Pygmy mammoth

Pygmy mammoth Temporal range: Late Pleistocene to Early Holocene, 0.06–0.011Ma PreЄ Є O S D C P T J K Pg N ↓
Skeleton
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Proboscidea
Family:	Elephantidae
Genus:	Mammuthus
Species:	M. exilis
Binomial name
Mammuthus exilis (Stock & Furlong, 1928)[1]

The pygmy mammoth or Channel Islands mammoth (Mammuthus exilis) is an extinct species of dwarf elephant descended from the Columbian mammoth (M. columbi) of mainland North America.^[2] This species went extinct during the Late Quaternary extinction where a lot of megafauna species went extinct because of certain conditions that were changing as the ice age was ending, along with other factors these large species could not handle and/or evolve around.^[3] A case of island or insular dwarfism, from a recent analysis in 2010 it was determined that M. exilis was on average, 1.72 m (5.6 ft) tall at the shoulders and 760 kg (1,680 lb) in weight.^[4] A stark contrast to its 4.3 m (14 ft) tall, 9,100 kg (20,100 lb) ancestor.^[5]

Discovery

Excavation in 1994

In July 1994, L. Agenbroad was called by the National Park Service to inspect an uncovered, unidentified skeleton found on the northeast coast of Santa Rosa Island.^[6] B. Agenbroad, L. Agenbroad, D. Morris, S. Morris, T. Rockwell and L. Roth ^[6] found bones of the axial skeleton of a large land vertebrate on the Santa Rosa Island. On August 1994, they decided there was enough evidence to start excavate and dig up the skeleton. They recovered 90% of a mature male Pygmy Mammoth's skeleton.^[7] The mammoth was about 50 years old when it died.^[6] The small bones were preserved in life position, which represented that it had died where it was found rather than being scattered around the island. The bones were returned to the Santa Barbara Museum of Natural History.^[7] After the discovery of the skeleton, a pedestrian survey of the island was started, which resulted the discovery of 160 new locations of mammoth remains, the vast majority being found on Santa Rosa Island.^[6] This was the first discovery of a nearly complete specimen of the pygmy mammoth. Fortunately enough, the skeleton of the mammoth was only missing a foot, a tusk, and a couple of vertebrae. The remains were covered by a sand dune, which prevented the bones from scattering and kept them intact. Since the bones were undisturbed, it is predicted he simply died of old age.^[8]

Habitat

Remains of M. exilis have been discovered on three of the northern Channel Islands of California since 1856: Santa Cruz, Santa Rosa, and San Miguel, which together with Anacapa were the highest portions of the now mostly submerged superisland of Santa Rosae. The late Pleistocene elephant may have lived on the islands until the arrival of the Chumash people during the early Holocene, between 10,800 and 11,300 years ago. Radiocarbon dating indicates M. exilis existed on the island for at least 47,000 years prior (which is the approximate limit of the dating method).^[7]

Modern elephants are excellent swimmers, and the ancestors of M. exilis most likely swam the 4 mi (6.4 km) to Santa Rosae. As the population of mammoths increased, the lack of large predators such as the dire wolves, smilodon and the american lion and the loss of habitat caused by the rise of sea levels at the end of the ice age as Santa Rosae split into four islands favored smaller animals. Because of this, the pygmy mammoths began evolving through generations as a survival mechanism to stay alive on the ever-shrinking Santa Rose Island, their body size became smaller in order to require less food and resources to keep them energized and alive. After this evolution is when the species became its own distinct species, the pygmy mammoth.^[5]

M. exilis should not be confused with the mammoths of Wrangel Island or Saint Paul Island, which were small races of the woolly mammoth (M. primigenius) and which died out around 1700 B.C. and 4000 B.C., respectively.

The Pygmy mammoth was able to thrive in all of the many different ecosystems found on Santa Rosae, such as high elevation plateaus to dune, grassland, riparian and steppe-tundra ecosystems.^[7]Their fossils are found in the Channel Islands and in the California Channel. The evidence of their habituation in all of those diverse dwellings is revealed by the pollen and plant pieces found in sediments and in dung. In addition, each habitat has a specific isotope mark from the unique types of soil, plants and water.^[9] These mammoths sometimes modified their habitats, specifically in Channel Islands, where they created more spacious grassland in result of their roaming.^[6]

Evolution

Land bridges have been theorized to be on the channel island during the time of ancestral elephants because it was assumed the Mammoths could not swim. However, evidence of there ever being land bridges is not very high. New evidence shows that elephants are skilled enough to swim and do it cross water gaps. Modern Elephants are believed to swim much faster at speeds that range from 0.96–2.70 km/hr. ^[6] Pygmy mammoths are thought to be descendants of Columbian mammoths. When the Ice Age caused the sea levels to lower the four Channel Islands formed a single island that was closer to the mainland and also larger in size. Therefore, given that the Columbian mammoths are good swimmers, they were able to cross the body of water and inhibit the new island and adapt to the new environment through evolution.^[10]

The pygmy mammoth is a descent of the Columbian mammoth and when the Columbian mammoth travelled to these islands, having to swim for a very long time to get there they had already experienced a 30% decrease in size which gave rise to the pygmy mammoth in the first place because of their dwarfed size.^[11] The pygmy mammoths lived on the Santa Rose island which was beginning to shrink because of the ice plates that were covering a lot of the ocean on the northern hemisphere were melting and the sea levels were rising. The pygmy mammoths began evolving through generations as a survival mechanism to stay alive on the ever-shrinking Santa Rosa Island, their body size became smaller in order to require less food and resources to keep them energized and alive and because the population of pygmy elephants stayed the same while the amount of land they had to roam became smaller and more habitable for smaller species.^[12] Also, their mainland predators such as the dire wolves, the smilodon and the american lion were not prominent on Santa Rose allowing them to evolve for multiple generations. After this evolution is when the species became its own distinct species, the pygmy mammoth.^[13] Despite popular belief, the evolution and dwarfing of the Pygmy mammoth took a long period of time. The process took over 30,000 years but as the mammoths adapted to the new land and restricted food supply they slowly became only as half as tall as their Columbian mammoth predecessors.^[10]

The sexual dimorphism of Pygmy Mammoths follows the norm for most elephants as the male shows dominance. The male species has grander limb bones and a larger skull. ^[7] In addition the male tusks are bigger and longer than those of the female. The female species has different pelvic bones that allow for childbirth. ^[14]

As the population of the mammoths increased on the island, the food supply became more and more limited as the island decreased in size due to climate changes of sea levels rising and glaciers melting. Smaller mammoths required less food which gave them a stronger advantage. Smaller mammoths survived also because larger predators were fighting for survival as well. Larger predators were decreasing just as larger mammoth. In less than 20,000 years, natural selection favored smaller mammoths and they became the norm over larger mammoths.^{[15] [16]}

Extinction

It is unknown how exactly the pygmy mammoth went extinct, although generally human interference is thought to have a greater impact on the extinction of island-dwelling species. It has been debated whether their extinction was caused by human overkill, climate change, or possibly even the aftermath of a comet impact. There does not happen to be any research connecting humans directly to the hunting and killing of the pygmy mammoth on the Santa Rosa island mostly because all the research points to the fact that humans on this island seemed to be mostly reliant on sea creatures for food because of the massive reorganization of the marine species in this area because of Santa Rose breaking up into four different islands when the sea levels began to rise.^[17] However, there were likely significant climate changes happening in the area during the existence of mammoths. After rising sea levels broke Santarosae into four landmasses sometime between 18,000 and 7,000 years ago, the land area of the northern Channel Islands shrank by almost eighty percent. Postglacial warming reduced the freshwater and food sources for the pygmy mammoth, which may have had a negative effect.^[18] A controversial explanation of the pygmy mammoth's extinction is that of a cosmic impact. Kennet Douglas hypothesized that the cosmic impact may have caused rapid ecosystem disruption thus might have triggered the megafaunal extinctions which lead to reductions in other animal populations such as the pygmy mammoth. These claims are disputed due to the lack of shocked minerals, tektites, and impact craters. However, there has been a discovery of shock-synthesized hexagonal nanodiamonds (lonsdaleite) in YDB sediments dating to 12,950 ± 50 cal BP at Arlington Canyon, Santa Rosa Island, California. On Earth lonsdaleite is known to occur in meteorites and impact craters, and its existence in Santa Rosa where the pygmy mammoth dwelled, supports the notion of a cosmic impact.^[19]

Notes

References

Agenbroad, L.D. (2001). "Channel Islands (USA) pygmy mammoths (Mammuthus exilis) compared and contrasted with M. columbi, their continental ancestral stock", World of Elephants, 473

Torben, Rick (2012). "Flightless ducks, giant mice and pygmy mammoths: Late Quaternary extinctions on California’s Channel Islands, 3

See also

• Pygmy elephant

Wikispecies has information related to: Mammuthus exilis