New research presented at the European Geosciences Union’s General Assembly reveals that extreme climate events during the mid-Cretaceous period led to the extinction of dominant marine predators, paving the way for new species to rise.
Key Points at a Glance
- Mid-Cretaceous climate volatility caused mass extinctions of apex marine predators.
- Ocean anoxia and high CO₂ levels disrupted marine ecosystems.
- Predatory niches were filled by emerging species like mosasaurs and sharks.
- Study utilized extensive phylogenetic and morphological data of marine reptiles.
- Findings highlight the impact of climate change on marine biodiversity.
The mid-Cretaceous period, approximately 94 million years ago, was a time of significant upheaval in Earth’s oceans. Dominant marine predators such as pliosaurids, ichthyosaurs, and thalattosuchian crocodyliforms, which had ruled the seas for millions of years, suddenly vanished from the fossil record. In their place, new predators like mosasaurs, plesiosaurs, and sharks emerged and diversified.
A recent study presented at the European Geosciences Union’s General Assembly attributes this dramatic shift to a combination of ocean anoxia (a depletion of oxygen) and extreme climate conditions during the Cenomanian/Turonian transition. This interval is recognized as the hottest period in the last 541 million years, characterized by elevated carbon dioxide levels and disruptions in essential oceanic nutrients like sulfur and iron. These environmental stresses created inhospitable conditions for many species at the top of the food chain.
As Earth’s climate system destabilized, massive algal blooms and changing ocean chemistry led to widespread zones of anoxia—“dead zones” where oxygen levels plummeted. Without sufficient oxygen, the survival of large, fast-moving marine reptiles became impossible. These creatures relied heavily on high metabolic rates and active hunting strategies, both of which became unsustainable under these new environmental constraints.
Dr. Valentin Fischer of the Université de Liège and his colleagues conducted a comprehensive analysis combining phylogenetic relationships of hundreds of marine reptile lineages with the largest dataset of 2D and 3D morphological data on marine reptiles assembled to date. By reconstructing the evolutionary history of these animals and mapping their anatomical traits, the team was able to identify the exact time window and characteristics of species that vanished versus those that survived and radiated.
Their findings indicate that extinction rates were markedly elevated during this transition and disproportionately affected long-bodied predators with elongated snouts and streamlined skulls—forms best suited for fast pursuit of prey in open waters. These adaptations, once considered evolutionary advantages, became liabilities in the rapidly changing ecosystem.
Meanwhile, species that survived and later dominated the seas often exhibited shorter skulls and more robust jaws, potentially reflecting greater ecological versatility and the ability to exploit different feeding strategies. For example, mosasaurs—the giant marine lizards that rose to prominence—possessed powerful jaws capable of crushing hard-shelled prey. This adaptability may have allowed them to thrive in environments that were too unstable or unpredictable for their predecessors.
Interestingly, the fossil record also reveals a burst of diversification in shark species during the same period. Their evolutionary success during this transitional epoch likely stems from their flexible physiology and reproductive strategies, which allowed them to colonize both shallow and deep marine environments and survive the ecological bottlenecks that wiped out their reptilian competitors.
Beyond anatomical changes, the study explored how entire marine ecosystems restructured. The disappearance of apex predators would have caused a cascading effect, altering predator-prey relationships throughout the food web. New balance points were established as ecosystems filled in the gaps left by extinction, with different species adapting to the newly available niches.
This deep-time case study offers sobering parallels to our modern world. Current anthropogenic climate change is also warming the oceans, disrupting nutrient cycles, and expanding zones of low oxygen. The ancient marine die-offs documented in this study serve as a cautionary tale: when environmental thresholds are crossed, even the most dominant species can vanish swiftly, leaving room only for the most adaptable to take their place.
“This kind of research helps us understand the evolutionary consequences of climate change,” said Dr. Fischer. “It’s not just about who survives or goes extinct—it’s about how ecosystems reinvent themselves in the wake of crisis.”
As scientists continue to examine the sedimentary layers and fossil remains from this turbulent period, they are piecing together not only a story of extinction, but one of transformation. The end of one reign beneath the waves marked the beginning of another—one shaped as much by catastrophe as by resilience.
Source: European Geosciences Union
