Sciuridae, Fischer de Waldheim, 1817

Pfaff, Cathrin, Martin, Thomas & Ruf, Irina, 2015, BSeptal compass ^ and Bseptal formula ^: a new method for phylogenetic investigations of the middle ear region in the squirrel-related clade (Rodentia: Mammalia), Organisms Diversity & Evolution (New York, N. Y.) 15 (4), pp. 721-730 : 724-726

Sciuridae

The middle ear region of the investigated sciurids shows similarities and differences in the occurrence of the bony septa in the epitympanic recess and the tympanic cavity at the genus level. Based on the ancestral character analysis, the ancestral character state of the intrabullar septa for Sciuridae is a primary septum I and a secondary septum A in the epitympanic recess and in the tympanic cavity the secondary septum a and the primary septa I and II (>Re A,I,II/a,I,II; Fig. 3 ).

Marmotini All investigated species of Marmotini possess a secondary septum a and the primary septa I and II in the tympanic cavity ( Fig. 3 ; Table S2). However, the distribution of septa in the epitympanic recess is not uniform. Notocitellus annulatus , Callospermophilus lateralis , Spermophilus citellus , and Spermophilus erythrogenys are characterized by the primary septum I and the secondary septum A in the epitympanic recess (>Re A,I/a,I,II). Additionally, S. citellus displays a primary septum III in the tympanic cavity (>Re A,I,II/a,I,II,III). Both extant and extinct U. undulatus and Spermophilus suslicus possess the secondary septum B in the epitympanic recess (>Re B,I/a,I, II). The epitympanic recesses of the investigated Cynomys species are characterized by an additional septum representing the primary septum II. These two species can be differentiated by the occurrence of the dorsal secondary septum A, which occurs in the middle ear of Cynomys leucurus (>Re A,B,I,II/a,I,II), but not in the middle ear of Cynomys ludovicianus (>Re B,I,II/a,I,II).

Tamiini The investigated species of Tamiini , Neotamias townsendii and Eutamias sibiricus , display a secondary septum A and a primary septum I in the epitympanic recess ( Fig. 3 ; Fig. S1 A, B ; Table S2). Additionally, N. townsendii possess a dorsally laying primary septum II. In the tympanic cavity, both species possess the secondary septum a and the primary septa I and II (>Re A,I/a,I,II).

Funambulini All investigated taxa are characterized by the dorsally positioned secondary septum A and primary septum I ( Fig. 3 ; Table S2). The ventrally located represent ancestral character states; asterisk character is stated as present and absent in the ancestral character state analysis (Mesquite 3.02). Source of splits: a Huchon et al. (2002), b Marivaux et al. (2004), c Blanga-Kanfi et al. (2009), d Vianey-Liaud (1974), e Mercer and Roth (2003), f Herron et al. (2005), g Harrison et al. (2003). Number of nodes correspond to septal ancestral character states (see supplementary material Table S3)

primary septum II is found in all funambulins (>Re A, I/a,I,II; Fig. S1 C, D ), except in Funisciurus anerythrus (>Re A,I/a,I).

Protoxerini All of the investigated taxa of Protoxerini possess the same number of septa in the middle ear region ( Fig. 3 ; Table S2). Species can be characterized by the secondary septum A and the primary septa I and II in the epitympanic recess and the secondary septum a and the primary septa I and II in the tympanic cavity (>Re A,I,II/a,I,II).

Xerini The investigated taxa of Xerini show the highest number of ventrally positioned primary septa in the middle ear cavity of all examined sciurids ( Fig. 3 ; Table S2). In the dorsal epitympanic recess, all members of this tribe are characterized by a secondary septum A. Additionally, Geosciurus inauris possesses a secondary septum B (>Re A,B,I/a,I,II,III,IV) and Spermophilopsis leptodactylus a primary septum II (>Re A,I,II/ a,I,II,III,IV). Except for Euxerus erythropus (>Re A,I/a,I,II,III), Xerini have four primary septa in the tympanic cavity. The epitympanic recess of the investigated specimen of † H. costatus is partly destroyed; a secondary septum a and a primary septum I can be identified ventrally in the tympanic cavity (?/a,I,?).

Pteromyini The epitympanic recess is enlarged in latero-occipital direction in all investigated taxa of Pteromyini . Based on this anatomy and the corresponding definition of the septal compass, no dorsal secondary septa can be found in gliding sciurids ( Fig. 3 ; Table S2; Fig. S1 E, F ). The spatial arrangement of the dorsal and ventral primary septa discriminates the investigated taxa. The epitympanic recess and the tympanic cavity of Pteromys volans and Petaurista petaurista are characterized by the primary septa I and II (<Re I,II/a,I,II), whereas the middle ear regions of Glaucomys volans and Iomys horsfieldii thompsoni display the dorsal primary septa I to III and ventral primary septa I and II (<Re I,II,III/a,I,II). Hylopetes sagitta displays the highest number of septa of all investigated pteromyines comprising four primary septa in the epitympanic recess and three primary septa in the tympanic cavity (<Re I,II,III,IV/a,I,II,III; Fig. S1 E, F ).

Sciurini In this tribe, the investigated taxa of the genus Sciurus show the secondary septum a and the primary septa I and II in the tympanic cavity ( Fig. 3 ; Table S2). The investigated specimen of Tamiasciurus hudsonicus possesses a secondary septum A and the primary septa I and II in the epitympanic recess. Additionally, a secondary septum a and the primary septa I to III are found in the tympanic cavity (>Re A,I,II/a,I,II,III).

Nannosciurini The investigated taxa of Nannosciurini have the lowest number of septa of all investigated species in this study (>Re I/a,I; Fig. 3 ; Table S2). Except for Nannosciurus melanotis , all studied species of this tribe show dorsally the secondary septum B and the primary septum I. A secondary septum a and the primary septum I are located within the tympanic cavity (>Re B,I/a,I).

Ratufini The investigated species of this tribe, Ratufa bicolor , possesses a secondary septum A and the primary septum I in the epitympanic recess ( Fig. 3 ; Table S2). In the tympanic cavity, the secondary septum a and the primary septum I are present (>Re A,I/a,I; Fig. S2 I, J ).

† Palaeosciurus feignouxi The auditory region of the investigated specimen of † P. feignouxi is characterized by the secondary septum B and the primary septum I in the epitympanic recess ( Fig. 3 ; Table S2). In the tympanic cavity, we observe the primary septa I and II and, additionally, the secondary septum a (>Re B, I/a,I,II).

Figures

Fig. 3 Patterns of Bprimary septa^ and Bsecondary septa^ in the epitympanic recess and the tympanic cavity in the squirrel-related clade, †Palaeosciurus feignouxi, †Ischyromys typus, and investigated outgroups: pmd posterior medial diverticulum, filled square character present, white square character absent, question mark character not preserved. Species of investigated glirids: group I:= Glis glis; group II= Dryomys nitedula intermedius, Eliomys quercinus, and Graphiurus parvus; group III=Muscardinus avellanarius. Schemas of septal compass

Fig. 1 Application of the septal compass. a Skull and right middle ear region in lateral view of Petaurista petaurista. b The right ear region of Euxerus erythropus in ventral view. The relative position of all septa is gathered by using several different orientated clipping planes. c Lateral

Fig. 2 Septal compass: general graphical illustration of the anatomy of the right middle ear region with the relative position of the primary septa (I–III) and the secondary septa (A, B, a). One asterisk (a*) indicates the Blateral secondary septum^ of MacPhee (1981), two asterisks (**) indicate the Banterior septum^ of Wible (2009). cty cavum tympani (=tympanic cavity), fsa fossa subarcuata (=subarcuate fossa), rec recessus epitympanicus (=epitympanic recess)