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Bizarre tail weaponry in a transitional ankylosaur from subantarctic Chile

Abstract

Armoured dinosaurs are well known for their evolution of specialized tail weapons—paired tail spikes in stegosaurs and heavy tail clubs in advanced ankylosaurs1. Armoured dinosaurs from southern Gondwana are rare and enigmatic, but probably include the earliest branches of Ankylosauria2,3,4. Here we describe a mostly complete, semi-articulated skeleton of a small (approximately 2 m) armoured dinosaur from the late Cretaceous period of Magallanes in southernmost Chile, a region that is biogeographically related to West Antarctica5. Stegouros elengassen gen. et sp. nov. evolved a large tail weapon unlike any dinosaur: a flat, frond-like structure formed by seven pairs of laterally projecting osteoderms encasing the distal half of the tail. Stegouros shows ankylosaurian cranial characters, but a largely ancestral postcranial skeleton, with some stegosaur-like characters. Phylogenetic analyses placed Stegouros in Ankylosauria; specifically, it is related to Kunbarrasaurus from Australia6 and Antarctopelta from Antarctica7, forming a clade of Gondwanan ankylosaurs that split earliest from all other ankylosaurs. The large osteoderms and specialized tail vertebrae in Antarctopelta suggest that it had a tail weapon similar to Stegouros. We propose a new clade, the Parankylosauria, to include the first ancestor of Stegouros—but not Ankylosaurus—and all descendants of that ancestor.

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Fig. 1: Skeletal anatomy of the S. elengassen holotype (CPAP-3165).
Fig. 2: Caudal weapon of S. elengassen holotype (CPAP-3165).
Fig. 3: Evolution of armoured dinosaurs and their tail weaponry.

Data availability

All data supporting the findings of this study are available in the paper and its Supplementary Information. Raw data from all CT scans are available online (https://www.morphosource.org/concern/biological_specimens/000382748). The LSID for this publication is urn:lsid:zoobank.org:pub:4D3ABFEE-5F21-4632-96F9-FE8C1542ACF7. The LSID ZooBank code for the new genus (Stegouros) is: urn:lsid:zoobank.org:act:5306DD47-2130-4866-8DB4-09092C632A4A. The LSID ZooBank code for the new species (Stegouros elengassen) is: urn:lsid:zoobank.org:act:992A259B-75EE-4206-89FB-6666FBCBDFA6. TNT files for phylogenetic analysis are provided on Zenodo (https://zenodo.org/record/5706422#.YZQ7Kr3Q8-R).

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Acknowledgements

We thank F. Suazo, E. Nuñez, D. Bajor, J. P. Guevara, D. Flores, S. Jiménez, I. Meyer-Navia, S. Davis and members of the J. Clark team for help in the field and/or fossil preparation; Estancia Cerro Guido and especially the Matetic, Simunovic and Reyes families for granting access and important logistical support in the field; the Consejo de Monumentos Nacionales (National Monuments Council) of the Chilean Ministry of Culture, Arts and Heritage for fieldwork permits; M. Reguero for access and help with the Antarctopelta holotype; J. Vidal for technical support in the acquisition of CT images; and staff at the Willi Hennig society for free distribution of TNT software. This research was supported by the Agencia Nacional de Investigación y Desarrollo ANID (National Agency for Research and Development) of the Chilean Ministry of Science, Technology, Knowledge and Innovation through grants PIA Anillo ACT172099 (to A.O.V.), FONDECYT 1190891 (to A.O.V.), FONDECYT 1151389 (to M.A.L.), and PhD scholarships for S.S.-A., J.A.-M., J.P.-L. and J.P.P.

Author information

Authors and Affiliations

Authors

Contributions

J.K., S.S.-A., H.O., B.A., J.A.-M., J.P.P. and V.M. extracted the fossil. J.K., S.S.-A., B.A. and J.A.-M. carried out laboratory preparation. J.P.-L., J.F.B., S.S.-A. and V.M.-W. processed and sampled CT and μCT data. S.S.-A. and A.O.V. described the material. S.S.-A. and A.O.V. scored phylogenetic matrices. S.S.-A. conducted the maximum-parsimony analyses. C.S.-G. and V.M. carried out taphonomic studies. M.A.L., L.M.E.M., R.A.F., J.P.P., H.M., C.T., D.R. and L.F.H. carried out geological and palaeoenvironmental studies. A.O.V. and S.S.-A. wrote the bulk of the manuscript; S.S.-A., C.S.-G., V.M. and J.P.P. made figures. All of the authors collected data and contributed to the writing, discussion and conclusions.

Corresponding authors

Correspondence to Sergio Soto-Acuña or Alexander O. Vargas.

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The authors declare no competing interests.

Additional information

Peer review information Nature thanks James Kirkland and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 Cranial bones of S. elengassen holotype (CPAP-3165).

a, skull in lateral view (with left? supraorbital reversed). b, posterior skull in dorsal view. c, premaxilla in ventral view. d, left maxilla lateral view. e, f, both maxillae in ventral view. g, left? supraorbital dorsal view. h, i, predentary in occlusal and ventral views. j, basisphenoid in palatal view. k, right dentary in lingual view. Scale bars, 10 cm.

Extended Data Fig. 2 Postcranial axial skeleton of S. elengassen holotype (CPAP-3165).

a-c axis in anterior and right lateral views. d-f, anterior cervical vertebra in anterior, right lateral and ventral views. g-i, posterior cervical vertebra in anterior, right lateral and dorsal views. j-l, anterior dorsal vertebra in anterior, right lateral and dorsal views. m, n, posterior dorsal vertebra in anterior and right lateral views. o, synsacral vertebrae in right lateral view. p, anterior caudal vertebrae in right lateral view. q, posterior caudal vertebrae in right lateral view. Scale bar 10 cm.

Extended Data Fig. 3 Sternal, pectoral girdle and forelimb bones of S. elengassen holotype (CPAP-3165).

a, b, sternal plates in ventral view. c-e, right coracoid in lateral, medial and glenoideal views. f-i left humerus in anterior, posterior, proximal and distal views. j, left radius in lateral view. k, left ulna in anterior view. l, m, left hand in proximal and dorsal views. n, originally semiarticulated right hand in dorsal view. o, fully prepared right hand in dorsal view. Scale bars, 10 cm.

Extended Data Fig. 4 Pelvic girdle, hindlimbs and dermal armour of S. elengassen holotype (CPAP-3165).

a, right ilium in lateral view. b, pelvis in ventral view. c, d, left pubis in lateral and medial views. eh, left femur in proximal, distal and posterior views. i, right femur in lateral view. j, left foot in dorsal view. k, right foot in lateral view. l, m, isolated mid-sized ovalate keeled osteoderm in dorsal and lateral views. n, o, sacral covering in dorsal and ventral views. p, keeled osteoderm with deeply excavated inner surface associated with right ulna in ventrolateral view. q, flat osteoderm associated with right radius in dorsal view. Scale bars, 10 cm (a–k, n, o) 2 cm (l, m, p, q).

Extended Data Fig. 5 Paired comparisons of the axial skeleton between S. elengassen holotype CPAP-3165 (left) and A. oliveroi holotype MLP 86-X-28-1 (right).

Synsacral complex in a, b, anterior, c, d, posterior and e, f, ventral views; anterior caudal vertebrae (6th in Stegouros elengassen) in h, i, posterior and j, k, right lateral views; posterior caudal vertebrae (17th in Stegouros elengassen) in l, m, dorsal and n, o, posterior views. Scale bars, 2 cm (a, c, e, h, j, l, n), 5 cm (b, d, f, g, i, k, m, o).

Extended Data Fig. 6 Anatomy of caudal weapon osteoderms in S. elengassen holotype (CPAP-3165) and A. oliveroi holotype (MLP 86-X-28-1).

a, digital reconstruction and b, photograph of Stegouros elengassen caudal weapon cross section (at level of first osteoderm pair and 14th caudal vertebra) in anterior and left lateral views. c, d, Antarctopelta oliveroi dorsal osteoderm of the first pair in left lateral and anterior views. e, Stegouros elengassen, 3D reconstruction of the left second osteoderm in dorsal and internal views. f, Antarctopelta oliveroi left second osteoderm fragment in dorsal and internal views. g, h, Stegouros elengassen, digital reconstruction of the caudal weapon cross section (at level of second pair and 17th caudal vertebra) in anterior and posterior views. i, j, proposed configuration of the caudal weapon of Antarctopelta oliveroi in anterior and posterior views. Scale bars 10 cm.

Extended Data Fig. 7 Comparison of cervical and pedal bones between S. elengassen holotype (CPAP-3165) and A. oliveroi holotype (MLP 86-X-28-1).

ad, Stegouros elengassen posterior cervical vertebra in anterior, right lateral, posterior, and dorsal views. eh. Antarctopelta oliveroi posterior cervical vertebra in anterior, right lateral, posterior, and dorsal views. i, j Stegouros elengassen right foot in dorsal and ventral views. k, l, Antarctopelta oliveroi right metatarsal in dorsal and ventral views. m, Antarctopelta oliveroi isolated pedal phalanx in dorsal, proximal and ventral views. Scale bars 5 cm.

Extended Data Fig. 8 Comparison of dermal skeleton between S. elengassen holotype (CPAP-3165) and A. oliveroi holotype (MLP 86-X-28-1).

a, Stegouros elengassen dermal ossicles in internal view. b, Antarctopelta oliveroi disarticulated dermal ossicles. c, Antarctopelta oliveroi dermal ossicle close-up exposed in internal view. df, sacral covering fragments of Antarctopelta oliveroi. Scale bars, 5 mm (1, b), 1 mm (c), 10 cm (d-f).

Extended Data Fig. 9 Comparison of teeth and dentary of S. elengassen holotype (CPAP-3165) and A. oliveroi holotype (MLP 86-X-28-1).

ad, Stegouros elengassen digital reconstruction of cheek tooth in labial, mesial, lingual, and distal views. eh, Antarctopelta oliveroi tooth (reversed) in labial, mesial, lingual, and distal views. ik, Stegouros elengassen right dentary (mirrored for better comparison) in labial, lingual and occlusal views. ln Antarctopelta oliveroi right dentary fragment in labial, lingual and occlusal views. Scale bars 5 mm (a-d), 10 mm (e-h), 10 cm (i-n).

Extended Data Table 1 Results of phylogenetic analysis

Supplementary information

Supplementary Information

(1) Taphonomical aspects of the holotype. (2) Geological and Palaeoenvironmental context. (3) Additional comparisons to Antarctopelta oliveroi. (4) Results of the phylogenetic analyses. (5) References.

Reporting Summary

Peer Review File

Supplementary Video 1

3D reconstruction of the tail weapon of Stegouros based on the digital segmentation of a CT scan volume.

Supplementary Video 2

Transversal view of the segmented volume for the tail weapon of Stegouros.

Supplementary Video 3

Digital endocast of the tail weapon of Stegouros.

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Soto-Acuña, S., Vargas, A.O., Kaluza, J. et al. Bizarre tail weaponry in a transitional ankylosaur from subantarctic Chile. Nature 600, 259–263 (2021). https://doi.org/10.1038/s41586-021-04147-1

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