The bats (order Chiroptera in infraclass Eutheria) are among the most widespread and speciose living mammals but have a very poor fossil record (Jones et al, 2021). Until recently, there was controversy as to whether the bats comprise a monophyletic group, but molecular evidence has established that they do represent a single clade (Amador et al, 2018).
Only a few fossils have been identified as stem bats and few phylogenetic analyses have been published. Recent analyses (e.g. Amador et al, 2020; Moyers Arévalo et al, 2020) are generally in agreement. The time tree shown below is based on Moyers Arévalo et al (2020):
Only a few fossils have been identified as stem bats and few phylogenetic analyses have been published. Recent analyses (e.g. Amador et al, 2020; Moyers Arévalo et al, 2020) are generally in agreement. The time tree shown below is based on Moyers Arévalo et al (2020):
Figure 1. Phylogenetic time tree of the stem-Chiroptera
The oldest known members of the stem-Chiroptera are Icaronycteris index and Onychonycteris finneyi, both described from the Early Eocene Green River Formation of Wyoming, USA (Jepsen, 1966; Simmons et al, 2008). These fossils are illustrated below, together with the other species shown in Figure 1 above:
Names in red indicate that the fossil is younger than the oldest known crown-group fossil.
Figure 2. Images of stem-group bats
The above images are ordered from most basal to most crownward, but no clear trends in development can be seen by comparing these images. They all look very like modern bats, and indeed the oldest known crown-bat, the Early Eocene (Early Ypresian) Eppsinycteris anglica (Álvarez-Carretero et al, 2022) is of the same or older age. No public-domain images are available of this species.
Taking into account the ghost lineage on the bat stem line (see Figure 1), the stem group of the Chiroptera must have appeared during the Paleocene, which implies a stem-to-crown transition of between 5.6 and 14 million years. That seems to be a short time in which to accommodate what Jones et al (2021) call a “large morphological disparity between bats and their closest living relatives”. Clearly, better elucidation of bat evolution requires the discovery of some more transitional fossils in the form of Paleocene stem-Chiroptera.
Taking into account the ghost lineage on the bat stem line (see Figure 1), the stem group of the Chiroptera must have appeared during the Paleocene, which implies a stem-to-crown transition of between 5.6 and 14 million years. That seems to be a short time in which to accommodate what Jones et al (2021) call a “large morphological disparity between bats and their closest living relatives”. Clearly, better elucidation of bat evolution requires the discovery of some more transitional fossils in the form of Paleocene stem-Chiroptera.
References
Álvarez-Carretero, S., Tamuri, A. U., Battini, M., Nascimento, F. F., Carlisle, E., Asher, R. J., ... & Dos Reis, M. (2022). A species-level timeline of mammal evolution integrating phylogenomic data. Nature, 602(7896), 263-267.
Amador, L. I., Moyers Arévalo, R. L., Almeida, F. C., Catalano, S. A., & Giannini, N. P. (2018). Bat systematics in the light of unconstrained analyses of a comprehensive molecular supermatrix. Journal of Mammalian Evolution, 25(1), 37-70.
Amador, L. I., Almeida, F. C., & Giannini, N. P. (2020). Evolution of traditional aerodynamic variables in bats (Mammalia: Chiroptera) within a comprehensive phylogenetic framework. Journal of Mammalian Evolution, 27(3), 549-561.
Jepsen, G. L. (1966). Early Eocene bat from Wyoming. Science, 154(3754), 1333-1339.
Jones, M. F., Li, Q., Ni, X., & Beard, K. C. (2021). The earliest Asian bats (Mammalia: Chiroptera) address major gaps in bat evolution. Biology Letters, 17(6), 20210185.
Moyers Arévalo, R. L., Amador, L. I., Almeida, F. C., & Giannini, N. P. (2020). Evolution of body mass in bats: insights from a large supermatrix phylogeny. Journal of Mammalian Evolution, 27(1), 123-138.
Simmons, N. B., & Geisler, J. H. (1998). Phylogenetic relationships of Icaronycteris, Archaeonycteris, Hassianycteris, and Palaeochiropteryx to extant bat lineages, with comments on the evolution of echolocation and foraging strategies in Microchiroptera. Bulletin of the AMNH; no. 235.
Simmons, N. B., Seymour, K. L., Habersetzer, J., & Gunnell, G. F. (2008). Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation. Nature, 451(7180), 818-821.
Amador, L. I., Moyers Arévalo, R. L., Almeida, F. C., Catalano, S. A., & Giannini, N. P. (2018). Bat systematics in the light of unconstrained analyses of a comprehensive molecular supermatrix. Journal of Mammalian Evolution, 25(1), 37-70.
Amador, L. I., Almeida, F. C., & Giannini, N. P. (2020). Evolution of traditional aerodynamic variables in bats (Mammalia: Chiroptera) within a comprehensive phylogenetic framework. Journal of Mammalian Evolution, 27(3), 549-561.
Jepsen, G. L. (1966). Early Eocene bat from Wyoming. Science, 154(3754), 1333-1339.
Jones, M. F., Li, Q., Ni, X., & Beard, K. C. (2021). The earliest Asian bats (Mammalia: Chiroptera) address major gaps in bat evolution. Biology Letters, 17(6), 20210185.
Moyers Arévalo, R. L., Amador, L. I., Almeida, F. C., & Giannini, N. P. (2020). Evolution of body mass in bats: insights from a large supermatrix phylogeny. Journal of Mammalian Evolution, 27(1), 123-138.
Simmons, N. B., & Geisler, J. H. (1998). Phylogenetic relationships of Icaronycteris, Archaeonycteris, Hassianycteris, and Palaeochiropteryx to extant bat lineages, with comments on the evolution of echolocation and foraging strategies in Microchiroptera. Bulletin of the AMNH; no. 235.
Simmons, N. B., Seymour, K. L., Habersetzer, J., & Gunnell, G. F. (2008). Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation. Nature, 451(7180), 818-821.
Image credits – stem-Chiroptera
- Header (Fruit bats, photographed near Katherine Gorge in the Northern Territory, Australia): shellac, Attribution 2.0 Generic (CC BY 2.0)
- Figure 2 (Onychonycteris finneyi, fossil): Matthew Dillon, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
- Figure 2 (Onychonycteris finneyi, life restoration): Nobu Tamura, under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 2 (Icaronycteris index, fossil): Erik Terdal from Tulsa, United States, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
- Figure 2 (Icaronycteris index, life restoration): Nobu Tamura, under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 2 (Archaeonycteris sp.): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 2 (Hassianycteris messelensis): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 2 (Paleochiropteryx tupaiodon, fossil): Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2 (Paleochiropteryx tupaiodon, life restoration): Nobu Tamura, under Creative Commons Attribution- ShareAlike (CC BY-SA) license