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A New Enantiornithine Bird with Unusual Pedal Proportions Found in Amber

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A New Enantiornithine Bird with Unusual Pedal Proportions Found in Amber Report A New Enantiornithine Bird with Unusual Pedal Proportions Found in Amber Highlights Authors d New fossil is first avian species recognized from amber Lida Xing, Jingmai K. O’Connor, Luis M. Chiappe, ..., Han Hu, Ming Bai, d Elektorornis is distinct from all other birds based on the Fumin Lei proportions of the foot Correspondence d Scutellae scale filaments on foot suggest probing function for elongated third toe [email protected] (L.X.), [email protected] (J.K.O.) In Brief Xing et al. describe an avian hindlimb preserved in 99 million-year-old Burmese amber. Its unusual pedal proportions support the erection of a new species, Elektorornis chenguangi gen. et sp. nov. The elongated third toe may have been a feeding adaptation, aided by scutellae scale filaments with possible tactile function. Xing et al., 2019, Current Biology 29, 2396–2401 July 22, 2019 ª 2019 Elsevier Ltd. https://doi.org/10.1016/j.cub.2019.05.077 Current Biology Report A New Enantiornithine Bird with Unusual Pedal Proportions Found in Amber Lida Xing,1,2,11,* Jingmai K. O’Connor,3,4,11,12,* Luis M. Chiappe,5 Ryan C. McKellar,6,7,8,11 Nathan Carroll,5 Han Hu,9 Ming Bai,10 and Fumin Lei10 1State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, China 2School of the Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China 3Key Laboratory of Vertebrate Evolution and Human Origins of the Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Beijing 100044, China 4CAS Center for Excellence in Life and Paleoenvironment, Beijing 10010, China 5Dinosaur Institute, Natural History Museum of Los Angeles County, Los Angeles, CA 90007, USA 6Royal Saskatchewan Museum, Regina, Saskatchewan S4P 4W7, Canada 7Biology Department, University of Regina, Regina, Saskatchewan S4S 0A2, Canada 8Department of Ecology & Evolutionary Biology, University of Kansas, Lawrence, KS 66045, USA 9Zoology Division, School of Environmental and Rural Sciences, University of New England, Armidale, NSW 2351, Australia 10Key Laboratory of Zoological Systematics and Evolution, Institute of Zoology, Chinese Academy of Sciences, Beijing, 100101, China 11These authors contributed equally 12Lead Contact *Correspondence: [email protected] (L.X.), [email protected] (J.K.O.) https://doi.org/10.1016/j.cub.2019.05.077 SUMMARY Holotype HPG-15-2 (Hupoge Amber Museum, Tengchong City Amber As- Recent discoveries of vertebrate remains trapped in sociation, China) is an incomplete avian right hindlimb with middle Cretaceous amber from northern Myanmar plumage from the left wingtip preserved encased in amber, [1, 2] have provided insights into the morphology of measuring 34.8 mm 3 34.4 mm 3 8.2 mm and weighing soft-tissue structures in extinct animals [3–7], in 5.51 g. The bird appears to have undergone significant decay particular, into the evolution and paleobiology of prior to resin polymerization. In many places, the skin from the early birds [4, 8, 9]. So far, five bird specimens have foot has sloughed off the bones and drifted short distances through the amber. been described from Burmese amber: two isolated wings, an isolated foot with wing fragment, and two Etymology partial skeletons [4, 8–10]. Most of these specimens Elektorornis,‘‘Elektor,’’ the word for amber; ‘‘-ornis,’’ Greek, contain the remains of juvenile enantiornithine birds meaning bird. The species name ‘‘chenguangi’’ is in honor of [4]. Here, we describe a new specimen of enantiorni- Chen Guang, a curator at the Hupoge Amber Museum. thine bird in amber, collected at the Angbamo locality in the Hukawng Valley. The new specimen includes a Diagnosis partial right hindlimb and remiges from an adult or Small enantiornithine (distal condyles of the tibiotarsus contact- subadult bird. Its foot, of which the third digit is ing medially, J-shaped metatarsal I, metatarsal IV mediolaterally much longer than the second and fourth digits, is reduced relative to metatarsals III and IV, metatarsal IV trochlea distinct from those of all other currently recognized reduced to a single condyle, and recurved pedal unguals) with Mesozoic and extant birds. Based on the autapo- the unique combination of following traits: pedal digit III, 20% morphic foot morphology, we erect a new taxon, longer than tarsometatarsus; hallux, more than 86% length of Elektorornis chenguangi gen. et sp. nov. We suggest pedal digit II ( 80% in pengornithids); and pedal digit II, 59% that the elongated third digit was employed in a length of digit III, proportionately shorter than that of any other Mesozoic bird (autapomorphic within the Enantiornithes; unique foraging strategy, highlighting the bizarre Table S1). morphospace in which early birds operated. Locality and Horizon RESULTS Late Albian-Cenomanian 98.8 ± 0.6 Ma [11] Angbamo locality, Hukawng Valley, Kachin Province (Tanai Township, Myitkyina Systematic Paleontology District), northern Myanmar. Aves Linnaeus 1758 Ornithothoraces Chiappe 1995 Ontogenetic Assessment Enantiornithes Walker 1981 The specimen is considered to be subadult to adult, based on Elektorornis gen. nov. the complete fusion of the proximal tarsals to the tibia and the Elektorornis chenguangi sp. nov. (Figures 1A–1C) distal tarsals to the metatarsals [12]. 2396 Current Biology 29, 2396–2401, July 22, 2019 ª 2019 Elsevier Ltd. Figure 1. Tarsal Structure and Integumentary Structures Preserved in Elektorornis chenguangi HPG-15-2 (A) HPG-15-2 overview, with inset providing greater detail on foot, arrowheads marking different apices of unguals and ungual sheathes where visible, and red arrow marking base of mt III 4 shared with (D). (B and C) Osteological details. (D) Tuft of elongated SSFs near apex of mt III ph 3, with horizontal arrowhead marking edge of reticulae from digital pad, inclined arrowhead marking edge of scute, white arrow marking sloughed reticulae, and red arrow marking base of ungual in (A). (E) Detail of lowermost SSFs in (D), showing hollow cores (arrowheads) and mottled outer walls, presumably due to feather oils. Fe, femur; fi, fibula; lc, lateral condyle; mc, medial condyle; mt, metatarsal and corresponding digit; ph, phalanx; tb, tibia. Scale bars, 5 mm in (A); 1 mm in (A) inset; 0.5 mm in (D); and 0.25 mm in (E). See also Figures S1, S2, and S4. Description enantiornithines [13, 19]. This trochlea is also slightly plantarly Osteology displaced relative to the coplanar trochleae III and IV. In plantar The distal end of the right femur is jointed with a fully articulated view, the medial condyle of the trochlea has greater plantar pro- tibiotarsus, fibula, and pes (Figures 1B and 1C). The femur is jection, and metatarsal III displays a slight depression just prox- obscured by dense soft tissue. A separate, poorly preserved imal to the trochlea. The trochlea of metatarsal IV appears element contacts the lateral surface of the knee (possibly the distal reduced to a single condyle as in other enantiornithines [20]. ends of the ulna-radius). The tibiotarsus seems to bear a circular Metatarsal I is approximately 20% of the length of metatarsal proximal articular surface that is expanded relative to the shaft, II, proximally tapered, and mediolaterally compressed. Similar as in many enantiornithines [13]. The tibiotarsal shaft has an oval to HPG-15-1 [4], the shaft of metatarsal I articulates with the cross-section with the long axis oriented mediolaterally. The distal medial surface of metatarsal II, somewhat extending onto the end of the tibiotarsus is expanded so that the articular surface plantar surface such that the lateral surface of the shaft of meta- formed by the condyles is much wider than the shaft. The medial tarsal I is deeply concave. The distal articular surface of meta- condyle is wider than the lateral condyle, a condition typical of tarsal I projects plantarly, nearly perpendicular to the longitudinal enantiornithines and other basal pygostylians [14–17]. The medial axis of the shaft (J-shaped in medial view), a condition similar to surface of the medial condyle appears somewhat excavated (Fig- some other enantiornithines [14, 21]. The hallux was fully ure 1B). The articular surface extends onto the caudal surface of reversed in the anisodactyl condition. the tibiotarsus. The fibula is mediolaterally compressed and The non-ungual pedal phalanges are gracile (Figures 1B and approximately three-quarters of the length of the tibiotarsus. 1C). The third digit is the longest, followed by the fourth, the sec- The proximal tarsometatarsus appears fully fused (Figures 1B ond, and the hallux, which is 86% of the length of the second and 1C). The proximal ends of the metatarsals are expanded digit. The foot is unusual in that the third digit is proportionately relative to their shafts, which remain unfused to one another— much longer than the other digits and in that the first digit ap- a condition typical even in adult enantiornithines. The dorsal proaches the length of the second digit (typically, the second surface of metatarsal III is convex, similar to the condition in avi- digit is proportionately longer) (Table S1). The penultimate pha- saurid enantiornithines [18, 19]. Typical of enantiornithines, lanx is the longest in each digit, as in most perching birds [22]. metatarsal IV is narrower than metatarsals II and III [13]. Meta- The first phalanx of digit I is very long, shorter only than the penul- tarsal III is the longest; metatarsals II and IV are subequal, ex- timate phalanx of the third digit. The ungual phalanx of digit I is tending to just above the trochlea of metatarsal III. The trochlea slightly more curved than that of the other digits. The proximal of metatarsal II is the widest, a proportion common among phalanx of digit II is short, approximately half the length of the Current Biology 29, 2396–2401, July 22, 2019 2397 Figure 2. Wing Fragment Plumage (A) Overview of primary and secondary feather exposure at polished edge of amber piece, with in- clined arrows marking primary rachises (P1 and P2 weakly distinguished from secondaries and marked in red); vertical arrows mark secondaries; horizontal arrows mark pale areas in wing; and lettered circles mark positions of (B) and (C).
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