ORDER DIDELPHIMORPHIA - AMERICAN OPOSSUMS Because of the diverse forms that have evolved (often through convergent evolution), there are few conspicuous features of marsupials that can reliably separate them from eutherian mammals. Unique anatomical characteristics include the presence of a masupium (and corresponding epipubic or marsupial bone) and a forked reproductive system resulting in two vaginae and two uteri in females. Correspondingly, the male usually has a forked penis. In addition, the testicles are located anterior to the penis. These features may be obscure or impossible to observe in museum specimens. The Didelphimorphs (American opossums) are characterized by an opposable (thumblike), clawless toe on the hind foot, a prehensile tail, and more teeth than any other land mammal. There is only one family, Didelphidae, with a single species represented in Wisconsin. Virginia Opossum, Didelphis virginiana Specimen The opossum is approximately the size of a house cat and resembles a giant rat with its long muzzle, large naked ears, and hairless tail. The fur on the body is rather shaggy and grizzled gray in color, contrasting with the white face. The lower legs and feet are blackish and the hairless toes are pink. There are five toes on each foot. The opossum is not likely to be confused with any other Wisconsin mammal; no other Wisconsin mammal of this size has a combination of the naked tail and thin, leathery, hairless ears. Skull The opossum skull is characterized by a long snout, large canines, tiny braincase (narrower than the rostrum), and a high sagittal crest. It also has more teeth than any other terrestrial mammal. Dental Formula: I: 5/4 C: 1/1 P: 3/3 M: 4/4 = 50 Similar Species: The opossum skull is commonly mistaken for a carnivore (especially for a canine). The very narrow braincase and greater number of teeth (50 in opossum versus 42 or fewer in carnivores) is the most obvious way to differentiate between the orders. You can make it easier on yourself by only counting the incisors; carnivores have 3 pairs of upper and 3 pairs of lower incisors whereas the opossum has 5 pairs of upper and 4 pairs of lower incisors. There are a couple of other skull characteristics that can be used as well: 1.) The opossum has palatine fenestrae (1A) in addition to incisive foramen (1B), forming at least four elongated slits on the upper palate; in carnivores the two incisive foramen are present (1C), but the palatine fenestrae are absent. 2.) The opossum lacks conspicuous auditory bullae (2A); auditory bullae are prevalent in carnivores (2B). Opossum Skull Carnivore Skull 1B 1A 1C 2B 2A Jeff
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