Pygmy Mammoths and Santarosae: The Drowned Superisland

During the last ice age, the sea level along this coast was low enough that the four northern Channel Islands you can see from the harbor were not four islands. They were one. Geologists call it Santarosae: a landmass large enough to hold forests and freshwater, separated from the mainland by a channel roughly five miles wide at its narrowest.

Columbian mammoths crossed that channel. They were twelve feet tall at the shoulder and among the largest land mammals on Earth at the time. What moved them to swim roughly five miles of open ocean is not known; perhaps a drought pushed them toward the water, perhaps they followed vegetation visible from the shoreline. What is known is that they made it, and that once they arrived, they were stranded.

Over hundreds of generations, isolated on Santarosae without predators large enough to threaten them and without the food supply to sustain Columbian-scale bodies, they shrank. Island dwarfism is a well-documented biological process, the same mechanism that produced pygmy elephants on Mediterranean islands. By the time the sea level rose and Santarosae broke into four separate islands, the mammoths on those islands stood roughly six feet tall. They were a new species: Mammuthus exilis, the pygmy mammoth.

The most recent pygmy mammoth remains date to approximately thirteen thousand years ago. That coincides closely with the first documented human arrival on the Channel Islands. Whether the mammoths were hunted to extinction, whether the habitat change of the rising sea finished them, or some combination of both, is a question the fossil record has not fully answered. The bones found on Santa Rosa and San Miguel Islands answered the smaller question: they were here, they were real, and they were extraordinary.