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| Alnashetri cerropoliciensis Makovicky, Apesteguía & Gianechini, 2012 in Makovicky, Mitchell, Meso, Gianechini, Cerda et Apesteguía, 2026. |
Abstract
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 America. Late Cretaceous alvarezsauroids possess specialized forelimbs adapted for digging, minute supernumerary teeth and heightened sensory capacities, and are interpreted as myrmecophagous. They are hypothesized to exhibit evolutionary miniaturization coupled to their dietary specialization. Fragmentary South American taxa are traditionally arrayed as a paraphyletic grade with respect to the Late Cretaceous Asian subclade Parvicursorinae, invoking dispersal to explain their disjunct distributions. Here we describe a skeleton of the alvarezsauroid Alnashetri cerropoliciensis 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.
Peter J. Makovicky, Jonathan S. Mitchell, Jorge G. Meso, Federico A. Gianechini, Ignacio Cerda and Sebastian Apesteguía. 2026. Argentine Fossil rewrites Evolutionary History of a baffling Dinosaur Clade. Nature. DOI: doi.org/10.1038/s41586-026-10194-3 [25 February 2026]
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