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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.

Main

Dinosauria Owen, 1842

Ornithischia Seeley, 1887

Thyreophora Nopcsa, 1928

Ankylosauria Osborn, 1923

Parankylosauria clade nov.

Stegouros elengassen gen. et sp. nov.

Etymology. Stegouros, after the Greek stego (roof) and the Greek uros (tail) in reference to the covered tail; elengassen, after an armoured beast in the mythology of the local Aónik’enk people8.

Holotype. CPAP-3165 (Colección de Paleobiología de Antártica y Patagonia, at the Instituto Nacional Antártico Chileno, INACH) consists of a mostly complete skeleton (Fig. 1) of which the posterior half (tail, sacrum, pelvic girdle and hindlimbs) is largely articulated, with the remaining disarticulated skeleton including axis, cervical and dorsal vertebrae, sternal plates, coracoids, forelimbs (including a partially articulated right manus), osteoderms and several disarticulated cranial bones. Preservation reflects a single individual (Supplementary Information) and the ontogenetic fusion of elements is complete (neural arches, occipital bones, anterior and greater trochanters of the femur), discarding juvenile status (that is, an animal with no signs of impending maturity9).

Fig. 1: Skeletal anatomy of the S. elengassen holotype (CPAP-3165).
figure 1

a, Preserved elements of the skeleton in CPAP-3165; the mandible is reversed, and the position of osteoderms in grey shade is uncertain. b, c, Dorsal (b) and right lateral (c) views of the rostral portion of the premaxilla. d, Caudal view of the occipital complex. e, f, Digital reconstruction of cheek tooth (labial (e) and lingual (f) views). g, Anterior view of the mid dorsal vertebra. h, i, Lateral (h) and anterior (i) views of the left humerus. j, Lateral view of the left ulna. k, Dorsal view of the partial right hand. l, Dorsal view of the pelvis. m, Anterior view of the right femur. n, Anterior view of the left tibia and fibula. o, Articulated right foot (ventral view). p, Articulated tail with caudal weapon of paired osteoderms. Scale bars, 50 cm (a), 1 cm (b, c, gk and mo), 5 mm (e and f) and 10 cm (l and p).

Locality and horizon. Río de las Chinas Valley, Estancia Cerro Guido, Magallanes Region, Chilean Patagonia (51° S). Lower section of the Dorotea Formation (upper Campanian—lower Maastrichtian), between 71.7 ± 1.2 million and 74.9 ± 2.1 million years ago10,11 (Supplementary Information).

Diagnosis. A small-sized (180–200-cm body length, including tail), slender-limbed, armoured dinosaur that differs from all other members of this clade by the presence of a short tail with no more than 26 vertebrae, covered distal to the 14th caudal vertebra by seven pairs of large osteoderms, the distalmost five of which are fused into a flat composite unit. Stegouros can be assigned to Ankylosauria on the basis of medially bowed dentary tooth rows; strongly medially inset maxillary tooth rows; maxilla with a medial process; and short cervical vertebrae. Otherwise, the postcranial skeleton lacks ankylosaurian synapomorphies. Stegouros differs from Antarctopelta in its smaller body size; proportionally larger neural canal; longer dorsosacral centra; higher and narrower sacral centra; teeth with six mesial denticles (versus seven to eight in Antarctopelta); and cingula lacking vertical furrows. Stegouros differs from Kunbarrasaurus in a more curved ulna and ischium (straight in Kunbarrasaurus) and a dorsal maxillo-lacrimal process that is narrower and posteriorly inclined.

Diagnosis of the genus. As described for the type species.

Description. Preserved skull elements imply a proportionally large head, although depth versus width cannot be established (Fig. 1b–d and Extended Data Fig. 1). The premaxillae and maxillae preserve rostral portions only. The premaxillae are toothless, completely fused at the midline (with no suture or indentation), narrow, short and high, with a deep palatal surface (Fig. 1b, c). The maxillae are seamlessly fused to the lacrimals, which are posteriorly inclined. As in Ankylosauria, the maxillae show a medial process (secondary palate) and strongly inset tooth rows12. The maxillary tooth rows begin shortly anterior to the lacrimal, extending under the orbit. The supraoccipital is large, forming the entire upper margin of the foramen magnum, thickening above it into a distinct dorsal shelf (Fig. 1d). The basisphenoid is short (less than the basioccipital length). Two unidentified circumorbital skull roof fragments show strongly rugose surfaces ornamented by foramina and grooves, and clear sutures between bones as in Kunbarrasaurus6 (not obliterated as in other Ankylosauria13). The right dentary is sinuous in lateral view, with a medially bowed row of 14 alveoli; the last two of which show erupted teeth in position (Extended Data Fig. 1). These are leaf-like with high crowns that are mesiodistally asymmetric, and denticles continuous to enamel flutings that reach down to a bulged cingulum. The cingulum is asymmetric14: horizontal in labial view, but an arch in lingual view that tilts towards basomesial (Fig. 1e, f). The predentary is short and deep, with thin dorsal processes, longer than the ventral processes (Extended Data Fig. 1).

The axis is short as in Ankylosauria13, bearing a prominent odontoid process (Extended Data Fig. 2). The cervical centra are wider than long as in ankylosaurs, but laterally concave as in some stegosaurs15. Towards the posterior, the upward projection of the transverse processes increases, reaching 60° above horizontal in the dorsal vertebrae. Dorsal vertebrae have tall neural arches with high pedicles, tall neural spines and prezygapophyses that are fused into a U shape (Fig. 1g). Four true sacral vertebrae fuse anteriorly to two dorsosacrals, the ‘presacral rod’13. The ribs of the dorsosacrals are short and contact the ilium without fusing to it. Sacrocaudals are absent (Extended Data Fig. 2). The 13 more-proximal caudal vertebrae comprise the more flexible portion of the tail, with the remaining vertebrae encased in the large osteoderms of the caudal weapon. The 18th caudal is broken, and all of the vertebrae distal to it are missing (Figs. 1p and 2, Extended Data Fig. 2 and Supplementary Videos 1 and 2). From the remaining space within the caudal weapon, we estimate that there are no more than 8 missing vertebrae, suggesting at most 26 tail vertebrae, which is lower than documented in any armoured dinosaur (the lowest being 35 in Scelidosaurus16). The caudal centra are amphiplatyan to platycoelous. The transverse processes are long (about twice the neural spine) and are present beyond mid-length of the tail. The neural spines of caudals 7–12 are slightly thickened distally, and shorter than the haemal arches. Posterior to the 12th caudal, the centra show a ventral groove and are equally long than wide, but also very low. A digital endocast of the caudal weapon reveals a notably flattened interior space (Supplementary Video 3). Computed tomography (CT) scans show that the prezygapophyses of caudals 15–18 are short, while the postzygapophyses extend caudally over the following centrum, fusing medially into a structure that is wedge-like in dorsal view, with a corresponding V-shaped space between the prezygapophyses of the following vertebra (Fig. 2d–f). No ossified tendons are preserved.

Fig. 2: Caudal weapon of S. elengassen holotype (CPAP-3165).
figure 2

ac, Three-dimensional reconstruction of the caudal weapon obtained through the digital segmentation of a CT scan volume, in dorsal, (a) ventral (b) and right lateral (c) views. df, Preserved distal caudal vertebrae (number 14 to 18) in dorsal (d), ventral (e) and right lateral (f) views, showing a ventral groove and elongate postzygapophyses. Scale bar, 10 cm.

The scapulae were not preserved. The coracoids have a well-preserved scapular margin, discarding fusion to the scapula. The sternal plates are unfused with a long tubular caudolateral process (Extended Data Fig. 3). The humerus has a slender diaphysis but a mediolaterally expanded epiphyses, and a well-developed, anteriorly directed deltopectoral crest (Fig. 1h, i). A well-defined descending ridge along the caudolateral margin of the humerus includes a weak tubercle at its proximal end, at the same position as the triceps tubercle of Stegosauria17. The radius is slender, whereas the ulna is bowed and proximally expanded, with a well-developed olecranon (Fig. 1j). The hand of Stegouros presents definitive hoof-like unguals (in contrast to the comparatively sharp unguals of Scelidosaurus16). The partially articulated right hand shows a reduction to only two phalanges in digit II, as in Stegosauria18, with a flattened disc-like non-ungual (Fig. 1k). Other flat, disc-shaped phalanges were found disarticulated but associated with both hands. The left hand preserves a little U-shaped carpal attached to the fifth metacarpal, of which the anatomical position and shape suggest that it is an ulnare (Extended Data Fig. 3). The ilium shows a long preacetabular process that is strongly anterolaterally deflected (Fig. 1l and Extended Data Fig. 4). The shape and relative positions of the supracetabular shelf (lateral process) and postacetabular process are very similar to Stegosauria, suggesting medial rotation of the latter during ontogeny19. The ischium is long and lacking an obturator process and ischial symphysis. It tapers distally, bending slightly at mid-length (Extended Data Fig. 4). No pubes were preserved. The femora are straight and only slightly longer than the tibiae (Fig. 1m, n), with a reduced ridge-like fourth trochanter, and an anterior trochanter fused to the greater trochanter (Extended Data Fig. 4). Both feet are complete and articulated. They do not spread distally, showing more extensive proximal contact surfaces between metatarsals III and IV than Stegosauria and Ankylosauria13,18 (Fig. 1o). There is no reduction in the pedal phalangeal formula, although the distalmost non-unguals of digits III and IV are flattened and disc-shaped. All of the pedal unguals are hoof-like.

No cranial osteoderms were found. A small (19 mm) flat osteoderm was found near the axis. Eight medium-sized (40–50 mm) elliptical and keeled osteoderms (Extended Data Fig. 4) resemble flank scutes of other armoured dinosaurs20 but were not clearly associated with skeletal elements, except for one that was preserved near the neural arch of a dorsal vertebra. No large cervical osteoderms were found; four small osteoderms (15–20 mm) with higher, acuminate keels were found clustered together near the left manus. Numerous ossicles (small, 4–5 mm osteoderms) were scattered around all of the skeletal elements. These are almost-square oblate spheroids ornamented by pitting in the external surface, and strong orthogonal fibres on the inner side. At the anterior sacrum, the dorsal space between the ilium and the tip of the sacral neural spines is covered by a continuous layer of thin dermal bone with vascular furrows and pits (Fig. 1l and Extended Data Fig. 4). Two pairs of small semiconical osteoderms with high, acuminate keels21 and concave inner surfaces were found associated with the anterior tail. The seven pairs of lateral osteoderms of the tail weapon are clearly in anatomical position. The first (most proximal) pair in the series has an acuminate keel with a caudo-laterally slanted apex, a flattened dorsal surface (more conical ventrally), and a markedly concave inner medial surface. Their posterior ventral aspect is fused to smaller semiconical osteoderms similar to those of the proximal tail, pointing ventrolaterally and posterior (Supplementary Video 1). The following pair of osteoderms is similar, but larger (covering two entire vertebrae), flatter and lacking the smaller ventral osteoderms (Fig. 2 and Supplementary Video 1). The next five pairs of osteoderms are flattened and fused to each other at their anterior–posterior contact surfaces, giving each osteoderm a roughly pentagonal appearance in the upper view, with laterally projecting apexes. They conform a large frond-like structure covering the tail dorsolaterally, and also ventrally towards its distalmost end (Supplementary Videos 1 and 2). Two small knob-like structures at the distal tip probably represent an eighth pair of very small osteoderms. At the appendicular skeleton, a small rounded keeled osteoderm with a concave inner surface was found appressed to the upper right ulna, along with a flat subtriangular osteoderm (Extended Data Fig. 4). Keeled osteoderms were found at the lateral side of both feet (three on the left foot, two on the right; Fig. 1o).

Discussion

Stegouros shows ankylosaurian skull characters, but slender limbs; most postcranial characters are ancestral for Eurypoda (Stegosauria + Ankylosauria), and a few resemble Stegosauria. We carried out phylogenetic analyses with five different datasets modified from recent studies focusing on Ornithischia22, armoured dinosaurs23, Stegosauria24 and Ankylosauria3,25. In all of the datasets, Stegouros was found to be closer to Ankylosauria than to Stegosauria, and further grouped with the basal ankylosaurs Kunbarrasaurus and Antarctopelta, forming a monophyletic clade that split earliest from all other Ankylosauria (Fig. 3, Extended Data Table 1 and Supplementary Information). Note that four out of the five modified datasets3,22,23,24 supported Stegosauria as sister of Ankylosauria (as in most studies2,17), including a dataset that had previously supported a different result23.

Fig. 3: Evolution of armoured dinosaurs and their tail weaponry.
figure 3

a, After including Stegouros, phylogenetic time-calibrated analyses based on different modified matrices support the monophyly of Parankylosauria (Gondwanan Ankylosaurs) and their early split from all other ankylosaurs (Euankylosauria). Most (but not all) analyses also supported Ankylosauria as sister to Stegosauria, and a monophyletic Nodosauridae. b, The distribution of Parankylosauria findings in Gondwana during the latest Cretaceous (map generated using the ODSN Plate Tectonic Reconstruction Service). c, Armoured dinosaurs are the only amniotes to have evolved three different specialized tail weapons in Stegosauria, Stegouros and Ankylosaurinae.

Before Stegouros, relationships among Gondwanan ankylosaurs have been enigmatic because only Kunbarrasaurus from the late Lower Cretaceous of Australia6,26,27 was represented by a well-preserved skeleton. Kunbarrasaurus includes a skull with ancestral characters6, but most of the tail and distal limbs are missing. Like Stegouros, Kunbarrasaurus is small-sized (around 2.5 m), maxillary tooth rows extend under the orbit6, osteoderms are present on the limbs26 and a thin layer of dermal bone covers the sacrum27. Stegosauria have a superficially similar sacral covering, but it is formed by the expanded transverse processes of the sacral vertebrae18, while the pelvic shield of other Ankylosauria is also different, formed by a mosaic of fused osteoderms that also covers the ilium28. Both Stegouros and Kunbarrasaurus show a slender humerus with a descending ridge, and a supracetabular process that is semi-circular in the dorsal view26 that are usually found in Stegosauria18 and could therefore be ancestral characters for Eurypoda. Antarctopelta from the late Campanian age of the Antarctic peninsula is a larger ankylosaur (around 4 m) known from a very partial skeleton (approximately 15%)7. Both Stegouros and Antarctopelta show ancestrally slender metatarsi and no sacrocaudals (Extended Data Fig. 7 and Supplementary Information). Some vertebrae of Antarctopelta are unusual for Ankylosauria, even leading to discussion that they could belong to marine reptiles3,29, but comparison to Stegouros confirms that they are caudal vertebrae. Both dinosaurs present uniquely specialized vertebrae with a flattened centrum and a ventral groove, which are found in the caudal weapon of Stegouros (Extended Data Fig. 5 and Supplementary Information). Large enigmatic osteoderms of Antarctopelta7 show a marked medial concavity and an acuminate keel with a slanted apex, resembling the large caudal weapon osteoderms of Stegouros (Extended Data Fig. 6 and Supplementary Information). Combined with the flattened distal caudal centra, we infer that Antarctopelta had a similar weapon. A close relationship of Antarctopelta with Stegouros is plausible given their similar age, palaeogeographical proximity, and evidence of intercontinental dispersal of flora and fauna between the Antarctic peninsula and southern South America during the late Cretaceous5,29.

Few flank osteoderms were recovered for both Stegouros and Antarctopelta7, which may reflect light trunk armour as documented in Kunbarrasaurus (where it is preserved in situ27). Flat plates of broken dermal bone in Antarctopelta may represent fragments of a sacral covering similar to that of Kunbarrasaurus and Stegouros (Extended Data Fig. 8), a potentially derived trait shared by all three taxa. All three also share the presence of numerous small ossicles with a pattern of orthogonal striae on the inner side that are unique among armoured dinosaurs7,27,30, and teeth that have denticles confluent with enamel ridges, reaching basally to a bulged and asymmetric cingulum (Extended Data Fig. 9). The North-American ankylosaur Edmontonia has similar teeth14, but this is probably convergent given the distant position of this ankylosaur in phylogenetic analyses.

Taking into account that Stegouros is probably related to other basal ankylosaurs from southern Gondwana, we propose the clade Parankylosauria (‘at the side of Ankylosauria’) to include the first ancestor of Stegouros—but not Ankylosaurus—and all descendants of that ancestor. Conversely, we propose the clade Euankylosauria (‘true Ankylosauria’) for the first ancestor of Ankylosaurus—but not Stegouros—and all of its descendants (Fig. 3). The evidence for slender limbs in Parankylosauria suggests that stout limbs and broad feet (the namesake of Eurypoda) are actually convergent between Euankylosauria and Stegosauria. The generally ancestral postcranium of Parankylosauria also implies that ankylosaurian specializations evolved first in the skull. Tail clubs of Ankylosaurinae must have evolved independently from the tail weapon of Stegouros, as closer relatives of Ankylosaurinae such as Nodosauridae and even basal Ankylosauridae had no specialized tail weapon31 (Fig. 3). In Ankylosauridae, long prezygapophyseal articulations stiffen the distal tail, which becomes the handle of the tail club in Ankylosaurinae31. The tail of Stegouros reflects a different evolutionary pathway, with short prezygapophyses, and a notably shorter tail that is stiffened through osteoderm fusion (Fig. 3b). Among amniotes, herbivores with osteoderms and stiff trunks are more likely to evolve specialized tail weapons1,32, and armoured dinosaurs in particular are the only clade to have evolved three different kinds of tail weapons: paired spikes (thagomizers) in stegosaurs, clubs in ankylosaurines and the ‘macuahuitl’ of Stegouros (our suggested term, after the Aztec war club). The Parankylosauria must have originated before the earliest record of Euankylosauria, some 167 million years ago, in the mid-Jurassic period (Fig. 3 and Supplementary Information). After the final separation of Laurasia and Gondwana in the late Jurassic, different clades of Ankylosauria may have prevailed in each supercontinent. These and other possibilities raised by Stegouros illustrate that much still remains unknown about the evolution of armoured dinosaurs, especially in Gondwana33.

Methods

CT scans

The tail of the fossil specimen was scanned with the Brilliance 64 X-Ray CT scanner (Phillips) housed at the Clinical Hospital of the Pontificia Universidad Católica de Chile using a voltage of 120 kV and a current of 0.17 mA generating 836 slices with a thickness of 0.4 mm and a pixel size of 0.249 mm. The tooth of the specimen was scanned using the Skyscan 1272 X-Ray μCT (Bruker) system housed at the engineering faculty of the Pontificia Universidad Católica de Chile using a voltage of 100 kV and a current of 100 μA, generating 1,316 slices with a pixel size of 13.397 μm. Both the tail and the tooth scan data were segmented, analysed and visualized in VGSTUDIO MAX v.3.2.5.

Phylogenetic analyses

Parsimony analysis

The phylogenetic relationships of thyreophoran dinosaur were investigated using TNT (v.1.5)34 through five different matrices available in literature3,22,23,24,25, following the same procedure described in the original data sources. The first data matrix is a modified version of that in ref. 22 and is composed of 383 characters and 74 taxa. Characters 2, 23, 31, 39, 125, 163, 196, 203, 204, 222, 227, 238, 243, 247, 268, 292, 296, 302, 306, 320 and 361 were treated as ordered22. The search for most parsimonious trees (MPTs) was performed using a traditional search (heuristic) with 1,000 replicas of tree bisection and reconnection (TBR) holding 100 trees per replica. The trees obtained were used as the starting point for a second round of TBR by traditional search. The second data is a modified version of that in ref. 23 and is composed of 127 characters (12 new characters were added) and 10 taxa. A traditional search was carried out with 10,000 replicates holding 10 trees. The third data matrix is a modified version of that from refs. 12,18,24,33 and is composed of 131 characters (10 new characters were added) and 28 taxa. Characters 1–24, 29, 112 and 113 were treated as ordered12,18. A new technology search analysis (sectorial, ratchet, drift and tree fusing) was made with ten random-addition sequences, and the trees obtained were used for a second round of TBR using traditional search. The fourth data matrix is a modified version of that from refs. 3,35 and is composed of 189 characters (12 new characters were added) and 65 taxa. In this matrix all of the characters were treated as unordered, and the search for MPTs was performed using TBR with 1,000 replicas, holding 10 trees per replica, and then a second search was performed with the trees saved in memory. The fifth data matrix is a modified version of that from refs. 4,25 and is composed of 306 characters (13 new characters were added) and 37 taxa. The characters 1, 2, 3, 7, 10, 13, 16, 18, 23, 25, 30, 36, 38, 48, 49, 54, 64, 87, 98, 101, 103, 104, 105 140, 141, 143, 145, 148, 149, 156, 162, 165, 174, 177, 194, 201, 205, 209, 217, 229, 231, 232, 236, 237, 238, 268 and 279 were treated as ordered. For the search of MPTs, traditional search was used, with 10,000 TBR replicas with 10 trees saved per replica. Character argumentation and list changes are available in the Supplementary Information. For all of the analyses, standard bootstrap values were calculated with 1,000 pseudoreplicates and Bremer support values were calculated using the script bremer.run36.

Statistical analysis

We used a Templeton test to assess the significance of whether the phylogenetic position of Stegouros was closer to Ankylosauria than to Stegosauria (Extended Data Table I). For this, we used randomly chosen MPTs from the tree space of the unconstrained analysis (A1) and a forced topology, where Stegouros is closer to Stegosauria than to Ankylosauria (A2), and applied a one-sided Wilcoxon signed-rank test to the differences in character transformations between these trees. Templeton tests were performed using TNT v.1.5 and the script is available online (http://www.anbg.gov.au/cpbr/tools/templetontest.tnt).

Estimation of divergence times

The stratigraphic adjustment and estimation of the divergence times of the calibrated phylogenies was performed by calculating the Manhattan stratigraphic measure and gap excess ratio37,38,39,40 using the TNT script implemented in ref. 41. Divergence times were not estimated for the data matrix of ref. 22, which comprises only a small sample of armoured dinosaurs.

Nomenclatural acts

This published Article and the nomenclatural acts it contains have been registered in ZooBank, the proposed online registration system for the International Code of Zoological Nomenclature. The LSID for this publication is urn:lsid:zoobank.org:pub:4D3ABFEE-5F21-4632-96F9-FE8C1542ACF7; the LSID for the new genus (Stegouros) is: urn:lsid:zoobank.org:act:5306DD47-2130-4866-8DB4-09092C632A4A, and the LSID for the new species (Stegouros elengassen) is: urn:lsid:zoobank.org:act:992A259B-75EE-4206-89FB-6666FBCBDFA6.

Reporting summary

Further information on research design is available in the Nature Research Reporting Summary linked to this paper.

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.

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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.

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

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The authors declare no competing interests.

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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.

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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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