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Evolution- Squirrels/Sciuridae

General Species Information

Squirrels are classified as small to medium sized rodents and are therefore part of the rodentia family. They belong to the sciuridae sub-family which is made up of tree squirrels, ground squirrels and flying squirrels, as well as chipmunks, marmots, and prairie dogs. Squirrels can be found virtually anywhere in the world where there is vegetation. There are 50 genera and 268 species in the squirrel family known at this time and more specifically, 22 genera and 122 species of tree squirrels which belong to the subfamily sciurinae (the squirrels we are most familiar with and see daily).

Abert's Squirrel (n.d) Retrieved October 30, 2015

Roles Squirrels Play in their Different Environments

11 MYA

36 MYA

3 MYA

30 MYA

Animalia Life. (n.d.). Retrieved October 31, 2015.

The typical tree squirrels that inhabit North America and that are so well known to us are more than just a cute site to watch or a minor nuisance in our lives. In fact, in urban areas squirrels help us greatly in several respects. First, squirrel populations can be great environmental indicators and can bring attention to potential public health problems. For example, a sick squirrel population can raise some red flags that there might be a environmental problem like high pesticide or chemical byproduct concentration in the environment. Also, squirrels frequently forget where they have hidden their nuts and seeds, and then these undisturbed nuts oftentimes take root. As a result, they can involuntarily plant trees or other plants, helping to add and to expand the ecosystem.

In a more general sense, all squirrel species play a very important role in their respective habitats. Whether it be the African ground squirrel or the Pallas's squirrel in China, squirrels fill ecological niches and help balance ecosystems by acting as prey and consuming other organisms.

Squirrel Archives - All Seeing Me. (n.d.). Retrieved October 31, 2015.

Evidence of squirrels in Europe. However, it is still unsure exactly how the squirrel ancestors arrived there. It is believed at that time the passage between North America and Eurasia was much more accessible and as a result, the squirrels were able to make their way to Europe.

Earliest fossil evidence found in the squirrel lineage dated back to 36 million years ago. The species of the fossil is the Douglassciurus Jeffersoni and was found in North America’s west.

The diversification of squirrels exploded within this period, branching off more and creating many new species, most likely to fill ecological niches. These species now inhabit South East Asia. The diversification corresponded with very low sea levels, suggesting that the squirrels could have crossed areas that are now submerged. As a result, they would have made their way to South East Asia as well as neighbouring islands and land masses. It has been hypothesized that when these islands once again became inaccessible and remote, the diversification of the squirrel species present in those areas would have accelerated as a result of evolution.

Squirrel Characteristics

South America had not been inhabited by squirrels before 3 million years ago. Then, a land bridge was believed to be formed as a result of geological shifts. Evidence has suggested that all squirrel species now inhabiting South America except for one, descended from one common ancestor that would have crossed the land bridge when it was formed.

Squirrels are known for their slim long bodies and bushy tails; however, variation can come between species. Squirrels in the scuirinae family are very skilled at climbing trees, but in order for them to effectively climb down trees, they possess very flexible ankle joints that enable them to rotate their feet. They have rounded ears and some species have tufts of fur at the corners. In addition, squirrels have strong curved claws. Colouring often varies extremely between species.

Common tree squirrels that are found in North America have very powerful jaws that enable them to crack open nuts which are a large part of their diet. Their diet consists of nuts, seeds, fruit, insects and fungi and sometimes bark, flowers, and even bird eggs.

The Evolution of Squirrels:

Picture Id: 1291112. (n.d.). Retrieved October 31, 2015.

18-20 Million Years Ago

7 Million Years Ago

31-36 Million Years Ago

31 Million Years Ago

11 Million Years Ago

36 Million Years Ago

3 Million Years Ago

31-36 MYA

18-20 MYA

7 MYA

Evolutionary Summary

In general, squirrels and their characteristics have stayed fairly consistent throughout their long history. Their evolutionary history consists mostly of traveling to new environments and continents where they then adapt as a result of natural selection. Their history does not consist of huge physical changes like some other species. Even the first full rodent fossil discovered looks very similar to rodents we know now and in addition, is structurally similar to squirrels as well. From this we can conclude that squirrels and rodents are well suited to, and do well in many different environments around the world. Their characteristics work well for them in all these environments, and as a result, huge amounts of adaptation are not necessary. This would explain why they have not physically evolved as much as other species have.

Fossil records show the appearance of squirrel lineage in Africa to have taken place 10-12 million years after their appearance in Europe. DNA evidence has shown that at this point, the diversification of squirrels had begun to speed up and the lineage of what is now the modern African tree and ground squirrels, began branching off.

Between what we know as Siberia and Alaska, evidence shows a forested connection of land was present during this period. This would have served as the passage way for many tree/ground squirrel lineages between the two continents.

In the following 5 million years, a rapid divergence of squirrel lineage into five main segments took place.

The Future of Squirrels (Continued)

(example of squirrel lineage branching off. The branches are all coming from the sciuridae sub-family)

Sciuridae. (n.d.). Retrieved October 25, 2015.

Tree squirrels have longer bodies and tails to help them when they are maneuvering throughout trees and to stay balanced. However, if in 10,000 years, trees are not readily available to serve as homes for squirrels, these traits would not necessarily be advantageous or needed. Squirrels would no longer have access to trees like they do now, and would have to find other way to get shelter rather than in trees. I considered that squirrels with stockier builds and traits more similar to ground squirrels would begin to possess the more advantageous characteristics because their features are specialized in maneuvering on the ground. Stronger hind legs would most likely become more common in tree squirrel populations and shorter tails would become common as well because long tails would not be needed as much for balancing on branches.

More about Rodentia and their Evolution.

Rogosodon: reconstruction and structure.

Earliest complete fossil from rodent-like ancient mammal discovered. (n.d.). Retrieved October 31, 2015.

Rodentia are the largest group of mammals that exist today with over 1500 different species. Squirrels are in the rodent family, and therefore under this heading. Like squirrels, rodents are very widespread and can be found on all continents except for Antarctica as well as most islands. Their vast range across the world is attributed to geological shifts and conditions millions of years ago. These shifts were what enabled them to cross to other continents, whereas today, crossing to other continents would not be possible due to bodies of water.

Evolutionary, rodents are considered very successful because of their amazing ability to adapt, and live in many different environments. Some adaptations credited for making them so successful are their flexible ankles and their strong and ridged jaws.

The earliest completed rodent fossil is estimated to be 160 million years old, and was found in China near Bejing. Pictured on the top right is the structure of the fossil (Rugosodon) and its reconstruction . Pictured on the bottom right is the fossil found.

The Future of Squirrels

Rogosodon: Fossil

Earliest complete fossil from rodent-like ancient mammal discovered. (n.d.). Retrieved October 31, 2015.

Based on the hypothesis that squirrels have not changed a huge amount in the past 36 million years because they possess characteristics that work so well with many environments, I have predicted that in the next 10,000 years, they will not change much more either. However, one reason that they would need to evolve was if their habitats were changed so dramatically, that their features were no longer advantageous. Since deforestation has become a large concern in our world, I considered how squirrels would adapt if trees became sparse and uncommon as a result of deforestation in 10,000 years.

Deforestation. (n.d.). Retrieved October 31, 2015.

Mammal Watching in Namibia. (n.d.). Retrieved October 31, 2015.

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