Miniaturization and Phylogenetic Radiation in Alvarezsauroids: Insights From Histological Analysis

Miniaturization of body size is a widespread and important phenomenon in animals, but among dinosaurs, miniaturization occurred only rarely, once in the lineage leading to birds and once in the Alvarezsauroidea, one of the most bizarre theropod groups. Miniaturization and powered flight are intimately linked in avialan theropods, but the causes and patterns of body size reduction are less clear in the non-volant Alvarezsauroidea. 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 million years ago, 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 the ecological changes during the Cretaceous Terrestrial Revolution, more specifically, by the shifts to myrmecophagous ecological niche.