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Dromiciops gliroides is the single extant representative of the marsupial family Microbiotheriidae. The importance of D. gliroides stems from its peculiar cranial anatomy (specifically the configuration of the tympanic region) and dentition and from its controversial position in the phylogenetic tree of marsupials-a South American form more closely related to Australasian marsupials. We studied the postnatal ontogeny of the skull in D. gliroides by analyzing qualitative and allometric aspects of the development of cranial structures. We compared recently weaned young individuals with adults and described the bivariate and multivariate allometric trends of 14 cranial dimensions for a sample of 37-51 specimens. Most cranial components develop in a way similar to didelphids studied so far. However, some trends (e.g., growth of the orbit) seem particular to D. gliroides. The microbiotheriid bulla of D. gliroides, a structure to which five basicranial bones contribute parts, is already present in its highly derived condition in the youngest specimens of our series. We conclude that except for the bulla, most of the cranial development in D. gliroides is highly conservative and that some peculiarities may be shared with other marsupials of similarly small body size. Data on australidelphians and small-size didelphids are needed to contrast these patterns.

The postcranial skeleton of the neotropical living marsupial Glironia venusta is described and compared in a functional framework. Osteological and myological characters of 19 species of living didelphids and some additional placentals were consulted as models to explain functional implications from the morphology. The skeleton of G. venusta provides evidence about locomotory behavior and specific capacities of movements, and reveals patterns comparable to arboreal didelphids and placentals with high capacity to climb. In general terms, G. venusta has few diagnostic characters in the context of the didelphid sample analyzed, which includes representatives of all recognized clades in the family (second sacral not fused to the ilium, humeral greater trochanter well developed, tibia shorter than femur). Most of the postcranial pattern in G. venusta is consistent with arboreal locomotion, but unlike Caluromys and Caluromysiops, it seems to have faster locomotion. The morphology of the vertebral column, at the thoracic and lumbar portions, shows features that allow powerful lateral and sagittal movements during different phases of locomotion. The patterns evidenced in the forelimbs, pelvic girdle, and hindlimbs point to arboreal habits as well, except for some features on the humerus, illium and fibula. Even if most didelphids have been cataloged as generalized with respect to their mode of gait, the skeletal morphology of G. venusta and the high variation existent in further neotropical marsupials with a variety of body sizes, reveal a diverse combination of features associated to specialized capacities of movements. This indicates a diversity of locomotory modes and postures in didelphids. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)