Two Late Cretaceous sauropods reveal titanosaurian dispersal across South America

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. Hechenleitner et al. describe two new titanosaurians and the finding of numerous accumulations of titanosaurian eggs in La Rioja, Argentina. This study suggests nesting site philopatry among Titanosauria and that this clade was spread throughout southern South America at the end of the Late Cretaceous.

By the Late Cretaceous, vast regions of South America remained flooded by epicontinental seas 18 , 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 Ecuador 19 and Saltasaurus 20 -plus a putative record of Neuquensaurus 21 -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 Brazil 11,22 , and the non-saltasaurid specimens in NW Argentina are too fragmentary 23 to allow determination of paleobiogeographic relationships. In addition to saltasaurids, the other high-level clade amongst titanosaurians is the Colossosauria, recently stembased defined as the most inclusive clade containing Mendozasaurus but not Saltasaurus, nor Epachthosaurus 9 . It includes the subclades Rinconsauria and Lognkosauria (plus a few related taxa), whose taxonomic composition has fluctuated over the years [2][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.  Table 3). The yellow ring corresponds to the record of the saltasaurid titanosaurian Yamanasaurus in Ecuador. Map modified from Scotese 17   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-R 28 ). 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 Malawisaurus 31 , Mendozasaurus 3 , Futalognkosaurus 32 , and Dreadnoughtus 33 than to Overosaurus 24 , in which the lamina is weakly developed, and the floor of the sprf is reduced. In Trigonosaurus 28 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 Overosaurus 24 and Trigonosaurus 28 (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 Overosaurus 24 ). 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., Trigonosaurus 28 , Lirainosaurus 34 ).
The pcdl and its anterior projection, the apcdl, plus the welldeveloped pcpl are the most conspicuous traits in the lateral aspects of these vertebrae (Fig. 2c), as seen in several titanosaurians, such as Malawisaurus 31 , Elaltitan 35 , Overosaurus 24 , Trigonosaurus 28 , and Dreadnoughtus 33 . 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 Xshaped 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 Overosaurus 24 (other dorsal vertebrae have no clear X-pattern) and Petrobrasaurus 36 , but not in other titanosaurians such as Malawisaurus 31 , Elaltitan 35 , Trigonosaurus 28 , Lirainosaurus 34 , and Dreadnoughtus 33 . 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 Overosaurus 24 , Trigonosaurus 28 , Muyelensaurus 37 , and Dreadnoughtus 33 .
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 Barrosasaurus 38 , and a posterior dorsal vertebra referred as to Trigonosaurus 39 , but they can correspond to accessory laminae rather than to the true sprl, which is usually seen in more anterior vertebrae 40 .
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 debated 40,41 ; it is here interpreted as the podl. This lamina is similar to the podl observed in dorsal vertebrae of Malawisaurus 31 , 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 vertebrae 41 . 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 Rapetosaurus 43  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., Dreadnoughtus 33 ) or squared (e.g., Choconsaurus 42 , Overosaurus 24 , Trigonosaurus 28 ) 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 titanosauriforms 45 .
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 centra 1 . The centra are dorsoventrally tall, differing from the depressed centra of saltasaurines 25,46 . 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 Baurutitan 47 , and also CRILAR-Pv 518c from Los Llanos, east La Rioja 23 . The neural arch of each caudal vertebra is situated over the anterior twothirds of the centrum, and each is relatively tall with welldeveloped 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 Aeolosaurus 30 . 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 Trigonosaurus 28 and Aeolosaurus 30 , contrasting with the most common condition amongst titanosaurians, i.e., vertical or posteriorly oriented neural spines (e.g., Baurutitan 47 , Dreadnoughtus 33 , Saltasaurus 25 ). The available haemal arches are opened Y-shaped, with no expanded pedicels, as are those reported for other derived titanosaurians 48 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)  , 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 Magyarosaurus 50 . 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. Diplodocus 51 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 Quaesitosaurus 52 , with the sulcus restricted to its anterior portion. In Bonitasaura 53 and Rapetosaurus 54 , the articular surface is not divided. The anterior process of the quadratojugal projects ventrally, whereas the posterolateral process barely extends ventrally, similar to Nemegtosaurus 52 , and much less developed than in Tapuiasaurus 55 and Sarmientosaurus 4 . 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).
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 death [56][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 Rinconsaurus 59 and Uberabatitan 29 . 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 Uberabatitan 13,29 . 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. Saltasaurus 25 and Rocasaurus 47 ), 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 Opisthocoelicaudia 60 , Alamosaurus 61 , Trigonosaurus 28 , and the "Series A" from Brazil 30 . 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., Trigonosaurus 28 ). 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 Overosaurus 24 . 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 Lirainosaurus 34 . 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, Overosaurus 24 , or Aeolosaurus 30 . The neural arches are on the anterior portion of the centra, as in most titanosaurians, and some other titanosauriforms (e.g., Wintonotitan 62 ). The neural spines are laminar and vertically directed, while the prezygapophyses are short and anteriorly projected. Such morphology shows many similarities with Rinconsaurus 59 and Muyelensaurus 37 , but even more so with the Brazilian Trigonosaurus 28 and Uberabatitan 13,29 . As for the centra, Bravasaurus differs from saltasaurines, in which they are depressed, with a ventral longitudinal hollow (e.g., Saltasaurus 25 ). Nor do they possess the ventrolateral ridges (Fig. 3i) present in other titanosaurians such as Aeolosaurus 30 , Overosaurus 24 , 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 Opisthocoelicaudia 60 , Diamantinasaurus 63 , and Savannasaurus 64 , much more than in Rinconsaurus 59 and Muyelensaurus 37 . The deltopectoral crest is markedly expanded distally (Fig. 4a), as in Saltasaurus 25 , Neuquensaurus 27 , Opisthocoelicaudia 60 , and Dreadnoughtus 33 . 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 Futalognkosaurus 32 or Opisthocoelicaudia 60 . The distal end is markedly expanded, as in several derived forms (e.g., Rapetosaurus 43 , Bonitasaura 44 , Muyelensaurus 37 ). The ilium of the specimen CRILAR-Pv 613 resembles the ilium of other derived titanosaurians, such as Rapetosaurus and Bonatitan 65 . The femur is straight, with the fourth trochanter placed at the proximal third (Fig. 4c, d), as in Uberabatitan 13 , Patagotitan 2 , Bonitasaura 44 , and Futalognkosaurus 32 , whereas in Rinconsaurus 59 , Muyelensaurus 37 , and Diamantinasaurus 63 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 Epachthosaurus 66 .
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 equation 67 (see "Methods" section). Estimates of <10 tons are few among titanosaurians. The European Magyarosaurus (750 kg), is interpreted as a case of insular dwarfism 50,68 . The mass of the European Lirainosaurus was less than two tons 50 , whereas that of the Argentinean Saltasaurus and Neuquensaurus was five and six tons 2 , respectively. Among colossosaurians, estimations for Rinconsaurus indicate just four tons 2 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 dataset 2,6 , although some taxa, such as Baurutitan and Trigonosaurus show  Supplementary Fig. 4). The former one is placed as the basalmost colossosaurian, and the latter is clustered together with Uberabatitan, Gondwanatitan, and Bravasaurus.
Both Punatitan and Bravasaurus are recovered within Colossosauria 9 . Punatitan shows three of the seven ambiguous synapomorphies that diagnose the newly erected clade 9 , 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 subspherical eggs, arranged in two superposed rows.
The QSD eggs are similar to some Late Cretaceous titanosaurian eggs 69 . Among the remarkable diversity of eggs worldwide, only Auca Mahuevo 70 (Argentina), Dholi Dungri 71 (India), and Toteşti 72 (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 Mahuevo 70 and Toteşti 72 , but slightly smaller than the ones from Dholi   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 specimens 70 . Outwards, the units end out in rounded nodes of 0.3-0.4 mm in diameter, forming densely packed ornamentation that is typical of the titanosaurian clade [69][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. 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 Australasia 69 . 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 titanosaurians 9 in NW Argentina. For 40 years, Saltasaurus remained as the only wellrepresented sauropod for this region. Saltasaurus is closely related with the Patagonian Rocasaurus and Neuquensaurus, as well as Yamanasaurus 19 , 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 Argentina 20,23,73 and Chile 74 suggested the occurrence of nonsaltasaurine 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 supported 11 , but recent phylogenetic analyses, including the one here presented, suggest the Brazilian species may represent a distinctive genus, other than Aeolosaurus 12,13 . 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 colossosaurians 2,9 (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 Magyarosaurus 68 , 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 climate 75 and vegetation 76 (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 Brazil 14 ), whereas Rinconsauria may contain a few Brazilian forms 2,6,9,77 . Besides, some taxa recovered within Rinconsauria are often included within Aeolosaurini, a group of titanosaurians with unstable interspecific phylogenetic relationships 12 . Our results suggest that Rinconsauria is much more diverse and widely distributed than previously thought 2,3,6,9,37 . 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 Mahuevo 70 and Toteşti 72 , allow their identification. The spherical shape of the eggs, the monostratified 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 QSD 78,79 . 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 Negro 80 , in Patagonia, as well as an isolated record from Bauru 81 . Eggs similar in diameter were attributed to dwarf Cretaceous titanosaurians from Toteşti 72 . 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 worldwide 69 . 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 titanosaurians 69,72,78,82,83 . 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 'Aeolosaurus' maximus, a taxon which has been recognised as a member of Aeolosaurini 84 , 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 analyses 13,14 . 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 study 85 . 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/covariance 67  where BM is body mass, and C H+F is the sum of circumferences of the humerus and femur. It has been used to estimate the body mass of gigantic (e.g., Patagotitan 2 ), as well as medium-sized titanosaurians (e.g., Rapetosaurus 86 ).
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., Sarmientosaurus 4 , Patagotitan 2 ).
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 rionegrinus 30 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 studies 6 , 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.1.1 87 . 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.1.1 87 , 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.
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.