Throughout most of the world today, the major predators of insects that live under the bark of trees are birds--the avian woodpeckers of the family Picidae. This has not always been true, however, and it is not the case in modern Madagascar and Australia/New Guinea where the role of wood-boring insect predators are filled by mammals. These extant "mammalian woodpeckers" are the aye-aye, a lemur from Madagascar, and possums of the genus Dactylopsila, found in northern Australia and New Guinea. While avian woodpeckers use their bills to excavate holes in wood and use a long tongue to retrieve insects, these mammals use large teeth at the front of their mouths to excavate and elongated third or fourth fingers to grasp insects. Surprisingly, the "mammalian woodpecker" body plan is fairly common in the Cenozoic fossil record. The early placental mammal family Apatemyidae almost certainly occupied this niche, as evidenced by nearly complete skeletons of genera like Apatemys and Heterohyus, which display the same chisel-like front teeth and elongated fingers as living "mammalian woodpeckers" (1). Other groups of Cenozoic mammals are known only from cranial and dental remains but share enough features with living "mammal woodpeckers" that we can be fairly confident they also occupied this niche. These include the primitive marsupial Yalkiparodon (2), the African lemur-like primates Plesiopithecus and Propotto (3), and the early primate-like mammal Chiromyoides (4)*. Of course, as long as a niche exists, given enough time something will exploit it. In that case, perhaps it is not terribly surprising that mammal woodpeckers appear to have been relatively common in the early Cenozoic, but who was occupying the niche during the Mesozoic? Avian woodpeckers in the family Picidae had almost certainly not evolved yet (5), and we have no evidence for apatemyids or other "mammalian woodpeckers" at that time either. A paper published last week in Current Biology might provide some clues (6). In the paper, Xing et al. describe the foot of a Cretaceous bird preserved in Burmese amber with a surprisingly long third toe--a trait unknown in any other living or fossil bird. The authors speculate that this bird may have used its unusual toe for extracting insects, much like living aye-ayes. Artist reconstruction of Elektorornis chenguangi. Credit Zhongda Zhang/Current Biology. If this bird, Elektorornis chenguangi, did in fact use its elongated toe like an aye-aye it would represent another convergent evolution of the woodpecking mode of life. In an unexpected twist, this bird may have behaved more like one of the "mammalian woodpeckers" than the more closely related avian Picidae. Living avian woodpeckers possess a number of specialized traits to help them cope with the extreme stress caused by rapidly drumming their bills on wood (7). The suite of changes that was necessary for modern avian woodpeckers to evolve was almost certainly more complex than those necessary for mammalian woodpeckers, and perhaps the group of enantiornithine birds that Elektorornis belonged to also opted for the shorter evolutionary path. Of course, since we currently lack a skull or bill of Elektorornis this mode of life remains speculative, but the elongated toe is oddly reminiscent of the digits of aye-ayes and apatemyids. Although Picidae are the most familiar woodpeckers to the vast majority of us alive today, evolutionarily they may be the exception rather than the norm!
*Stay tuned for an upcoming paper about this bizarre little mammal References 1) Kenigswald, W. V., Rose, K. D., Grande, L., & Martin, R. D. (2005). First apatemyid skeleton from the lower Eocene Fossil Butte Member, Wyoming (USA), compared to the European apatemyid from Messel, Germany. Palaeontographica, Abteilung A: Palaozoologie - Stratigraphie, 272: 149-169. 2) Beck, R. M. D. (2009). Was the Oligo-Miocene Australian metatherian Yalkaparidon a 'mammalian woodpecker'? Biological Journal of the Linnean Society, 97: 1-17. 3) Gunnell, G. F., Boyer, D. M., Friscia, A. R., Heritage, S., Manthi, F. K., Miller, E. R., Sallam, H. M., Simmons, N. B., Stevens, N. J., & Seiffert, E. R. (2018). Fossil lemurs from Egypt and Kenya suggest an African origin for Madagascar's aye-aye. Nature Communications, 9: 1-12. 4) Szalay, F. S. & Delson, E. (1979). Evolutionary History of the Primates. Academic Press, New York, 580 pp. 5) Shakya, S. B., Fuchs, J., Pons, J.-M., & Sheldon, F. H. (2017). Tapping the woodpecker tree for evolutionary insight. Molecular Phylogenetics and Evolution, 116: 182-191. 6) Xing, L., O'Connor, J. K., Chiappe, L. M., McKellar, R. C., Carroll, N., Hu, H., Bai, M., & Lei, F. (2019). A new enantiornithine bird with unusual pedal proportions found in amber. Current Biology, 29: 1-6. 7) Wang, L., Cheung, J. T.-M., Pu, F., Li, D., Zhang, M., & Fan, Y. Why do woodpeckers resist head impact injury: a biomechanical investigation. PLOS One, 6: e26490.
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