The modern study of bat evolutionary relationships is full of discoveries and debate. Despite them being a well know group of mammals, early evolution of the group Chiroptera is still poorly understood. Their geographic origin is unknown and the fossils record is rare because of their fragile bones and tropical habitat. The earliest bats are recorded from the Early Eocene of North America, Europe, North Africa and Australia where they seem to appear suddenly and simultaneously (Smith, 2007).
Below: Fossil of Icaronycteris index, earliest known bat until the 2003 discovery of Onychonycteris finneyi
Fossil of Icaronycteris index, earliest known bat until the 2003 discovery of Onychonycteris finneyi
Up until 2003, the oldest known fossil bat (around 50 million years ago) was Icaronycteris index, which is almost identical to extant bats. (above) When Onychonycteris finneyi (literally, the “clawed bat”) was turned up in Wyoming (Simmons et al, 2008), it took its place as the earliest known and most primitive bat, dating back to about 52 million years ago. Extensive skeletal analysis suggests that O. finneyi was capable of flight, albeit in an “undulating, gliding/fluttering” style, similar to that of the mouse-tailed bats (Rhinopomatidae), but did not possess the inner ear structure necessary for echolocation. This is very strong evidence that flight evolved before echolocation. Echolocation may have evolved only once and was later lost in the Pteropodids, or it possibly evolved twice separately (Simmons, 2005).
The clawed digits from which this bat got its name suggest that in addition to flying, this bat could also crawl about and might have been a very good climber as well. The mode of flight suggests an intermediate, incremental stage between gliding and flying by wing flapping. The tail bones of O.finneyi suggest that it had a broad caudal membrane that most likely functioned as an airfoil. The use of the tail membrane to capture insects, which is seen in many modern bats, would not be likely to arise until after echolocation evolved to enable more sophisticated insect capture. The dentition indicates that this bat was insectivorous. Since neither fossil skull retains intact ocular orbits, the activity patterns (i.e. time of day when they were most active) could not be inferred (Simmons et al, 2008).
Oldest known bat fossil.
Bats make up 20% of mammals; they have a great diversity comprised of 1240 species. They are divided into two suborders, informally known as megabats and microbats. Megabats eat fruit, nectar or pollen. They are extremely important to their ecosystem for distributing seeds and pollinating flowers. Many plants rely solely on bats for this function. Other traits distinguish them such as a claw on their second toe, diurnal sleep pattern and good vision. Microbats are specialized for echolocating which they rely on for navigation and finding prey (Moratelli, 2013). This allows them to utilize a nocturnal niche where they mostly eat insects; however, vampire bats are hematophagous, famously linking the species with myths of vampires and evil spirits. Microbats serve an important role as pest control. Kitti’s hog-nosed bat is the smallest species weighing less than .1 oz.; it is contender for the world’s smallest living mammal.
Kitti's Hog-Nosed Bat, the smallest bat and the smallest mammal!
Teeling et al (2005) confirmed the monophyly theory but did reveal, to some surprise, that some Microchiroptera are more closely related to “Megabats” or fruit bats (Pteropodidae), than previously thought (Almeida, 2011). This led to a revision in bat phylogeny strongly supported by statistical tests (Teeling, 2005). The order Chiroptera has been divided into two new suborders, Yinpterochiroptera and Yangochiroptera. Yinpterochiroptera includes the magabats of Pteropodidae and their close relatives of the superfamily Rhinolophoidea. Yangochiroptera is only species that use laryngeal echolocation.
Phylogenetic Tree of Chiroptera, reproduced from Simmons et al, 2008.
Phylogenetic Tree of Chiroptera, reproduced from Simmons et al, 2008.
New Guinea and Melanesia Islands are possible ancestral area for fruit bats. Even with evidence of closely spaced cladogenetic events it is difficult to pinpoint the evolutionary transitions of these animals. Almeida et al find that Pteropodidae has apparently been distinct from other bat lineages since at least the early Eocene. Around 26 million years ago pteropodids experienced an explosive radiation. Resulting in all the extant lineages that live today. They are one of the most diverse bat families, ranking second with over 45 genera and 180 species (Almeida, 2011).
Bats are an amazing example of convergent evolution of flight. Convergent evolution is when species of different lineages independently evolve similar features. Flight has evolved independently multiple times. First in insects over 330 million years ago, then in pterosaur reptiles 225 million years ago, then in birds 150 million years ago. Bats are the most recent to independently develop this a mere 50-60 million years ago, and are the only mammals to do so (Simmons, 2008).
References
Almeida, F., Giannini, N., DeSalle, R., Simmons, N. (2011) Evolutionary relationships of the old world fruit bats (Chiroptera, Pteropodidae): Another star phylogeny? BMC Evolutionary Biology, 11(1), 281-297. doi:10.1186/1471-2148-11-281
Agnarsson I, Zambrana-Torrelio CM, Flores-Saldana NP, May-Collado LJ. A time-calibrated species-level phylogeny of bats (Chiroptera, Mammalia). PLOS Currents Tree of Life. 2011 Feb 4. Edition 1. doi: 10.1371/currents.RRN1212.
Moratelli, R. (2013). Evolutionary history of bats: fossils, molecules and morphology. Journal of Mammalogy, 94(2), 520-521.
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. doi:10.1038/nature06549
Smith, T., Rana, R., Missiaen, P., Rose, K., Sahni, A., Singh, H., & Singh, L. (2007). High bat (Chiroptera) diversity in the Early Eocene of India. Naturwissenschaften, 94(12), 1003-1009.
Teeling, E.C.; Springer, M. S. Madsen, O. Bates, P. O'Brien, S. J. Murphy, W. J. (2005). "A Molecular Phylogeny for Bats Illuminates Biogeography and the Fossil Record". Science 307 (5709): 580–584.
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