The middle and inner ears of the Palaeogene golden mole Namachloris: A comparison with extant species

Abstract

Many living species of golden moles (Chrysochloridae) have greatly enlarged middle ear ossicles, believed to be used in the detection of ground vibrations through inertial bone conduction. Other unusual features of chrysochlorids include internally coupled middle ear cavities and the loss of the tensor tympani muscle. Our understanding of the evolutionary history of these characteristics has been limited by the paucity of fossil evidence. In this article, we describe for the first time the exquisitely preserved middle and inner ears of Namachloris arenatans from the Palaeogene of Namibia, visualised using computed tomography, as well as ossicles attributed to this species. We compare the auditory region of this fossil golden mole, which evidently did not possess a hypertrophied malleus, to those of three extant species with similarly sized ear ossicles, Amblysomus hottentotus, Calcochloris obtusirostris, and Huetia leucorhinus. The auditory region of Namachloris shares many common features with the living species, including a pneumatized, trabeculated basicranium and lateral skull wall, arteries and nerves of the middle ear contained in bony tubes, a highly coiled cochlea, a secondary crus commune, and no identifiable canaliculus cochleae for the perilymphatic duct. However, Namachloris differs from extant golden moles in the apparent absence of a basicranial intercommunication between the right and left ears, the possession of a tensor tympani muscle and aspects of ossicular morphology. One Namachloris skull showed what may be pneumatization of some of the dorsal cranial bones, extending right around the brain. Although the ossicles are small in absolute terms, one of the Huetia leucorhinus specimens had a more prominent malleus head than the other. This potentially represents a previously unrecognised subspecific difference.

The middle ear of the pink fairy armadillo Chlamyphorus truncatus (Xenarthra, Cingulata, Chlamyphoridae): comparison with armadillo relatives using computed tomography

“…This would contribute to a higher acoustic compliance, promoting low‐frequency sound transmission. However, in many mammals usually regarded as having ‘freely mobile’ ossicles, the anterior process actually retains a bony connection with the skull at its tip (Mason, , ; Mason et al ). The bone of the anterior process tends to be very thin, and the articulation is flexible in fresh specimens.…”

The ossicular chain of Cainotheriidae (Mammalia, Artiodactyla)

“…Inter‐ and intraspecific variability of the shape of the ossicles has been described for a few groups of mammals, such as the African mole‐rats (Bathyergidae; Lange et al ., 2005), the hominoid primates (Stoessel et al, 2016) and the golden moles (Chrysochloridae; Mason et al ., 2018). The sample from DAM1 composed of 16 mallei and 16 incudes, all referred to P. valdense , allows the consideration of intraspecific variability of these ossicles in this small extinct artiodactyl.…”

Neurobiology: Crowdsourcing CO2 to Conserve Brain Energy

“…Various mammals show adaptations to a life underground, from the mechanosensory star's tactile fovea of the star-nosed mole (Condylura cristata) that replaces sight with touch for finding prey [9], to the evolution of a hypertrophied malleus in many extant golden moles (Chrysochloridae) that likely supports detection of ground vibrations [10]. Naked mole-rats have several adaptations to subterranean life: lack of external ears enables smooth traversing of tunnels, five longitudinally oriented rows of vibrissae run down each side of the body and mediate orientation to mechanical stimuli (Figure 1) [11], and there are numerous factors suggesting adaptation to a hypoxic and hypercapnic environment, such as high O 2 affinity haemoglobin [12], reduced CO 2 affinity of the connexin26 gap junction hemichannel protein [13], ability to withstand prolonged hypoxia and hypercapnia [2,14], and absence of somatic, but not visceral, acidsensitivity (CO 2 induces tissue acidosis) [3,7,15].…”