CAREER: Investigating the deep origin and evolution of the bird beak by synthesizing the fossil record and comparative embryology of archosaurian reptiles

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Our planet teems with life in a vast array of shapes and sizes, interconnected in myriad ways by ecology and by descent. One way to understand how and why Earth’s biodiversity came to be is to trace the history of a great evolutionary radiation deteremine the key events that contributed to its success. Among land vertebrates, birds stand out in terms of species number, engineering, and geographic spread. This project will explore both the history and the embryonic development of one of the signal innovations that underpins bird diversity: the beak. In doing so, the project will discover the way in which the beak was built during evolution and the way in which it is assembled anew during the early existence of each individual bird. The avian beak is a “key innovation,” a sort of technological advance that enabled subsequent success. It is a remarkably effective and versatile biological tool whose complex and efficient architecture has been modified for taking in food as disparate as nectar (hummingbirds) and flesh (birds of prey). The diversity of birds is, in many ways, a diversity of beaks. The beak was also integral to the evolution of bird flight: the first beaks, in the dinosaurian ancestors of birds, seem to have acted as “surrogate hands” to replace the fingers fused and incorporated into the wing. This project will use fossils that document 300 million years of reptile evolution to determine the steps that led to the evolution of the bird beak. They will, in particular, seek to discover how pre-existing anatomical structures, especially the predatory jaws of ancient archosaurs were co-opted and rearranged to make the beak, and will search for signatures of the classic evolutionary (and engineering) phenomena of tradeoffs and co-dependencies (integration and modularity). The project also investigates how the embryonic beak is formed in modern birds and how its development differs from that of the reptilian snout — in particular, whether comparisons of modern embryos with fossils and fossilized embryos can tell us about which parts of the bird beak form by late “tweaking” of, or addition to, a fundamentally reptilian embryonic anatomy and which are truly “new.” The grand narrative of the way in which birds, living dinosaurs, came to be so diverse and abundant in the modern world is naturally compelling, and the data to be generated are often striking and beautiful. The project will incorporate the findings of their work, including digital images and 3D prints of these data, into the courses they teach at Yale and into kits and curricula designed for K-12 classrooms, with a focus on low-income urban schools. Beyond exchange of materials and knowledge, the project's investigators will visit classrooms and will, moreover, conduct on-campus tours of research and collections facilities for K-12 students, many from underrepresented groups in STEM, and their teachers, with the intention of building long-term relationships with schools, teachers and students. The project also will lead to the design of exhibits featuring the discoveries from this work in part of the newly renovated Yale Peabody Museum. Finally, the project team will conduct a series of online and in-person public lectures and question-and-answer sessions throughout the duration of the project. The avian beak is a marvel of biological engineering and a key innovation that enabled the radiation of birds. This project will examine the morphological assembly of the beak from stem reptiles through to crown-clade birds and, simultaneously, the embryonic assembly of the beak as compared to that of the reptilian snout. In particular, the project will test the hypothesis of co-option followed by further novelty and constraint release in the origin of the avian head from the ancestral archosaurian head, then the origin of the neognath jaw apparatus from the ancestral avian kinetic system. This will be accomplished using a combination of 3D CT imaging of fossils and extant taxa, and reconstruction of soft tissue anatomy using phylogenetic bracketing. Analyses of shape and transformation will be three-dimensional and quantitative. The project will further test hypotheses of integration and modularity of skeletal elements surrounding major soft tissue structures using a variety of methods, and in particular will elucidate constraint and tradeoffs resulting from spatial packing or competition for space. This will be accomplished by quantitatively analyzing evolutionary trajectories and developmental trajectories (in part reconstructed using high-fidelity confocal imaging of entire immunostained embryos). Finally, the project will seek to unravel the relative contributions of the distinct ontogenetic or developmental processes of terminal addition and early transformation in the evolutionary origin of the avian skull, using an integrative approach that incorporates embryos, fossils, and fossil embryos. In particular, the project will include both evolutionary sequences of reconstructed ancestors (from fossil data) and ontogenetic sequences (fossil and extant) in combined analyses and will quantitatively assess the correspondence or non-correspondence of these sequences. Ultimately, the project will present a comprehensive picture of the sequence of evolution and development leading to the distinctive avian condition, and will gain insight into the transformational phenomena and evolutionary dynamics in operation during its genesis.This project was jointly funded with the Sedimentary Geology and Paleobiology Program in the Division of Earth Sciences.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.