North American Marsupials

North America serves as the primary source of fossils revealing early marsupial evolution. Marsupials first appear in the Mid-Cretaceous. Kokopellia, from Utah, USA may be the oldest marsupial at 100 Ma (Kemp, 2005, p. 196). Alphadon represents the first undisputed marsupial. Alphadon from the Upper Cretaceous of North America is known mostly from teeth and jaws. Alphadon is a member of the family Didelphidae and is often considered a model archetype marsupial. Alphadon had the marsupial dental formula of three premolars and four molars (placental mammals have three molars). Like living marsupials these early forms exhibited a tooth replacement related to their nursing habits. Only the last premolar is replaced, anterior dentition is not because of extended nursing (Benton, 2005, p 309).

In the late Cretaceous marsupials underwent an adaptive radiation in North America. These early marsupials were small and adapted to various niches. Teeth indicate that various marsupials were specialized as insectivores, carnivores, and omnivores. Although more common than placentals, they were not as diverse or numerous as the multituberculate mammals. The K/T mass extinction event put a halt to the diversification of marsupials in North America. In the Cenozoic North American marsupials would be represented only by the didelphids (opossums).

South American Marsupials

It is not clear if marsupials originated in North America or Asia. In either case, it is clear they migrated to South America and Australia. Today Opossums and their South American relatives represent marsupials of the Western hemisphere; however, in the past their diversity was much greater. South American marsupials underwent an adaptive radiation in the Early Paleocene, which resulted in three lineages. Multiple niches were filled by this adaptive radiation.

Didelphimorphs (Order Didelphimorphia) radiated into small to medium sized opossums specialized as insectivores and omnivores. Didelphids went extinct in both Europe and North America during the Miocene. South American forms migrated back to Central and North America in the Pliocene/Pleistocene during the Great American Biotic Interchange.

Sparassodontia (Borhyaenoids) is an extinct order of specialized South American carnivores. Many of these carnivores evolved body shapes and lifestyles like those of placental dogs, bears, and cats (Benton, 2005, p. 315). These organisms serve as an excellent example of convergent evolution. Cladosictis from the Late Oligocene to the Early Miocene of Patagonia was similar to an otter. Borhyaena also from the Late Oligocene to Early Miocene of Patagonia was jaguar sized and rather bear-like (Palmer, 2006, pp. 346-347). Thylacosmilus from the Late Miocene to Early Pliocene of Argentina was like the placental saber-toothed cats of North America and Europe. The upper canines, like their placental counterparts, were enormous. Unlike its placental counterparts the canines grew continuously and were protected by a vertical extension of the lower jaw when the mouth was closed. Thylacosmilus was one of the last borhyaenids to survive (Kemp, 2005, p. 204).

Paucituberculata is the order that includes living caenolestids or shrew-opossums. Many extinct forms of this order had teeth adapted to a rodent-like diet. Argyrolagus from the Late Miocene to Late Pliocene of Patagonia had the body form and lifestyle like a modern day placental kangaroo rat (Palmer, 2006, p. 347).

Australian Marsupials

Today, marsupials comprise the dominant native fauna of Australia. Most people are familiar with wombats, possums, koalas, and kangaroos. However, throughout the Cenozoic Australia played host to a great diversity of marsupials. Marsupials first appear in Australia in the Eocene. Australian marsupials of the Early Cenozoic evolved without competition from placental mammals and, unlike South America, their adaptive radiation included the evolution of middle to large herbivores as well as carnivores (Kemp, 2005, p. 208). However, as in South America convergent evolution resulted in Australian marsupials with body forms and lifestyles similar to placental mammals on other continents. Marsupial equivalents of rodents, wolves, tapirs, anteaters, and cats are easily recognized.

Australian Megafauna

Let’s examine a few of the prehistoric megafauna of Australia. Thylacoleonidae comprise a family of lion-like marsupials. Thylacoleo from the Late Pliocene to the Late Pleistocene of Australia had a leopard-like form. The incisors were canine-like, while the cheek teeth formed large shearing blades. The front paws were equipped with large claws on semi-opposable thumbs. This marsupial family is unusual in that the carnivorous forms evolved from a specialized herbivorous ancestor (Kemp, 2005, p. 213). Diprotodon of the Pleistocene represents the largest known marsupial to ever have evolved. The largest specimens were hippo-sized at 3 m long, 2.6 m high at the shoulder and weighing in at 2.78 metric tonnes. Diprotodon had dentition specialized for browsing on relatively soft vegetation (Kemp, 2005, p. 213). Palorchestes was a horse-sized herbivore resembling a tapir that ranged from the Miocene to the Pleistocene. Large powerful claws may have been used to dig for tubers and roots. The snout seems to have been elongated into a short trunk. During the Pleistocene kangaroos evolved into medium to large sized browsers and grazers. Procoptodon was the largest kangaroo and stood 3 m tall. This genus of kangaroo had a short face with eyes pointing forward. The hind foot had one functional toe that ended in a hoof-like structure. The forelimbs were adapted for securing branches.

Australian marsupials flourished and did not experience significant competition from placental mammals until aborigines arrived during the Pleistocene 40,000 years ago. Europeans introduced more invasive placental mammals during the 1780’s. Today, habitat destruction and competition from invasive species endanger Australia’s marsupial populations.

Benton, M.J. (2005). Vertebrate Palaeontology [3rd edition]. Main: Blackwell Publishing.

Kemp, T.S. (2005). The Origin and Evolution of Mammals. New York: Oxford University Press.