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This study assessed the occurrence of sexual size dimorphism (SSD) and sexual shape dimorphism (SShD) in the skull and mandible of representatives of most species within the 3 orders of living New World opossums, Didelphimorphia, Paucituberculata, and Microbiotheria, using geometric morphometrics. Centroid sizes and partial warps were extracted from landmarks set on images of the dorsal, ventral, and lateral views of the skull and lateral view of the mandible, and were compared between sexes to estimate SSD and SShD. Specimens totaling 2,932 from 71 species of Didelphidae, 5 species of Caenolestidae, and 1 species of Microbiotheriidae were analyzed. SSD was variable in the Didelphimorphia and the Paucituberculata and absent in Microbiotheria. Similar results were found for SShD, but SSD and SShD are not clearly coupled. I also evaluated the validity of Rensch’s rule—the widely observed phenomenon of correlated increases in SSD and body size for male-biased sexual dimorphism, or correlated decreases in SSD in body size for female-biased sexual dimorphism—in the Didelphidae. Didelphids span 2 orders of magnitude in body size, and, when present, sexual dimorphism is male-biased. Regressions of SSD and SShD estimators onto size, using phylogenetic independent contrasts, showed either no significant relationship between SSD or SShD with size in any of the structures and views analyzed, or a trend contrary to Rensch’s rule (smaller species more dimorphic, but with male-biased dimorphism). Lack of adherence to Rensch’s rule in Didelphimorphia may relate to a lack of social interactions and male territoriality, usually associated with such a trend via sexual selection. If the trend contrary to Rensch’s rule is real, an explanation may lie in the increasing amount of smallbodied species that recently have been found to be semelparous and thus subject to stronger selection for larger males. DOI: 10.1644/09-MAMM-A-018.1.

The sequence of molar eruption, along with the timing of replacement of the 3rd deciduous premolar by a permanent premolar, has been documented in several species in the family Didelphidae, and routinely used to establish age classes. We documented the dental eruption and replacement patterns in the 3 species of the neotropical marsupial genus Caluromys, which constitute one of the most basal groups within the Didelphidae. We examined 258 specimens of Caluromys derbianus, 54 specimens of C. lanatus, and 386 specimens of C. philander. These species have a dental eruption pattern that is similar to that of Marmosops, with replacement of the 3rd deciduous premolar after eruption of the 4th molar. However, we sometimes found incompletely erupted P3 and M4 occurring simultaneously as part of a M4–P3 eruption sequence. We suggest that the occurrence of this intermediate pattern in other taxa should be verified by examination of large series of specimens. Because teeth erupting in sequence can be erroneously considered as fitting an intermediate pattern, eruption sequence and time of completion of development should be used separately when using dental eruption in phylogenetic analyses. Because of the high degree of intraspecific variation observed in several taxa in Didelphidae, a system of age classes based solely on eruption of upper molars may be more useful than those that combine information on molar eruption sequence and timing of premolar replacement.