• apes;
  • embolomere;
  • otic notch;
  • hearing;
  • tetr

Middle ear structure has been of interest for a long time in studies of the origins and relationships of early tetrapod groups. The model of a dorsally-directed, rod-like stapes with a tympanum, thought common to labyrinthodont amphibians, was taken to be primitive for tetrapods. The stapes of embolomeres and other early anthracosaurs were assumed to be of this form, but difficulties resulted if the middle ear structure of fossil and living reptiles was considered ultimately derived from this source.

The embolomere stapes has been identified and does not conform to the predicted model. It most closely resembles that of Greererpeton, an early notchless temnospondyl. The stapes is compared with those of other tetrapods in terms of the theoretical five processes. An interpretation is put forward in which all but the opercular are seen as potentially present. The embolomere stapes is compared with that of Greererpeton in terms of recent theories of mechanical function and is seen to weaken them. They are then compared as part of a possible acoustic mechanism. The embolomere middle ear structure is reinterpreted as a receiver for low-frequency sound and the ‘otic notch’ is not considered to have housed a tympanum.

The resemblance between the stapes of these two animals seems best explained by their closeness to the plesiomorphic condition for tetrapods, a conclusion which forces the abandonment of the concept of a ‘labyrinthodont middle ear’. The middle ear structure of later groups can be interpreted as having evolved from one similar to that seen in these two animals. The conclusion supports those reached in other recent papers that tympana were not primitive for tetrapods but have been independently derived in several groups.