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Theropoda Marsh, 1881

Tetanurae Gauthier, 1986

Chilesaurus diegosuarezi gen. et sp. nov.

Etymology. In reference to Chile, and honoring Diego Suárez, who at the age of 7, discovered the first bone remains in the Toqui Formation.

Holotype. Servicio Nacional de Geología y Minería, Chile (SNGM)-1935 consists of a nearly complete, articulated skeleton, approximately 1.6 m long (Fig. 1, Supplementary Information and Extended Data Fig. 1). Holotype specimen was skeletally immature at the time of its death, as evidenced by the incomplete fusion of neurocentral sutures. This ontogenetic inference agrees with the size of the holotype, which represents 50% the length of the larger specimen SNGM-1888 (ref. 3).

Figure 1: Skeletal anatomy of Chilesaurus diegosuarezi gen. et sp. nov.
figure 1

a, Reconstructed skeleton (SNGM-1935). b, Fourth cervical vertebra (SNGM-1935) in right lateral view, with a close-up of tabicated anterior pleurocoel. c, d, Composite reconstruction of right hand (carpals, metacarpals, and non-ungual phalanges of digits I and II are from specimen SNGM-1935; ungual phalanges I and II are from specimen SNGM-1937; metacarpal III is from specimen SNGM-1887) in dorsal (c) and medial (d) views. e, Pelvic girdle (SNGM-1936) in right lateral view. f, Articulated ischia (SNGM-1936) in posterior view. g, Articulated pubes (SNGM-1936) in anterior view. h, Proximal tarsals (SNGM-1888) in distal view. i, j, Left astragalus (SNGM-1936) in proximal (l) and anterior (m) views. k–m, Left pes (SNGM-1937) in dorsal (k), medial (l) and proximal (m) views. ap, ascending process; ast, astragalus; b, basin; c, carpal; calc, calcaneum; ia, ischiadic apron; is, ischium; mtc, metacarpal; p, pubis; poz, postzygapophysis; pps, posterior pleurocoel; prz, prezygapophysis; ps, pubic symphysis; I, II, III, IV, digits first to fourth.

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Paratypes. Postcranial skeletons of four individuals, corresponding to different ontogenetic stages, ranging approximately from 1.2 to 3.2 m in total length (Extended Data Table 1). Several specimens referred to as indeterminate theropods and tetanurans previously3 are here referred to as Chilesaurus diegosuarezi.

Locality and horizon. Central Patagonian Cordillera, Aysén (Chile; approximately 46° S); Toqui Formation3,4, Tithonian, latest Jurassic.

Diagnosis. Chilesaurus differs from other dinosaurs in the following combination of autapomorphies: premaxilla short and deep, with prominent plate-like postnarial process; teeth leaf-shaped, being finely denticulate only on the crown apex of erupting teeth; coracoid subquadrangular in side view and with transversely thick margins; manual digit II with short pre-ungual phalanges; manual digit III atrophied; iliac blade with posterodorsal prominence; ischiadic peduncle of ilium robust; supracetabular crest absent; pubis fully retroverted; pubic shaft rod-like and distally unexpanded; femoral mediodistal crest absent; tibia without fibular crest. In addition, Chilesaurus shows the following unique combination of characters: dentary deeper anteriorly than posteriorly; cervicals with septate and paired pleurocoels; pubic apron transversely narrow; ischia connected through a proximodistally extended medial lamina (‘ischial apron’); femoral greater trochanter anteroposteriorly expanded, astragalar ascending process lower than astragalar body; calcaneum subtriangular in distal view; metatarsal I robust, elongate, and proximally compressed transversely; metatarsal II transversely wider than the other metatarsals; pedal digit I large.

Isolated skull material (including premaxilla, maxillae, frontals, postorbital, squamosal, basicranium, ectopterygoid and dentary; Extended Data Fig. 2), suggests a proportionally small head for Chilesaurus. The premaxilla is short and deep, with a rugose rostral margin that suggests a ramphotheca (Fig. 2). Frontals are elongate and narrow and participate extensively in the orbital margin. The basisphenoidal recess is deep. The dentary is short and deep, with a downturned symphyseal region but a straight alveolar margin. Dentary teeth are tall, leaf-shaped, and procumbent, with small serrations restricted to the crown apex (Fig. 2).

Figure 2: Selected cranial bones and teeth of Chilesaurus diegosuarezi gen. et sp. nov. (SNGM-1935).
figure 2

a, Partial right (?) maxilla in lateral view. b, Left premaxilla in medial view. c, Right dentary in lateral view. d, Details of dentary teeth in lingual view. e, Crown of unerupted dentary tooth. f, Detail of the carina of an unerupted tooth (arrows indicate denticle positions). alv, alveoli; cr, crown tooth; ro, root tooth.

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Cervical vertebrae are long and low, forming a slender neck. Cervical and anterior dorsal vertebrae possess a pair of septate pleurocoels, which are absent posterior to the pectoral region (Fig. 1g). ‘Pectoral’ vertebrae bear prominent hypapophyses.

The scapular blade (Extended Data Fig. 3) is elongate and slightly anteroposteriorly expanded distally, as in basal averostrans5. The coracoid is subquadrangular and lacks theropod characteristics such as the posteroventral process and biceps tuberosity1. It is notably thick transversely, contrasting with the delicate anterodorsal and dorsal margins of most dinosaurs. The limb bones are stout, as in sauropodomorphs, and forelimb length is 56% that of hind limbs. The humerus is proximodistally short and transversely wide (Extended Data Fig. 3). The single proximal carpal is large, with a transversely convex proximal articular surface. Metacarpals I–III are present, but only manual digits I and II are well developed (Fig. 1d–f). Metacarpal I is stout, and phalanx 1-I is short and strongly twisted along its main axis, as in basal sauropodomorphs6. The ungual of digit I is shorter than metacarpal II and less curved than most basal tetanurans5,7. Metacarpal II is the longest, and its digit presents strongly shortened pre-ungual phalanges, as in some ceratosaurians8. Metacarpal III is much more slender than in basal theropods, and its digit comprises a single minute phalanx.

The ilium is dolichoiliac, typical for Theropoda1 (Fig. 1b). The pubic pedicle is elongate (as in sauropods, ornithischians and therizinosaurs2,9), and the ischiadic peduncle is bulbous, as in ornithischians and alvarezsaurid coelurosaurs10. A prominent supratrochanteric process is present on the posterodorsal corner of the ilium, similar to those of sauropods, therizinosaurs, paravians, and some ornithischians11,12,13. The acetabular roof is transversely narrow and a supracetabular crest is absent, as occurs in derived coelurosaurs and ornithopods1,11,13. The pubis (Fig. 1b, c) closely resembles that of basal ornithischians, therizinosaurs and dromaeosaurid paravians in being fully retroverted, with a reduced proximal end bearing a posteriorly open obturator notch1,2. The pubic apron is transversely narrow and the pubis has a rod-like shaft. It is distally unexpanded with a rounded contour, sharply contrasting with the prominent distal ‘foot’ of other theropods1. The ischium (Fig. 1a, b) is proximally expanded and lacks an obturator process. Notably, both ischia are connected through a proximodistally extended medial lamina (ischial apron), a feature reported in some megalosauroids14.

The femur is robust (Extended Data Fig. 4). The greater trochanter is anteroposteriorly expanded, similar to coelurosaurs. The anterior trochanter is wing-like and proximally projected, and the fourth trochanter is semicircular. The distal third of the femur resembles sauropodomorphs in that it lacks distinctive theropod features, such as an anteromedial elliptical muscle scar, a mediodistal crest and its associated medial adductor fossa. The cnemial crest of tibia (Extended Data Fig. 4) is rounded in lateral view, as in basal sauropodomorphs and basal ornithischians, and differs from the subtriangular crest present in most basal theropods1,15. As in sauropodomorphs, and unlike most theropods, the proximal end of the tibia lacks a fibular crest1. Like theropods, the distal end of the tibia is anteroposteriorly compressed with a laterally extending malleolus, more transversely expanded than in coelophysoids, but less than in tetanurans1. The fibula lacks a proximomedial pocket and iliofibular tubercle. The tarsus (Fig. 1k–m) resembles basal saurischians16. The astragalar ascending process is low and broad, unlike tetanurans, in which it is laminar, tall, and transversely wide1,6,17. The proximal astragalar surface possesses a deep, well-defined basin posterior to the ascending process, as in basal dinosaurs. As in early saurischians (for example, Herrerasaurus, basal sauropodomorphs), the calcaneum of Chilesaurus is transversely wide and subtriangular in distal view, rather than rectangular and disc-shaped as in theropods1,16,17. The foot is wide and proximodistally short (Fig. 1h–j). The proximal half of metatarsal I is transversely compressed but anteroposteriorly expanded, unlike most theropods, in which it tapers proximally. Metatarsal I of Chilesaurus represents more than 50% of the length of metatarsal II, in contrast with most other theropods (25–33%)1,17. Metatarsal III is the longest, but metatarsal II is the thickest, as in basal sauropodomorphs. Digit I is large, only slightly shorter than digit II, approaching the tetradactyl condition of early sauropodomorphs, ornithischians18 and derived therizinosaurs2.

The bizarre anatomy of Chilesaurus raises interesting questions about its phylogenetic relationships. We scored Chilesaurus into four different integrative archosauriform, theropod and sauropodomorph data sets9,19,20,21. Remarkably, all these analyses placed Chilesaurus as a member of Theropoda, near the origin of tetanurans5 (Fig. 3a), dismissing conceivable positions near Therizinosauria, Sauropodomorpha or Ornithischia. The theropodan position of Chilesaurus is supported by pleurocoels in cervical and anterior dorsal vertebrae; hypapophyses on ‘pectoral’ vertebrae; preacetabular process of ilium dorsoventrally expanded; femoral fourth trochanter semicircular; and tibia distally expanded and with lateral malleolus extending strongly laterally. Tetanuran affinities are supported by scapular blade elongate and strap-like; distal carpal semilunate; and manual digit III reduced (Supplementary Information). For a basal tetanuran, Chilesaurus possesses a number of surprisingly plesiomorphic traits on the hindlimbs, especially in the ankle and foot, which resemble basal sauropodomorphs7,9,12. These features are here considered as secondary reversals that might be related to a less-cursorial mode of locomotion. Furthermore, derived features of the dentary and teeth shared by Chilesaurus, sauropodomorphs and therizinosaurs are interpreted as homoplasies related to herbivorous habits22,23,24,25. In this context, pubic retroversion of Chilesaurus may be related to an increased gut capacity for processing plant material25.

Figure 3: Phylogenetic relationships of Chilesaurus diegosuarezi gen. et sp. nov. among main dinosaur clades and its plots in the theropod morphospaces.
figure 3

a, Time-calibrated simplified strict consensus tree, in which the green bars indicate herbivorous dinosaur lineages28. E, Early; L, Late; M, Middle. Aal, Aalenian; Alb, Albian; Apt, Aptian; Baj, Bajocian; Ber, Berriasian; Brm, Barremian; Bth, Bathonian; Clv, Callovian; Crn, Carnian; Cmp, Campanian; Cnm, Cenomanian; Con, Coniacian; Hau, Hauterivian; Het, Hettangian; Kim, Kimmeridgian; Maa, Maastrichtian; Nor, Norian; Oxf, Oxfordian; Plb, Pliensbachian; Rh, Rhaetian; Sin, Sinemurian; Stn, Santonian; Tth, Tithonian; Toa, Toarcian; Tur, Turonian; Vlg, Valanginian. Numbers indicate millions of years ago. b–d, Chilesaurus diegosuarezi gen. et sp. nov. plotted in the theropod morphospace (principal coordinate (PCO)1 versus PCO2) based on axial skeleton (b); scapular girdle and forelimb (c); and pelvic girdle (d). Red dots in b–d indicate the position of Chilesaurus.

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The discovery of Chilesaurus lends support to the interpretation22,23 that dietary diversification towards herbivory was more commonplace among basal theropods than previously thought. Independent evolution of herbivory has been recognized for several major coelurosaurian subclades23, but for just a single probable example outside Coelurosauria (that is, the toothless ceratosaurian Limusaurus26,27). Chilesaurus expands the list of non-coelurosaurian theropods that shifted their diet from carnivore to herbivore.

Chilesaurus represents an extreme case of mosaic evolution among dinosaurs, owing to the presence of dental, cranial and postcranial features that are homoplastic with multiple disparate groups. Using quantitative morphospace analysis, we explored morphospace occupation of different skeletal regions in Chilesaurus with respect to a variety of avian and non-avian theropods. This shows that Chilesaurus has a ceratosaur-like axial skeleton, a ‘basal tetanuran’ forelimb and scapular girdle, a coelurosaur-like pelvis, and a tetanuran-like hindlimb (Fig. 3b–d and Extended Data Fig. 5). General ankle and foot construction does not group with any theropod clade, probably as a result of the characters shared by Chilesaurus, sauropodomorphs and herrerasaurids.

Chilesaurus is the numerically dominant taxon in the Aysén tetrapod fossil assemblage, and represents an unusual case of a theropod having the palaeoecological role of a preeminent small-to-medium sized herbivore in a Jurassic ecosystem. This is in sharp contrast with other Late Jurassic dinosaur assemblages (for example, Tendaguru and Morrison formations11,27), in which ornithischian dinosaurs are the most abundant small-to-medium sized herbivores. Available evidence indicates that Chilesaurus is a unique dinosaur lineage known only from southern South America, suggesting an outstanding case of endemism among otherwise relatively cosmopolitan worldwide Jurassic dinosaur faunas28.

Chilesaurus illustrates how much relevant data on the early diversification of major dinosaur clades remain unknown. It also provides an important cautionary benchmark in our attempts to gain a reliable view of the overall evolutionary history of Dinosauria.