- The oldest known ancestor to present day bats, " Icaronycteris index ", apparently did not have a calcar or spur as evidenced by fossil remains.
- Novacek ( 1987 ) reanalyzed morphology of " Icaronycteris " and " Palaeochiropteryx " and concluded that they are more closely related to Microchroptera than to Megachiroptera.
- Sears et al . ( 2006 ) also studied the relative proportion of bat forelimb digits from several extant species and compared these with a fossil of " Icaronycteris index ", an early extinct species from approximately 50 million years ago.
- Most recently, Simmons and Geisler ( 1998 ) found that " Icaronycteris ", " Archaeonycteris ", " Hassianycteris ", and " Palaeochiropteryx " represent a series of consecutive sister-taxa to extant microchiropteran bats.