Ontogenetic Variation in the Bony Labyrinth of Monodelphis domestica (Mammalia: Marsupialia) Following Ossification of the Inner Ear Cavities

Abstract

Ontogeny, or the development of an individual from conception to death, is a major source of variation in vertebrate morphology. All anatomical systems are affected by ontogeny, and knowledge of the ontogenetic history of these systems is important to understand when formulating biological interpretations of evolutionary history and physiology. The present study is focused on how variation affects the bony labyrinth across a growth series of an extant mammal after ossification of the inner ear chambers. Digital endocasts of the bony labyrinth were constructed using CT data across an ontogenetic sequence of Monodelphis domestica, an important experimental animal. Various aspects of the labyrinth were measured, including angles between the semicircular canals, number of turns of the cochlea, volumes of inner ear constituents, as well as linear dimensions of semicircular canals. There is a strong correlation between skull length and age, but from 27 days after birth onward, there is no correlation with age among most of the inner ear measurements. Exceptions are the height of the arc of the lateral semicircular canal, the angular deviation of the lateral canal from planarity, the length of the slender portion of the posterior semicircular canal, and the length of the canaliculus cochleae. Adult dimensions of several of the inner ear structures, such as the arcs of the semicircular canals, are achieved before the inner ear is functional, and the non-ontogenetic variation in the bony labyrinth serves as an important source for behavioral, physiological, and possibly phylogenetic information. Anat Rec, 293:1896Rec, 293: -1912Rec, 293: , 2010. V V C 2010 Wiley-Liss, Inc.Key words: Monodelphis; bony labyrinth; ontogeny; inner ear; CT Variation is a phenomenon that affects all morphological systems. Phenotypic variation is the result of many factors, including phylogenetic history, gender, geography, and ontogeny. Ontogeny, or the development of an individual from conception to death, is a major source of variation that has garnered much attention in scientific literature. Ontogenetic variation is a special complication for systematists, particularly paleontologists, when only one specimen or a small sample of specimens of a taxon is known, which can lead to vexing systematic problems (Brinkman, 1988;Tykoski, 2005;Wiens et al., 2005). For example, two specimens that differ slightly in morphology might represent two separate taxa, or they might represent different ontogenetic stages of a single species, or the variation might indicate any of a series of other possibilities (see Bever, 2006). Because of this, the effect of ontogenetic transformations on anatomy has significant implications for physiological and evolutionary studies.

Ear Structures of the Naked Mole-Rat, Heterocephalus glaber, and Its Relatives (Rodentia: Bathyergidae)

“…It seems to be a common trend among mammals that the bony labyrinth changes little, post-ossification (see e.g. [65–67]). The membranous semicircular canals of Fukomys mole-rats have also been found not to expand postnatally [31].…”

Comparative study of notoungulate (Placentalia, Mammalia) bony labyrinths and new phylogenetically informative inner ear characters

“…Hyrtl, 1845;Gray, 1907Gray, , 1908MacIntyre, 1972;Fleischer, 1973;Luo & Ketten, 1991;Meng & Fox, 1995;S anchez-Villagra & Schmelzle, 2007;Schmelzle et al 2007;Horovitz et al 2008Horovitz et al , 2009Ladev eze et al 2008;Ruf et al 2009;Ekdale, 2010Ekdale, , 2011Ekdale, , 2013Ni et al 2010;Ekdale & Rowe, 2011;Luo et al 2011;Billet & de Muizon, 2013). Fig.…”

Digital reconstruction of the inner ear of Leptictidium auderiense (Leptictida, Mammalia) and North American leptictids reveals new insight into leptictidan locomotor agility

“…The cochlear length was measured along the outer curvature from the anterior border of the fenestra vestibuli to the apex of the cochlea ). Measurements for the cochlea aspect ratio (cochlear height divided by cochlear width) and bony labyrinth volume were conducted according to Ekdale (2010Ekdale ( , 2013. The inner ear height (IEH) was calculated according to , and measured as the linear distance between the dorsal apex of the crus commune and the ventral apex of the cochlea.…”

“…Angular measurements were conducted following the protocol of Ekdale (2010). However, the points for the ObliqueSlice tool in Avizo Ò were modified in that they were set on the outer curvature of the semicircular canals and the basal turn of the cochlea, respectively.…”