Microchiroptera is diagnosed by the following synapomorphies: a free premaxilla, only two lower premolars on each side of the jaw, and a xiphisternum with a medial keel (Simmons and Geisler, 1998). However, these features are troublesome as many lineages within Microchiroptera have apparently secondarily evolved different conditions.
Several other soft-tissue characteristics are found in Microchiroptera, but not Megachiroptera. These characteristics include the presence of a tragus, a small or absent aquaeductus cochleae, the presence of a sophisticated echolocation system, the m. styloglossus originating from the ventral surface of the midpoint of the stylohyal, the clavicle articulating with the coracoid process of the scapula, the m. spinotrapesius clearly differentiated from the trapezius complex, the m. acromiodeltoideus not originating from the thoracic vertebra, the angle of the spinal cord between the dorsal horns from 0 to 25°, and the inferior colliculus larger than the superior colliculus (Simmons and Geisler, 1998). As these features can not be examined in the fossil material collected to date, it is not known whether they also occured in fossils which are closely related to Microchiroptera. Thus, these characteristics can only indirectly support microchiropteran monophyly.
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