Which Groups Survived Better During Mass Extinctions: Reptiles, Mammals, or Both?

Both mammals and reptiles have shown remarkable resilience during past mass extinctions, even as they have faced the widespread extinction of numerous species. Despite the vastly different survival trajectories of reptiles and mammals, the question remains: which group fared better during these pivotal moments in Earth's history?

The Context of Mass Extinctions

Over the course of Earth's history, there have been five mass extinctions, with the Holocene, the epoch we are currently living in, marking the likely beginning of a sixth. A mass extinction is defined as the loss of a significant percentage of species within a relatively short period in geologic time. The minimum threshold for a mass extinction typically ranges from 60 to 75 percent of species in a few million years or less. Each of these extinctions has left an indelible mark on the course of evolution, reshaping the landscape of life on Earth.

Survival Post-Mass Extinctions

Survival, rather than 'better' survival, is the key factor to consider when evaluating the outcome of mass extinctions. In the immediate aftermath of a mass extinction, the situation is dire for all life. The term 'better' does not apply; there is only survival, albeit through varied means. However, as tens of millions of years pass, some groups emerge victorious, diversifying and spreading into newly vacated ecological niches.

Data and Analysis: Survivable Groups

Reptiles, being a vastly older lineage, have weathered three major mass extinctions: the Permo-Triassic (252 million years ago), the Triassic-Jurassic (200 million years ago), and the Cretaceous-Paleogene (66 million years ago). In each case, reptiles were 'whittled down' but then diversified into new species. This process led to the rise of iconic groups such as crocodilians, snakes, and lizards. Post-extinction, these groups thrived, building upon their resilience to adapt to new ecological opportunities.

Mammals, though more recently evolved, faced their own challenges during the end-Cretaceous extinction (66 million years ago). This event, believed to have been caused by a massive meteor impact, wiped out the dinosaurs, allowing mammals to expand into new ecological niches. Mammals, while initially minor in comparison to reptiles and dinosaurs, began to diversify and take over terrestrial ecosystems. The Eocene, particularly, saw a "mammal's coming-out party," where they exploded in diversity and spread across multiple ecosystems.

Comparison and Conclusions

Imposing a standard of 'better survival' is complex and often subjective. The immediate aftermath of a mass extinction is harsh, and the term 'better' is misleading. Survival, however, is the ultimate test. While reptiles have thrived through multiple mass extinctions, mammals are relatively newcomers in comparison. Both groups have demonstrated remarkable adaptability, but the course of subsequent evolution has favored mammals more significantly.

Mammals have successfully out-competed reptiles in many terrestrial niches, leading to their dominance in the post-extinction era. This dominance is not a measure of inherent superiority but a direct result of the ecological opportunities created by the extinctions. After each mass extinction, the group that survives and diversifies expands into vacated niches, driving the overall increase in biodiversity.

Mass extinctions, therefore, serve as significant turning points in the evolution of life. They are catalysts for reshaping ecosystems, opening doors for previously minor groups to dominate. In this sense, both reptiles and mammals have been central to the story of life's evolutionary history, each playing a crucial role in shaping the course of biodiversity over millions of years.