A frequent misconception about bats is that the oldest bats are known from the early Eocene (~52.5 million years ago) Green River Formation of western North America. In fact, there are earlier fragmentary bats known from Eocene deposits of both Australia and Portugal. Australonycteris clarkae and Archaeonycteris? praecursor both date to likely earliest Eocene sediments (1,2), while the complete skeletons of Icaronycteris index and Onychonycteris finneyi from the Green River Formation date to slightly younger late early Eocene deposits (3,4). In a new paper out today in Biology Letters, colleagues and I describe the first early Eocene bat from Asia, which is also—along with Australonycteris and Archaeonycteris? praecursor—among the oldest bats currently known (5). Bats actually occur nearly worldwide during the early Eocene (spanning 56-48 million years ago), being found in Africa, Australia, Europe, North America, South America, and the Indian subcontinent--which likely hadn’t completely collided with Asia at that point—by the end of the early Eocene (6). Until now, however, the oldest bats from mainland Asia dated to the middle Eocene ~43-44 million years ago. The new bat from the Junggar Basin of northwestern China, which we have named Altaynycteris aurora, likely dates to the first million years of the early Eocene based on biostratigraphic correlation with fossils from Inner Mongolia (5). This places Altaynycteris approximately contemporary with Australonycteris and Archaeonycteris? praecursor from Australia and Europe. Holotype upper first molar of Altaynycteris aurora. SEM photograph by Ni Xijun, IVPP. Finding early Eocene bats in central Asia is important, since we know several other groups of animals originated in Asia around this time period. The origin of both rodents and lagomorphs (rabbits, hares, and pikas) can confidently be traced to Asia, while many other groups of placental mammals have been hypothesized to originate there (7). Morphologically, Altaynycteris also appears primitive enough to represent a very early branch of the bat tree.
Only two teeth of Altaynycteris are currently known, but they possess enough similarity to other early Eocene bats (e.g., exaggerated W-shaped crests, lack of a mesostyle) that we are confident they belong to bats. These teeth, however, possess other similarities to non-bat insectivorous mammals from the Paleocene and early Eocene. They lack the common hook-shaped parastyle of most bats, and possess small cusps and crests (the metaconule and postparaconule crista) that are absent in most bats. The fact that Altaynycteris appears so primitive lends credence to the idea that bats may have a central Asian origin. 1) Hand, S., Novacek, M., Godthelp, H., & Archer, M. (1994). First Eocene bat from Australia. Journal of Vertebrate Paleontology, 14: 375-381. 2) Tabuce, R., Antunes, M.T., & Sigé, B. (2009). A new primitive bat from the earliest Eocene of Europe. Journal of Vertebrate Paleontology, 29: 627-630. 3) Jepsen, G.L. (1966). Early Eocene bat from Wyoming. Science, 154: 1333-1339. 4) 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. 5) 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: 20210185. 6) Smith, T., Habersetzer, J., Simmons, N.B., & Gunnell, G.F. (2012). Systematics and paleobiogeography of early bats. In Evolutionary History of Bats: Fossils, Molecules, and Morphology (G.F. Gunnell & N.B. Simmons, eds.), 23-66. 7) Bowen, G.J., Clyde, W.C., Koch, P.L., Ting, S., Alroy, J., Tsubamoto, T., Wang, Y, & Wang, Y. (2002). Mammalian dispersal at the Paleocene/Eocene boundary. Science, 295: 2062-2065.
0 Comments
A recent paper by Mazin and Poeuch in the journal Geobios describes the first known trackways of non-pterodactyloid (formerly called “Rhamphorynchoid”) pterosaurs (1). I began my master’s degree with the intention of studying pterosaur locomotion and trackways by using bats as living comparisons. “Rhamphorynchoid” pterosaurs – a hodgepodge group that contains all pterosaurs that don’t belong to the superfamily Pterodactyloidea – were particularly interesting to me due to the lack of known trackways and the amount of debate existing about how they walked. In the end, my interest in filling the gaps in our collective knowledge about how bats walk trumped my interest in non-pterodactyloid locomotion and I ended up abandoning that portion of my thesis. I remain interested in the terrestrial capabilities of pterosaurs, however, so I was very excited to read this new paper. Recognized trackways of pterodactyloid pterosaurs have become increasingly common since the 1990s (2), demonstrating that those late Jurassic to late Cretaceous animals were terrestrially adept quadrupeds with erect gaits. The absence of trackways belonging to the Triassic through Jurassic non-pterodactyloids, however, has sparked debate as to how terrestrially competent these animals were and whether they walked on two or four legs. This new paper describes trackways belonging to three new ichnospecies of non-pterodactyloid pterosaurs from an intertidal region in the late Jurassic Period of France. It suggests that non-pterodactyloids were not only quadrupedal on the ground, but that they held their hands with fingers aligned with the body, unlike pterodactyloids which held their fingers perpendicular. In 2015, Mark Witton presented a number of reasons that he felt suggested the terrestrial abilities of non-pterodactyloids were underestimated (3). Two of the points he focused on were whether the uropatagium (the part of the flight membrane that extends between the two hind legs) restricted movement of the legs, and whether the limbs were sprawled and how that would influence terrestrial locomotion. Witton observed that many living animals with extensive uropatagia are nonetheless adept walkers. My master’s research on living bats underscored this point as we documented walking and bounding gaits in the common vampire bat (Desmodus rotundus) and Seba’s short-tailed bat (Carollia perspicillata), while noting that other bats with relatively small patagia, like the greater sac-winged bat (Saccopteryx bilineata), appeared incapable of anything but a sprawling crawl (4). Other studies have shown bats with even larger uropatagia, such as the pallid bat (Antrozous pallidus), are capable of rapid and coordinated gaits (5). Walking postures of two non-pterodactyloid pterosaurs: sprawling gait of Dorygnathus (A); erect gait of Dimorphodon (B). Credit Mark Witton/PeerJ. Bats can inform Witton’s point about sprawling, too. Due to hindlimbs that are rotated 90⁰ from those of a typical mammal, bats cannot stand erect and must sprawl when on the ground. Their forelimbs--like those of pterosaurs--dwarf their hindlimbs, resulting in trackways in which the tracks produced by their hands are located outside those produced by the feet (4, 6). Nonetheless, Desmodus, Antrozous, and many other bat species are quite capable on the ground. Witton points out that not all non-pterodactlyoids appear to have had a sprawling posture, and those like Dimorphodon and Darwinopterus may have had terrestrial abilities similar to pterodactyloid pterosaurs. The trackways illustrated in the paper by Mazin and Poeuch seem to confirm this, as there is little evidence that the trackmaker possessed notably sprawled forelimbs.
Ultimately, it seems that the rarity of non-pterodactyloid pterosaur tracks is not a result of a lack of terrestrial ability, but may instead stem from differences in habitat and behavior. Not all environments are uniformly suited to preserve fossil trackways, and if non-pteryodactyloid pterosaurs were not occupying those favorable environments their trackways will not frequently be preserved. It is also unlikely that even the most terrestrially inept pterosaurs were completely helpless on the ground, and sprawling pterosaurs may have been able to employ a breaststrokelike crawl (similar to what we have documented in many phyllostomid bats and Saccopteryx). Ever since pterodactyoid tracks were first recognized in the fossil record they have been identified with increasing regularity all over the world. While non-pterodactyoid tracks may not prove to be as common as those of pterodactyoids, it is extremely likely that there are more out there, waiting to tell us more about how these animals moved and lived. References 1) Mazin, J.-M., & Pouech, J. (2020). The first non-pterodactyloid pterosaurian trackways and the terrestrial ability of non-pterodactyloid pterosaurs. Geobios https://doi.org/10.1016/j.geobios.2019.12.002 2) Lockley, M., Harris, J. D., & Mitchell, L. (2008). A global overview of pterosaur ichnology: tracksite distribution in space and time. Zitteliana, B28: 185-198. 3) Witton, M. P. (2015). Were early pterosaurs inept terrestrial locomotors? PeerJ, 3:e1018; DOI 10.7717/peerj.1018 4) Jones, M. F., & Hasiotis, S. T. (2018). Terrestrial behavior and trackway morphology of Neotropical bats. Acta Chiropterologica, 20: 229-250. 5) Dietz, C. L. (1973). Bat walking behavior. Journal of Mammalogy, 54: 790-792. 6) Lawrence, M. J. (1969). Some observations on non-volant locomotion in vespertilionid bats. Journal of Zoology, 157: 309-317. |
Matthew JonesMusings on evolution and paleontology-related research and news. Archives
June 2021
Categories
All
|