Introduction

Titanosaurs are an anatomically specialised clade of sauropod dinosaurs that predominated during the Cretaceous (Powell 2003; Curry Rogers 2005). Until recently, titanosaurs have been regarded as Gondwanan, enigmatic sauropods, due to early perceptions of their relative rareness in North America, Asia, and Europe, and the relative incompleteness of skeletal remains attributed to them (Romer 1956; McIntosh 1990). The recent proliferation of well preserved titanosaur and titanosaur-related genera on both northern and southern landmasses has led to a better understanding of their anatomy and to the recognition that they formed major components of the diversity and biomass of many Cretaceous terrestrial ecosystems (Curry Rogers 2005; Wilson 2006).

Although cladistic analyses of titanosaurs share many areas of agreement, these analyses suffer from highly disparate taxonomic content (Wilson 2006: fig. 7) and relatively incomplete taxonomic coverage. The most comprehensive analysis of Titanosauria to date was unable to resolve a stable framework for future work (Curry Rogers 2005). Thus, the potential power that titanosaurs have to address research questions in a variety of fields relevant to dinosaur palaeontology (e.g., palaeobiogeography, body size evolution, growth rates, extinction) is currently hindered by a lack of a comprehensive, resolved, and well-supported cladistic analysis of the group. Phylogenetic analyses of titanosaurs are in turn hindered by a number of concerns with alpha-taxonomy that must be resolved before including these taxa in a cladistic analysis, especially for taxa that were discovered early, such as saltasaurines. Revisions of alpha-level taxonomy have long been a part of sauropod systematics. McIntosh's detailed study of sauropods from the Morrison Formation of North America, which involved examination of quarry maps, field notes, museum collections, and original localities, led to a stable taxonomy for these taxa (Ostrom and McIntosh 1966; McIntosh and Berman 1975; Berman and McIntosh 1978; McIntosh and Williams 1988; McIntosh and Carpenter 1998; McIntosh 2005). Such revisions are ongoing, particularly within the Titanosauriformes, preparing the ground for future analyses of the group (e.g., revisions of “Titanosauridae”, Salgado 2003; “Titanosaurus”, “Titanosauridae”, Wilson and Upchurch 2003; Phuwiangosaurus, Suteethorn et al. 2009; Jainosaurus, Wilson et al. 2009; Euhelopus, Wilson and Upchurch 2009; Mongolosaurus, Mannion in press). Although revision is occurring at a slower rate than new titanosaur discoveries are being made (see taxon lists in Curry Rogers 2005; Wilson and Upchurch 2009), many of the new discoveries include or have been followed by extensive documentation of field associations (e.g., González Riga and Astini 2007; Curry Rogers 2009; Pérez et al. 2009).

As discussed below, the original holotypes of Neuquensaurus and Saltasaurus do not overlap anatomically, complicating evaluation of their validity as separate taxa. Below, we revise the holotype of the Late Cretaceous South American saltasaurine Neuquensaurus australis in order to evaluate its validity and referral of new materials to it. We begin by providing a review of the taxonomic history of Neuquensaurus and justification for augmenting the holotype, followed by a redescription of the holotypic remains. We close with a discussion of remains referred to Neuquensaurus and implications for the regional identity of the biconvex vertebra present in many titanosaurs.

Institutional abbreviations.—BYU, Brigham Young University Museum of Paleontology, Provo, USA; MACN, Museo Argentino de Ciencias Naturales “Bernardo Rivadavia”, Buenos Aires, Argentina; MCS, Museo Cinco Saltos, Cinco Saltos, Argentina; MCT, Museu de Ciências da Terra, Departmento Nacional de Producão Mineral, de Rio de Janeiro, Brazil; MLP, Museo de La Plata, La Plata, Argentina; MPCA, Museo Provincial “Carlos Ameghino”, Cipolletti, Rio Negro, Argentina; PVL, Instituto Miguel Lillo, Tucumán, Argentina.

Other abbreviations.—cpol, centropostzygapophyseal lamina; cprl, centroprezygapophyseal lamina; pcdl, posterior centrodiapophyseal lamina; posl, postspinal lamina; prsl, prespinal lamina; spol, spinopostzygapophyseal lamina; sprl, spinoprezygapophyseal lamina. (Abbreviations for vertebral laminae follow Wilson 1999).

Taxonomic history and original type series of Neuquensaurus australis

The British palaeontologist Richard Lydekker traveled to Argentina in 1893 to describe mammalian and reptilian fossils housed at the then newly founded Museo de La Plata. Lydekker had spent 1874–1882 describing Indian fossils and conducting fieldwork while appointed at the Geological Survey of India in Kolkata, followed by a decade in England writing a series of papers describing European fossils (Anonymous 1915). When Lydekker (1894: 4) began his work at the Museo de La Plata, he was staggered by the quantity and quality of fossil material present in the museum, remarking that “upon my arrival the reality far exceeded my most eager expectations … I was absolutely lost in astonishment and admiration at the number and beauty of its palaeontological treasures.”

Among other fossils, Lydekker (1893) described remains of sauropod dinosaurs that he attributed to new species of the genus “Titanosaurus”, the type species of which he described from India in 1877. Lydekker's southern representative of that genus, “Titanosaurus” australis, comprised bones collected from a single locality on the right bank of the Río Neuquén near the city of Neuquén. Lydekker did not participate in the excavation, and it is unlikely that he visited the locality. It is likely that any information he had about field associations was passed to him by expedition members. No maps or quarry photographs are known to exist, and for this reason subsequent researchers have had a limited range of options to further examine or document associations.

Regarding the Museo de La Plata's collection of sauropods, Lydekker (1893: 1–2) mentioned that “By far the great majority of the bones were found at a single spot in Neuquen … mostly found in association,” but he also noted that “the majority [of the bones] were picked up by the members of the expedition lying loose on the surface of the country.” Later, he stated that “… the name Titanosaurus australis is proposed … represented by a large series of associated vertebrae from Neuquen mostly belonging to a single individual, together with the bones of the fore and hind limbs, and some fragments of the pectoral and pelvic girdles. The caudal vertebrae represented in plate I may, however, be taken as the actual types.” Lydekker probably chose caudal vertebrae as the types of “Titanosaurus” australis to make them directly comparable with “Titanosaurus indicus”, whose type series included distal caudal vertebrae (Lydekker 1877).

Lydekker (1893) separated two vertebrae from the holotypic locality as a separate species, which he called “Titanosaurus” nanus for its small size. The validity of the species “T.” nanus has not been recognised by most authors (e.g., Bonaparte and Gasparini 1978; Powell 2003; Wilson 2002; Wilson and Upchurch 2003; Upchurch et al. 2004).

German palaeontologist Friedrich von Huene visited the Museo de La Plata between 1923 and 1926 and undertook a revision of Lydekker's work. Huene was also tasked by the Museum's then-director, Luis Torres, with the description of new Patagonian dinosaur material that was collected from Río Negro Province by the museum in 1921–1922 under the direction of palaeontologist Santiago Roth and geologist Walter Schiller. Among these new materials and those Lydekker (1893) had included in “T.” australis, Huene (1929) separated some out as the new species “Titanosaurus” robustus and others as the new genus and species Laplatasaurus araukanicus. Huene (1929: 23, translated from the Spanish) recognised the difficulty in identifying individuals in the Cinco Saltos quarry: “The numerous bones at Cinco Saltos had been discovered in a way that does not allow determination of which bones pertain to each individual, with the exception of the few series of caudal vertebrae. Various species and various genera are completely intermixed. The separation, sadly, I had to do by examination, and in these cases errors could not be excluded…All the material had to be ordered by me in such a way that consideration of its shape, preservation, and comparison resulted in the most natural correlation possible.” Huene's criteria for separating species and genera were often not transparent or testable (e.g., unspecified proportional differences).

Huene's taxonomic decisions were not commented upon for almost 50 years, until they were formalised by Bonaparte and Gasparini (1978). In their study of the sauropods of Neuquén and Chubut Provinces, Bonaparte and Gasparini (1978) designated the type caudal vertebrae listed by Lydekker (1893: pl. 1) as the holotype of “Titanosaurus” australis. Importantly, Bonaparte and Gasparini (1978) placed each taxon into a standardised stratigraphic framework. “T.” australis and “T.” robustus were listed by Bonaparte and Gasparini (1978) as coming from the Río Colorado Formation (now regarded as a subgroup; Leanza et al. 2004), possibly the Bajo de la Carpa Member (now regarded as a formation; Leanza et al. 2004), as well as possibly the Allen Formation. The provenance of these materials is now regarded as the Anacleto Formation (Leanza et al. 2004; Salgado et al. 2005).

Powell (1986, 1992, 2003) provided the first exhaustive revision of South American titanosaurs, in which he re-evaluated the collection of the Museo de La Plata. In his Ph. D. thesis, Powell (1986) coined the name Neuquensaurus as a new genus for “T.” australis and “T.” robustus, which was first formally published in Powell (1992). Powell (2003: 40) stated that the sacrum (MLP Ly 7), as well as other caudal vertebrae (MLP Ly 66 and 48), also belong to the holotype of Neuquensaurus australis. The assignment of the two caudal vertebrae to the holotype of N. australis was established by their articulation with the holotypic caudal vertebra that preserves a fragment of the preceding neural spine (MLP Ly 5; Powell 2003), but no evidence was presented justifying inclusion of the sacrum in the holotype. In addition to these augmentations, Powell (2003: 40) suggested that two vertebrae be removed from the holotype of N. australis: “… MLP Ly 6 [a caudal centrum] should be discarded from the holotype, since it clearly is a caudal vertebral centrum of the same species but a different individual. MLP Ly 1 [the biconvex vertebra] should be excluded from the material corresponding to the holotype as well since its morphology clearly indicates it belongs to a titanosaurid closely related to Titanosaurus.” As we discuss below, comparisons with the more complete Museo Cinco Saltos specimen (MCS-5) suggest that the caudal centrum (MLP Ly 6) likely pertains to the holotypic individual (see Caudal vertebrae below). The biconvex vertebra (MLP Ly 1), has a snap-fit to the sacrum (MLP Ly 7). Therefore, they belong to the same individual, which must be either included or excluded from the holotype.

McIntosh (1990) considered Saltasaurus and Neuquensaurus to be congeneric, based on his view that observed differences between them were minor. This opinion has not been followed by other authors, who typically recognise them to be distinct genera (e.g., Salgado and Azpilicueta 2000; Martinelli and Forasieppi 2004; Upchurch et al. 2004).

The most recent taxonomic review of Neuquensaurus came with the referral of new materials (MCS-5) to the genus by Salgado et al. (2005: 623), who stated that “The type specimen of ‘Titanosaurus’ australis (= Neuquensaurus australis) was thus artificially constructed by Huene on the bas [is] of mostly isolated materials.” Although Huene (1929) referred numerous remains to ”T.” australis, it was Lydekker (1893) who designated the caudal vertebrae as the types, a decision formalised by Bonaparte and Gasparini (1978). The material referred to Neuquensaurus by Salgado et al. (2005) was referred to that genus on the basis of autapomorphies shared with both the holotype and hypodigm. Salgado et al. (2005) did not discuss Powell's (2003) modification of the holotype of Neuquensaurus australis, but they provided a revised diagnosis of the species, which is discussed below.

Augmentation of the holotype of Neuquensaurus australis

Both of the classic saltasaurine taxa, Neuquensaurus and Saltasaurus, were discovered disarticulated in bonebeds with mostly undocumented field associations (Lydekker 1893; Bonaparte et al. 1977). The hypodigms of Neuquensaurus australis and Saltasaurus loricatus consist of numerous elements that have substantial overlap postcranially. In contrast, the holotypes of each of these species are limited and do not overlap—the holotype of Neuquensaurus consists of six caudal vertebrae, one of which we interpret as a sacral vertebra (see below), and part of a seventh caudal vertebra (Lydekker 1893), whereas that of Saltasaurus is represented by a complete sacrum (Bonaparte and Powell 1980). Although Saltasaurus and Neuquensaurus are thought to represent closely related genera that define Saltasaurinae (McIntosh 1990; Salgado et al. 1997; Wilson 2002; Powell 2003; Upchurch et al. 2004; Curry Rogers 2005; Calvo et al. 2008), there is some uncertainty surrounding their taxonomy (see above). The lack of consensus regarding the distinctiveness of Saltasaurus and Neuquensaurus and uncertainty about their constituency hinders assessment of their phylogenetic relationships to other saltasaurines (e.g., Rocasaurus, Salgado and Azpilicueta 2000; Bonatitan, Martinelli and Forasiepi 2004) and referral of new specimens (e.g., MCS-5, Salgado et al. 2005).

While examining the collections of the Museo de la Plata, we attempted to articulate axial remains attributed to Neuquensaurus and discovered that the sacrum MLP Ly 7 and the biconvex vertebra MLP Ly 1 articulate well (i.e., their outlines are identical and the convexity of the former conforms to the concavity of the latter) and matrix remaining on each vertebra snaps together when the vertebrae are articulated (Fig. 1). This indicates that the sacrum and biconvex vertebra were articulated prior to collecting and pertain to the same individual. Based on this new information, we argue that the sacrum should be included in the holotype of Neuquensaurus, which makes it directly comparable to the holotype of Saltasaurus.

The case for regarding the remaining six caudal vertebrae as belonging to the same individual as the sacrum is not as clear. Excluding the partial neural spine cemented to one of them, none of the caudal vertebrae articulate with one another. Few serially homologous anatomical features or features of preservation tie these vertebrae to one another, and we identify morphological differences between them that could be either regional or taxonomic. Fortunately, Lydekker's type series can be compared to a more complete skeleton referable to Neuquensaurus australis from Cinco Saltos in Río Negro Province (MCS-5; Salgado et al. 2005; see below), which includes a sacrum and 15 caudal vertebrae. Based on comparisons that we detail below, the sacrum and caudal vertebrae of the type series of Neuquensaurus australis can be regarded as a single individual. We could not evaluate the claim by Powell (2003: 40) that the caudal vertebrae MLP Ly 66 and 48 belong to the holotype, because they are missing from the collections of the MLP.

Systematic palaeontology

Fig. 1.

Holotypic sacrum of the sauropod Neuquensaurus australis (Lydekker, 1893), MLP Ly 1 and 7, from the Late Cretaceous of Neuquén, Argentina. Posterior view of sixth sacral vertebra and anterior view of seventh sacral vertebra showing corresponding matrix, which snaps together when the vertebrae are articulated. The abbreviations (s6–s7) indicate sacral vertebral identity. Dashed lines indicate missing bone.

Fig. 2.

Holotypic sacrum of the sauropod Neuquensaurus australis (Lydekker, 1893), MLP Ly 1 and 7, from the Late Cretaceous of Neuquén, Argentina. Stereophotographs and line drawings in ventral view. The abbreviations (s1–s7) indicate sacral vertebral identity. Dashed lines indicate missing bone, and dotted lines indicate intervertebral sutures.

Fig. 3.

Holotypic sacrum of the sauropod Neuquensaurus australis (Lydekker, 1893), MLP Ly 7, from the Late Cretaceous of Neuquén, Argentina. Close-up stereophotographs of sutures between sacral vertebrae 3–6 in ventral view.

Fig. 4.

Holotypic sacrum of the sauropod Neuquensaurus australis (Lydekker, 1893), MLP Ly 1 and 7, from the Late Cretaceous of Neuquén, Argentina in right lateral view. The abbreviations (s1–s7) indicate sacral vertebral identity. Dashed lines indicate missing bone.

Fig. 5.

Holotypic seventh sacral vertebra of the sauropod Neuquensaurus australis (Lydekker, 1893), MLP Ly 1, from the Late Cretaceous of Neuquén, Argentina in left lateral (A), ventral (B), anterior (C), right lateral (D), dorsal (E), and posterior (F) views.

Fig. 6.

Holotypic caudal vertebrae of the sauropod Neuquensaurus australis (Lydekker, 1893) from the Late Cretaceous of Neuquén, Argentina. A. MLP Ly 2, ?second or third caudal vertebra in left lateral (A₁), anterior (A₂), posterior (A₃), and ventral (A₄) views. B. MLP Ly 3, ?seventh caudal vertebra in left lateral (B₁), anterior (B₂), posterior (B₃), and ventral (B₄) views. C. MLP Ly 5a, b, parts of the ?ninth and tenth caudal vertebrae in left lateral (C₁), anterior (C₂), posterior (C₃), and ventral (C₄) views. D. MLP Ly 4, mid-caudal vertebra in left lateral (D₁), anterior (D₂), posterior (D₃), and ventral (D₄) views. E. MLP Ly 6, mid-posterior caudal vertebra in left lateral (E₁), anterior (E₂), posterior (E₃), and ventral (E₄) views. Abbreviation: sprl, spinoprezygapophyseal lamina.

Discussion

Referred materials.—The distalmost caudal vertebra of the type series of Neuquensaurus australis (MLP Ly 6) has a particular preservational style in which the condyle is stained deep red (Fig. 6E). Although other caudal vertebrae in the Museo de La Plata collections share the morphology and identical preservation of MLP Ly 6 (e.g., MLP Ly 71), the absence of a quarry map or other information linking these to the holotype precludes regarding them as the same individual. We follow Lydekker (1893), Huene (1929), and Bonaparte and Gasparini (1978) in not regarding these other caudal vertebrae as part of the holotype.

Abundant axial, appendicular, and dermal materials in the collections of the Museo de La Plata have been referred to Neuquensaurus. None of these elements were found in direct association with the holotypic remains, and overlap between these referred specimens and the holotype is limited. The criteria used to assign these non-overlapping materials to Neuquensaurus in the past are not clear, but referral of specimens to Neuquensaurus must be made via materials that anatomically overlap with the holotype—the sacrum and six caudal vertebrae mentioned above—and share autapomorphies with it. A strong case can been made for referral of the specimen MCS-5 to Neuquensaurus australis, which overlaps with the holotypic remains and shares diagnostic features such as the presence of seven sacral vertebrae (see below).

The vertebral “series 1–4” described by Huene (1929) were considered by him to represent individuals, or mainly individuals, but there was little discussion of the criteria that led to this assessment (see above). Apart from several short articulated series of 2–6 caudal vertebrae, individuals cannot be confidently recognised among the Huene's “series”. Consequently, referral to Neuquensaurus must be justified on a case-by-case basis using autapomorphies. We can refer three caudal vertebrae in the Museo de La Plata to Neuquensaurus australis (MLP CS 1400, 1402, 1407; see Diagnosis and Referred specimens above). Three other caudal vertebrae (MLP CS 1429, 1432) appear to be referable to N. australis based on the figures of Huene (1929: pl. 5), but the relevant parts of those vertebrae are now missing. Although other caudal vertebrae in the Museo de La Plata resemble those of Neuquensaurus australis, in the absence of autapomorphies and of definitive associations of individuals, we cannot confidently refer them to the taxon.

Diagnostic features.—The diagnosis for Neuquensaurus australis given by Powell (2003) was revised by Salgado et al. (2005: 625) to include six characters. Four of these characters can observed in the emended holotype, two in the sacrum and two in the caudal vertebrae: (i) seven sacral vertebrae; (ii) third to fifth sacral centra narrowed; (iii) mid- and posterior caudal vertebrae with parasagittal ventral ridges that bound a non-keeled ventral fossa culminating in chevron facets; (iv) lateral walls of caudal centra parallel to one another (i.e., not visible in ventral view).

The ventral narrowing of the third to fifth sacral centra was cited as an autapomorphy of Neuquensaurus australis by Powell (2003) and Salgado et al. (2005). This feature requires further refinement, because mid-sacral vertebrae are narrower than the first or last in several neosauropods, including Diplodocus (Hatcher 1901: fig. 9), Brachiosaurus altithorax (Riggs 1904: pl. 73: 2), an indeterminate titanosaur from Brazil (MCT 1536-R; Campos and Kellner 1999), Trigonosaurus (Campos and Kellner 1999: fig. 16; Campos et al. 2005: fig. 24), the topotype partial sacrum of Alamosaurus (PMU R 172; Mateer 1976; note this author reversed anterior and posterior on the sacrum), and Isisaurus (Jain and Bandyopadhyay 1997: fig. 10). In the above-listed taxa, this narrowing can be subtle (e.g., Brachiosaurus, Diplodocus) or more dramatic (e.g., Trigonosaurus, Alamosaurus), but in Neuquensaurus australis, this narrowing is autapomorphically extreme, such that the mid-sacral centra are between one-third and one-half of the widths of the first and seventh sacral centra.

All of the type series caudal vertebrae have parallel lateral walls of the centra except the anteriormost preserved centrum (MLP Ly 2; Fig. 6), which has angled lateral faces. This condition—anterior caudal vertebrae with ventromedially sloping faces that give way to vertically-oriented lateral faces in more posterior caudal vertebrae in the series—is similar to that found in several other titanosaurs, including Laplatasaurus araukanicus, MUCPv-204, Gondwanatitan, and the referred Cinco Saltos Neuquensaurus australis (MCS-5; Salgado and Garcia 2002; Salgado et al. 2005). This suggests that morphology of the centrum changes along the caudal series in the holotype of N. australis. Likewise, the presence of parasagittal ventral ridges bounding a fossa that is not subdivided by a keel in middle and posterior caudal vertebrae is not unique to Neuquensaurus, but is present in other titanosaurs as well (e.g., Alamosaurus).

The Cinco Saltos individual (MCS-5) is much more complete than the holotype, preserving 15 complete caudal vertebrae, a sacrum and both ilia, an ischium, both femora, and an articulated tibia, fibula, and astragalus, allowing a fuller diagnosis of the taxon (Salgado et al. 2005). Based on this material, we identify three autapomorphies in the middle caudal vertebrae of N. australis: prezygapophyses with a nonarticulating anterior process, longitudinal ridge extending from base of transverse process, and elongate postzygodiapophyseal lamina (podl) oriented nearly parallel to the spinoprezygapophyseal lamina (sprl) and spinopostzygapophyseal lamina (spol). The fibula of Neuquensaurus australis is autapomorphically rotated anteromedially and translated slightly so that the lateral trochanter is visible in anterior view (e.g., Salgado et al. 2005: fig. 7H, I).

Titanosaur sacra from the El Brete quarry, Salta, Argentina

All described remains of Saltasaurus loricatus (Bonaparte and Powell 1980) were collected from a single quarry in El Brete, in Salta Province, northwestern Argentina. Based on duplication of elements (femora in this case), and the presence of two unfused, small, juvenile cervical vertebrae that may or may not pertain to the same individual, the minimum number of individuals in this quarry is six (five were listed by Bonaparte et al. 1977, plus the juvenile material; MDD and JAW personal observations). A full quarry map is not available for the El Brete locality, but a partial map was given in Bonaparte et al. (1977: fig. 2) that illustrates the disarticulated nature of the specimens.

There are four titanosaur sacra from the El Brete quarry: PVL 4017-92 is the holotype of Saltasaurus loricatus (Bonaparte and Powell 1980); PVL 4017-93 is a similar sacrum; PVL 4017-18 is a more fragmentary, slightly longer sacrum; PVL 4017-142 is three coosified sacral centra. The holotypic sacrum of S. loricatus (PVL 4017-92) and the one that closely resembles it (PVL 4017-93) are subequal in size and have six coossified vertebrae that bear firmly fused contacts with the ilia. Centrum widths are subequal along both sacra, which are roughly as wide across their sacral ribs as they are long anteroposteriorly. In contrast, the slightly longer sacrum (PVL 4017-18) has seven co-ossified sacral vertebrae and unfused ilia (Powell 2003: pl. 55: 8; Figs. 7, 8). It has a length-to-width ratio of nearly 1.5 (74.2 cm long, ca. 50 cm wide). Sacral centra decrease to half their width by the fifth sacral vertebrae and then increase in size again posteriorly to reach their greatest width by seventh sacral centrum. The elongate El Brete sacrum (PVL 4017-18) matches the holotype of Neuquensaurus australis and the specimen MCS-5 in the number of constituent vertebrae, vertebral proportions, and sacral proportions, but it differs in that its seventh sacral centrum is fused to the remainder of the sacrum. This difference may indicate a later ontogenetic stage for the El Brete specimen, or it may be an actual taxonomic difference. The holotypic and Cinco Saltos (MCS-5) Neuquensaurus sacra and the El Brete sacrum (PVL 4017-18) also differ in the degree of ventral curvature of both the first and last vertebrae of the sacrum (Figs. 4, 8), but we regard this variation as minor and possibly attributable to dorsoventral compression during preservation.

Fig. 7.

Sauropod sacrum PVL 4017-18 from the Saltasaurus loricatus Bonaparte and Powell, 1980 quarry at El Brete, Argentina in dorsal view. Note the presence of seven sacral vertebrae. The abbreviations (s1–s7) indicate sacral vertebral identity. Dashed lines indicate missing bone.

Fig. 8.

Sauropod sacrum PVL 4017-18 from the Saltasaurus loricatus quarry at El Brete, Argentina. Stereophotographs in right lateral view, with anterior towards the top. The abbreviations s1 and s7 indicate sacral vertebral identity.

Differences in the number, shape, and sacral centra proportions between the elongate El Brete sacrum (PVL 4017-18) and the holotype of Saltasaurus loricatus (PVL 4017-19) can be interpreted in several ways. Regarding these differences as variation within the species preserves the monospecificity of the El Brete quarry (with regard to titanosaurs), but it reaches beyond our ability to defend intraspecific variation and would render homoplastic the features shared between PVL 4017-18 and Neuquensaurus australis. Alternatively, we could interpret these differences as indications of sexual dimorphism, which likewise would preserve the monospecificity of El Brete. However, sexual dimorphism has not been demonstrated in any titanosaur, and there does not appear to be bimodal variation in other parts of the anatomy found at El Brete. If, on the other hand, we interpret these differences as taxonomic, it suggests that a second titanosaur genus is present at El Brete. We suggest that the similarities between PVL 4017-18 and Neuquensaurus australis indicate that they belong to the same species: sacrum approximately 1.5 times as long anteroposteriorly as wide, seven sacral vertebrae, and sacral centra four and five narrowed to less than half the width of the first and seventh sacral centra.

Saltasaurus is typically regarded as stratigraphically younger than Neuquensaurus (late Campanian—Maastrichtian for the former, early to middle Campanian for the latter; Dingus et al. 2000; Powell 2003; Leanza et al. 2004), but radiometric ages for the quarry-bearing formations of either taxon do not exist (Leanza et al. 2004). Biostratigraphy in these formations is largely based on tetrapods (Leanza et al. 2004), and is therefore somewhat coarse. The presence of a sacrum attributable to Neuquensaurus australis in the El Brete quarry (PVL 4017-18) suggests that these two saltasaurines coexisted in the Late Cretaceous of South America, raising the possibility that some of the other materials attributed to Saltasaurus loricatus actually belong to Neuquensaurus australis, and vice versa.

Mechanism of sacral vertebral count increase in Neuquensaurus australis

The number of sacral vertebrae varies systematically within Sauropodomorpha. Basal sauropodomorphs (i.e., “prosauropods”) possess three sacral vertebrae, to which a fourth is added in all sauropods (Wilson and Sereno 1998). Five sacral vertebrae characterise Patagosaurus and more derived sauropods—one dorsal vertebra is incorporated into the sacrum in that its sacral rib touches the ilium, and it is fused to the other sacral vertebrae (Wilson 2002). Somphospondylan sauropods (e.g., Euhelopus, Isisaurus) are characterised by the addition of another dorsosacral vertebra, which both touches the ilium and is fused to the other sacral centra (Wilson 2002). In almost all titanosaurs for which a complete sacrum is known (Epachthosaurus, Malawisaurus, Isisaurus, Saltasaurus, MCT 1489-R, MLP 46-VIII-212), all centra are fused in adults and the last sacral vertebra has a posteriorly convex face. The only exceptions are Opisthocoelicaudia and an indeterminate titanosaur sacrum from Brazil (MCT 1536-R), in which the last sacral centrum has a concave posterior face.

Neuquensaurus australis has seven sacral vertebrae, which is autapomorphic (Salgado et al. 2005). Theoretically, this increase in sacral vertebral count may have been accomplished by incorporation of a dorsal or caudal vertebra or by duplication of an existing sacral vertebra. We rule out the hypothesis that N. australis incorporated a dorsal vertebra into the sacrum, because what is preserved of the first and second sacral vertebrae is morphologically and topologically identical to those in other titanosaur sacra. Moreover, the second sacral rib of Neuquensaurus and other titanosaurs is identical to the first sacral rib in those sauropods that have only five sacral vertebrae (e.g., Apatosaurus, BYU 1252). It is more likely that a seventh vertebra was added into the sacrum of N. australis via incorporation of a caudal vertebra by or duplication of one of the existing sacral vertebrae. How the addition of a seventh sacral vertebra evolved is informed by the sacral anatomy of outgroups to Neuquensaurus. However, this is complicated by the ambiguous identity of the sacro-caudal vertebrae of other titanosaurs, as discussed below.

Table 1.

Biconvex sacral or anterior caudal vertebrae attributed to titanosaurs.

The biconvex vertebra of titanosaurs

Two exemplars of Neuquensaurus australis bear biconvex vertebrae that are part of the sacrum but are unfused to other sacral centra (MLP Ly 1, MCS-5/16). In addition, two biconvex vertebrae collected from Cinco Saltos were identified by Huene (1929) as caudal vertebrae of Neuquensaurus (= “Titanosaurus”) australis (MLP CS 1390) and N. (= “T.”) robustus (MLP CS 1389). These vertebrae could pertain to Neuquensaurus, but we were unable to identify autapomorphies defending this assertion. Several other titanosaurs have biconvex vertebrae from the sacro-caudal region (Table 1): Pellegrinisaurus (Salgado 1996), Baurutitan (Kellner et al. 2005), Alamosaurus (Gilmore 1946; BYU 11392, MDD and JAW personal observations), unpublished materials from the Pab Formation of Pakistan (GSP-UM 6772, MDD and JAW personal observations), an indeterminate titanosaur from Argentina (MLP 52-XI-1-14), and a vertebra that was found in the Antarctosaurus wichmannianus quarry (MACN 16432). Like N. australis, these biconvex vertebrae have posterior condyles that are much more strongly developed and dorsoventrally shorter than their anterior ones.

The biconvex vertebra in these other titanosaurs have been identified as the first caudal vertebra, but its inclusion in the sacrum in Neuquensaurus australis raises the possibility that some of them may actually be sacral vertebrae. In two of the three exemplars of N. australis, the biconvex seventh sacral centrum was not fused to the sixth sacral centrum. In one case, this vertebra was firmly incorporated into pelvis (MCS-5/16); in the holotype it was articulated to the remainder of the sacrum but that contact was nearly lost (MLP Ly 1), and in the El Brete exemplar it was fused to the preceding vertebra (Figs. 7, 8). Thus, the biconvex vertebra could be preserved as an isolated element in specimens that are incompletely fused laterally or are damaged (e.g., Pellegrinisaurus, Antarctosaurus, and the Pakistani biconvex centrum (GSP-UM 6772). Other biconvex titanosaur vertebrae can confidently be identified as caudal vertebrae, because their transverse processes resemble those of anterior caudal vertebrae in other titanosauriforms and are devoid of signs of fusion with a preceding vertebra or ilium (e.g., Baurutitan).

Future discoveries are needed to resolve the ambiguity over the regional identity of the less complete biconvex titanosaur vertebrae and to more fully understand the mechanism by which a seventh vertebra was incorporated into the sacrum of Neuquensaurus australis.

Conclusions

Saltasaurine titanosaurs are regarded as derived sauropods, but more detailed knowledge of their interrelationships has been hindered by taxonomic problems dating back to their early discovery. We have demonstrated that an incomplete sacrum in the collections of the Museo de la Plata belongs to the same individual as the holotypic caudal vertebrae of the saltasaurine Neuquensaurus australis, making this genus comparable to, and demonstrably distinct from, Saltasaurus loricatus. Autapomorphies present in the sacral and caudal vertebrae confirm referral of specimens held in the Museo Cinco Saltos and Museo La Plata to N. australis. A sacrum of N. australis is also present in the El Brete quarry, which has produced all remains of Saltasaurus, indicating that these genera coexisted in the Late Cretaceous of South America. Neuquensaurus is unique among sauropods in possessing seven sacral vertebrae, the last of which is biconvex. The biconvex vertebrae of some other titanosaurs may later be demonstrated to belong to the sacrum, rather than the caudal series, as currently presumed.