Cranial endocasts from a growth series of Monodelphis domestica (Didelphidae, Marsupialia): A study of individual and ontogenetic variation

Abstract

Intraspecific variation (e.g., ontogenetic, individual, sexual dimorphic) is rarely examined among cranial endocasts (infillings of the braincase cavity) because of the difficulty in obtaining multiple specimens of a species, particularly fossil taxa. We extracted digital cranial endocasts from CT scans of a growth series of skulls of Monodelphis domestica, the gray short-tailed opossum, as a preliminary assessment of the amount of intraspecific variation in mammalian endocranial morphology. The goals of this study were 1) to provide an anatomical description to document developmental changes in endocranial morphology of M. domestica and 2) to examine ontogenetic and individual variation with respect to phylogenetic characters of endocranial cavities that are known to be variable between different mammalian taxa. In this study, "ontogenetic variation" refers to variation between specimens of different ages whereas "individual variation" (i.e., polymorphism) is restricted to variation between specimens of comparable age. Aside from size, changes in shape account for the greatest amount of morphological variation between the endocasts of different ages. Endocast length, width, and volume increase with age for the growth series. Relative olfactory bulb cast size increases with age in the growth series, but the relative size of the parafloccular casts shows a slight negative allometric trend through ontogeny. More than one-third of the phylogenetic characters of the endocranial cavity we examined showed some sort of variation (ontogenetic, individual, or both). This suggests that although endocasts are potentially informative for systematics, both ontogenetic and individual variation affect how endocranial characters are scored for phylogenetic analysis. Further studies such as this are necessary to determine the taxonomic extent of significant intraspecific variation of these endocranial characters.

Exceptional Skull of Huayqueriana (Mammalia, Litopterna, Macraucheniidae) From the Late Miocene of Argentina: Anatomy, Systematics, and Paleobiological Implications

“…Anatomical terminology generally conforms to, or is adapted from, nomina recognized in Terminologia Anatomica (e.g., Schaller, 1992); for more specialized terms, we utilized MacPhee (1981MacPhee ( , 2014 for the caudal cranium, Soria (2001) for the dentition, and Macrini et al (2007aMacrini et al ( , 2007b for the cerebral endocast. The description of the inner ear assumes that the lateral semicircular canal is oriented horizontally and centrally within the surrounding array of anatomical structures (Graf and Klam, 2006;Ekdale, 2013).…”

“…The surfaces of the cerebellar hemispheres c are smooth. Meninges, venous sinuses, and other structures (e.g., arachnoid granulations) affect endocast topography and consequently there is no direct correlation between the apparent degree of lissencephaly seen on hemisphere casts and actual, in vivo surfaces (Macrini et al, 2007a). The paraflocculus c , caudodorsal to the internal acoustic meatus on the lateral face of the cerebellar hemisphere c , is less protruding than in Te.…”

“…Asterisk (*) indicates depression on right lateral piriform lobe. tory tracts projecting from the cortex (Macrini et al, 2007a(Macrini et al, , 2007b.…”

“…23A). The hypophyseal region c , the space occupied in vivo by a portion of the pituitary gland (Macrini et al, 2007a), is represented by a small, shallow bulge, about 2.5 % of total endocast volume. It is situated in the middle of the ventral floor, level with the anterior border of the piriform lobe c and slightly anterior to foramen ovale ( fig.…”

Comparative Morphology of the Internal Nasal Skeleton of Adult Marsupials Based on X-ray Computed Tomography

“…I also examined individuals of a growth series of Monodelphis domestica, the gray short-tailed opossum. This sample consists of 18 opossums ranging in age from 27 days old to breeding adults (Macrini et al, 2007b). A handful of extant placental and monotreme taxa were also examined for outgroup comparison; these specimens are listed in table 1.…”