2026-02-25 ミネソタ大学
Alnashetri was rapidly covered by an advancing sand dune that preserved it almost intact for 90 million years. Photo provided by Peter Makovicky, University of Minnesota
＜関連情報＞
- https://cse.umn.edu/college/news/tiny-dinosaur-big-impact-90-million-year-old-fossil-rewrites-history
- https://www.nature.com/articles/s41586-026-10194-3
アルゼンチンの化石が不可解な恐竜系統の進化史を書き換える Argentine fossil rewrites evolutionary history of a baffling dinosaur clade
Peter J. Makovicky,Jonathan S. Mitchell,Jorge G. Meso,Federico A. Gianechini,Ignacio Cerda & Sebastian Apesteguía
Nature Published:25 February 2026
DOI:https://doi.org/10.1038/s41586-026-10194-3
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 America1,2,3. 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.
