It's unlikely that a single significant cool spell alone caused the demise of megafauna. While climatic changes might have played a role in the decline of some megafaunal populations, it was likely the combination of multiple complex and interrelated factors that ultimately contributed to their extinction.

Key Contributing Factors:

1. Climate Change: Fluctuations in climate, including the cooling and drying phase known as the Younger Dryas, did likely have an impact on the megafauna. These climatic shifts led to changes in vegetation and ecosystems, affecting the availability and quality of food sources for large herbivores.

2. Overhunting: Human activities, such as hunting and overexploitation of resources, are widely considered a significant factor in the demise of megafauna. As humans spread across the globe and encountered megafauna, they competed for resources and engaged in hunting practices that impacted these populations.

3. Habitat Destruction and Disruption: Human activities also resulted in the modification of natural habitats, further reducing the available resources and space for megafauna to thrive. The alteration of ecosystems through human-induced changes, including vegetation loss and disruption of migratory routes, affected the survival of megafaunal species.

4. Disease Transmission: The introduction of diseases from humans and their livestock is another potential factor. Novel diseases, to which megafauna had no immunity, could have spread rapidly through populations and contributed to their decline.

5. Trophic Cascade Effects: The disappearance of megafauna could have triggered ecological cascading effects, where interconnected species in the ecosystem were affected by their loss. Large herbivores played essential roles in shaping plant communities and influencing predator-prey relationships. Their decline would have disrupted ecosystem dynamics, potentially leading to further population declines and imbalances.

It's important to note that the extinction of megafauna likely varied among different regions and species based on local conditions, environmental factors, and the interactions with the aforementioned factors. The demise of megafauna was not solely attributed to a particular cool spell but rather the combination of multiple stressors, both climatic and human-related, that ultimately led to their population declines and eventual extinction.