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Two Late Cretaceous sauropods reveal titanosaurian dispersal across South America

Abstract

South American titanosaurians have been central to the study of the evolution of Cretaceous sauropod dinosaurs. Despite their remarkable diversity, the fragmentary condition of several taxa and the scarcity of records outside Patagonia and southwestern Brazil have hindered the study of continental-scale paleobiogeographic relationships. We describe two new Late Cretaceous titanosaurians from Quebrada de Santo Domingo (La Rioja, Argentina), which help to fill a gap between these main areas of the continent. Our phylogenetic analysis recovers both new species, and several Brazilian taxa, within Rinconsauria. The data suggest that, towards the end of the Cretaceous, this clade spread throughout southern South America. At the same locality, we discovered numerous accumulations of titanosaurian eggs, likely related to the new taxa. With eggs distributed in three levels along three kilometres, the new site is one of the largest ever found and provides further evidence of nesting site philopatry among Titanosauria.

Introduction

Titanosaurian sauropods are a group of large, long-necked, herbivorous dinosaurs with a complex evolutionary history1,2,3,4,5,6. During the Late Cretaceous, they underwent an extensive evolutionary radiation worldwide. Most of their record in South America is restricted to Argentine Patagonia (e.g., Neuquén, Golfo San Jorge and Austral basins) and the Bauru Basin of SW Brazil7,8,9 (Fig. 1a). Some studies have attempted to establish paleobiogeographic links between these regions10,11, although there are remarkable faunistic differences between Patagonian and Brazilian titanosaurians12,13,14,15. Similarly, other contemporaneous tetrapods, such as pleurodiran turtles and notosuchian mesoeucrocodylians, also show heterogeneous distributions16,17.

Fig. 1: Titanosaurian record in South America, map of the study area and skeletal reconstructions of the new titanosaurian species.
figure1

a Percentage diversity of Cretaceous titanosaurian sauropods in three main regions of South America: Patagonia (purple), NW Argentina (green), and SW Brazil (yellow) (Supplementary Table 3). The yellow ring corresponds to the record of the saltasaurid titanosaurian Yamanasaurus in Ecuador. Map modified from Scotese17. b Location of the discoveries. c Punatitan coughlini gen. et sp. nov. d Bravasaurus arrierosorum gen. et sp. nov. Preserved elements are coloured in red in c, d. Scale bar: 100 km in b, and 1 m in c, d.

By the Late Cretaceous, vast regions of South America remained flooded by epicontinental seas18, and although there are high-rank taxonomic similarities, the evidence of eventual connections between northern and southern terrestrial faunas are still scarce. The ubiquity of the clade Titanosauria in a geographically intermediate area is validated by the occurrence of the saltasaurids Yamanasaurus from Ecuador19 and Saltasaurus20—plus a putative record of Neuquensaurus21—from NW Argentina (Fig. 1a), along with fragmentary accounts of sauropod dinosaurs in the latter region. However, saltasaurids have not been documented so far in the Bauru Basin nor other units in Brazil11,22, and the non-saltasaurid specimens in NW Argentina are too fragmentary23 to allow determination of paleobiogeographic relationships. In addition to saltasaurids, the other high-level clade amongst titanosaurians is the Colossosauria, recently stem-based defined as the most inclusive clade containing Mendozasaurus but not Saltasaurus, nor Epachthosaurus9. It includes the subclades Rinconsauria and Lognkosauria (plus a few related taxa), whose taxonomic composition has fluctuated over the years2,3,4. The fossil record of colossosaurians has, so far, a disparate distribution, with most of its members reported in Patagonia and SW Brazil.

Herein, we report the discovery of new dinosaurs from the Upper Cretaceous red beds of the Quebrada de Santo Domingo locality (QSD) in the Andes of La Rioja, NW Argentina (Fig. 1b). We recovered three partial skeletons that belong to two new derived titanosaurian dinosaur species (Fig. 1c, d) in different stratigraphic positions of the Ciénaga del Río Huaco Formation. Moreover, we found titanosaurian egg clutches and eggshells in an intermediate stratigraphic position, distributed in three levels. With an overwhelming abundance of eggs, QSD is one of the largest nesting sites documented worldwide. The results of our phylogenetic analysis incorporating the two new taxa suggest that they have Patagonian and Brazilian affinities, reinforcing the hypothesis of a close relationship between the titanosaurian sauropod faunas from northern and southern South America during the Late Cretaceous.

Results

Systematic palaeontology

Sauropoda Marsh, 1878

Titanosauria Bonaparte and Coria, 1993

Colossosauria González Riga et al., 2019

Punatitan coughlini gen. et sp. nov.

Etymology.Puna’ is the local name that distinguishes the oxygen-depleted atmosphere typical of the high Andes, and ‘coughlini’ refers to the geologist Tim Coughlin, who reported the first dinosaur fossils in the area.

Holotype. CRILAR-Pv 614 (Paleovertebrate Collection of Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja, Argentina), partial skeleton composed of the anterior portion of posterior cervical vertebra (likely C12), two middle dorsal vertebrae (likely D6–D7), partial sacrum, 13 articulated caudal vertebrae (some with articulated haemal arches), right pubis, left ischium, and several dorsal ribs.

Horizon and type locality. Sandstone levels 170 m above the base of the Ciénaga del Río Huaco Formation (Campanian-Maastrichtian) at QSD, La Rioja, NW Argentina (Geological Setting in Supplementary Information).

Diagnosis. A medium-sized titanosaurian sauropod characterised by the following combination of features (autapomorphies marked with an asterisk): (1) middle dorsal vertebrae (likely D6–D7) with anterior and posterior spinodiapophyseal laminae (spdl) forming wide and flat surface, between aliform and transverse processes*; (2) accessory posterior centrodiapophyseal lamina (apcdl) crossed over by the posterior centroparapophyseal (pcpl) lamina, forming a X-shaped intersection in D6–D7; (3) pcpl reaches the bottom of posterior centrodiapophyseal lamina (pcdl) in D6–D7*; (4) extra-depression ventrally to intersection of pcpl and apcdl in D6–D7*; (5) deep postzygodiapophyseal centrodiapophyseal fossa (pocdf) in D6–D7; (6) neural spine of D6 tapering dorsally, forming an inverted-“V” profile in anterior/posterior view; (7) caudal transverse processes persist beyond Ca15; (8) slightly anteriorly inclined neural spines in anterior-middle caudal vertebrae (Ca5–6 to Ca10); and (9) distally expanded prezygapophyses in anterior-middle caudal vertebrae.

Description and comparisons of Punatitan

Most diagnostic features are in the axial skeleton of Punatitan (Fig. 2), allowing us to distinguish the new taxon from other titanosaurians. The holotype CRILAR-Pv 614 represents a medium-sized individual, larger than the holotypes of Overosaurus24, Saltasaurus25, Neuquensaurus26,27, and Trigonosaurus28, about the same size as the holotype of Uberabatitan29, and smaller than Aeolosaurus30, ‘Aeolosaurus11, Mendozasaurus3 and giant taxa (e.g., Argentinosaurus, Patagotitan).

Fig. 2: Punatitan coughlini gen. et sp. nov. (CRILAR-Pv 614).
figure2

a, b Cervical vertebra (C12) in dorsal a and anterior b views. c, d Dorsal vertebra (D6) in right lateral c and posterior d views. e Dorsal vertebra (D7) in anterior view. f Articulated series of caudal vertebrae (Ca5–Ca17). g Detail of Ca8–Ca12. acpl anterior centroparapophyseal lamina, apcdl accessory posterior centrodiapophyseal lamina, dp diapophysis, nc neural canal, ns neural spine, pcdl posterior centrodiapophyseal lamina. pcpl posterior centroparapophyseal lamina, pocdf postzygapophyseal centrodiapophyseal fossa, posdf postzygapophyseal spinodiapophyseal fossa, posl postspinal lamina, poz postzygapophysis, pp parapophysis, prsl prespinal lamina, prz prezygapophysis, spdl spinodiapophyseal lamina, spol spinopostzygapophyseal lamina, sprl spinoprezygapophyseal lamina, tprl interprezygapophyseal lamina. Circled numbers correspond to apomorphies numbered in the text. Measurements in Supplementary Table 1. Scale bars: 100 mm.

A cranial portion of a posterior cervical vertebra is only available (Fig. 2a, b). It may correspond to C12, based on Overosaurus and Trigonosaurus (MCT 1499-R28). The centrum is shorter dorsoventrally than it is wide transversely, with its anterior surface strongly convex. The base of the right parapophysis is level with the ventral border of the centrum and ventrally delimits the deeply concave lateral surface of the centrum. The prezygapophyses are anterolaterally projected and well separated from each other. Their anterior edge is placed slightly anterior to the level of the articular surface. Both are medially connected by a sharp interprezygapophyseal lamina (tprl) that forms an opened U-shaped edge in dorsal view. The right base of a rounded dorsomedially projected spinoprezygapophyseal lamina (sprl) is preserved. Although the neural arch is incomplete, the position and development of the prezygapophyses, together with the position, orientation, and robustness of the sprl, suggest a wide and concave spinoprezygapophyseal fossa (sprf). Overall, the cervical vertebra of Punatitan is similar to that of most titanosaurians. The robust sprl is more similar to that of Malawisaurus31, Mendozasaurus3, Futalognkosaurus32, and Dreadnoughtus33 than to Overosaurus24, in which the lamina is weakly developed, and the floor of the sprf is reduced. In Trigonosaurus28 the sprl is also conspicuous but relatively short, thus defining a small sprf.

Two dorsal vertebrae are known for Punatitan, interpreted as D6 (Fig. 2c, d) and D7 (Fig. 2e), based on comparisons with Overosaurus24 and Trigonosaurus28 (e.g., the relative position of parapophysis and diapophysis, orientation of neural spine). The centra are opisthocoelous, almost as high as wide. Laterally, they show deep and partitioned pleurocoels that have tapering, acute caudal margins. They are located dorsally, near the neurocentral junction. The neural arches are fused to the centra, without a sign of suture.

The diapophyses are robust and well projected laterally, while the parapophyses are more anteriorly and slightly ventrally positioned, as occurs in middle dorsal vertebrae (e.g., D5–D7 of Overosaurus24). Below these processes, the neural arches are notably intricate, showing a broad, deeply excavated fossa (Fig. 2c) with a conspicuous asymmetry in both lateral sides, as seen in other sauropods (e.g., Trigonosaurus28, Lirainosaurus34).

The pcdl and its anterior projection, the apcdl, plus the well-developed pcpl are the most conspicuous traits in the lateral aspects of these vertebrae (Fig. 2c), as seen in several titanosaurians, such as Malawisaurus31, Elaltitan35, Overosaurus24, Trigonosaurus28, and Dreadnoughtus33. The pcdl projects posteriorly to reach the posterodorsal border of the centrum. The apcdl projects anteriorly from the dorsal edge of this lamina, contacting the anterodorsal border of the centrum. The accessory lamina is crossed over by the pcpl, forming an X-shaped intersection that is evident on the right side of D6 and D7 (on left sides of both, the pcpl finishes when contacting the apcdl, forming a Y-shaped pattern). The pattern observed in D6–D7 of Punatitan is roughly observed in D7 of Overosaurus24 (other dorsal vertebrae have no clear X-pattern) and Petrobrasaurus36, but not in other titanosaurians such as Malawisaurus31, Elaltitan35, Trigonosaurus28, Lirainosaurus34, and Dreadnoughtus33. Conspicuously, these laminae define deep fossae in Punatitan. The deep, subtriangular fossa, dorsally delimited by the pcdl and apcdl is identified as posterior centrodiapophyseal fossa (pcdl-f)33. It is deeper in Punatitan than in Overosaurus24, Trigonosaurus28, Muyelensaurus37, and Dreadnoughtus33.

The anterior centroparapophyseal lamina (acpl) and pcpl project ventrally and posteroventrally, respectively, from the parapophysis. The pcpl is truncated on the left side of D6–D7 when touching the apcdl; consequently, on this side, the pcdl-f is much larger than on the right side. In both dorsal vertebrae, the acpl and pcpl also define a deep but small fossa.

The oval-shaped prezygapophyses are connected medially by transversely short tprl (Fig. 2e). They are detached from the diapophyseal body by a marked step that dorsally elevates their articular surface. In anterior view, the centroprezygapophyseal lamina (cprl) has a sharp border, and it widens dorsally. This lamina and the acpl define a deep fossa that faces anterolaterally. The sprl in these dorsal vertebrae are present as blunt structures that are poorly preserved. They connect the prespinal lamina (prsl) medially, without obstructing its path. A similar condition was inferred for Barrosasaurus38, and a posterior dorsal vertebra referred as to Trigonosaurus39, but they can correspond to accessory laminae rather than to the true sprl, which is usually seen in more anterior vertebrae40.

The postzygapophyses are higher than the lateral tip of the diapophysis in D6–D7, and there is no direct contact between the postzygapophyses and the diapophyses. Instead, there is a lamina that starts at the postzygapophysis and projects anterodorsally to connect to the spdl, closer to the base of the spine than to the base of the diapophysis. The homology of this lamina is debated40,41; it is here interpreted as the podl. This lamina is similar to the podl observed in dorsal vertebrae of Malawisaurus31, Choconsaurus (D6?42) and Dreadnoughtus (D6?33), and its unusual connection with the spdl may be related to changes of the neural spine inclination and the relative position of the postzygapophyses and diapophyses in middle dorsal vertebrae41. At this point, this short podl delimits ventrally a very small postzygapophyseal spinodiapophyseal fossa (posdf), which faces laterally (Fig. 2c). A similar small fossa is present in the anteriormost dorsal of Rapetosaurus43 and the mid-posterior dorsal of Bonitasaura44. It differs from the condition seen in Lirainosaurus and Neuquensaurus, in which the posdf is well developed and faces more posteriorly. The postzygapophyses in D6 slope dorsally to the neural spine without a spinopostzygapophyseal lamina (spol), differing from the condition of Dreadnoughtus33, Mendozasaurus3 and Elaltitan35, which have a sharp lamina. The centropostzygapophyseal lamina is also well developed, contacting the pcdl near the level of the neural canal. Both laminae define a large and deep pocdf.

The neural spine is complete in D6 of Punatitan. It is somewhat inclined posteriorly, with the tip extending as far posteriorly as the posterior border of the centrum (Fig. 2c). It is anteroposteriorly narrow and tapers dorsally. In anterior view, the contour of the tip is rounded, without any expansion, forming an inverted V-shaped profile, with a slightly sigmoid outline owing to the presence of aliform processes. The neural spine bears a prsl and a postspinal lamina (posl). The prsl is sharp in the basal half of the spine, separating two deep, wide fossae, laterally delimited by the prominent spdl. The posl is also sharp and expands over almost all the neural spine, delimiting two deep, narrow fossae, laterally bordered by the postzygapophyses, and the aliform processes (Fig. 2d). The neural spine of D6 in Punatitan differs from that of most titanosaurians, which have expanded (e.g., Dreadnoughtus33) or squared (e.g., Choconsaurus42, Overosaurus24, Trigonosaurus28) neural spines.

The still unprepared sacrum of Punatitan is incomplete and will be described elsewhere. However, it was possible to observe an ossified supraspinous rod placed over the preserved neural spines (two or more). This structure is known for Epachthosaurus, Malawisaurus, and basal titanosauriforms45.

The holotype of Punatitan also preserves 13 articulated caudal vertebrae as well as several haemal arches (Fig. 2f). The first preserved caudal possibly represents Ca5. As in most titanosaurians, these caudal vertebrae have strongly procoelous centra1. The centra are dorsoventrally tall, differing from the depressed centra of saltasaurines25,46. Their anterodorsal border is anteriorly displaced from the anteroventral one, resulting in an oblique profile in lateral view. They have slightly concave lateral surfaces, with transversely thin ventrolateral ridges that delimit a deeply concave ventral surface that is devoid of fossae. The internal tissue of the caudal centra is spongy, and the neural arches are apneumatic.

In the anterior caudal vertebrae, a suture is present above the base of the transverse processes (Fig. 2g). It forms a conspicuous ridge, which is not evident in related taxa, although it resembles the dorsal tuberosity described for Baurutitan47, and also CRILAR-Pv 518c from Los Llanos, east La Rioja23. The neural arch of each caudal vertebra is situated over the anterior two-thirds of the centrum, and each is relatively tall with well-developed prezygapophyses and neural spines. The transverse processes are sub-triangular to laminar and gradually change from laterally to posterolaterally projected along the vertebral column. The prezygapophyses are long and project anterodorsally. The postzygapophyses contact the neural spine via a short spol and are located almost at the midline of the centra. This condition differs from the much more anteriorly placed postzygapophyses of the Patagonian Aeolosaurus30. The neural spine is rectangular in cross-section and anteroposteriorly longer than transversely wide (including prsl and posl). The spines are tall in the anterior caudal vertebrae and become shorter and square in the posterior ones. They also project slightly anteriorly, especially in Ca8–Ca10 (Fig. 2g). Some degree of anterior inclination of the neural spines is also reported for Trigonosaurus28 and Aeolosaurus30, contrasting with the most common condition amongst titanosaurians, i.e., vertical or posteriorly oriented neural spines (e.g., Baurutitan47, Dreadnoughtus33, Saltasaurus25). The available haemal arches are opened Y-shaped, with no expanded pedicels, as are those reported for other derived titanosaurians48.

Bravasaurus arrierosorum gen. et sp. nov.

Etymology. Bravasaurus, referred to the Laguna Brava, a lake that gives name to the Laguna Brava Provincial Park, and arrierosorum, refers to the people who crossed the Andes carrying cattle during the 19th century.

Holotype. CRILAR-Pv 612, right quadrate and quadratojugal, four cervical, five dorsal, and three caudal vertebrae, few dorsal ribs, three haemal arches, left humerus, fragmentary ulna, metacarpal IV, partial left ilium with sacral ribs, right pubis, partial ischium, left femur, and both fibulae.

Paratype. CRILAR-Pv 613, isolated tooth, right ilium, right femur, and dorsal ribs.

Horizon and type locality. Sandstone levels 34 m above the base of the Ciénaga del Río Huaco Formation (Campanian-Maastrichtian) at QSD, La Rioja, NW Argentina (Geological Setting in Supplementary Information).

Diagnosis. A small-sized titanosaurian sauropod characterised by the following association of features (autapomorphies marked with an asterisk): (1) quadrate with articular surface entirely divided by medial sulcus*; (2) sprl forms conspicuous step between neural spine and prezygapophyses, in middle cervical vertebrae*; (3) strongly depressed centra (up to twice as wide as tall) in posterior dorsal vertebrae; (4) robust dorsal edge of pneumatic foramen in dorsal centra, forming prominent shelf that extends laterally, beyond the level of the ventral margin of the centum*; (5) posterior dorsal vertebrae with a rough posl, ventrally interrupted by middle spinopostzygapophyseal laminae (m.spol) that contact the postzygapophyses; (6) posterior dorsal vertebrae with small ventral spinopostzygapophyseal fossa (v.spof) delimited dorsally by the m.spol and ventrally by the interpostzygapophyseal lamina (tpol); (7) humerus with narrow midshaft, with midshaft/proximal width ratio of 0.36; (8) deltopectoral crest of the humerus expanded distally; (9) slender fibula (Robustness Index [RI]49 = 0.15); (10) distal condyle of the fibula transversely expanded, more than twice the midshaft breadth.

Description and comparisons of Bravasaurus

The holotype of Bravasaurus (Figs. 3 and 4), as well as the referred specimen, indicates a small-sized titanosaurian, much smaller than Punatitan (Fig. 1c, d) and other medium-sized sauropods, such as Trigonosaurus, Overosaurus, and Bonitasaura. Considering that both specimens could be adults (see below), they would be similar to Neuquensaurus or Magyarosaurus50. Cranial elements include partial right quadrate and quadratojugal (Fig. 3a, b). The quadrate is anteroventrally directed and bears part of the quadrate fossa. The articular surface for the mandible is transversely elongated. It shows two condyles that separate from each other by a longitudinal sulcus (Fig. 3b). The medial condyle is round, whereas the lateral is anteroposteriorly elongated. Diplodocus51 also has a sulcus but restricted to the posterior region of the articular surface. Among titanosaurians, the articular surface of the quadrate has a kidney shape in Nemegtosaurus and Quaesitosaurus52, with the sulcus restricted to its anterior portion. In Bonitasaura53 and Rapetosaurus54, the articular surface is not divided. The anterior process of the quadratojugal projects ventrally, whereas the posterolateral process barely extends ventrally, similar to Nemegtosaurus52, and much less developed than in Tapuiasaurus55 and Sarmientosaurus4. Unlike in these latter taxa, the posterolateral process reaches the articular condyle of the quadrate, which can only be seen behind (and not below) the quadratojugal in lateral view (Fig. 3a).

Fig. 3: Axial elements of Bravasaurus arrierosorum gen. et sp. nov. (CRILAR-Pv 612).
figure3

a, b Quadrate and quadratojugal with interpretative drawing in right lateral a, and ventral b views (anterior to the right). c Middle cervical vertebra in right lateral view. d Anterior dorsal vertebra (D2) in posterior view. e Middle dorsal vertebra (D7) in right lateral view. fg Posterior dorsal vertebra (D8) in posterior f and left lateral g views. h, i Middle caudal vertebra in left lateral h and ventral i views (anterior towards left). acpl anterior centroparapophyseal lamina, ap anterior projection, dp diapophysis, itf infratemporal fenestra, m.spol middle spinopostzygapophyseal lamina, nc neural canal, ns neural spine, pcdl posterior centrodiapophyseal lamina, pcdlf posterior centrodiapophyseal fossa, pcpl posterior centroparapophyseal lamina, pl pleurocoel, podl postzygodiapophyseal lamina, posdf postzygapophyseal spinodiapophyseal fossa, posl postspinal lamina, poz postzygapophysis, pp parapophysis, prz prezygapophysis, q quadrate, qj quadratojugal, spdl spinodiapophyseal lamina, tpol interpostzygapophyseal lamina, and v.spof ventral spinopostzygapophyseal fossa. Circled numbers correspond to apomorphies numbered in the text. Measurements in Supplementary Table 2. Scale bars: 10 mm in a, b, and 50 mm in ci.

Fig. 4: Appendicular elements of Bravasaurus arrierosorum gen. et sp. nov. (CRILAR-Pv 612).
figure4

a Left humerus in anterior view. b Right pubis in ventrolateral view. c, d Left femur in anterior c and posterior d views. e, f Right fibula in lateral e and medial f views. dc deltopectoral crest, fh femoral head, ft forth trochanter, gt greater trochanter, lb lateral bulge, lt lateral tuberosity, and vc ventral crest. Circled numbers correspond to apomorphies numbered in the text. Measurements in Supplementary Table 2. Scale bars: 100 mm.

The holotype of Bravasaurus preserves cervical, dorsal, and caudal vertebrae. The neural arches of all elements are completely fused to their respective centra, which may indicate that it had reached somatic maturity before death56,57,58.

We recovered four anterior-middle cervical vertebrae less than half a meter away from the cranial material. Three of them are articulated and associated with ribs. They are opisthocoelous, with sub-cylindrical and relatively elongated centra (Fig. 3c). The neural arches have low neural spines, as observed in Rinconsaurus59 and Uberabatitan29. The diapophyses have posterior extensions, and the prezygapophyses are placed beyond the articular condyle of the centrum, as seen in the latter taxa. In Bravasaurus the postzygodiapophyseal lamina (podl) splits into a diapophyseal and a zygapophyseal segment, which become parallel with each other. Previous studies identified this feature as exclusive of Uberabatitan13,29. In derived titanosaurians, the neural spines contact the prezygapophyses via the sprl, which is straight or slightly curved ventrally in lateral view. In the anterior cervical vertebrae of few titanosaurians (e.g. Saltasaurus25 and Rocasaurus47), the sprl curves dorsally, forming a step close to the prezygapophysis. This step disappears beyond the first cervical vertebrae but remains present in middle cervical vertebrae of Bravasaurus (C5?–C6?; Fig. 3c).

The dorsal vertebrae of Bravasaurus have relatively short, opisthocoelous centra (Fig. 3d–g). The well-developed pleurocoels are located just below the dorsal margin of the centrum, which forms a shelf that extends laterally, beyond the limits of the centrum, in middle and posterior dorsal vertebrae. Except for D10, the preserved dorsal centra are strongly dorsoventrally depressed (Fig. 3d, f), as in Opisthocoelicaudia60, Alamosaurus61, Trigonosaurus28, and the “Series A” from Brazil30. The neural arches of the dorsal vertebrae are tall, but not as tall as in Punatitan, in which the pedicels are particularly long. The orientation of the preserved neural spines follows the same pattern as in other derived titanosaurians, i.e., vertical in anterior and posterior-most dorsal vertebrae, and inclined (as much as 40°) in middle dorsal vertebrae (e.g., Trigonosaurus28). The prsl and posl are robust along their entire length (especially in the posterior dorsal vertebrae).

The anterior dorsal (D2) shows a low, laterally expanded neural arch (Fig. 3d). Although poorly preserved anteriorly, this vertebra exhibits a broad prespinal fossa with a weak prsl. It has rounded, ventrolaterally inclined postzygapophyses that reach the diapophyses though long podl. Medially, the postzygapophyses join each other by small laminae (tpol?) that intersect at the height of the dorsal edge of the neural canal. The junction between these laminae and the dorsal edge of the neural canal forms two small fossae, as seen in the posterior cervical vertebrae of Overosaurus24. The neural spine is relatively low, and the postspinal fossa is particularly deep compared with the other dorsal vertebrae. The posl is weak. On the lateral aspect, the pcdl and the apcdl are the most conspicuous laminae. The diapophysis is eroded, and the parapophysis is located on the centrum above the pleurocoel.

The middle dorsal (D7) shows a slightly higher neural arch than D2, and its neural spine is inclined posteriorly, beyond the posterior articular surface of the centrum (Fig. 3e). The parapophysis is missing, but the orientation of acpl and pcpl suggests a position slightly below and anterior to the diapophysis. In D8 and D10, a pair of m.spol interrupts the path of the posl, ventrally limiting a single, small fossa, here interpreted as v.spof (Fig. 3f). Its ventral limit corresponds to the tpol. A similar structure is present in Lirainosaurus34. The podl is present in all the posterior dorsal vertebrae (D8–D10).

The anterior and middle caudal vertebrae of Bravasaurus are procoelous. The centra are as tall dorsoventrally as they are wide transversely, without any concavities on their ventral surfaces (Fig. 3h, i). The anterior margin of the centra does not appear to be anteroventrally inclined, as occurs in Punatitan, Overosaurus24, or Aeolosaurus30. The neural arches are on the anterior portion of the centra, as in most titanosaurians, and some other titanosauriforms (e.g., Wintonotitan62). The neural spines are laminar and vertically directed, while the prezygapophyses are short and anteriorly projected. Such morphology shows many similarities with Rinconsaurus59 and Muyelensaurus37, but even more so with the Brazilian Trigonosaurus28 and Uberabatitan13,29. As for the centra, Bravasaurus differs from saltasaurines, in which they are depressed, with a ventral longitudinal hollow (e.g., Saltasaurus25). Nor do they possess the ventrolateral ridges (Fig. 3i) present in other titanosaurians such as Aeolosaurus30, Overosaurus24, and Punatitan. Bravasaurus also differs from the latter taxa by the orientation of the neural spine in the anterior caudal, which is vertical rather than anteriorly directed. None of the preserved caudal vertebrae shows signs of distal expansion in the prezygapophyses, as seen in Punatitan.

The morphology of the humerus is compatible with that of many colossosaurian titanosaurians. Its robustness is high (RI = 0.35), as in Opisthocoelicaudia60, Diamantinasaurus63, and Savannasaurus64, much more than in Rinconsaurus59 and Muyelensaurus37. The deltopectoral crest is markedly expanded distally (Fig. 4a), as in Saltasaurus25, Neuquensaurus27, Opisthocoelicaudia60, and Dreadnoughtus33. All pelvic elements are represented in the holotype, although only the pubis (Fig. 4b) allows comparisons. It is proximodistally elongate and less robust than in Futalognkosaurus32 or Opisthocoelicaudia60. The distal end is markedly expanded, as in several derived forms (e.g., Rapetosaurus43, Bonitasaura44, Muyelensaurus37). The ilium of the specimen CRILAR-Pv 613 resembles the ilium of other derived titanosaurians, such as Rapetosaurus and Bonatitan65. The femur is straight, with the fourth trochanter placed at the proximal third (Fig. 4c, d), as in Uberabatitan13, Patagotitan2, Bonitasaura44, and Futalognkosaurus32, whereas in Rinconsaurus59, Muyelensaurus37, and Diamantinasaurus63 it is located in the middle third. The humerus-to-femur length ratio in Bravasaurus is 0.75, similar to Opisthocoelicaudia, higher than Neuquensaurus and Saltasaurus, but lower than Patagotitan and Epachthosaurus. The fibula (Fig. 4e, f) markedly contrasts with the rest of the appendicular elements, as it is particularly gracile. Its distal condyle is transversely expanded, as observed in Epachthosaurus66.

The known specimens of Bravasaurus indicate a small adult size. We estimate a body mass of 2.89 tons (2.17–3.61 tons, considering 25% error), based on a calibrated equation67 (see “Methods” section). Estimates of <10 tons are few among titanosaurians. The European Magyarosaurus (750 kg), is interpreted as a case of insular dwarfism50,68. The mass of the European Lirainosaurus was less than two tons50, whereas that of the Argentinean Saltasaurus and Neuquensaurus was five and six tons2, respectively. Among colossosaurians, estimations for Rinconsaurus indicate just four tons2 and at least some other genera (e.g., Overosaurus, Trigonosaurus, Baurutitan), lacking appendicular bones, are small-sized forms, slightly larger than Bravasaurus, based on their vertebral size.

Phylogenetic analysis

The result of our phylogenetic analysis nests Punatitan and Bravasaurus as derived titanosaurians in all most parsimonious trees. The topology of the strict consensus tree is similar to that obtained in previous studies using the same dataset2,6, although some taxa, such as Baurutitan and Trigonosaurus show noticeable changes in their position (Fig. 5; Supplementary Fig. 4). The former one is placed as the basalmost colossosaurian, and the latter is clustered together with Uberabatitan, Gondwanatitan, and Bravasaurus.

Fig. 5: Phylogenetic relationships of Punatitan and Bravasaurus within Lithostrotia.
figure5

Phylogeny of derived titanosaurians, based on the data set of Carballido et al. 6 (see “Methods” section and Supplementary Fig. 4). Time ranges for each terminal were obtained from published data. Colours in South American taxa are based on their palaeolatitudinal position. Both time ranges and palaeolatitude are given in Supplementary Table 4. 1. Lithostrotia, 2. Eutitanosauria, 3. Saltasauridae, 4. Colossosauria, 5. Lognkosauria, 6. Rinconsauria, and 7. Aeolosaurini.

Both Punatitan and Bravasaurus are recovered within Colossosauria9. Punatitan shows three of the seven ambiguous synapomorphies that diagnose the newly erected clade9, and Bravasaurus five. Furthermore, the new Riojan species are placed within the clade Rinconsauria, along with several titanosaurians from SW Brazil and Patagonia (Fig. 5). Punatitan is nested with the Argentinean Aeolosaurus, by sharing the presence of distally expanded prezygapophyses in posteriormost anterior and middle caudal vertebrae. Other features of the caudal vertebrae, such as the dorsal edge of the anterior articular surface of the centrum ahead of the ventral margin, and the neural spines anteriorly oriented in the posteriormost anterior and middle caudal vertebrae, relate the latter taxa with the Brazilian ‘Aeolosaurus’ and Overosaurus, as successive sister taxa. Bravasaurus is included in a collapsed clade comprising the Brazilian Trigonosaurus, Uberabatitan, and Gondwanatitan. The clade is supported by a single synapomorphy: height/width ratio smaller than 0.7 in the posterior articular surface of cervical centra.

QSD nesting site

We documented three egg-bearing levels in the lower section of Ciénaga del Río Huaco Formation at QSD. The egg clutches and eggshells are included in an interval of floodplain deposits in at least three distinct but closely spaced horizons at 59.2, 62.8 and 63.9 m above the base of the unit (Supplementary Fig. 1). Fossil-bearing rocks are siltstones and sandy siltstones with horizontal lamination and graded and massive bedding that form thin tabular sheets, extending for tens to hundreds of metres. The fossiliferous layer is laterally traced over more than three kilometres, and the egg clutches and eggshells (CRILAR-Pv 620–621) are exposed regularly all along with it. Nineteen egg clutches were spotted, one with up to 15 sub-spherical eggs, arranged in two superposed rows.

The QSD eggs are similar to some Late Cretaceous titanosaurian eggs69. Among the remarkable diversity of eggs worldwide, only Auca Mahuevo70 (Argentina), Dholi Dungri71 (India), and Toteşti72 (Romania) preserve titanosaurian embryos. Therefore, these sites are the most reliable to correlate eggs with their producers. At QSD, the eggs are cracked, slightly compressed and flattened by the sedimentary load (Fig. 6a, b). We estimate an egg size of 130–140 mm, similar to the eggs from Auca Mahuevo70 and Toteşti72, but slightly smaller than the ones from Dholi Dungri71 (160 mm). The eggshells are mono-layered, measuring 1.67 ± 0.31 mm (n = 30). The thickness is similar to the eggshells from layers 1–3 of Auca Mahuevo. The eggshells from Toteşti and layer 4 of Auca Mahuevo are slightly thicker, measuring 1.7–1.8 mm, whereas in Dholi Dungri they reach 2.26–2.36 mm. The QSD shells are composed of densely packed shell units of calcite crystals, which radiate from nucleation centres (Fig. 6c, d). They flare out at 50°, and their lateral margins become parallel at the inner third of the shell, like in the Auca Mahuevo specimens70. Outwards, the units end out in rounded nodes of 0.30.4 mm in diameter, forming densely packed ornamentation that is typical of the titanosaurian clade69,70,71,72. Multiple straight pore canals run through the eggshell, between the shell units. They have funnel-shaped external apertures that form round depressions between the surficial nodes. Among titanosaurian eggshells, those from Dholi Dungri and Auca Mahuevo (layers 1–3) also have straight pore canals, whereas, in those from Toteşti and the layer 4 of Auca Mahuevo, the pore canals ramify in a Y-shaped pattern.

Fig. 6: Quebrada de Santo Domingo nesting site.
figure6

a Part of a titanosaurian egg clutch, CRILAR-PV 620/001. b Partial egg and the surrounding matrix. c, d Eggshell micrographs under SEM c and TLM d. Note the straight pore canal with a funnel-shaped aperture in c. Scale bars: 100 mm in a, and 0.5 mm in c, d.

As in Auca Mahuevo and other Cretaceous nesting sites, the QSD specimens are preserved in a floodplain palaeoenvironment. The occurrence of compact accumulations of whole eggs is consistent with the hypothesis of incubation within the substrate, as currently do the megapode birds from Australasia69. Along with the egg clutches, hundreds of shells also appear scattered within the egg-bearing levels. Such an arrangement could be a consequence of the local transport of exposed shells during floods, but also the product of local removal during subsequent nesting episodes. Soft sediment deformation and dislocation are frequent, and could also have contributed to their dispersion. These features suggest that each of the three egg-bearing levels could constitute a time-averaged assemblage.

Discussion

As far as we know, Punatitan and Bravasaurus represent the first confirmed occurrence of colossosaurian titanosaurians9 in NW Argentina. For 40 years, Saltasaurus remained as the only well-represented sauropod for this region. Saltasaurus is closely related with the Patagonian Rocasaurus and Neuquensaurus, as well as Yamanasaurus19, from Ecuador. There is a consensus regarding the close relationship of these taxa, which constitute the Saltasaurinae, a clade of small-sized titanosaurians from the Late Cretaceous that is also supported by our phylogenetic result. The phylogenetic data also suggest that saltasaurines may not have a close relationship with other Late Cretaceous titanosaurians from South America (Fig. 5). Fragmentary findings in NW Argentina20,23,73 and Chile74 suggested the occurrence of non-saltasaurine titanosaurians between Patagonia and Bauru, but the hitherto known evidence was insufficient to conjecture about their phylogenetic affinities. The new phylogenetic analysis recovers Punatitan within a clade of typically “aeolosaurine” taxa, such as Aeolosaurus and Overosaurus, whereas Bravasaurus is nested in a collapsed clade with Brazilian species. The Patagonian and Brazilian Aeolosaurus species show a close relationship as previously supported11, but recent phylogenetic analyses, including the one here presented, suggest the Brazilian species may represent a distinctive genus, other than Aeolosaurus12,13. Both Riojan species expand the diversity of the clade Rinconsauria, and its geographical distribution.

Based on a combination of direct observations and body mass estimation, Bravasaurus was a small-sized titanosaurian, though not as small as the dwarf Magyarosaurus or Lirainosaurus. Although it had probably reached its maximum size, it is much smaller than Punatitan (Fig. 1c, d). The largest titanosaurians ever known are placed within colossosaurians2,9 (e.g., Argentinosaurus, Patagotitan), but others are relatively smaller, such as Rinconsaurus, Overosaurus, Trigonosaurus, Baurutitan, and Gondwanatitan. In this context, the available evidence suggests that Bravasaurus (~3 tons) is the smallest colossosaurian yet recorded, followed by the taxa mentioned above. In contrast to Magyarosaurus68, Bravasaurus appears to have inhabited inland territories. By the latest Late Cretaceous, there is an evident reduction in size in saltasaurids and rinconsaurians across South America, which may be related to fluctuations in climate75 and vegetation76 (e.g., grassland), as a result of more temperate conditions and influence of remnant epicontinental seas during the dynamic aperture of the Atlantic.

The new findings from La Rioja reduce the paleobiogeographic gap of Late Cretaceous colossosaurians in South America, which were previously restricted to Patagonia and SW Brazil. Colossosauria is divided into the gigantic Lognkosauria (e.g., Patagotitan, Futalognkosaurus), plus some related forms, and the Rinconsauria. So far, the former clade is mostly limited to Patagonia (although there are few putative non-rinconsaurians in Brazil14), whereas Rinconsauria may contain a few Brazilian forms2,6,9,77. Besides, some taxa recovered within Rinconsauria are often included within Aeolosaurini, a group of titanosaurians with unstable interspecific phylogenetic relationships12. Our results suggest that Rinconsauria is much more diverse and widely distributed than previously thought2,3,6,9,37. The oldest representatives of this clade would be in northern Patagonia, for the earliest Late Cretaceous. By the Campanian–Maastrichtian, the Rinconsauria increased their diversity and spread geographically northward, through La Rioja, to SW Brazil.

Comparison of the QSD eggs with confirmed occurrences of titanosaurian eggs, such as Auca Mahuevo70 and Toteşti72, allow their identification. The spherical shape of the eggs, the mono-stratified shells and the nodular external ornamentation indicate that the QSD eggs belong to titanosaurian sauropods. More specific features (e.g., egg size, shell thickness, and straight vertical pore canals), associate the QSD specimens with the Auca Mahuevo eggs (layers 1–3). La Rioja Province is already known for its titanosaurian nesting sites in the Los Llanos region, several hundred kilometres southeast of QSD78,79. There, two localities preserve Late Cretaceous nesting sites that show distinct palaeoenvironmental conditions. The eggs from these sites markedly differ in their shell thicknesses but share the same egg diameter, around 170 mm, larger than the 140 mm eggs from QSD. In South America, the only eggs to match that size are those from Auca Mahuevo and Río Negro80, in Patagonia, as well as an isolated record from Bauru81. Eggs similar in diameter were attributed to dwarf Cretaceous titanosaurians from Toteşti72. The QSD eggs are relatively small, so either Bravasaurus or Punatitan may have been the producers. Further specimens are required to evaluate each scenario.

Both the oological and sedimentological data suggest a distinct nesting strategy from other sites of La Rioja. Unlike the sites in Los Llanos, the titanosaurian eggs of QSD appear in successive floodplain deposits, as occurs in Auca Mahuevo and other nesting sites worldwide69. Each of the egg-bearing levels contains multiple egg accumulations that were not necessarily laid contemporaneously. The several episodes interspersed in the sedimentary sequence allow us to infer nesting site philopatry, a behaviour that seems to have been frequent among Cretaceous titanosaurians69,72,78,82,83. This evidence and egg morphological features advocate a nesting strategy similar to that displayed at Auca Mahuevo. The QSD site provides further evidence on the plasticity of Late Cretaceous titanosaurian sauropods regarding their nesting strategies. Although it is still necessary to better understand the nesting conditions in other regions, such as Brazil, it seems increasingly evident that the adaptation to different nesting strategies could have been crucial in the diversification and dispersal of titanosaurians across South America.

Methods

Specimens

All material described in this study is housed at the Paleovertebrate Collection of CRILAR (La Rioja, Argentina).

Taxa and systematic definitions

For the sake of simplicity, we used generic names when they are monotypic. The only exception corresponds to Aeolosaurus. The data set already included ‘Aeolosaurusmaximus, a taxon which has been recognised as a member of Aeolosaurini84, although it does not exhibit the diagnostic features of the genus (see Martinelli et al. 12 for further discussion) and is not grouped with the Patagonian species in some analyses13,14. Consequently, we refer to it as ‘Aeolosaurus’. We followed the systematic definitions provided by Carballido et al.2 and González Riga et al.9.

Eggshell micro-characterisation

We selected several eggshell fragments from QSD for microscopic imaging. Thin sections were carried out in the Petrology Lab at CRILAR, La Rioja, using the standard protocol for petrographic sectioning. We cut and mounted six eggshell fragments for their observation under a scanning electron microscope, following the protocol described in a previous study85. We used a LEO 1450VP equipment in the Laboratorio de Microscopía Electrónica y Microanálisis (Universidad Nacional de San Luis, San Luis, Argentina).

Body mass

We estimated the body mass of Bravasaurus using a scaling equation adjusted for phylogenetic correlation/covariance67. The equation

$${\rm{log}}\ {\rm{BM}} = 2.754 \cdot {\rm{log}}\ {{\rm{C}}_{{\rm{H}}+{\rm{F}}}}−1.097$$

where BM is body mass, and CH+F is the sum of circumferences of the humerus and femur. It has been used to estimate the body mass of gigantic (e.g., Patagotitan2), as well as medium-sized titanosaurians (e.g., Rapetosaurus86).

Phylogenetic analysis

We tested the phylogenetic position of Bravasaurus and Punatitan amongst 30 derived titanosaurian terminals using a modified version of the data matrix of Carballido et al.6. This matrix has been used to assess the phylogenetic position of derived titanosaurians and related taxa (e.g., Sarmientosaurus4, Patagotitan2).

Data on several South American titanosaurians was added in order to expand the representation of their diversity. We added scorings for Gondwanatitan and Uberabatitan to increase the information on Brazilian taxa. We also included Aeolosaurus rionegrinus30 and the saltasaurine Rocasaurus, from Patagonia to the data set.

We added five characters (four from previous studies and one new) and modified few scorings (Supplementary Tables 4, 5; Supplementary Data 1). This resulted in a data set of 96 taxa and 421 characters (Phylogenetic Analysis in Supplementary Information, and Supplementary Data 2). As in previous studies6, 24 characters were considered as ordered (14, 61, 100, 102, 109, 115, 127, 132, 135, 136, 167, 180, 196, 257, 260, 277, 278, 279, 280, 300, 304, 347, 353, 355).

Statistics and reproducibility

We performed a parsimony analysis of the modified data matrix using TNT v.1.187. We did a heuristic search with 1000 replicates of Wagner trees and two rounds of tree bisection-reconnection branch swapping. Branch support was quantified using decay indices (Bremer support values). They were calculated with TNT v.1.187, and are given in the Supplementary Fig. 4. A TNT file containing raw data for the parsimony analysis is available in the Supplementary Data 2.

Nomenclatural acts

This published work and the nomenclatural acts it contains have been registered in ZooBank, the proposed online registration system for the International Code of Zoological Nomenclature (ICZN). The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix “http://zoobank.org/”. The LSIDs for this publication are: urn:lsid:zoobank.org:pub:CDA87D24-50DA-415A-9FAF-54FB7CF26D73; urn:lsid:zoobank.org:act:18840DCF-33EF-465D-8F69-0B38BB601BF7; urn:lsid:zoobank.org:act:658B5D64-1432-46BC-B543-DFF1155EC71E; urn:lsid:zoobank.org:act:336215DA-56AB-4B69-8059-C1FFA564D58A; urn:lsid:zoobank.org:act:84B7ECE6-60B4-4324-B983-CD86C8952E8A.

Reporting summary

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

Data availability

Additional information, including the dataset analysed in this study, is available in the Supplementary Information, and Supplementary Data 1, 2 files. CRILAR-Pv 612-614 and 620-621 are deposited at the Paleovertebrate Collection of CRILAR (Anillaco, La Rioja), and are available upon request.

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Acknowledgements

We thank Secretaría de Cultura, Subsecretaría de Patrimonio, and Gobierno de La Rioja. We are grateful to Sergio de la Vega, Carlos Bustamante, Julia Desojo, Hernán Aciar, Leonel Acosta, Marcelo Miñana, Victoria Fernandez Blanco, Tomaz Melo, Jimena Trotteyn, Mariano Larrovere, Marcos Macchioli, Tatiana Sánchez, Gabriel Hechenleitner, Eugenio Sanchez, and Walter Bustamante for their help during fieldwork and preparation of the specimens. Special thanks to Grupo Roggio, and Alberto Acevedo, Germán Brizuela, Rubén Brizuela, and Carlos Verazai, for providing us with accommodation at their facilities during fieldworks (2016–2019). We thank Tim Coughlin, Rod Holcombe, and Andrea Arcucci for sharing information about QSD. Comparative data were collected in previous visits thanks to Pablo Ortiz (Instituto Miguel Lillo, Tucumán), Ignacio Cerda (Museo Carlos Ameghino, Río Negro), Diógenes de Almeida Campos and Rodrigo Machado (MCT), and Luiz Carlos Borges Ribeiro (CPPLIP-UFTM). We thank Jose Carballido for his help with the phylogenetic analysis. The Willi Henning Society for TNT software. Research supported by CONICET, The Jurassic Foundation (E.M.H.), and the Palaeontological Society PalSIRP (E.M.H.). Fieldwork permits (2015–2019): RES S.C. 135, 238, 109, 124.

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E.M.H. coordinated the project (including fieldwork); E.M.H., A.G.M., L.E.F. were involved in study concept. E.M.H., L.L., A.G.M., and S.R. wrote the paper, and with J.R.A.T., L.E.F., and L.S. gathered the data. L.L., L.E.F., and S.R. made the figures. All authors discussed the results and commented on the manuscript.

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Correspondence to E. Martín Hechenleitner.

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Hechenleitner, E.M., Leuzinger, L., Martinelli, A.G. et al. Two Late Cretaceous sauropods reveal titanosaurian dispersal across South America. Commun Biol 3, 622 (2020). https://doi.org/10.1038/s42003-020-01338-w

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