Comparative Postnatal Ontogeny of the Skull in Dromiciops gliroides (Marsupialia: Microbiotheriidae)

Abstract

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.

Ontogeny of the Early Triassic Cynodont Thrinaxodon liorhinus (Therapsida): Cranial Morphology

“…This may be explained by the fact that the zygoma in T. liorhinus is less flared laterally than in the other three species. Isometric trends in the maximum width of the skull were also reported in small to medium-sized extant marsupials such as Didelphis albiventris and Dromiciops gliroides where the expansion of the temporal fenestrae was a consequence of the interaction between the isometry of the skull width and the negative allometry of the braincase (Abdala et al, 2001;Giannini et al, 2004). This suggests that the temporal fenestra expanded medially to accommodate the further development of the adductor musculature during growth; however, the relative reduction in braincase width is much more extreme in extant mammals than in Thrinaxodon.…”

Skull Growth of the Korean Water Deer, <i>Hydropotes inermis argyropus</i>

“…In addition to the above analyses, we performed allometric analyses. To describe patterns of shape change accompanied by growth, allometry has often been employed [1,11,12,20]. We used bivariate allometry for each trait for each sex.…”

Postnatal Ontogeny of the Skull in the Extant North American Turtle Sternotherus odoratus (Cryptodira: Kinosternidae)

“…Postnatal transformations in discrete cranial characters are inferred here based largely on the qualitative description of differences observed between the smallest (presumed youngest) and largest (presumed oldest) specimens in the sample following the methods of Giannini et al (2004). However, because such differences may reflect variation more accurately explained by phenomena other than ontogeny (e.g., sexual, individual, teratological, or stochastic patterns of structural complexity), variation throughout the sample is noted with the frequency of each variant presented as a percentage of the sample.…”

“…P . 0.01; Abdala and Giannini, 2000;Giannini et al, 2004). t-scores were calculated under the null hypothesis that b 1 5 1.0 using the equation t 5 (b 1 21)/s b (where s is the standard error; Zar, 1999) with isometry being the condition in which the allometry coefficient is statistically indistinct from the null hypothesis (unity; Giannini et al, 2004).…”

“…Least squares and RMA are correlated arithmetically through the correlation coefficient with their differences representing shifts in scale between b 1 (Niklas, 1994). The size of these shifts depends on the amount of variation explained by size (the correlation coefficient [R value]); these techniques were applied to cranial growth in marsupial mammals by Abdala and Giannini (2000), Abdala et al (2001), Flores et al (2003Flores et al ( , 2006, and Giannini and Abdala (2004).…”