The ear region of the marsupial sabertooth,Thylacosmilus: Influence of the sabertooth lifestyle upon it,and convergence with placental sabertooths

The mastoid auditory bulla of the extinct marsupial sabertooth, Thylacosmilus, has an enlarged, complex hypotympanic sinus but lacks an alisphenoid contribution. These are marked departures from the usual marsupial condition. Details of the ear region of Thylacosmilus are compared with those of the convergent, extinct placental sabertooth, Smilodon, and each is compared with less specialized related forms to define differences and similarities of the evolutionary paths that led to the striking overall convergence. Functional factors such as pressure transformer ratio (PTR), impedance transformer ratio (ITR), acoustic impedence at the eardrum, and the fraction of the sound energy theoretically transmitted to the inner ear cannot be estimated for Thylacosmilus because certain critical measures are still unknown (tympanum size, ossicle lever arm ratios). However, in both sabertooths enlarged complex hypotympanic sinuses, characterized by expansions and contractions, are greatly developed. They vastly increase middle ear space (volume) and must have influenced these factors. In both, the sinuses provide the large air volume needed to prevent excessive damping of sound energy transmission (Hunt and Korth, '80), and both are believed to have achieved a further modulation of the system from the cushioning or “pillow” effect of the confined air as it directly damps the tympanum itself. Thylacosmilus has still another feature that may have given greater control over damping of sound energy transmission: the direct opening (probably membrane covered) of one of the sinus cavities into the side of the meatal tube. In this feature, as in others noted earlier (Turnbull, '76, '78), we see a greater degree of specialization in this marsupial sabertooth than in a placental counterpart.