Abstract: Some of the smallest examples of dinosaurian body size are from alvarezsaurians, an enigmatic group of maniraptoran coelurosaurians with a peculiar combination of anatomical features unique among theropods. Despite the large number of alvarezsaurian species described worldwide and the increased understanding this has provided, the body-size macroevolution of alvarezsaurians has received little attention. Here we reconstruct and analyse directional trends of alvarezsaurian body-size evolution through an integrated analysis of body mass, ontogenetic age, and morphological rate data enabled by a comprehensively revised phylogeny. Our analyses identify four periods of high morphological rate evolution (Bathonian-Callovian, Hauterivian-early Berriasian, early Cenomanian, and late Cenomanian-Turonian) that we link to the key effects of animal body-size changes for the first time, including morphological novelty, structural reduction and simplification, elevated homoplasy, and behavioral changes associated with miniaturization. In doing so, this study provides a holistic example of miniaturization in a Mesozoic vertebrate group that offers a framework for other detailed studies of animal body-size evolution, including in more disparate groups.

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