The bony labyrinth is the bony capsule of the inner ear, which is the center of hearing and balance in the skull of vertebrates. In mammals this osseous structure ossifies early in the development being fully formed largely before birth. This means that it does not grow after full ossification while the animal continues to grow until full adult size well after birth. At the level of an individual, the bony labyrinth thus shows a negative ontogenetic allometric relationship with the surrounding skull and with the animal’s body mass. At the evolutionary level, between species, such a negative allometry has already been evidenced for the middle ear, a component of the ear region made of three tiny bones, the ossicles, in contact with the inner ear. Herein, we test the allometric relationship between the bony labyrinth and skull length as well as body mass on a large sample in the ruminant clade (Mammalia, Ruminantia) using micro computed tomography as the imaging method of choice. We find a strong negative allometry paralleling the ontogentic allometry described earlier. This evolutionary relationship related to the timing of ossification of the bony labyrinth is probably critical in explaining the large hearing frequency range of mammals as well as their particular ecological adaptations.
The innervation of the inner ear has been thoroughly investigated in humans and in some animal models such as the
guinea pig, the rabbit, the cat, the dog, the rat, the pig and some monkeys. Ruminant inner ears are still poorly known
and their innervation was never investigated despite its potential interest in phylogenetic reconstructions. Following
earlier works on the ontogeny of the cow’s ear, we expand our understanding of this structure by reconstructing the fine
innervation pattern of the inner ear of the cow in two ontogenetic stages, at 7 months gestation and at an adult age. Since
we work on dry skeletal specimens, only the endocast of the innervation inside the petrosal bone was reconstructed up to
the internal acoustic meatus. The paths of the facial and vestibulocochlear nerves could be reconstructed together with
that of the spiral ganglion canal. The nerves have a very fibrous pattern. The bony cavities of the ampular and utricular
branches of the vestibulocochlear nerve could also be reconstructed. Our observations confirm that not all bony
structures are present in foetal stages since the branch of cranial nerve VII is not visible on the foetus but very broad on
the adult stage. The fibrous pattern within the modiolus connecting the spiral canal to the cochlear nerve is also less
dense than in the adult stage. The shape of the branch of cranial nerve VII is very broad in the cow ending in a large
hiatus Fallopii; this, together with the above-mentioned particularities, could constitute relevant observations for
phylogenetical purposes when more data will be made available.
The timing of ossification of middle ear ossicles has been extensively studied in humans. This is an exception since it is vastly unknown in the +5000 extant species of placentals. As a preliminary approach, a cow foetus (around 115 days of gestation) was visualized using X-ray microtomography (μCT) and the ossicles including stapes, incus, and malleus could be extracted from the data set. All three bones have already undergone substantial ossification, which allow comparison to adult middle ear bones. Their ossification at this stage parallels ossification in humans at a comparable stage of gestation. While full ossification is not yet achieved almost all the morphological characters of the ossicles are observed. Bone tissue is still very porous, the stapes does not have the characteristic plate-like footplate, the lenticular process of the incus is missing and the manubrium of the malleus is very thin and not yet complete. Despite all this, the ossicles are articulate with each other and perfectly with the bony labyrinth. The stapes footplate is positioned on the oval window but is smaller than the latter while it should perfectly fit to transmit sound vibrations to the cochlea. All ossicles, especially the stapes, have not yet reached adult size, while the bony labyrinth already has. This is the first detailed description of a set of middle ear bones in a placental at mid-gestation based on high-resolution μCT. Similarities in ossification timing with humans encourage more work to be done on foetuses to understand if a general rule for placental mammals exists.
X-ray imaging in the absorption contrast mode is an established method of visualising calciﬁed tissues such as bone and teeth. Physically soft tissues such as brain or muscle are often imaged using magnetic resonance imaging (MRI). However, the spatial resolution of MRI is insuﬃcient for identifying individual biological cells within three-dimensional tissue. X-ray grating interferometry (XGI) has advantages for the investigation of soft tissues or the simultaneous three-dimensional visualisation of soft and hard tissues. Since laboratory microtomography (μCT) systems have better accessibility than tomography set-ups at synchrotron radiation facilities, a great deal of eﬀort has been invested in optimising XGI set-ups for conventional μCT systems. In this conference proceeding, we present how a two-grating interferometer is incorporated into a commercially available nanotom m (GE Sensing and Inspection Technologies GmbH) μCT system to extend its capabilities toward phase contrast. We intend to demonstrate superior contrast in spiders (Hogna radiata (Fam. Lycosidae) and Xysticus erraticus (Fam. Thomisidae)), as well as the simultaneous visualisation of hard and soft tissues. XGI is an imaging modality that provides quantitative data, and visualisation is an important part of biomimetics; consequently, hard X-ray imaging provides a sound basis for bioinspiration, bioreplication and biomimetics and allows for the quantitative comparison of biofabricated products with their natural counterparts.
High-resolution X-ray computed tomography has become a vital technique to study fossils down to the true
micrometer level. Paleontological research requires the non-destructive analysis of internal structures of fossil
specimens. We show how X-ray computed tomography enables us to visualize the inner ear of extinct and
extant ruminants without skull destruction. The inner ear, a sensory organ for hearing and balance has a rather
complex three-dimensional morphology and thus provides relevant phylogenetical information what has been to
date essentially shown in primates. We made visible the inner ears of a set of living and fossil ruminants using
the phoenix x-ray nanotom®m (GE Sensing and Inspection Technologies GmbH). Because of the high absorbing
objects a tungsten target was used and the experiments were performed with maximum accelerating voltage of
180 kV and a beam current of 30 μA. Possible stem ruminants of the living families are known in the fossil
record but extreme morphological convergences in external structures such as teeth is a strong limitation to our
understanding of the evolutionary history of this economically important group of animals. We thus investigate
the inner ear to assess its phylogenetical potential for ruminants and our first results show strong family-level