Powered flight—that is, the ability to remain aloft by flapping wings—has only evolved three times in vertebrates: once in mammals (bats), and twice in archosaurian reptiles (birds and pterosaurs). Major breakthroughs in bird origins occurred as long ago as the 1860s, with the discovery of Archaeopteryx and its mix of avian and reptilian features. Subsequent decades have reinforced the connection between birds and reptiles, and the past 30 years have introduced dozens of feathered theropods that each add a piece to the puzzle of bird origins. Pterosaurs and bats, however, have not had the same luck. The origins of both groups have remained some of the greatest mysteries in vertebrate paleontology. The first bats are known from the earliest Eocene (~55 Ma) and those with complete skeletons look effectively like modern bats (1). Similarly, the first pterosaurs date to the late Triassic (~220 Ma) and are fully flighted when they appear in the fossil record (2). Both groups are clearly very distinct from their closest relatives, although even those relatives have been debated. The only consensus being that bats belong within the mammalian superorder Laurasiatheria (including cows, whales, rhinos, hedgehogs, bears, and others) (3) while pterosaurs are archosaurs, likely closely related to dinosaurs (4, 5). There is hope on the horizon for pterosaurs, however, as a recent paper claims to have identified the closest fossil relatives of pterosaurs (6). A group of poorly-understood, nonflying Triassic reptiles known as lagerpetids are reconstructed as the sister-group to pterosaurs in a phylogenetic analysis based on new specimens and may shed light on the acquisition of features leading to pterosaurs and, in turn, to powered flight. The Brazilian lagerpetid Ixalerpeton. Art by Rodolfo Nogueira. Among the features shared by pterosaurs and lagerpetids are uniquely shaped semicircular canals of the inner ear. Several more distantly related living animals (including birds and primates) also possess enlarged anterior semicircular canals, which help to maintain equilibrium (6). Other endocranial and postcranial features also indicate the capacity for coordinated movements, and—although the authors only allude to this—may suggest an arboreal lifestyle for at least some lagerpetids.
While lagerpetids may not represent the partially-flighted holy grail of pterosaur evolution, their recognition as close pterosaur relatives bridges many gaps in our understanding. That many adaptations indicate highly coordinated movement suggests that the most recent common ancestor of both pterosaurs and lagerpetids occupied a niche where these skills were necessary, perhaps as arboreal insectivores. Do these traits explain why the direct ancestors of pterosaurs have remained so elusive? We won’t be certain until we add more pieces to the puzzle, but at least now the picture is starting to take shape. Now, excuse me while I go to work on this bat mystery… References 1) 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: 818-821. 2) Kellner, A.W.A. (2015). Comments on Triassic pterosaurs with discussion about ontogeny and description of new taxa. Anais da Academia Brasileira de Ciências, 87: 669-689. 3) Doronina, L., Churakov, G., Kuritzin, A., Shi, J., Baertsch, R., Clawson, H., & Schmitz, J. (2017). Speciation network in Laurasiatheria: retrophylogenomic signals. Genome Research, 27: 997-1003. 4) Nesbitt, S.J. & Hone, D.W.E. (2010). An external mandibular fenestra and other archosauriform character states in basal pterosaurs. Palaeodiversity, 3: 225-233. 5) Hone, D.W.E. (2012). Pterosaur research: recent advances and a future revolution. Acta Geologica Sinica, 86: 1366-1376. 6) Ezcurra, M.D., Nesbitt, S.J., Bronzati, M., Dalla Vecchia, F.M., Agnolin, F.L., Benson, R.B.J., Egli, F.B., et al. (2020). Enigmatic dinosaur precursors bridge the gap to the origin of Pterosauria. Nature, 588: 445-449.
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Matthew JonesMusings on evolution and paleontology-related research and news. Archives
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