Bighorn sheep went extinct on Tiburon Island hundreds of years ago, long before 1975, when they were first thought to populate the sparsely-populated island. This secret was revealed during examination of ancient poop.

In 1975, a population of 16 females and four males was "introduced" to the island to create a breeding stock. This was undertaken to replace animals being wiped out by large-scales land waste. What might now be considered the introduction of alien species, was a chance for the species to survive forty years ago. 

"Introduction success was expected on Tiburón Island, given the suitable habitat, lack of predators, absence of domestic sheep and their diseases, and minimal human disturbance. Indeed, by the mid-1990s, the Tiburón herd had grown to a stable population of 500 animals, one of the most successful large mammal introductions in the world," Benjamin Wilder, of the University of California, said

Fossilized dung discovered on the island shows the species, Ovis canadensis, lived on the island centuries before people brought them there, and may be native to the island. 

Wilder found the fossilized dung pile, partly covered with a type of natural concrete created from urine. Jim Mead, who owns a large dung collection, compared the pile to other species, making the first analysis of the find. Carbon dating of the ancient feces dated the droppings to between the years AD 384 and 544.

Discovery of bighorn sheep on Tiburon Island took researchers by surprise. There does not even seem to be stories handed down through native people, telling of the animals. 

Extinction of Bighorn sheep on Tiburon Island happened sometime between the 6th Century - around the final fall of Rome, and the 19th century. DNA was sampled from the fossilized manure, and compared the sequence to modern animals. Analysis confirmed the droppings were from bighorn sheep, similar to those in California and Arizona today. 

Tiburon Island is located in the Gulf of California, off the coast of Sonora, Mexico. The name translates to "Shark Island." 

Bighorn sheep may have reached the island during the Pleistocene era, the last series of ice ages. At that time, a land bridge connected Tiburon Island to the mainland. 

"They [disappeared] sometime in the last 1500 years, probably due to inherent dynamics of isolated populations, prolonged drought, and/or human overkill. The reintroduced population is vulnerable to similar extinction risks," researchers wrote in the article, published in Plos One. 

This is a rare example of a time when the introduction of an "alien species" serves to reintroduce an animal back to its native habitat. This raises questions about the definition of native vs. invasive species. The answer to that question could have significant scientific and legal consequences.  

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