The pygmy mammoth or Channel Islands mammoth (Mammuthus exilis) is an extinct species of dwarf mammoth native to the northern Channel Islands off the coast of southern California during the Late Pleistocene. It was descended from the Columbian mammoth (M. columbi) of mainland North America, which are suggested to have colonised the islands around 250–150,000 years ago. At only tall at the shoulder, it was around 17% the size of its mainland ancestor. The species became extinct around 13,000 years ago, coinciding with major environmental change and the arrival of humans in the Channel Islands.
Discovery
thumb|left|Excavation in 1994, Santa Rosa
Mammoth remains have been known on the northern Channel Islands of California since 1856. They were first reported in scientific literature in 1873.
In 1994 the National Park Service called in scientists to inspect an uncovered, unidentified skeleton found on the northeast coast of Santa Rosa Island. They found bones of the axial skeleton of a large land vertebrate and decided to excavate and dig up the skeleton. They recovered 90% of a mature male pygmy mammoth's skeleton.
After the discovery of the skeleton, a pedestrian survey of the island began. This resulted in the discovery of 160 new locations of mammoth remains, the vast majority being found on Santa Rosa Island.
Distribution
left|thumb|Northern Channel Islands which M. exilis inhabited shown in dark green, with maximum extent of Santa Rosae shown in light blue surrounding the islands
Remains of M. exilis have been discovered on three of the northern Channel Islands of California: Santa Cruz, Santa Rosa, and San Miguel, which together with Anacapa were the highest portions of the now mostly submerged superisland of Santa Rosae which existed during times of lowered sea level. The area of the combined island has been estimated at , though there was never a land bridge as has often historically been suggested. Mammoths, like living elephants, were probably good swimmers and able to swim this distance.
Description
thumb|Size of the mostly complete Santa Rosa specimen discovered in 1994 compared to a human
M. exilis was on average, tall at the shoulders and in weight, making it around 17% of the body size of its tall, ancestor. Like other mammoths, the species exhibited sexual dimorphism, with males being larger than females. The limb bones of the species show isometric growth (preserving length-width ratio) from juveniles to adults, similar to those of living African elephants. In comparison to Columbian mammoths, the femur has a rounded rather than elliptical cross-section, and lacks a lateral tuberosity. The tusks are relatively straight and are around 50% the length and diameter of those of Columbian mammoths.
Extinction
The youngest records of the species date to around 13,000 years ago. This time coincides with the reduction of the area of Santa Rosae as a result of rising sea level, the arrival of humans in the Channel Islands (as evidenced by Arlington Springs Man) and climatic change resulting in the decline of the previously dominant conifer forest ecosystems and expansion of scrub and grassland, possibly induced by wildfires. It is therefore difficult to disentangle the precise causes of its extinction, though the pygmy mammoths had likely survived a greater reduction in island area during the preceding Last Interglacial/Sangamonian.