“Septal compass” and “septal formula”: a new method for phylogenetic investigations of the middle ear region in the squirrel-related clade (Rodentia: Mammalia)

Abstract

Here, we introduce the “septal compass” and the “septal formula” as a new method for phylogenetic investigations of the middle ear region in squirrel-related clade. The middle ear cavity is characterized by bony septa that divide the dorsally lying epitympanic recess and the ventrally lying tympanic cavity into several segments or diverticula. The distribution patterns of these septa are conservative among the squirrel-related clade and are restricted to the species, genus, and family level. In the studied outgroups represented by †Ischyromys typus and lagomorphs, no septa are found in the epitympanic recess and tympanic cavity. Therefore, the “septal compass” and the “septal formula” provide a new approach for phylogenetic interpretations of the middle ear region. It is user-optimized and can be modified for other rodent families and mammalian taxa and will facilitate phylogenetic assumptions in future investigations. Additionally, this method will enable the allocation of isolated tympanic bullae to the respective genus level.

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Acknowledgments

We are grateful to all people who provided access to collections and material: C. Beard, A. Henrici and M. Dawson (all Carnegie Museum of Natural History, Pittsburgh); L. Costeur (Naturhistorisches Museum, Basel); R. Hutterer (Zoologisches Forschungsmuseum Alexander Koenig, Bonn); D. Kalthoff and U. Johansson (both Naturhistoriska riksmuseet, Stockholm); F. Mayer (Museum für Naturkunde, Berlin); P. Mein (Claude Bernard University, Lyon); D. Möricke (Staatliches Museum für Naturkunde, Stuttgart); K. Rauscher (Paläontologische Sammlung der Universität Wien, Wien); G. Rößner (Bayrische Staatssammlung für Paläontologie und Geologie, München); S. van der Mije (NCB Naturalis, Leiden); G. Weber and J. Rößinger (both Zoologische Schausammlung, Tübingen); and F. Zachos and A. Bibl (both Naturhistorisches Museum, Wien). Thanks to people from NCB Naturalis (Leiden), K. Hermes, and M. Scheske (both Steinmann-Institut, Bonn) for technical support. We also thank J. Kriwet (University Vienna, Vienna), M. Laumann (University of Konstanz, Germany), J. A. Schultz, and R. Schellhorn (both Steinmann-Institut, Bonn) for helpful discussions. Many thanks to two anonymous reviewers whose comments helped us to substantially improve the manuscript. This research is funded by the Fazit-Stiftung, Frankfurt (to CS) and Deutsche Forschungsgemeinschaft (DFG RU 1496/4-1 to IR).

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Fig. S1
figure4

μCT slices through middle and inner ear region and the respective septal compass of selected Sciuridae. A. + B. Eutamias sibiricus; C. + D. Paraxerus cepapi. E. + F. Hylopetes sagitta. Asterisk (*) indicates meshwork of bony septa. Cross (=†) refers to extinct taxa. Abbrevations: art stap – arteria stapedialis (= stapedial artery), bull aud - bulla auditiva (= auditory bulla), cc - crus commune, co – cochlea, inc – incus, LSC - lateral semicircular canal, mas – mastoid, mall – malleus, sta – stapes, pro – promontorium. (GIF 297 kb)

Fig. S2
figure5

μCT slices through middle and inner ear region and the respective septal compass of selected Rodentia. A. Aplodontia rufa; B. + C. Glis glis; D. + E. Graphiurus parvus. Abbreviations see Fig. S1. (GIF 243 kb)

Fig. S3
figure6

μCT slices through middle and inner ear region of selected Rodentia. A. + B. Muscardinus avellanarius; C. + D. †Palaeosciurus feignouxi; E. †Ischyromys typus. Abbreviations see Fig. S1. (GIF 327 kb)

Fig. S4
figure7

μCT slices through middle and inner ear region of selected Lagomorpha. A. Ochotona alpina, B. Oryctolagus cuniculus. Abbreviations see Fig. S1. (GIF 135 kb)

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Pfaff, C., Martin, T. & Ruf, I. “Septal compass” and “septal formula”: a new method for phylogenetic investigations of the middle ear region in the squirrel-related clade (Rodentia: Mammalia). Org Divers Evol 15, 721–730 (2015). https://doi.org/10.1007/s13127-015-0222-x

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Keywords

  • Rodentia
  • Sciuromorpha
  • Middle ear
  • Epitympanic recess
  • Tympanic cavity
  • Bony septa