Sometimes we refer to bizarre animal forms as having been made from a combination of spare parts from other creatures, and such is the case for Macrauchenia, literally ãlarge camel.ä Here we have an animal with rhinoceros-like feet, featuring three toes and a very simple ankle joint. Macrauchenia had a short cranium mismatched with a long muzzle, perched on a long, camel-like neck; hence its name.

Macrauchenia had a mouth brimming with a full complement of 44 teeth, unusual for herbivores, which usually lack some. Its upper molar teeth were ridged, while its lower molars were shaped like two crescents, as if they came from two different animals. But the most distinctive feature of the animal was its nasal openings, which were near the top of the skull, indicative of a proboscis (trunk) that was long and flexible. This prehistoric herbivore was the perfect patchwork of spare parts.

But the large camel of Patagonia is not the only strange beast that graced the prehistoric Earth. Documenting the changes that occurred when proboscidians colonized islands has led me to investigate island biogeography and the indelible evolutionary ãstampä islands place on their residents. Isolation from other land masses offers impunity for evolutionary innovation, and an abundance of unfilled niches fuels a rapid radiation of forms. Thinking on a grander scale, I considered the similar impact of ãisland continents,ä which offer many of the advantages of smaller islands, and even more diversity of habitats. Like Australia, South America was once a unique island continent, with its concomitant evolutionary opportunities, and produced a distinctive mammalian fauna. The large camel of Patagonia was one product of these special evolutionary forces.

The South American mammalian fauna is even less studied than the distinctive animals of Australia, the other widely known island continent. South Americaâs isolation for nearly 55 million years led to the evolution of a number of distinctive animals, most of which became extinct only in the last few million years. The original fauna was composed entirely of marsupials, edentates (the ancestors of modern sloths, anteaters, and armadillos), and condylarths (ancient ungulates, like Macrauchenia). For more than 44 million years, these original stocks evolved on their own, giving rise to 6 distinctly South American orders, 25 families, and numerous species. They were joined about 40 million years ago by primates and rodents, both of which were probably introduced as ãwaif migrants,ä castaways from other continents that arrived on floating materials.

But South Americaâs isolation, and the evolutionary abundance it created, was finally eliminated late in its history by the formation of the Panamanian land bridge. The continental link spurred colonization by North American and Eurasian forms, which rather quickly destroyed the ãisland continentä fauna by better exploiting the available resources, or by preying on naive species that had no history of such predation. All herbivores native to South America became extinct following the Plio-Pleistocene invasion from North America. Not one species survived. One group of herbivores almost made it, dying out only in the early Pleistocene. This group included the patchwork Macrauchenia patachonia.

Macrauchenia missed being part of our modern South American fauna by a mere few million years. Extinct or alive, creatures made from spare parts like Macrauchenia often represent the unique forces of evolution on islands and island continents where a ãclean slateä affords evolutionary experimentation. Similar conditions prevail following mass extinctions, when the lack of competition spurs the evolutionary radiations responsible for most of the Earthâs biological diversity.