The Recursive Chicken is an ancient species which has developed a unique method of self-preservation. The shells of its eggs are formed from a hyper-dense material, and this allows them to warp space-time in accordance with Einstein’s theory of General Relativity.
Hence the eggs produced by each generation of recursive chicken contain the chicks of the previous generation. This means that any predator which successfully killed a recursive chicken would cause an immense temporal paradox. The species is thus completely immune from predation.
From the biologist’s point of view, the recursive chicken is a particularly valuable discovery because it allows a simple answer to the question “Which came first, the chicken or the egg?” It is now believed that the first chickens were recursive chickens, and that they laid themselves after hatching. Today’s ordinary chickens are the mutant offspring of their recursive ancestors.
The special defense developed by the recursive chicken is not without cost, however. Since only a single creature is required to successfully hatch several of the previous generation, the chicken is rapidly dwindling in numbers. Nevertheless, a few specimens still survive.
To explain the numbers seen today, it is necessary to hypothesize that there was an exponentially larger number of recursive chickens at some time in the distant past. The earliest fossilized remains of a recursive proto-chicken have been dated to the early Cretaceous period, and simple calculations show that at that time the first generation of recursive chickens must have covered the entire planet to a depth of 18.4 metres.
It is now believed that this is what caused the extinction of the dinosaurs.