CRANIAL KINESIS AND THE MORPHOLOGY OF THE MIDDLE EAR: TWO POSSIBLY RELATED FEATURES

The problem of the evolution of the tetrapod middle ear rates as one of the classical problems of comparative anatomy. In the early times of comparative morphology attention was focused on the problem of the homologies of the mammalian ossicula auditus. This aspect of the problem is now solved by the general acceptance of Reichert's (1838) and Gaupp's (1913) theory (the only debated point being now that of the homology of the ossiculum accessorium mallei), and the main interest has gradually shifted to that of the evolution of cavum tympani and of the eardrum. The attention of morphologists has been focused on the evolution of the mammalian middle ear, yet an approach to the problem from a broader viewpoint may be useful, as the available evidence points to a connection between changes in the cavum tympani and those of the kinetic structures of the whole skull. The hypothesis of a connection between cranial kinesis and the morphology of the middle ear was originally proposed (Simonetta, 1956) to explain the structures of some reptiles and birds, later extended to mammals (Simonetta, 1957a), and finally generalized for all tetrapods (Simonetta, 1957b) . Its prior publication in Italian does not seem to have removed the need for the present discussion, particularly as new evidence has become available. The present hypothesis is that the structure of the middle ear, and particularly of the cavum tympani, found in such reptiles as Lacerta, Gecko, etc., and generally assumed to be comparatively primitive, is constantly associated with the existence of a metakinetic line of flexing in the skull, and that the middle ear cavity will show some modification whenever the metakinetic line of flexure is missing and the skull is either mesokinetic or akinetic. Such modification in the cavum tympani will be either toward reduction or toward expansion, although there is, apparently, no connection between the type of kinesis (or the loss of kinesis) and the type of specialization of the middle ear. As will be apparent from further discussion, the present hypothesis integrates the basic contributions by Westoll (1943, 1945) and by Watson (1953), while it does not seem to support the hypotheses of Tumarkin (1955) or Shute (1956). Cranial kinesis has received, since the classic paper by Versluijs (1912), a good deal of attention, but its evolution and functional significance are not yet quite clear. Typically it is conceived as an active movement, by which the upper jaw is lifted relative to the braincase. In metakinetic and amphikinetic skulls, the skull table also takes part in the movement, thanks to a dorsal line of flexure caudal to the parietals. In amphikinetic skulls there is an additional line of flexure in front of the parietals called the "mesokinetic line"; this line of flexure occurs alone in mesokinetic skulls, in which the skull table is rigidly fixed to the braincase. Active cranial kinesis is classically conceived as useful in order to increase the oral gape. However, in an increasing number of instances, cranial kinesis is considered to function primarily as a shock absorber; movement might in some instances be passive, the muscular action serving basically to oppose excessive movement of the bones during biting, the skull thus acquiring a functionally useful elasticity. Cranial kinesis is probably primitive in vertebrates, but, among tetrapods, there are a number of instances in which kinetic