Marsupials

Marsupials include all of your favorite animals from Australia along with the Virginia possum and a few animals in South America. They are the inferior cousins of the placental mammals, and I don’t say this just because I am a placental mammal. They are generally wiped out in almost all ecological niches by placental mammals when the two clades come into contact by the merging of continents.

Cretaceous metatherians were the ancestors of Paleocene marsupial mammals. Scientists think that the true marsupials branched off from the other metatherians approximately 85 Ma. In the End Cretaceous extinction, approximately 90% of metatherian taxa went extinct. For example, Didelphodon (Figure 12‑39) was a 5 kg metatherian carnivore that lived just before the End Cretaceous extinction in Colorado (68 – 66 Ma) but went extinct. It probably fed on small vertebrates such as lizards and small mammals, which would not have provided sufficient food resources after the extinction.

Figure 12‑39. Left. Cast of Didelphodon vorax in Late Cretaceous (68 Ma). Rocky Mountain Dinosaur Resource Center, CO. Credit: MCDinosaurhunter. Used here per CC BY-SA 3.0. Right. life restoration of Didelphodon vorax. Nobu Tamura. Used here per CC BY-SA 3.0.

Figure 12‑40. The monito del monte (Dromiciops gliroides) is the only remaining member of the microbiotheria in South America. Credit: Jose Luis Bartheld. Used here per CC BY 2.0

The microbiotheria were metatherian insectivores that survived the End Cretaceous extinction. The oldest microbiotheriidae in the fossil record are Khasia and Pucadelphys from Tiupampa in Bolivia, an early Paleocene (64 Ma) paleontological site. Khasia is represented in the fossil record by a single tooth. However, scientists can determine its size and that it is likely that it climbed trees. The tooth links it to the marsupials that live in Australia, the Australidelphia. Monito del monte is the only surviving species of the microbiotheria (Figure 12‑40).

Pucadelphys was a marsupialiformes, which means that it was not as close to the ancestral tree of marsupials as Khasia, which was within the clade marsupialia. Scientists link Pucadelphys with the nonmarsupial metatherian carnivores called sparassodonts, which were the dominant carnivores in South America for most of the Cenozoic Era.

Figure 12‑41. Marsupial thylacines (Tasmanian tigers) in National Zoo, Washington D.C. in 1906, prior to their extinction in 1933. Credit: E.J. Keller. No ©.

The first sparassodont was Mayulestes, which was the size of a rat. It had teeth that sliced meat. They evolved into species that were similar to placental carnivores on other continents. The ranged from the size of a weasel to that of a bear. They died out approximately 3 million years ago, probably due to the cooling climate. This was prior to the invasion of placental mammals from North America when the continents of South and North America connected at the Isthmus of Panama. One controversial aspect of the sparassodonts is that they had small epipubic bones. Possibly, they had cartilage epipubic bones like the thylacenes (Tasmanian tigers) in Tasmania (Figure 12‑41) or possibly they had some placental characteristics. After the extinction of the sparassodonts, there were no nonmarsupial metatherians.

Figure 12‑42. Marsupial cladogram. Credit: Wikipedia.

Marsupials did not occupy herbivore niches in South America due to the presence of large placental ungulates, Meridiungulata, that evolved in South Ameica after the End Cretaceous extinction but went extinct at the end of the Pleistocene (when humans arrived). The marsupials in Australia are in the Australidelphia superorder (Figure 12‑42), which includes the Diprotodonda (kangaroos), Notoryctemorphia (marsupial moles), Dasyuromorphia (Tasmanian devil and other carnivores), and Paramelemorphia (bandicoot).

The first marsupial in the fossil record in Australia was in the Early Eocene (55 Ma). Marsupials traveled to Australia in the Eocene when South America and Australia were connected to Antarctica. The next marsupial in the Australian fossil record was in the Oligocene (30 Ma). There are now 120 species of marsupials in Australia. They were able to survive in Australia because they were isolated due to the fact that Australia never connected to a continent with placental mammals. In contrast, when South America connected to North America at the Isthmus of Panama, approximately 3 Ma, the North American placentals entered South America during the Great American Interchange and wiped out almost all South American marsupial mammals. The didelimorphs, originated in the Cretaceous, survived in South America, and then migrated to North America in the Great American Interchange (3 Ma). The Virginia opossum is the only marsupial that currently inhabits in North America. Another didelimorph, the gray short-tailed opossum, still survives in South America.

Placental mammals are more intelligent and more social than marsupial mammals, which means that these characteristics evolved in the eutherians of the Cretaceous. This is not due to the size of the brain but due to the structure of the brain. The primary difference between the placental mammal brain and the marsupial brain is that the marsupial brain does not have a corpus callosum, which connects the two cerebral halves in placental mammals and enables communication between the two sides of the brain. It consists of 200 – 300 million axonal projections in humans (Figure 11‑1). The corpus calossum must have evolved after the divergence of the eutherians (ancestors of placentals) and the metatherians (marsupials), which took place at the beginning of the Cretaceous.

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Koala bear mother and joey. Credit: Benjamint444. Used here per CC BY-SA 3.0

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