RESEARCH NEWS & VIEWS fields emit ultra­violet radiation that energizes observatories7 will allow astronomers to study elements is something of a cosmic treasure, the planet’s atmosphere. the escaping atmospheres of planets as small providing ample scientific opportunities to Spake et al. observed WASP-107b using a as Neptune, which has a four times that study planetary formation and . camera on board the Hubble Space Telescope, of . Spake and colleagues’ detection of helium in and concluded that the planet’s atmosphere Theorists have predicted that the atmos- WASP-107b will enable astronomers to look escapes to form a comet-like (Fig. 1). pheres of Neptune-sized exoplanets could be for atmospheres that are rich in helium, and Astronomers have long known that plan- rich in helium8, owing to differences in the perhaps in heavier elements, thereby opening ets can lose their atmospheres in this fashion5, rates at which hydrogen and helium are lost a new subfield of exoplanetary . ■ so this aspect of Spake and colleagues’ work is to space. Like other giant planets, these bodies not surprising. But the authors have added a are thought to start out with atmospheres of Drake Deming is in the Department key twist to the story. Until now, only hydro- predominantly hydrogen, abundant helium of Astronomy, University of Maryland, gen (the main component of giant planets) and smaller amounts of elements heavier College Park, Maryland 20742, USA. and a few elements with low abundances6 than helium. As their atmospheres escape, e-mail: [email protected] have been identified in eroding exoplanetary hydrogen is lost fastest, leading to a gradual 1. Alpher, R. A., Bethe, H. & Gamow, G. Phys. Rev. 73, atmospheres. relative enrichment in the helium content of 803–804 (1948). Atoms in the gaseous tail of an exoplanet are the atmosphere. 2. Seager, S. & Sasselov, D. D. Astrophys. J. 537, 916–921 (2000). most easily detected when they absorb stellar Heavier elements such as carbon and oxygen 3. Moutou, C., Coustenis, A., Schneider, J., Queloz, D. & light during a transit — a passage of the planet would be slow to escape, and could in prin- Mayor, M. Astron. Astrophys. 405, 341–348 (2003). in front of its host star. However, atoms in such ciple be present in exoplanetary atmospheres 4. Spake, J. J. et al. 557, 68–70 (2018). 5. Vidal-Madjar, A. et al. Nature 422, 143–146 (2003). a tenuous tail have a tendency to relax to their in concentrated amounts. These heavier ele- 6. Ben-Jaffel, L. & Ballester, G. E. Astron. Astrophys. lowest-energy (ground) state. In this state, ments are key to understanding both how 553, A52 (2013). most atoms absorb mainly light, planets form and how they acquire their 7. Liske, J., Padovani, P. & Kissler-Patig, M. Proc. SPIE 8444, 84441I (2012). and measuring such absorption is difficult for atmospheres. For planetary astronomers, 8. Hu, R., Seager, S. & Yung, Y. L. Astrophys. J. 807, 8 two reasons. an escaping atmosphere that is rich in heavy (2015). First, Earth’s atmosphere is opaque to most ultraviolet light, which means that absorp- tion measurements must be made from space. PALAEONTOLOGY Currently, only Hubble has the capability for ultraviolet studies of exoplanetary atmos- pheres, and this telescope could reach the end of its mission lifetime in the next decade. Evolutionary insights Second, the pattern of how much ultraviolet stellar light is absorbed by transiting planets as a function of time or wavelength tends to from an ancient be complex. Such complexity makes it difficult to interpret ultraviolet measurements of a dispar is a key extinct bird from when were shedding transiting planet’s atmosphere. characteristics of their ancestors and evolving their current features. A Fortunately, helium atoms have a long-lived reconstructed of I. dispar now illuminates this transition. See Letter p.96 (metastable) state, in addition to the ground state. Metastable helium atoms absorb near- infrared stellar light, which has a wavelength received a copy of the monograph from Marsh, only slightly beyond the limits of human the letter that he wrote back to Marsh said: vision. Measurements at this wavelength are he distinctive features of birds, from “Your work on these old birds and on the many much easier to interpret than those at ultra- to , provide a stark of N. America has afforded the violet wavelengths. separation between avians and other best support to the theory of evolution, which Spake and colleagues observed a transit of Tanimal groups. But how did the features of has appeared within the last 20 ” (see WASP-107b, and measured the amount of the bird skull evolve? On page 96, Field et al.1 go.nature.com/2hhjxrd). near-infrared stellar light that was transmitted present a computerized reconstruction of the The specimens Marsh presented in through the planet’s eroding atmosphere as a skull of a pivotal early bird that brings avian were predominantly from function of wavelength. The authors identified evolution into sharper focus. two contrasting bird genera: , a narrow absorption feature that they associ- In the late 1800s, the palaeontologist Othniel which was flightless and essentially wingless, ated with metastable helium atoms (see Fig. 1 C. Marsh and his field crews made many of the standing 1.3–1.8 metres tall and comparable of the paper4). This signal is more than five first reported discoveries of ancient to today’s , and a -like bird called times greater than any false signal that could and from western , Ichthyornis, which had an average be produced by stellar activity. amassing a fossilized ‘bestiary’ that dwarfed of about 60 centimetres (ref. 3). However, Detecting helium in the escaping atmos- what was then known from Europe2. Marsh’s neither was closely related to living loons or pheres of other exoplanets will be difficult treasures were constantly in the headlines, per- . Both birds had many sharp, curved because the absorption signal is intrinsically haps never more so than when he published3 teeth, which were absent only from the front weak, especially for planets smaller than his monograph Odontornithes in 1880, which part of the upper jaw, and their beaks were WASP-107b. However, astronomers will reported several previously undescribed fossil covered by a horny sheath. Unfortunately, the eagerly rise to the challenge. The near-infrared birds of the mid- period (around excavated , being small, fragile and of an signature of metastable helium is readily trans- 80 million to 87 million years ago) from the elaborate architecture, were badly crushed, and mitted through Earth’s atmosphere, which shores of and nearby states. Familiar proved challenging to prepare. The restoration, means that eroding exoplanetary atmos- yet strange in many ways, these creatures were mounting and illustration of the specimens pheres could be probed using ground-based so archaic that they retained teeth and sub- were, shall we say, somewhat overenthusi- telescopes. The advent of a new generation of stantial bony , thus providing clues to the astic. The specimens could be convincingly extremely large telescopes at ground-based reptilian . When described only after the mounts had been

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disassembled and prepared afresh more than a century later1,4. a Fast forward to the twenty-first century, and in the past 20 years some of the most sen- sational dinosaur discoveries have been the seemingly endless reports of ‘feathered’ dino- saurs and newly identified early birds, mainly from Cretaceous deposits in . These specimens are closer to (the earliest known bird, from the Late of about 145 million years ago) than to Hesperornis and Ichthyornis5. These discover- ies have shown that the evolution of feathers, b from hair-like down to feathers, broadly paralleled the sequence of development of the features of a single in living birds6. Such insights suggest a plausible sequence for the evolution of and flight in birds, whereby newly hatched ancient dinosaurs flapped their incipient wings as a way of boosting their ability to scale steep inclines when evading Figure 1 | Skull of the bird Ichthyornis dispar. Field et al.1 report the reconstruction of the skull of predators7. an extinct species. Their reconstruction fills in some structures missing from previously available But many questions persist about the , thereby illuminating the transition between the loss of ancient dinosaur features and the anatomical changes in early bird evolution, and evolution of characteristics found in present-day birds. The sections in yellow are newly identified this is where the work of Field and colleagues fossil material, whereas the grey structures have been described previously. a, A side view of the skull. b, A view from above the skull ( positioned on the left) showing cross-sections in two focal planes. comes in. Present-day birds have that are The section above the black line is closer to the top of the skull than the region below the black line. different in many ways from those of all other Scale bar, 1 centimetre. (Adapted from Extended Data Fig. 2 of ref. 1.) animals, including the dinosaurs from which they evolved. Bird snouts are lightweight, usu- ally narrow and sometimes quite long. Indeed, tomography, in which a reconstruction of each invertebrates in the water column, whereas the bones of the bird snout are relatively light is compiled by taking extremely thin Ichthyornis seems to have been more a surface and fragile compared with those of other - cross-sectional images all the way through skimmer or perhaps a shallow plunger like a mals, and these structures are covered by a the bone, like slicing a salami sausage and tern or gull2–4. How did these different preda- strong beak made of the , which re­assembling it. This enables the internal tory approaches favour the same pattern of enables birds to access various , such as and outer shape to be visualized. The reduction, which also happened inde- or carcasses. Inside the beak is a complex images of all the separate bones are assembled, pendently in other early bird groups? How did of bones that corresponds to the human palate; and a computer program enables the bone the mobility of the bones of the palate against but unlike ours, the bird bones have mobile images to be manipulated, allowing analysis of adjacent skull bones in Ichthyornis compare connections to each other and to the surround- how the bones might have moved. with the ranges of motion in the palates of ing skull and jaw bones. This system of mobil- The resulting skull images (Fig. 1) show that dinosaurs and living birds, and what might ity is an elaboration of the basic dinosaurian the beak of Ichthyornis has some features that these evolutionary changes suggest about the one, and is key to accommodating the diverse place it between the earliest birds and living diet and mode of feeding of Ichthyornis? feeding habits of birds. birds: the beak was small, had not yet evolved a Whatever the answers to these questions Moreover, ‘bird brain’ is not the insult you bony shelf structure in the palate and was lim- turn out to be, Field and colleagues’ beauti- might think. Bird brains are larger relative to ited to the tip of the jaw. However, the probable fully rendered 3D scans and reconstructions their body size than is the case for , and mobility of the Ichthyornis skull seems to be of this iconic fossil avian, along with their the relative size of bird brains is comparable to more like that of living birds. The brain would comparisons of these structures with those of that of placental mammals. As birds evolved have been much like those of today’s birds, but earlier and later birds, provide an important from their dinosaur ancestors, the bones that the cheek region, bounded by bones of the resource to aid our understanding of early bird protect the brain enlarged to keep pace with and the side of the skull, has charac- evolution. ■ the changes in brain size. The bones of the skull teristics that are closer to those of dinosaurs, roof and cheek region are also comparatively such as the retention of a large bony chamber Kevin Padian is in the Department of larger than the equivalent structures in their for the adductor muscles that close the jaw. Integrative and the of dinosaur ancestors, whereas the adductor Therefore, several key features of the brain and , University of , muscles of the bird jaw are reduced. But in palate evolved before the jaw muscles became Berkeley, Berkeley, California 94720, USA. what did these features evolve, and how reduced and the familiar features of the beak e-mail: [email protected] did they shape avian evolution? of living birds evolved. 1. Field, D. J. et al. Nature 557, 96–100 (2018). Ichthyornis is closely related to living This study raises many questions that 2. Plate, R. The Dinosaur Hunters (McKay, 1964). birds, but retains many features of the remain to be answered. For example, were 3. Marsh, O. C. Odontornithes: A Monograph on the earliest birds. No Ichthyornis skull material had there functional changes that went along with Extinct Toothed Birds of North America (US Govt been uncovered since Marsh’s discoveries in the reducing the jaw muscles from the ancestral Printing Office, 1880). 4. Clarke, J. A. Bull. Am. Mus. Nat. Hist. 286, 1–179 1870s. But Field et al. describe four new three- dinosaurian condition? Did this change reflect (2004). dimensionally preserved specimens with skull a change in diet? And what ecological habits 5. Chiappe, L. M. & Meng, Q. Birds of Stone: Chinese remains, and they image them in 3D, along are correlated with the loss of teeth from the Avian Fossils from the Age of Dinosaurs (Johns Hopkins Univ. Press, 2016). with some overlooked skull bones from Marsh’s front part of the upper jaw and the evolution 6. Prum, R. O. & Brush, A. H. Sci. Am. 288, 84–93 original specimens. The authors used a stand- of the horny beak that covers it? Hesperornis (2003). ard technique called high-resolution computed was probably a diver that hunted and 7. Dial, K. P. Science 299, 402–404 (2003).

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