Evolution and systematics

This group of mammals was traditionally included in the marsupials until 1993 when it was placed in its own order, Didelphimorphia, together with two other New World opossum-like families, the Microbiotheriidae (one species) and the Caenolestidae (five species). Here Didelphidae is treated in its own order, Didelphimorphia, and Microbio-theriidae and Caenoletidae are each covered in an order as well. The fossil record suggests that didelphids are relatively primitive, unspecialized mammals that emerged some 75-100 million years ago (mya) in North America. Their evolutionary and biogeographical history is complex; they colonized Europe, Asia, and Africa some 60 mya, but disappeared from those continents some 20 million years later, presumably due to competition and predation with the appearance of placental mammals. Twenty mya they were restricted to South America, where they underwent an impressive radiation in the absence of other ecologically similar or predatory placental mammals. It was not until the Isthmus of Panama rose and North and South America joined, some three million years ago at the beginning of the

Pleistocene, that the didelphids again entered North America. Throughout this time they retained a remarkably stable morphology. The connection between North and South America also allowed the entrance of other placental mammals into South America. For the first time in 20 million years, marsupials again faced the faster, larger-brained pla-cental competitors and predators. Groups like the Borhyaenids (wolf- or hyena-like marsupials) and Thylacos-milids (marsupial saber-toothed cats) disappeared and gave way to true canids and felids. Some factors that may have contributed to their disappearance are the smaller en-cephalization quotient, lower metabolic rate and overall speed, and lower cursorial abilities of the marsupials compared to their placental counterparts. Didelphids have relatively low evolutionary rates and a strong stabilizing selection that prevents greater morphological diversification in the group.

The family Didelphidae is arranged into two subfamilies, Caluromyinae and Didelphinae. As of 2002, 15 genera are recognized: woolly opossums (Caluromys), black-shouldered

The bare-tailed woolly opossum (Caluromys philander) is a marsupial. (Photo by Jany Sauvanet/Photo Researchers, Inc. Reproduced by per-

like Metachirus, have relatively long legs, whereas others like Monodelphis and Lestodelphys have relatively short legs. Except for a few species such as Monodelphis dimidiata, there is no sexual dimorphism.

Coloration varies widely. Some species are uniformly blackish, while others are almost completely whitish; other species are rusty reddish, gray, brown, tan, or yellowish brown. Un-derparts are nearly always paler than the dorsum. The venter of the water opossum is silvery white. Two genera have distinct dark blotches above the eyes; they are called four-eyed opossums. Some genera have characteristic dark and pale patterns, sometimes broad, dark saddle-like bands across the back, sometimes a longitudinal stripe along the dorsal spine and continuing along the top of the snout and to the tip of the nose. The hair can be short or long depending on the species, but it is always dense. In females of some genera, there is a distinct ventral pouch where young are kept in the developmental stages. The pouch opens circularly and can be almost completely closed. Mammae number 12 to 18 and are arranged in a circle with one in the center. One species, the water opossum, has a pouch that seals hermetically with an oily substance so that females can dive under the water surface without drowning the young that are attached to the nipples. Likewise, males of this species have a pouch, which protects the scrotum and testicles from contact with the water.

opossums (Caluromysiops), water opossums (Chironectes), common opossums (Didelphis), bushy-tailed opossums (Glironia), gracile opossums (Gracilinanus), Patagonian opossums (Lestodelphys), lutrine opossums (Lutreolina), mouse opossums (Marmosa), slender mouse opossums (Marmosops), brown four-eyed opossums (Metachirus), woolly mouse opossums (Mi-coureus), short-tailed opossums (Monodelphis), gray four-eyed opossums (Philander), and fat-tailed mouse opossums (Thy-lamys). In 1993, Gardner recognized a total of 61 species, but there have been at least three additional species described since then.

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