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Argentine fossil rewrites evolutionary history of a baffling dinosaur clade

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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.

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Fig. 1: Anatomy of A. cerropoliciensis based on the new specimen MPCA Pv 377.
Fig. 2: Phylogenetic relationships, inferred body size evolution and biogeographical history of alvarezsauroids.
Fig. 3: Long bone histology of A. cerropoliciensis MPCA-Pv 477 and MPCA-Pv 377.

Data availability

The phylogenetic data matrix and data files used in the macroevolutionary and biogeographical analyses are available at Figshare57 (https://doi.org/10.6084/m9.figshare.30569894).

Code availability

Analytical code written for this study relied on published tools available as packages for the freely available statistical computing language R50. Scripts and files used for analyses in R are available at Figshare57 (https://doi.org/10.6084/m9.figshare.30569894).

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Acknowledgements

We thank J. Kaluza and J. Pazo for preparation of the Alnashetri specimens; G. Rougier, A. Shinya, P. Gallina and many others for assisting with fieldwork over the years; the members of the Avelas and Mariluan families for site access; the staff at the Agencia Cultura de Río Negro and R. Juarez-Valieri for assistance with permits issued to S.A.; and the entire Cerro Policía community, especially the Pincheira family, for years of hospitality. J. Gonzalez and V. Radermacher provided the silhouettes in Figs. 1 and 2, respectively. Funding for this research was provided by CONICET (to J.G.M., F.A.G., I.C. and S.A.), Field Museum (to P.J.M.), National Geographic (to S.A.), University of Minnesota (to P.J.M.), US National Science Foundation (to P.J.M.) and the Fulbright US Scholar program (to P.J.M.).

Author information

Authors and Affiliations

  1. Department of Earth and Environmental Sciences, University of Minnesota, Minneapolis, MN, USA

    Peter J. Makovicky

  2. The Field Museum, Chicago, IL, USA

    Peter J. Makovicky

  3. Department of Anatomical Sciences, Health Sciences Center, Stony Brook University, Stony Brook, NY, USA

    Peter J. Makovicky

  4. Biology Department, Coe College, Cedar Rapids, IA, USA

    Jonathan S. Mitchell

  5. Instituto de Investigación en Paleobiología y Geología, Universidad Nacional de Río Negro, General Roca, Argentina

    Jorge G. Meso & Ignacio Cerda

  6. Instituto Multidisciplinario de Investigaciones Biológicas de San Luis, Universidad Nacional de San Luis, San Luis, Argentina

    Federico A. Gianechini

  7. Museo Provincial Carlos Ameghino, Cipolletti, Argentina

    Ignacio Cerda

  8. Área de Paleontología, Fundación Azara, Universidad Maimónides, Buenos Aires, Argentina

    Sebastian Apesteguía

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  1. Peter J. Makovicky
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  2. Jonathan S. Mitchell
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  3. Jorge G. Meso
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  4. Federico A. Gianechini
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  6. Sebastian Apesteguía
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Contributions

P.J.M. funded fieldwork, collected data, performed analyses, designed figures, wrote the draft and revised the manuscript. J.S.M. conducted analyses and drafted figures. J.G.M. collected data, drafted figures and edited the draft and revised manuscripts. I.C. performed histological laboratory work, collected data, drafted figures and wrote relevant sections of the manuscript. F.A.G. edited the draft and revised manuscripts. S.A. organized and funded fieldwork and collection of the specimen, supervised fossil preparation, and edited the draft and revised manuscripts. All of the authors approved the final version of the manuscript.

Corresponding author

Correspondence to Peter J. Makovicky.

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The authors declare no competing interests.

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Nature thanks Luis Chiappe, Martin Ezcurra and Xing Xu for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Geographic, stratigraphic, and taphonomic information for specimens of Alnashetri cerropoliciensis.

a, Approximate location of Neuquén Basin (star) in Argentina (red), b, Google Earth capture of the Embalse Ezequiel Ramos Mexía reservoir on the Limay River and the Meseta de Rentería to its east. The stars represent the two sites that produced skeletons of Alnashetri cerropoliciensis. MPCA Pv 377 was found at the more southerly locality of Cerro Policía, named for the characteristic hill shown in c. c, Cerro Policía hill exhibiting aeolian Candeleros Fm. rocks at its base conformably overlain by Huincul Formation fluvial deposits. A thick ash at the base of the Huincul Fm. has been dated to 88 ± 3.9 MYA using fission track counts58. d, Stratigraphic succession of formations within the Upper Cretaceous Neuquén Group and their alvarezsauroid diversity, modified from refs. 7,45. e, Overview of the preserved parts of MPCA Pv 377 prior to disarticulation during preparation. The skull, neck, and tail were staged for photography and do not reflect their fossilized position. Other parts of the skeleton remain articulated in the state they were collected. Map of Argentina in a created with https://www.mapchart.net/, CC BY-SA 4.0, https://creativecommons.org/licenses/by-sa/4.0/. Satellite image in c downloaded from Google Earth© and available at https://earth.google.com/web/@-39.54689334,-68.68837868,-69656.55518752a,160604.11129246d,35y,1.53955224h,1.73974003t,1.5548r/data=CgRCAggBOgMKATBCAggASg0I____________ARAA?utm_source=earth7&utm_campaign=vine&hl=en;. In panel d: stratigraphic column adapted from ref. 59, CC BY 3.0, https://creativecommons.org/licenses/by/3.0/; silhouette of Alnashetri reproduced with permission from Jorge Antonio Gonzalez; larger silhouettes by J.G.M.

Extended Data Fig. 2 Comparison of hindlimb anatomical traits between specimens of Alnashetri cerropoliciensis and other Argentinean alvarezsaurids.

Proximal end of left femur in lateral view for MPCA Pv 377 (a) and MPCA Pv 477 (b); Right (c) and left (d) ankles of MPCA Pv 477 in anterior view; e) distal left tibia of MPCA Pv 377 in anterior view; f) right ankle of Patagonykus puertai in anterior view; g) ankle of Alvarezsaurus calvoi in anterior view; Right feet of MPCA Pv 477 in (h) posterior and (i) anterior views; Right feet of MPCA Pv 377 (j) and Alvarezsaurus calvoi (k) in posterior views. Abbreviations: clb, craniolateral buttress; lt, lesser trochanter; mif, insertion for M. Ischiofemoralis; mt, metatarsal; ts, trochanteric shelf. Scale bars, all bars = 1 cm, no scale bars shown for f-g, k. All photos ©P. Makovicky.

Extended Data Fig. 3 Rostrum and braincase of Alnashetri cerropoliciensis MPCA Pv 377.

Rostrum in a) right lateral, and b) left lateral views. Braincase of Alnashetri cerropoliciensis MPCA Pv 377 in c, left oblique dorsolateral view, and d, dorsal views. Abbreviations: atr, anterior tympanic recess; CN V, lower rim of trigeminal nerve exit; CN VII, exit of facial nerve; de, dentary; flr, floccular recess; fm, foramen magnum; hf, hypophysial fossa; la, lacrimal; mr, medial ridge on internal aspect of braincase; mxf, maxillary fenestra; na, nasal; pal, palatine; pmx, premaxilla; scx, anterior semicircular canal cross section. Scale bar 1 cm in a, b, 5 mm in c, d. Photos ©P. Makovicky.

Extended Data Fig. 4 Comparisons of axial and forelimb skeletal features of Alnashetri cerropoliciensis MPCA Pv 377 to other alvarezsaurids.

a, cervical 7 of Alnashetri cerropoliciensis in left lateral view; b, sacrum of Alnashetri cerropoliciensis in ventral view; c, left sternal plate and anterior gastralia of Alnashetri cerropoliciensis in dorsal view; d, posterior cervical of Xiyunykus pengi (IVPP 22783) in left lateral view; e, sacrum of Patagonykus puertai holotype in ventral view; f, forelimb and boat shaped sternum of the Mononykus olecranus holotype in oblique left lateral view; g, articulated right pectoral girdle and humerus of Alnashetri cerropoliciensis MPCA Pv 377 in lateral view. h, proximal half of left humerus of Alnashetri cerropoliciensis MPCA Pv 377 in lateral view. i, cast left humerus of Mononykus olecranus holotype in medial view. j-k, right and left partial humeri of Bannykus wulatensis in medial and anterior views respectively; l, articulated manus of Alnashetri cerropoliciensis MPCA Pv 377 in palmar view; m, Right preacetabular blade in lateral view and proximal part of ischium (inverted); n, acetabulum and postacetabular blade in lateral view; o, left and right pubic shafts in oblique anterior view. Abbreviations: I-2, phalanx 2 of digit I; cf, cuppedicus fossa; ct, coracoid tubercle; dpc, deltopectoral crest; ect, ectepicondylar tubercle; ft, flexor tubercle; hu, humerus; is, ischial shaft; it, internal tuberosity; lpb, left pubic boot; mc I/III, metacarpal I/III; prz, prezygapophysis; rpb; right pubic boot; s5, sacral 5; sar, supraacetabular ridge;; slc, semilunate distal carpal; spl, scapholunare; st, sternum; vs, ventral sulcus. Elements not to scale. All photos ©P. Makovicky.

Extended Data Fig. 5 Cervical vertebrae of the Morrison Formation alvarezsauroid (a-f).

More anterior cervical vertebra YPM [Yale Peabody Museum, New Haven, U.S.A] 1997 in a, left lateral, b, ventral, c, dorsal views. More posterior cervical vertebra YPM 1996 in d, left lateral, e, ventral, and f, posterior views. Abbreviations: acd, anterior central depression; cpf, caudal peduncular foramen; ep, epipophysis; nc, neural canal; ns, neural spine; pnf, pneumatic foramen; pp, parapophysis; ppf, parapophysial fossa (autapomorphy); prz, prezygapophysis; poz, postzygapophysis; vs, ventral sulcus. Scale bar applies to all panels. All photos ©P. Makovicky.

Extended Data Fig. 6 Cervical vertebrae comprising the holotype of Calamosaurus foxi (NHM UK [Natural History Museum UK, London, U.K.] R901).

More anterior cervical vertebra in a, lateral, b, dorsal, c, ventral, and f, posterior views. Centrum of more caudally positioned cervical vertebra in d, lateral and e, ventral views. Abbreviations: dp, diapophysis; ns, neural spine; pnf, pneumatic foramen; pp, parapophysis; prz, prezygapophysis; pz,postzygapophysis; vs, ventral sulcus. Scale bar applies to all panels. All photos ©P. Makovicky.

Extended Data Fig. 7 Additional phylogenetic results.

a, reduced consensus of 100000 most-parsimonious trees from an equal-weights parsimony analysis in TNT with a tree length of 3752 steps showing coelurosaurian relationships recovered from analysis of our cladistic dataset. Positions of wildcard taxa indicated with stippled lines. Major clades outside of Alvarezsauroidea are collapsed. Support values on branches are marked as Bremer support below internodes, bootstrap percentage above, and were all calculated following exclusion of wildcard taxa from trees. Lower case lettering indicates alternate positions of wildcard taxa: n = Calamosaurus, m = Morrison taxon, o= Kol; b, reduced consensus of 100000 most-parsimonious trees under IW (K = 15) recovered when Alvarezsaurus and Achillesaurus and Ceratonykus and Parvicursor are combined into two Operational Taxonomic Units, following prior7 literature suggestions. Only alvarezsauroid relationships are shown. EW results are identical when pruned to the same set of taxa.

Extended Data Fig. 8 Results of biogeographic analyses of alvarezsauroid distributions.

S-DEC (a-b) and S-DIVA (c) results from analysis of alvarezsauroid biogeography in RASP 4.2 (ref. 43). ‘V’ marks nodes where vicariance is reconstructed as part of the most optimal history; ‘E’ marks nodes where local extinction is reconstructed; a, S-DEC results with no time constraints enforced; b, shows results for time constrained analysis with dispersal between South America and Laurasian continents prevented following the Albian; c, S-DIVA results; Letters at nodes represent most likely ancestral area reconstructions; d, results of stratified BioGeoBEARS on the MPT with the highest GER score and lowest MIG. Area abbreviations for S-DEC and S-DIVA: A, Asia; B, North America; C, South America; D, Europe; abbreviations for BioGeoBEARS result A, Asia; B, South America; C, North America; D, Europe.

Extended Data Fig. 9 Results of macroevolutionary analyses of alvarezsauroid body mass evolution.

a, Bar plot showing relative support for how well each of 4 evolutionary models fits body mass evolution on each of the time scaled 41200 time calibrated trees. The x-axis is each of the 41200 time-scaled most parsimonious trees arranged by their increasing support for the dominant WN model. The y-axis indicates the Akaike weights for each of the 4 models for each tree; b, time calibrated body-mass phylomorphospace showing body size evolution in Alvarezsauroidea across all 41200 timescaled trees. Despite the observed uncertainty, a general pattern of initial size increase among early diverging taxa followed by independent size reduction in Alnashetri, and a broader expansion into BM phylomorphospace by alvarezsaurids is evident. Among the latter, only some parvicursorines exhibit a strong degree of reduction, but some non-parvicursorines and parvicursorines independently evolve large sizes suggesting there is no clear trend of body mass evolution in the clade.

Extended Data Fig. 10 Bone histology of Alnashetri cerropoliciensis MPCA Pv 477 (a-g) and MPCA Pv 377 (h-m).

Lines of arrested growth (LAGs) shown with white arrowheads. Numerals in arrowheads indicate LAG order from inner to outer cortex, black arrowheads mark resorption lines. a, b, femoral cortical bone details showing highly vascularized primary bone tissue and variation in intrinsic fibre orientation (prevalence of parallel- fibred bone in (a), presence of woven-fibred bone in (b)); c, overview and detail (inset) of femoral cortex at the lateral region showing high density of osteocyte lacunae. d-g, tibial cortex showing lower density of vascular canals in comparison with the femur, possible compacted coarse cancellous bone is observed in the inner cortex at the lateral region (f). h, i, overview (h) and detail (i) of the cortex. Note the higher density of vascular spaces in the primary bone preserved in the inner cortex. The perimedullary region exhibits remains of endosteal woven-fibred bone, an inner circumferential layer, secondary osteons and remains of compacted coarse cancellous bone tissue. j, overview of the cortex showing variable orientation of the vascular canals; k, detailed view of the inner cortex showing secondary osteons, remains of compacted cancellous bone, an inner circumferential layer and remains of endosteal woven-fibred bone. l, m, overview (l) and detail (m) of the endosteal woven fibred bone formed around the medullary cavity. a, b, d-f, h, k, m, cross polarized light with lambda compensator. c, i, j, l, plane polarized light. Abbreviations: cccb: coarse compacted cancellous bone tissue; cv: circumferentially oriented vascular canal; ewb: endosteal woven fibred bone; icl: inner circumferential layer; lb: lamellar bone tissue; lv: longitudinally oriented vascular canal; mc: medullary cavity; ov: obliquely oriented vascular canal; rv: radially oriented vascular canal; so: secondary osteon; vs: vascular space; wfb: woven fibred bone. All micrographs © I. Cerda.

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Makovicky, P.J., Mitchell, J.S., Meso, J.G. et al. Argentine fossil rewrites evolutionary history of a baffling dinosaur clade. Nature (2026). https://doi.org/10.1038/s41586-026-10194-3

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