a significant role in shaping evolutionary changes for dinosaurs and other organisms.
Extinction is a natural process that occurs regularly on Earth. Even in the absence of major catastrophic events, species become extinct due to factors such as competition, habitat changes, and evolutionary turnover. These gradual disappearances, known as background extinction, contribute to the continuous evolution of life on our planet.
In contrast to the well-known mass extinction events, episodic extinctions are smaller-scale events driven by specific environmental changes. These targeted extinctions are often regional rather than global, altering ecosystems and creating opportunities for new species to thrive.
Various factors can trigger episodic extinction events, including volcanic eruptions, ocean anoxia, climate shifts, sea level changes, meteor impacts, the emergence of new predators or competitors, and vegetation turnover. These events can disrupt ecosystems and lead to the decline or loss of certain species unable to adapt or compete in the changing environment.
Throughout the Cretaceous period, multiple extinction events occurred, with notable impacts on specific groups of organisms. Two significant extinction pulses around 94 million years ago, during the Cenomanian–Turonian transition, had a profound effect. The culmination of the Cretaceous period saw the demise of the dinosaurs due to a combination of the Chicxulub asteroid impact and environmental changes resulting from volcanic eruptions in the Deccan Traps of India.
Scientists study these smaller extinction events using various methods and tools to understand their causes and effects on biodiversity and ecosystems.
Computers have expedited the process of identifying extinction events for scientists. Large extinction events, characterized by extensive fossil evidence, are easily recognizable. However, smaller events necessitate more advanced tools and techniques. Researchers utilize sophisticated data analysis methods, molecular clock modeling, and Bayesian birth-death models sourced from comprehensive fossil databases to accurately reconstruct speciation and extinction rates. Molecular dating techniques combined with well-dated fossil discoveries enable the determination of extinction trends for certain species. By analyzing fossil shapes, locations, and evolutionary relationships, researchers can track extinction pulses, revealing patterns that traditional fossil counting may overlook. These innovative methods have transformed our understanding of extinction as a dynamic process.
The ichthyosaur, an apex predator that once dominated the oceans, serves as a prime example of a marine reptile group that faced episodic extinction events. Existing from approximately 250 to 90 million years ago during the Mesozoic era, ichthyosaurs coexisted with dinosaurs and bore resemblances to modern dolphins. Ranging in size from a few feet to colossal creatures exceeding 60 feet, ichthyosaurs evolved diversely over time. By the middle of the Cretaceous period, ichthyosaurs and other major marine predators, like pliosaurids, underwent declines. Around 90 million years ago, ichthyosaurs became extinct, paving the way for the rise of other marine predators. Paleontologists now view the ichthyosaur extinction as a two-phase process, with the first phase in the early Cenomanian reducing diversity significantly and the second phase marking their ultimate extinction around the Cenomanian-Turonian boundary.
These mid-Cretaceous extinctions are attributed to climate and oceanic instability by researchers. Factors such as ocean anoxia and elevated atmospheric COâ‚‚ levels, leading to warmer ocean temperatures, disrupted marine food chains. This environmental upheaval contributed to the extinction events, allowing predators like mosasaurs to evolve and occupy the ecological niche left by ichthyosaurs.
Land-dwelling creatures, including insects, also experienced extinctions during this period. Notably, studies indicate spikes in extinction among various species of darkling beetles (Tenebrionidae) in the mid-Cretaceous era. This decline coincided with the Cretaceous Terrestrial Revolution, characterized by the proliferation of flowering plants and shifting climates. While darkling beetles were well-adapted to arid environments, the spread of angiosperms and changing ecosystems posed challenges to their habitats.
The shrinking habitat of beetles was worsened by the effects of climate change and ecological disturbances caused by rapidly changing plant life. This led to a reduction in the niches available to these beetles, ultimately contributing to the decline and extinction of certain beetle species. The Deccan Traps, a series of massive volcanic eruptions in the region of present-day west-central India that began approximately 66 million years ago, played a significant role in the extinction of dinosaurs. These eruptions released vast amounts of lava, carbon dioxide, and sulfur gases over thousands of years, resulting in global climate changes such as cooling from aerosols and warming from COâ‚‚. The environmental strain caused by these eruptions likely played a part in the mass extinction event, alongside the impact of the Chicxulub asteroid, affecting ecosystems worldwide and weakening species already facing challenges. While it is difficult to solely attribute the extinction of specific dinosaur species to the Deccan Traps eruptions due to numerous factors at play during the end of the Cretaceous period, it is believed that species like Hadrosaurs, Ceratopsians (such as Triceratops), and Theropods (including raptors and large carnivores like Tyrannosaurus) may have disappeared in part due to the ecological pressures caused by the eruptions. Understanding these smaller extinction events provides valuable insights into the mechanisms of evolution, highlighting the selective nature of extinction where highly specialized or poorly migratory species are most at risk. These events also contribute to shaping future biodiversity, as new lineages emerge to fill the ecological niches left vacant by extinct species. By acknowledging these less-known events, we gain a more comprehensive understanding of evolution as a process shaped by pressures, disruptions, and cycles of loss and renewal, rather than a smooth and continuous progression.
