Bizarre tail weaponry in a transitional ankylosaur from subantarctic Chile

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 Ankylosauria2–4. 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. Stegouros elengassen, an ankylosaur from the late Cretaceous of Chile, has a large tail weapon, named a macuahuitl after the Aztec club, with a frond-like structure formed by seven pairs of laterally projecting osteoderms encasing the distal half of the tail.


Article
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 rows 12 . 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 Kunbarrasaurus 6 (not obliterated as in other Ankylosauria 13 ). 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 asymmetric 14 : 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 Ankylosauria 13 , 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 stegosaurs 15 . 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 Scelidosaurus 16 ). 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 Fibula Nasal process of premaxilla  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. 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 Stegosauria 17 . 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 Scelidosaurus 16 ). The partially articulated right hand shows a reduction to only two phalanges in digit II, as in Stegosauria 18 , 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 ontogeny 19 . 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 Ankylosauria 13,18 (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 dinosaurs 20 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 keels 21 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 Ornithischia 22 , armoured dinosaurs 23 , Stegosauria 24 and Ankylosauria 3,25 . 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 datasets 3,22-24 supported Stegosauria as sister of Ankylosauria (as in most studies 2,17 ), including a dataset that had previously supported a different result 23 .
Before Stegouros, relationships among Gondwanan ankylosaurs have been enigmatic because only Kunbarrasaurus from the late Lower Cretaceous of Australia 6,26,27 was represented by a well-preserved skeleton.
Kunbarrasaurus includes a skull with ancestral characters 6 , 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 orbit 6 , osteoderms are present on the limbs 26 and a thin layer of dermal bone covers the sacrum 27 . Stegosauria have a superficially similar sacral covering, but it is formed by the expanded transverse processes of the sacral vertebrae 18 , while the pelvic shield of other Ankylosauria Article is also different, formed by a mosaic of fused osteoderms that also covers the ilium 28 . Both Stegouros and Kunbarrasaurus show a slender humerus with a descending ridge, and a supracetabular process that is semi-circular in the dorsal view 26 that are usually found in Stegosauria 18 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 reptiles 3,29 , 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 Antarctopelta 7 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 Cretaceous 5,29 .
Few flank osteoderms were recovered for both Stegouros and Antarctopelta 7 , which may reflect light trunk armour as documented in Kunbarrasaurus (where it is preserved in situ 27 ). 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 dinosaurs 7,27,30 , 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 teeth 14 , 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 weapon 31 (Fig. 3). In Ankylosauridae, long prezygapophyseal articulations stiffen the distal tail, which becomes the handle of the tail club in Ankylosaurinae 31 . 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)  weapons 1,32 , 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 Gondwana 33 .

Online content
Any methods, additional references, Nature Research reporting summaries, source data, extended data, supplementary information, acknowledgements, peer review information; details of author contributions and competing interests; and statements of data and code availability are available at https://doi.org/10.1038/s41586-021-04147-1.

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 literature 3,22-25 , 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 ordered 22 . 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 ordered 12,18 .
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

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 ratio 37-40 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.

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 Last updated by author(s): Sept 28, 2021 Reporting Summary Nature Portfolio wishes to improve the reproducibility of the work that we publish. This form provides structure for consistency and transparency in reporting. For further information on Nature Portfolio policies, see our Editorial Policies and the Editorial Policy Checklist.

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Data
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Study description
The present study comprises the description, taphonomic assessment and phylogenetic analysis of a new Ankylosaur fossil specimen. Given the limited nature of the data no replicates were included.

Research sample
The Sampling strategy A single holotype specimen was available

Data collection
The morphological data of the fossil specimen was collected and codified as discrete characters by Sergio Soto-Acuña. Morphology of the specimen was assessed directly and through 3D reconstructions based on the digital segmentation of CTscan volumes generated by José Palma-Liberona.
Timing and spatial scale The timing of data collection is irrelevant for the present study.

Data exclusions
No data was excluded from this study.

Reproducibility
The present study does not include experimental procedures. As such, no measures were taken to verify the reproducibility of experimental findings.

Randomization
Given that the present study considers the description and phylogenetic analysis of a single fossil specimen, no randomization procedure was applicable besides the bootstrap methods applied in the phylogenetic analyses.

Blinding
Since the present study describes a single fossil specimen, no blinding procedure was used.
Did the study involve field work?

Disturbance
Only transient disturbance was carried out by digging in soils and rocks not covered by vegetation (the study area consists of cold steppe, with sparse vegetation cover of altitudinal cushions and grasslands). Excavation sites were covered to protect them from erosion and left with a similar appearance as before the intervention Reporting for specific materials, systems and methods We require information from authors about some types of materials, experimental systems and methods used in many studies. Here, indicate whether each material, system or method listed is relevant to your study. If you are not sure if a list item applies to your research, read the appropriate section before selecting a response.