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Facial and Masticatory Muscles |
Authors: Sarah Ogburn and Linda Brogdon Classification: Common name: Grey short-tailed opossum Evolution and Phylogeny: Monodelphis is a living marsupial native to South America. Living marsupials are the surviving branch of metatherian mammals. Metatheria is an infraclass within the animal class of Mammalia. It includes marsupials and their closest relatives. The earliest fossil metatherian, Sinodelphys szalayi, dates to 125 million years old and was found in the Liaoning Province of northeast China [1]. Its morphology suggests that it was an arboreal animal. An early eutherian mammal, Eomaia, was found in the same geologic layer as Sinodelphys. Eutheria is also an infraclass within the class Mammalia, including placental mammals and their closest ancestors-which are known only from the fossil record. The presence of these two early mammals indicates that the split between Eutheria and Metatheria must have occurred before 125 million years ago. Other data points to a split after 144 million years ago, at the beginning of the Cretaceous [2]. Other basal metatherians include Holoclemensia and Deltatheridium[1]. All living marsupials are restricted to Australia and South America, with the exception of Didelphis virginiana, which immigrated to North America in the Pleistocene. This distribution represents a dramatic switch, as many early fossil metatherians from the Cretaceous are found only in Eurasia and North America[1]. Scientists are uncertain exactly when or why this switch occurred, but it is believed that metatherians moved into South America sometime in the late Cretaceous or early Paleocene, and reached Australia via what is now Antarctica[3]. Other scientists contend that metatherians and eutherians arose in the southern supercontinent of Gondwana, along with monotremes, or that eutherians arose in North America and metatherians and montremes in Gondwana. More fossil evidence would help elucidate the problem of metatherian origins [4]. Protodidelphids evolved from primitive ameridelphians in the Paleocene. Didelphids appear in the middle Miocene in South America and did not migrate to North America until the Plio-Pleistocene. The fossil record of the didelphids is, unfortunately, very poor [3]. Modern opossums have retained many primitive metatherian characteristics, and have not evolved much since the original didelphids. It has been calculated that the rate of karyotypic evolution (as seen in number of changes per million years) in metatherians is far less than that of placental mammals. Estimates are that marsupials have had .13 karyonumber changes per million years versus the .51 seen in placentals [5]. Selective pressures have been few due to the extremely generalist nature of their diet and locomotion. Marsupials are also subject to specific evolutionary constraints due to their unique reproductive system. The breast and shoulder regions of marsupials are particularly constrained because the extremely underdeveloped young must climb from the birth canal to their mother's pouch [3]. Monodelphids' closest relatives are in the genus Marmosa, which includes mouse opossums. Appearance: Monodelphis species are small, short-tailed, rat-like opposums. They are 110-200 mm in length, with a tail about half as long as the body [6]. M. domestica males weigh 105-125g; females weigh 75-90g [7]. They are grayish brown and the tail is prehensile and hairless. Monodelphis species lack a pouch and have 8-14 mammae. They have pointed muzzles with a 5/4, 1/1, 3/3, 4/4 dentition. The ears are large and thin. They have claws on all 5 of their toes, except on the first digit of the hind foot (hallux) which is opposable [7, 8]. Geography & Habitat: M. domestica is one of 63 species in the family Didelphidae found exclusively in the New World [9]. Monodelphids are endemic to eastern and central Brazil, Bolivia, and Paraguay, but can also be found in Chile, Argentina [10]. M. domestica isn't very well adapted for arboreal life and typically inhabits rocky areas. Despite this fact, M. domestica is a very good climber. Relatives of M. domestica, including Didelphis virginiana, have spread throughout the eastern part of the United States [9]. Activity & Behavior General: M. domestica is a nocturnal, solitary species. It was named M. domestica because natives of Brazil allowed them to live in residences because they destroyed rodents and insects. Some people even kept them as pets [6]. However, most short-tailed opossums in North America are lab specimens. Since M. domestica is solitary, the males in captivity must be introduced to the females with great care in order to prevent injury or death [7]. Diet: M. domestica is omnivorous and typically eats insects, seeds, carrion, fruit, as well as many pest species of small rodents [6]. Larger species in Didelphidae are known to eat lizards, bird's eggs, and berries [10]. Reproduction & Development: One of the defining features of metatherians is their pouch, but M. domestica is actually pouchless [7]. The males induce estrus, which is approximately a 28-day cycle in the females. Short-tailed opossums will breed throughout the year in their tropical habitats. In captivity these cycles are often carefully controlled. Typically the females will have 8-14 young [6]. Eleven days after birth there is already evidence of development of a forebrain, a cervical spinal cord, and several somites. By day 14 there is digit formation and appearance of eyelids; birth typically occurs around day 15 [7]. Newborns will cling to the nipples on the mother until they are able to move to her back around 45 days after birth [5]. References: 1. Luo, Z-X.; Ji, Q., Wible, J.; Yuan, C-X. 2003. An Early Cretaceous
Tribosphenic Mammal and Metatherian Evolution. Science. 302: 1934-1940.
2. Cifelli, R.; Davis, B. 2003. Marsupial Origins. Science. 302: 1899-1900. 3. Szalay, F. 1994. Evolutionary History of The Marsupials and an Analysis
of Oseteological Characters. Cambridge University Press, Cambridge. 4. Weil, A. 2001. Relationships to chew over. Nature. 409: 28-31. 5. Klima, M. 1987. "Early Development of the Shoulder Girdle and
Sternum in Marsupials (Mammalia: Metatheria). Vol. 109 in Beck, F.; Hild,
W.; Kriz, W.; Ortmann, R.; Pauly, J.; Schiebler, T. Advances in Anatomy
Embryology and Cell Biology. Springer-Verlag, Berlin. 6. Nowak, R. 1997."Short-tailed Opossums". Walker's Mammals
of the World Online 5.1. The Johns Hopkins University Press. 7. The Smith Lab. 2002. "Possum Page". Accessed 2004 at http://www.duke.edu/~kksmith/index.html. 8. Thomas, O. 1888. Catalogue of the Marsupialia and Monotremata in the
Collection of the British Museum (Natural History). British Museum, London. 9. Whitaker, John Jr. & Hamilton, William J. "Mammals of the
Eastern United States." Cornell University Press: Ithica, 1998. 10. Clutton-Brock, Juliet. "Dorling Kindersley Handbook of Mammals." Dorling Kindersley: New York, 2002.
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Artwork: Weil, from Stubbs' 1776
"Anatomy of the Horse."
Background free from Eos Development, with
slight color modification.
