Wild Animals/Arctic Fox Evolution
In my grade 11 biology class, we have to make an evolutionary flow chart of a mammal. We have to create the flow chart from the first mammalian ancestor about 210 million years ago. We have to include the uninterrupted linneage of our animal along with the time (MYA) they occured, and significant branches of other closely related mammals. I chose the arctic fox and I'm having trouble finding information and filtering out the bad stuff.
Thank you for your question. I also wish to thank the authors of the websites and books I used.
It is only the second time I've had a question like yours. The first time was almost 10 years ago and the person wanted "to know the evolutionary history from mammal-like reptiles to the present day artic fox". This is a bit strange, but it means that I will use a lot of the information I compiled for the previous answer (http://en.allexperts.com/q/Wild-Animals-705/Artic-Fox-Alopex-lagopus.htm
). Since this answer, there have been a few new discoveries, so I have updated it.
Please note that there have been many divergences from the path leading to the Arctic fox, so I have tried to concentrate on those animals close to the direct line. I have only summarised events before mammals diverged from therapsids about 225 million years ago.
Synapsids and reptiles diverged after they evolved from a group of amphibians. The earliest synapsids are called 'pelycosaurs' and include the sphenacodontians, which gave rise to the second group of synapsids - the therapsids. Some of the more advanced synapsids were called cynodonts (http://en.wikipedia.org/wiki/Cynodontia
), which appeared about 260 millio years ago. Some of these probably were warm-blooded and had hairy coats.
The dividing line between therapsids and mammals is usually based on the number of bones in the lower jaw and the way in which the lower jaw is attached to the skull. Basically, if there is only one bone, the dentary, in the lower jaw and this is attached to the squamosal, then the animal is a mammal. Scientists are uncertain which group of cynodonts were the ancestors of mammals; different groups have their advocates. In fact some people think that monotremes developed from a different group of cynodonts compared with the marsupials and placentals.
Mammals are included in a group called the Mammaliformes (http://en.wikipedia.org/wiki/Mammaliaformes
), which also includes earlier forms with more than one bone in each side of the lower jaw. They date from the Middle Triassic (245-228 million years ago).
The Theria (http://en.wikipedia.org/wiki/Theria
) included the common ancestors of marsupials placentals and probably originated in the Early Jurassic (http://onlinelibrary.wiley.com/doi/10.1111/j.1096-3642.1984.tb00543.x/abstract
), 200-175 millio years ago.
One of the early ancestors of placental mammals is Eomaia (http://en.wikipedia.org/wiki/Eomaia
), which lived about 125 million years ago. There were primitive placentals in the late Cretaceous, with various orders evolving, but the mammals really came into their own with the extinction of the dinosaurs. Until a few decades ago, insectivores were considered to be the most primitive placentals and to be the ancestors of other placentals. It is now known that the Cretaceous placental mammal community was far more varied than previously thought.
says that carnivores are among the mammals that probably originated on the northern continent of Laurasia.
Carnivores began about 65 million years ago as squirrel-sized creatures, such as Cimolestes, that ate insects in the shadows of the dominant dinosaurs. About 58 million years ago, two lineages of mammals independently developed the carnassial scissor teeth. These were creodonts and carnivores. The creodonts dominated the world from 55 to 35 million years ago and the carnivores became top predators on the northern continents from 20 to 30 million years ago, as they were more efficient at grinding vegetation, so could have a more mixed diet.
The earliest true carnivores date from the Late Palaeocene and early Eocene. These forms were called miacids and lived on the ground and in trees, where they hunted small mammals. About 55-60 million years ago, early arboreal carnivores gave rise to two main branches – dog-like and cat-like carnivores. Cat-like forms became hunters by stealth and ambush, maintaining a dependence on carnivory. Dog-like forms opted for a mobile, generalist and opportunistic lifestyle.
The dog-like forms, or Canoidea, evolved in the New World, while the cat-like forms evolved in the Old World. The dog branch first appeared as tree-climbing, superficially marten-like carnivores in the Eocene around 42 million years ago (http://en.wikipedia.org/wiki/Caniformia
) and gave rise to dogs, bears, raccoons and weasels. The dog and cat branches evolved independently until 30 million years ago, in the early Oligocene, when they could migrate across the Bering Strait between America and Eurasia.
The Cynodictis was thought to be an ancestor of the dog, but the Amphicyonidae, or bear-dogs, which dominated Eurasia for million of years, are now considered to be unrelated to dogs and foxes. Prohesperocyon seems to have been the first member of the dog family (Canidae) and lived 40 million years ago in North America (http://en.wikipedia.org/wiki/Canidae
). The Canidae soon subdivided into three subfamilies, each of which diverged in the Eocene: Hesperocyoninae (about 39.74-15 million years ago), Borophaginae (about 34-2 million years ago) and the Caninae (from about 34 million years ago). The Caninae led to wolves, foxes and other modern members of the dog family (http://en.wikipedia.org/wiki/Canidae
). Osbornodon was the basal ancestor of modern dogs and foxes.
Around 9-10 million years ago in the Late Miocene the ancestors of the genera Canis, Urocyon and Vulpes expanded from southwestern North America, where the canine radiation began. Around 8 million years ago, they reached Eurasia via the Bering Strait (http://en.wikipedia.org/wiki/Canidae
The common ancestor of the Vulpes group of foxes originated in North America and diverged from other dogs, the lupine or wolf-like dogs, about 5-7 million years ago. Ancestral dogs crossed the Bering Strait into Eurasia, about 5-7 million years ago, while most of the rivals, including the last creodonts, became extinct. Early fox populations spread north and south. Foxes are opportunistic, small-scale foragers. Different environmental conditions led to different selective problems. The red fox originated in Eurasia 1.5 million years ago. The northern populations developed to form the arctic fox, which diverged 1-2 million years ago.
The arctic fox is related to the red fox and the Cape fox. The arctic fox is a recent Ice Age introduction. Its range forms a circumpolar ribbon to the north of the red fox. The two species overlap in the Eurasian and Canadian tundra. The red fox is larger and stronger than the arctic fox, and may kill it, but the red fox cannot find enough food in the north, where the arctic fox survives. Severe environmental selection pressures in the arctic resulted in the effect of accelerating evolution of many arctic species, such as the Arctic fox. The thick fur of the arctic fox has the highest insulation value of any mammal. The fox had a wider range during the Ice Age and has the most prolific breeding rate of any member of the dog family.
I hope this makes sense. There were a few contradictions in the text. For example, one article (http://en.wikipedia.org/wiki/Canidae
) said that Vulpes existed around 9-10 million years ago, but also said that Vulpes divereged from other dogs around 5-7 million years ago. I suspect these pieces of information were written by different authors. The first volume of the 'Handbook of Mammals of the World' says that the fox- and dog-like canids diverged 5-9 million yers ago.
This is a very abridged account of the story of the Arctic fox. I hope it has helped and that you will continue your interest in this area.
All the best