Image: Wikimedia Commons

Super-sized animals dominated the Pleistocene era and dated back as far as the dinosaurs — but hardly exist on Earth today.

Skeletons of dinosaurs, mastodons, and mammoths fill archaeology museums around the world in awe-inspiring displays of massive size. These animals are recognized as megafauna, some boasting more than a metric ton in body weight. 

The fossil record reveals accurate evidence of a mass extinction of Pleistocene megafauna near the end of the last ice age. A few of these species included the cave bear, the straight-tusked elephant, wooly rhinoceros, mammoth, the giant ground sloth, and the giant deer.

Scientists have long theorized the reasoning behind the impressive size of these animals and their subsequent extinction. Some surmise these animals boasted large sizes due to a greater concentration of oxygen in the atmosphere and more topographical space attributed with massive undeveloped land masses. Many scientists equate their massive size to evolutionary adaptations due to increasing competition, an idea that was transformed into a widely accepted theory.

Image: Bryan Kemp, Flickr

A paleontologist named Edward Cope is responsible for the officially recognized Cope’s Rule. The concept suggests that competition between members of the same species evolutionarily creates larger animals over time. 

Cope’s Rule also states that subsequent extinction of the same species is due to the same evolutionary adaptation — a limitation of resources created as larger animals dominate environments and require higher energy consumption. In addition, inevitably extended reproductive cycles made them more susceptible to environmental changes and a concurrent lack of ability for adaptation.

Scientific opposition to Cope’s Rule is centered around unexplained lineages that persist in the opposite direction. 

Other likely theories for the extinction of these massive prehistoric animals include the negative impact of humans, climatic change, and widespread epidemics.