Petrosal and Inner Ear of the Thylacine, Thylacinus cynocephalus (Harris, 1808) (Mammalia, Marsupialia, Dasyuromorphia)

The petrosal and inner ear are described for the thylacine, Thylacinus cynocephalus (Harris, 1808), based on µCT datasets. Comparisons are made with a sample of six extant marsupials, also from µCT datasets: the didelphid Monodelphis sp. (short-tailed opossum), the notoryctid Notoryctes caurinus Thomas, 1920 (northern marsupial mole or kakarratul), the peramelid Isoodon macrourus (Gould, 1842) (northern brown bandicoot), the myrmecobiid Myrmecobius fasciatus Waterhouse, 1836 (numbat), and the dasyurids Dasyurus hallucatus Gould, 1842 (northern quoll) and Sminthopsis macroura Gould, 1842 (stripe-faced dunnart). Dural sinuses and veins are reconstructed based on sulci, canals, and foramina as well as the few studies in the literature on cranial veins in marsupials. The most striking observations with the thylacine concern the pattern of venous drainage. Nearly all of its dural sinuses in the middle and caudal cranial fossae are enclosed in canals, whereas in the remaining taxa most of the sinuses run in open sulci. The thylacine has a novel canal through the substance of the pars canalicularis of the petrosal for the sigmoid sinus. It also has a novel canal that runs successively through the substance of the parietal, interparietal, and supraoccipital and then between the petrosal and exoccipital to join the sigmoid sinus that is called here the parieto-occipital diploic vein. In the two thylacines studied, this venous channel is paired in one but is only on the right side in the other. The exoccipital has an unusual canal that runs from the jugular foramen to the condyloid canal and connects the sigmoid sinus and inferior petrosal sinus; this arrangement is also present in Notoryctes caurinus. Another striking feature concerns the area of petrosal occupied by the inner ear, which appears to differ with body size: in the smaller taxa (Monodelphis sp. and Sminthopsis macroura), the inner ear occupies much of the petrosal volume; in the medium-sized taxa (Isoodon macrourus, Myrmecobius fasciatus, and Dasyurus hallucatus), there is considerably more petrosal than the area occupied by the inner ear; and lastly, in the largest (Thylacinus cynocephalus), the petrosal is roughly twice the area of the inner ear. Much of the “extra” petrosal in the thylacine is in the robust paroccipital process and in the endocranial floor of the impression of the cerebellar hemisphere. The inner ear of T. cynocephalus has an overall similar morphology to other marsupials. One notable difference is the lack of a secondary common crus in T. cynocephalus, as in its close relatives My. fasciatus and S. macroura, while it is present in D. hallucatus and other published dasyuromorphians. As previously reported, this feature appears to be relatively variable among marsupials.