A neglected part of the mammalian skull: The outer nasal cartilages as progressive remnants of the chondrocranium

Title: A neglected part of the mammalian skull: The outer nasal cartilages as progressive remnants of the chondrocranium

Creators: Wolfgang Maier

Submitted April 27, 2020.
Accepted July 13, 2020.
Published online at www.senckenberg.de/vertebrate-zoology on September 9, 2020.
Published in print Q3/2020.

DOI: 10.26049/VZ70-3-2020-09
PDF/A 8.6 MB

Published by Senckenberg Gesellschaft für Naturforschung

Date (Publication Year): 2020

Resource Type (General): TEXT

Resource Type (optional): Vertebrate Zoology, Scientific Article

Description (en): A rostrum equipped with a rhinarium, diverse mechanoreceptors and a set of vibrissae are apomorphic characters of therian mammals. Together, they constitute a tactile sensory organ that has also been named ‘rostral organ’. The rhinarium and the outer nasal openings are supported by a well-defined system of outer nasal cartilages; these cartilages can be actively moved by facial muscles. Because the structures of the rostrum are mainly composed of cartilage and soft tissues, fossils cannot contribute much information on its phylogenetic origin. Therefore, one has to rely on comparative anatomy of extant mammals. Because the cartilages are known to grow and differentiate well into juvenile and even subadult stages, embryological evidence is not too rewarding either. However, the microscopic anatomy of the rostrum of postnatal stages is difficult to study for various technical reasons, and it is hoped that modern imaging techniques will improve our knowledge.
Here, I provide a preliminary overview of the structural diversity of the outer nasal cartilages in selected taxa of placental mammals. The outer nasal cartilages of moles (Talpidae) and shrews (Soricidae) are relatively well-known and can serve as references. The soricids present a unique mode of retracting and telescoping the nasal cartilages; these peculiar structures are differentiated only postnatally by a sort of apoptosis. Several Afrotheria show a peculiar commissura alatransversalis and a specialized processus alaris superior, which are considered to be synapomorphies of Afroinsectiphilia (Afrotheria) at least. The peculiar trachea-like structure of the outer nasal cartilage of macroscelidids is also postnatally realized by an apoptotic morphogenetic mode.
Finally, the presented data are interpreted within the context of evolutionary biology of early mammals: First it is shown, that the paired outer nares of early synapsids fuse into a wide nasal aperture within true mammaliaforms, but only after the reduction of the prenasal processes of the premaxillaries. It is important to note that hatchlings of the monotreme Tachyglossus still retain the prenasal process, because it supports the egg-tooth. It is only after the reduction of this membrane bone, that the rostral organ could expand and diversify – and to become the new and functionally important tactile organ (‘rostral organ’) of therian mammals. Whereas small arboreal mammals show relatively simple outer nasal cartilages, they tend to become a specialized probe in terrestrial and fossorial taxa.

Key words: Afrotheria, external nose cartilage, insectivores, postnatal ontogeny, rostral organ.

Citation: Maier, W. (2020). A neglected part of the mammalian skull: The outer nasal cartilages as progressive remnants of the chondrocranium. Vertebrate Zoology, 70(3): 367-382. https://doi.org/10.26049/VZ70-3-2020-09