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Poultry Through Time is in a metastable state. The simulations also 3. Brandon, D. G. & Wald, M. Phil. Mag. 6, 1035–1044 Phys. Rev. Lett. 110, 255502 (2013). showed that the metastable domino phase (1961). 9. Kaur, I., Mishin, Y. & Gust, W. Fundamentals of Grain and 4. Sutton, A. P. & Balluffi, R. W. Interfaces in Crystalline Interphase Boundary Diffusion 3rd edn (Wiley, 1995). is stabilized when stress is applied perpen- Materials (Oxford Univ. Press, 1995). 10. Rittner, J. D. & Seidman, D. N. Phys. Rev. B 54, 6999–7015 dicularly to the plane of the simulated grain 5. Rabkin, E. I., Semenov, V. N., Shvindlerman, L. S. & (1996). boundary, so that its energy matches that of Straumal, B. B. Acta Metall. Mater. 39, 627–639 (1991). 11. Han, J., Vitek, V. & Srolovitz, D. J. Acta Mater. 104, 259–273 the stable pearl phase — thereby establishing 6. Cantwell, P. R. et al. Acta Mater. 62, 1–48 (2014). (2016). 7. Maksimova, E. L., Shvindlerman, L. S. & Straumal, B. B. 12. Watanabe, T. & Tsurekawa, S. Acta Mater. 47, 4171–4185 a true thermodynamic equilibrium between Acta Metall. 36, 1573–1583 (1988). (1999). the two phases. 8. Frolov, T., Divinski, S. V., Asta, M. & Mishin, Y. 13. Homer, E. R. Comp. Mater. Sci. 161, 244–254 (2019). Meiners and colleagues’ work clearly proves that phase transformations occur in the grain Palaeontology boundaries of pure metals, and thus opens up fresh opportunities for materials design. The number of possible polymorphs of bulk metals is generally limited, but the variety of Poultry through time grain-boundary structures and their poss- ible metastable polymorphs (sometimes Kevin Padian referred to as complexions6) is essentially boundless10,11. One can therefore envisage A newly discovered 66.7-million-year-old fossil bird excavated a processing technique that optimizes the in Belgium provides us with the best evidence so far for overall performance of a material by produc- understanding when the living groups of birds first evolved ing grain-boundary phases (either stable or and began to diverge. See p.397 meta stable) that maximize the positive effects of boundaries, but minimize their negative effects. For example, if one could produce grain-boundary polymorphs in aluminium The living groups of birds are amazingly and relatives of the other living bird groups). that efficiently block dislocation glide (to diverse, numbering some 10,000 species, By contrast, the stem-group birds are those maximize mechanical strength) and minimize and endlessly fascinating. But when did they placed outside the living groups of birds but electron scattering (minimizing electrical first evolve? The answer depends on how nevertheless still closer to them than they are resistivity), the resulting metal would be a you define a bird, how you recognize the to other major related groups, such as extinct ‘dream material’ for making wire conductors most basal (earliest diverging) birds in the dinosaurs: in other words, birds from Archaeo­ in overhead power lines — eliminating the fossil record, and how you account for the pteryx (the most basal known bird) up to, but need for more-expensive aluminium-based palaeontological and genetic gaps in our not including, living bird groups. The question composite wires. knowledge of bird evolution. On page 397, is whether Field et al. are reporting on another However, it remains to be seen whether the Field et al.1 report the discovery of a fossil stem-group bird or the first well-established full potential of engineering phase transfor- bird from 66.7 million years (Myr) ago, during crown-group bird, and what the age of their mations at grain boundaries can be realized the Late Cretaceous period, the most recent fossil discovery tells us about the timing of in practice. One reason is that it is not clear point in time currently known when the first avian evolution. how processing methods could be designed representatives of today’s birds evolved. In All available evidence indicates that birds that produce desired grain-boundary phases. the process, the authors offer what might be evolved from a group of carnivorous dinosaurs Moreover, a similar concept known as a useful corrective to genetics-based estimates called theropods during the Jurassic period grain-boundary engineering12 — the use of pro- of the timing of bird diversification. (about 200–145 Myr ago), and that bird flight cessing methods to obtain grain boundaries Most narratives of bird evolution begin with had evolved by then, at least considering that have a desired geometry and properties, the pioneer Archaeopteryx, which first took Archaeopteryx2. Through the Cretaceous without using phase transformations — has wing in the Late Jurassic period (some 150 Myr period (145–66.5 Myr ago) there was consid- so far yielded only modest practical results. ago) in present-day Germany2. Archaeopteryx erable evolutionary experimentation in the Another issue is that the large number of is a bird in the broad sense of the term — it had early offshoots of bird lineages (in such diverse possible grain-boundary polymorphs will a full complement of feathers and flew by flap- groups as the Enantiornithes, Hesperornithes make it difficult to systematically determine ping its wings — but it’s far from having the and Ichthyornithes)4. But these ancient birds polymorph properties. High-throughput hallmarks needed to group it with members are outside the crown group because they lack computational methods based on machine of any living birds. Archaeopteryx has features the structural and physiological features char- learning and big data will be of help here1,13. that are so unspecialized that they don’t pre- acteristic of living birds. These stem-group Indeed, Meiners and colleagues’ work is a vent it from being the ancestor of all later bird birds seem to have grown much like small promising example of how the synergistic groups, but they don’t tell us whether living dinosaurs had done ever since the Triassic combination of high-resolution microscopy birds arose from this exact lineage. period (about 250–200 Myr ago) — faster than techniques and computational methods can When considering the relationships typical reptiles but slower than today’s birds, lead to conceptual breakthroughs in the study between any large group of organisms, such reaching maturity in a few years, on the basis of grain boundaries. as birds, a key split on the evolutionary tree of examination of their bone tissues5. (Fig. 1) is the distinction between what are However, sometime during the latest Creta- Eugen Rabkin is in the Department of Materials called crown-group and stem-group mem- ceous, a stem-group lineage of birds evolved Science and Engineering, Technion – Israel bers3. For birds, the crown group includes that had much higher growth rates than Institute of Technology, 3200003 Haifa, Israel. all living birds (from ostriches to warblers, these more basal lineages, and that generally e-mail: [email protected] including quail, gulls, finches, woodpeckers, matured within a year or even sooner2,5. These crows and all their relations), plus all descend- became the crown-group birds. Their relation- 1. Meiners, T., Frolov, T., Rudd, R. E., Dehm, G. & Liebscher, C. H. Nature 579, 375–378 (2020). ants of their most recent common ancestor ship to the closely related stem-group birds 2. Rosenhain, W. & Ewen, D. J. Inst. Metals 10, 119–148 (1913). (that is, all the ancient ostriches, warblers remains fuzzy, partly because fossil birds are Nature | Vol 579 | 19 March 2020 | 351 ©2020 Spri nger Nature Li mited. All rights reserved. ©2020 Spri nger Nature Li mited. All rights reserved. News & views assume that the rate of molecular evolution Other living birds is relatively constant over time. The very Crown-group effort that such work takes to resolve these birds Asteriornis maastrichtensis? evolutionary divergences with such indirect Anseriformes molecular evidence is heroic. Why should palaeontological evidence be Asteriornis Stem-group maastrichtensis? ‘smarter’ than molecular evidence in cases birds such as this? Fossil remains are a crucial test Galliformes of molecular-based projections of evolution- Birds ary divergence times. If a molecular study Ratites estimates a given age of origin for a group, then fossils from the corresponding time- frame should have the diagnostic features Ichthyornithes that identify those groups. If fossils from the time of interest provide no evidence of the Hesperornithes expected newly evolved features, then the Enantiornithes molecular projections are not supported. The evidence for Asteriornis reported by Field Archaeopteryx and colleagues implies that crown-group birds first evolved when the Cretaceous period was Figure 1 | An evolutionary tree for birds. Birds can be divided into crown-group birds (all living birds plus nearly over. That places a strong constraint on all relatives of their most recent common ancestor) and stem-group birds, which fall outside this group but hypotheses for basal divergence times, but are closer to it than they are to other major related groups, such as the dinosaurs ancestral to birds. Fossils of there will always be more fossils to find. stem-group birds include specimens of Archaeopteryx, Enantiornithes, Hesperornithes and Ichthyornithes. 1 Such stem-group creatures had wings, but lacked some hallmarks of crown-group birds. Field et al. report Kevin Padian is in the Department of the discovery of a 66.7-million-year-old crown-group fossil bird that they call Asteriornis maastrichtensis. Integrative Biology and the Museum of This fits on the tree near Anseriformes (duck- and goose-like birds) and Galliformes (chicken- or quail-like Paleontology, University of California, birds), but the fossil remains are insufficient to determine whether it is closer to the Galliformes than to Berkeley, California 94720, USA. the Anseriformes, or whether it is outside the group formed by Galliformes and Anseriformes. Regardless of this, the fossil reveals that the duck and chicken lineages, together with the mostly flightless birds called e-mail: [email protected] ratites (such as ostriches) plus other living bird lineages, had evolved by at least 66.7 million years ago.
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