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Alvarezsauroids are an enigmatic clade of predominantly small-bodied theropod dinosaurs that are known mainly from the Jurassic to Cretaceous periods of Asia and South America1–3. Late Cretaceous alvarezsauroids possess specialized forelimbs adapted for digging4,5, minute supernumerary teeth and heightened sensory capacities6, and are interpreted as myrmecophagous. They are hypothesized to exhibit evolutionary miniaturization coupled to their dietary specialization2. Fragmentary South American taxa are traditionally arrayed as a paraphyletic grade with respect to the Late Cretaceous Asian subclade Parvicursorinae2,3, invoking dispersal to explain their disjunct distributions. Here we describe a skeleton of the alvarezsauroid Alnashetri cerropoliciensis7 representing to our knowledge the most complete and smallest South American taxon to date. We also recognize two alvarezsauroids among historic taxa from the Northern Hemisphere. Phylogenetic analysis recovers Alnashetri among basal non-alvarezsaurids rendering South American taxa polyphyletic. Combined with the new taxa recognized here, our biogeographical analyses infer a Pangaean ancestral distribution for Alvarezsauroidea, with vicariance dominating the early history of the clade. The early branching position of Alnashetri among larger-bodied relatives revises best-fit models of body size evolution in alvarezsauroids—we find no support for evolutionary miniaturization but, rather, find support for repeated evolution within a narrow body size range. 

Abstract Feathers are complex structures exhibiting structural/functional disparity across species and plumage. Flight was lost in >30 extant lineages from ~79.58 Ma–15 Ka. Effects of flight loss on senses, neuroanatomy, and skeletomusculature are known. To study how flightlessness affects feathers, we measured 11 feather metrics across the plumage of 30 flightless taxa and their phylogenetically closest volant taxa, with broader sampling of primaries across all orders of crown birds. Our sample includes 27 independent flight losses, representing nearly half of extant flightless species. Feather asymmetry measured by barb angle differences between trailing and leading vanes decreases in flightless lineages, most prominently in flight feathers and weakest in contour feathers. Greatest changes in feather anatomy occur in older flightless lineages (penguins, ratites). Comparative methods show that many microscopic feather traits are not dramatically modified after flightlessness compared to body mass increase and relative wing and tail fan reduction. Changes involved with greater vane symmetry show stronger shifts, however. Relaxing selection for flight does not rapidly modify feather flight adaptations, apart from asymmetry. Developmental constraints and relaxed selection for novel feather morphologies may explain some observed changes. Macroscopic changes to flight apparati (skeletomusculature, airfoil size) are more evident in recently flightless taxa and could more reliably detect flightlessness in fossils, with increased feather symmetry as a potential microscopic signal. We observed apical modification in later stages of feather development (asymmetric displacement of barb loci), while morphologies arising during early developmental stages are only altered after millions of years of flightlessness.