Icaronycteris index (replica)
GRI #792-R
This replica is based on a cast of the original holotype (the specimen on which the description of a new species is based) of Icaronyterix index (Jepsen, 1966). This was a small bat of the suborder Microchiroptera (microbats), measuring about 14 cm in length. Several specimens of this bat have been discovered in the lower Eocene deposits of the Fossil Lake Basin (Grande, 2013), including some that Rietbergen et al. (2023) classified as a separate species (I. gunnelli). These fossils are among the stratigraphically lowest bat skeletons found in the geologic record (Rietbergen et al., 2023).
The basal Eocene marks the abrupt appearance of fossil bats (order Chiroptera), already widely distributed across continents and fully capable of powered flight (Tabuce et al., 2009). Some, including I. index, exhibit specialized adaptations in their ear bones, indicating that they used echolocation—similar to many living bat species that navigate in the dark and locate insect prey (Novacek, 1985; Springer et al., 2001). Echolocation requires the sophisticated integration of mechanisms for sending, receiving, and interpreting sonic signals. The sudden appearance of bats in the fossil record follows a pattern observed in other organisms (see, for example, the introductions to the Ediacaran Biota and Ordovician Crinoids collections). The lack of known ancestral intermediates and the presence of fully functional complex adaptations from their earliest stratigraphic occurrence pose a challenge for evolutionary scenarios (Gunnell & Simmons, 2005; Speakman, 2008; Springer et al., 2001).
I. index was different from living microchiropteran bats in a few features, such as lacking a membrane connecting the hind legs, having a complete phalangeal formula in the manual digits II-V, and displaying a clawed index finger (Gunnell & Simmons, 2005; Jepsen, 1966).
References:
Grande, L., 2013. The lost world of Fossil Lake: Snapshots from deep time. University of Chicago Press. p. 425.
Gunnell, G.F. and Simmons, N.B., 2005. Fossil evidence and the origin of bats. Journal of Mammalian Evolution, 12, pp. 209-246. doi:10.1007/s10914-005-6945-2.
Jepsen, G.L., 1966. Early Eocene bat from Wyoming. Science, 154(3754), pp. 1333-1339. doi:10.1126/science.154.3754.1333.
Novacek, M.J., 1985. Evidence for echolocation in the oldest known bats. Nature, 315(6015), pp. 140-141. doi:10.1038/315140a0.
Rietbergen, T.B., van den Hoek Ostende, L.W., Aase, A., Jones, M.F., Medeiros, E.D. and Simmons, N.B., 2023. The oldest known bat skeletons and their implications for Eocene chiropteran diversification. PLoS One, 18(4), p.e0283505. doi:10.1371/journal.pone.0283505.
Speakman, J., 2008. A first for bats. Nature, 451(7180), pp.774-775. doi:10.1038/451774a.
Springer, M.S., Teeling, E.C., Madsen, O., Stanhope, M.J. and de Jong, W.W., 2001. Integrated fossil and molecular data reconstruct bat echolocation. Proceedings of the National Academy of Sciences, 98(11), pp. 6241-6246. doi:10.1073/pnas.111551998.
Tabuce, R., Antunes, M.T. and Sigé, B., 2009. A new primitive bat from the earliest Eocene of Europe. Journal of Vertebrate Paleontology, 29(2), pp. 627-630. doi:10.1671/039.029.0204.