Functional interpretation of the archaic bat Onychonycteris

GUNNELL, GF; AMADOR, L.I.; GIANNINI, NP; SIMMONS, NB

Durban

Congreso; 17th International bat Research Conference; 2016

Onychonycteris finneyi, from the late early Eocene (52.5 mya) of the Green River Formation,Wyoming, USA, is thought to be the most primitive bat discovered to date. Twospecimens are known, both nearly complete, articulated skeletons. Anatomicalevidence strongly suggests that Onychonycteris was capable of poweredflight, and for this reason the species has been at the center of a debate concerningthe relative timing of the evolution of flight versus echolocation in bats.Here we analyze the functional anatomy of Onychonycteris from theperspective of hypotheses of the origin of mammalian flight. We refinedprevious estimations of basic aerodynamic parameters (aspect ratio, wingloading, wing tip indices), and compared these values with those of other batsand with gliding mammals. In addition, we re-examined the anatomy of thepostcranium, including structures of the axial skeleton, and fore- and hindlimbcharacters. The two specimens are remarkably similar in all aspects. As inprevious reports, Onychonycteris appears in the lower end of observedparameters that estimate bat flight performance, indicating very high cost oftransport. This is most similar to locally-commuting species among extant bats,and corresponds with a strategy of short and direct fast flights interspersed withprolonged (foraging?) bouts of perching. The handwing accounts for 57% of theaspect ratio, and reduces wing loading by 28% over exploratory models withoutthe handwing. Presence of claws on all five manual digits and rigidity of allphalanges from digit base to the wing tips in Onychonycteris greatly differs from the condition seen in extantspecies, and certainly affected flight performance. Other structural detailsindicate great similarity with extant bats, including hindfoot suspension andderived wing folding. Onychonycteris is confirmed as a mosaic of advancedand primitive character states involved in the transition to powered flight.