Growth and miniaturization among alvarezsauroid dinosaurs

Sustained miniaturization, here defined as a drop in body size of at least two orders of magnitude from ancestors to descendants, is a widespread and important phenomenon in animals,1–3 but among dinosaurs, miniaturization occurred only rarely, once in the lineage leading to birds and once in the Alvarezsauroidea,1,3–5 one of the most bizarre theropod groups.1,5–7 Miniaturization and powered flight are intimately linked in avialan theropods,3,5,6,8–11 but the causes and patterns of body size reduction are less clear in the non-volant Alvarezsauroidea.1,5,6,12,13 Here, we present results from analyses on a comprehensive dataset, which not only includes new data from early-branching alvarezsauroids but also considers the ontogenetic effect based on histological data. Our analyses show that alvarezsauroid body mass underwent rapid miniaturization from around 110 to 85 mya and that there was a phylogenetic radiation of small-sized alvarezsauroids in the Late Cretaceous. Our analyses also indicate that growth strategies were highly variable among alvarezsauroids, with significant differences among extremely small taxa. The suggested alvarezsauroid miniaturization and associated phylogenetic radiation are coincident with the emergence of ants and termites, and combining previous functional morphological data, our study suggests that alvarezsauroid miniaturization might have been driven by ecological changes during the Cretaceous Terrestrial Revolution, more specifically by a shift to the myrmecophagous ecological niche. [Display omitted] •How and why the bizarre alvarezsauroid dinosaurs miniaturized is poorly known•We reveal a rapid Late Cretaceous alvarezsauroid miniaturization and radiation•Osteohistological data show highly variable growth strategies among alvarezsauroids•Alvarezsauroid miniaturization is possibly associated with adaptations to digging Qin et al. report their broadly sampled osteohistological analyses and quantitative body size macroevolution reconstruction for a bizarre theropod dinosaur group, the Alvarezsauroidea. Highly variable growth strategies and quick miniaturization are found in alvarezsauroids, possibly associated with a dietary switch to feeding on social insects.