Systematic palaeontology

Dinosauria Owen, 1842

Saurischia Seeley, 1887

Sauropoda Marsh, 1878

Titanosauriformes Salgado, Coria and Calvo, 1997

Somphospondyli Wilson and Sereno, 1998

Titanosauria Bonaparte and Coria, 1993

Lithostrotia Upchurch, Barrett and Dodson, 2004

Notocolossus gonzalezparejasi gen. et sp. nov.

Etymology

From the Greek notos (southern) and the Latin colossus, in reference to the gigantic size and Gondwanan provenance of the new taxon. Species name honours Dr. Jorge González Parejas, who has collaborated and provided legal guidance on the research, protection and preservation of dinosaur fossils from Mendoza Province for nearly two decades. In so doing, he has advised researchers on the creation of a natural park that serves to protect dinosaur footprints in Mendoza.

Holotype

UNCUYO-LD 301, an associated partial skeleton of a very large individual consisting of an anterior dorsal vertebra, an anterior caudal vertebra, the right humerus and the proximal end of the left pubis (Figs 1b, 2a–e, 3a,c,e,g and 4a–c; Supplementary Figs S1, S3). We consider these elements to represent a single titanosaurian individual because they were found within an area of 8 m by 8 m at the same stratigraphic level and are of the appropriate size and morphology to have been derived from a single skeleton.

Figure 2 Vertebral morphology of Notocolossus gonzalezparejasi. Anterior (second or third) dorsal vertebra of the holotype (UNCUYO-LD 301) in (a) anterior and (b) left anterolateral views. Anterior caudal vertebra of the holotype (UNCUYO-LD 301) in (c) anterior, (d) posterior and (e) right lateral views. Anterior caudal vertebra of the referred specimen (UNCUYO-LD 302) in (f) anterior, (g) posterior and (h) left lateral views. Abbreviations: al1, ‘accessory’ lamina 1; al2, ‘accessory’ lamina 2; cd, condyle; ct, cotyle; dp, diapophysis; nc, neural canal; ns, neural spine; pacdf, parapophyseal centrodiapophyseal fossa; posl, postspinal lamina; poz, postzygapophysis; pp, parapophysis; ppdl, paradiapophyseal lamina; prdl, prezygodiapophyseal lamina; prpl, prezygoparapophyseal lamina; prsl, prespinal lamina; prz, prezygapophysis; spdl, spinodiapophyseal lamina; spol, spinopostzygapophyseal lamina; sprl, spinoprezygapophyseal lamina; tp, transverse process; tpol, intrapostzygapophyseal lamina; tprl, intraprezygapophyseal lamina; vasl, ‘V-shaped’ anterior spinal lamina. Scale bars, 20 cm (a,b), 10 cm (c–h). Full size image

Figure 3 Comparison of anterior caudal vertebrae of Notocolossus gonzalezparejasi. Photographs (a,b,e,f) and interpretive drawings (c,d,g,h) of the anterior caudal vertebra of the holotype (UNCUYO-LD 301) (a,c,e,g) and the first five anterior caudal vertebrae of the referred specimen (UNCUYO-LD 302) (b,d,f,h) in dorsal (a–d) and left lateral (e–h) views (e and g reversed). Abbreviations, avr, anteroventral ridge; ns, neural spine; poz, postzygapophysis; prz, prezygapophysis; tp, transverse process. Scale bar, 10 cm. Full size image

Figure 4 Appendicular skeletal morphology of Notocolossus gonzalezparejasi. (a) Right humerus of the holotype (UNCUYO-LD 301) in anterior view. Proximal end of the left pubis of the holotype (UNCUYO-LD 301) in lateral (b) and proximal (c) views. Right tarsus and pes of the referred specimen (UNCUYO-LD 302) in (d) proximal (articulated, metatarsus only, dorsal [=anterior] to top), (e) dorsomedial (articulated) and (f) dorsal (disarticulated) views. Abbreviations: I–V, metatarsal/digit number; 1–2, phalanx number; ast, astragalus; cbf, coracobrachialis fossa; dpc, deltopectoral crest; hh, humeral head; ilped, iliac peduncle; of, obturator foramen; plp, proximolateral process; pmp, proximomedial process; rac, radial condyle; ulc, ulnar condyle. Scale bars, 20 cm (a–c), 10 cm (d–f). Full size image

Referred specimen

UNCUYO-LD 302, an associated partial skeleton of a second, smaller-bodied individual that includes an articulated anterior caudal series (consisting of seven partial vertebrae and haemal arches) and the complete and articulated right astragalus and pes (Figs 1b, 2f–h, 3b,d,f,h and 4d–f; Supplementary Figs S2, S4–S8). As with the holotype, we consider these elements to represent a single titanosaurian individual because they were found within an area of 5 m by 5 m at the same stratigraphic level and are of the appropriate size and morphology to have come from a single skeleton. See Supplementary Information for justification of the referral of this specimen to Notocolossus gonzalezparejasi.

Type locality and horizon

Cerro Guillermo, Malargüe Department, southern-most Mendoza Province, Argentina (Fig. 1a; coordinates on file at UNCUYO-LD). The holotype and referred specimen were collected 403 m apart in the basal-most bed of the Upper Cretaceous (upper Coniacian–lower Santonian, ~86 Ma) Plottier Formation of the Neuquén Group (see Supplementary Information for details).

Diagnosis

Large titanosaurian sauropod dinosaur diagnosed by the following autapomorphies: (1) anterior dorsal vertebra with parapophyseal centrodiapophyseal fossa subdivided by two ‘accessory’ laminae (one subvertical and visible in anterior and lateral views, the other anterodorsally oriented and visible only in lateral view); (2) anterior caudal vertebrae with laminae that converge ventrally on the anterior surface of the neural spine, not reaching the prezygapophyses and forming a ‘V-shaped’ conformation in anterior view; humerus with (3) greatly expanded proximomedial process, the proximal apex of which lies well medial to the humeral midshaft, (4) proportionally wide proximal end (proximal width: midshaft width ≈ 2.9) and (5) proximolaterally–distomedially oriented ridge bounding distal edge of ‘coracobrachialis fossa’; and pes with (6) metatarsal I with proximal dorsoventral diameter greater than the proximodistal length of the bone, (7) relatively short metatarsal III (only 1.2 times the length of metatarsal I), (8) proximal phalanges more than half as wide as their corresponding metatarsals are long and (9) pedal unguals reduced, rugose and distally truncated. These characters are associated with a unique combination of synapomorphies of the anterior caudal vertebrae that is observable in both known specimens: centra with (1) deeply concave anterior articular cotyles and strongly convex posterior articular condyles; (2) circular anterior articular surfaces and slightly quadrangular posterior articular surfaces; (3) anteroposteriorly concave lateral surfaces; (4) multiple vascular foramina on the lateral surfaces, ventral to the transverse processes; and (5) anteroposteriorly narrow, slightly concave ventral surfaces; transverse processes that are (6) robust, elongate and posteroventrally directed, nearly reaching the anteroposterior level of the posterior condyle of the centrum; (7) wide and rounded at their lateral ends; and (8) ornamented by longitudinal ridges on their anteroventral margins at the approximate midlength of the process; and (9) neural arches that are anteriorly placed. N. gonzalezparejasi also exhibits the following distinctive morphologies: (1) humerus with markedly asymmetrical proximal margin in anterior view (nearly straight laterally but strongly expanded and rounded proximomedially); metatarsal V (2) 90 percent the length of metatarsal IV and (3) longer than metatarsal I; and (4) pedal phalangeal formula 2-2-2-2-0, with digits I–III bearing unguals.

Description

The holotypic specimen of Notocolossus (UNCUYO-LD 301) preserves an almost complete anterior dorsal vertebra (Fig. 2a,b; Supplementary Fig. S1) that is missing only the lateral end of the right diapophysis and most of the right side of the neural spine. The bone is very large; if complete, it would measure approximately 1500 mm in maximum transverse dimension (i.e., width across the diapophyses), only 180 mm less than in dorsal vertebra 2 of the gigantic southern Patagonian titanosaur Puertasaurus11 (Supplementary Table S1). Moreover, the width across the diapophyses is substantially greater than in anterior dorsal vertebrae of another colossal titanosaur, Argentinosaurus (generally regarded as the most massive known terrestrial animal12), which reach only 1290 mm5. When considered in light of the exceptionally long, robust humerus of UNCUYO-LD 301, as well as the femoral length and body mass estimates generated from that bone (see below and Supplementary Information), the size of the dorsal vertebra of this specimen suggests that it represents an exceptionally large-bodied titanosaurian individual. Based on the positions of the parapophyses and prezygapophyses, as well on comparisons with anterior dorsal vertebrae of other titanosaurs (e.g., Futalognkosaurus6, Mendozasaurus9, Rapetosaurus13), we identify the Notocolossus dorsal vertebra as the second or third in the series. The centrum is opisthocoelous with a strongly convex, hemispherical anterior articular condyle, proportionally anteroposteriorly short and considerably wider than tall (anterior transverse width/dorsoventral height = 1.36). Damage to a few areas of the anterior condyle reveals that it is internally comprised by camellate (i.e., ‘spongy’ or ‘cancellous’) bone; this is also the case for the left parapophysis and diapophysis. The posterior cotyle is strongly concave. The ventral surface of the centrum is smoothly convex, lacking a keel or fossa. A small, deep lateral pneumatic fossa (‘pleurocoel’) is located anterior to the parapophysis.

The parapophyses extend from the dorsal end of the centrum to the base of the neural arch, with their dorsoventral midline positioned at the approximate dorsoventral level of the ventral margin of the neural canal. They are large, well developed, strongly concave in anterior view and much taller dorsoventrally than wide anteroposteriorly. Their articular facets face ventrolaterally. The parapophyseal articular facets are proportionally larger than in anterior dorsal vertebrae of Mendozasaurus (pers. obs.), though this discrepancy may well be due to serial variation along the dorsal column. The neural canal is large and subcircular in anterior view, slightly wider than tall. It is bordered dorsally by the intraprezygapophyseal lamina. The prezygapophyses are strongly developed, extending far anteriorly. Their facets are ovoid in dorsal contour, more than two times wider mediolaterally than long anteroposteriorly and flat dorsally. They face dorsomedially and their lateral ends slightly dorsally surpass the level of the diapophyses, as in the first dorsal vertebra of Rapetosaurus13. The diapophyses extend laterally well beyond the lateral margins of the prezygapophyses. The neural spine is slightly incomplete dorsally but was almost certainly low and subtriangular in anterior view.

Several neural arch laminae and fossae are evident. The parapophysis is linked to the diapophysis by the anteroposteriorly thin paradiapophyseal lamina. The laterally concave prezygoparapophyseal lamina comprises the anteromedial margin of a deep, teardrop-shaped fossa (here regarded as the parapophyseal centrodiapophyseal fossa following Wilson et al.14) that embays the anterior surface of the diapophysis and is better defined dorsally than ventrally. A thin, subvertical ‘accessory’ lamina subdivides this fossa, which extends posteromedially beyond the prezygoparapophyseal lamina, forming a deep, probably pneumatic cavity. This cavity is in turn subdivided by a second, anterodorsally–posteroventrally oriented ‘accessory’ lamina that is visible only in lateral view. The prezygapophyses are connected to the lateral margins of the diapophyses by the robust prezygodiapophyseal laminae and to each other by the much lower, boomerang-shaped intraprezygapophyseal lamina. At least the ventral part of the anterior face of the neural spine is bisected by the thick, rugose prespinal lamina, whereas the lateral margins of the spine are comprised by the spinodiapophyseal laminae.

UNCUYO-LD 301 also includes an anterior caudal vertebra (Figs 2c-e and 3a,c,e,g), probably the third or fourth in the series based on comparisons with titanosaurs with complete, well-preserved anterior caudal sequences (e.g., Alamosaurus, Baurutitan, Dreadnoughtus, Epachthosaurus). The posterior face of its anteroposteriorly short, strongly procoelous centrum is as tall as wide, whereas the anterior face is slightly wider than tall. The subcircular anterior cotyle is substantially larger than the subquadrangular posterior condyle (Supplementary Table S1), suggesting that, in Notocolossus, the anterior-most caudal centra rapidly decreased in diameter posteriorly. There is no evidence of pneumatic fossae on either lateral surface of the centrum, but these surfaces are anteroposteriorly concave and pierced by several vascular foramina, as in many other sauropods. The ventral surface is anteroposteriorly narrow and gently concave. There are no ventrolateral ridges extending between the haemal arch facets, nor is there an associated midline sulcus. The transverse processes are powerfully developed and curve ventrally and posterolaterally; the complete, club-shaped right process sweeps far posteriorly, with its end approaching the anteroposterior plane of the posterior margin of the centrum. The lateral extent of the transverse process is approximately 60 percent the posterior width of the centrum. A low, rugose ridge—possibly for attachment of the M. caudofemoralis longus15—extends across much of the anteroventral surface of the transverse process.

The prezygapophyses have large, subcircular, dorsomedially-directed articular facets, but they are comparatively anteroposteriorly shorter than in many other titanosaurs. The postzygapophyses are correspondingly elongate, with their articular facets connected to spinopostzygapophyseal laminae that extend posteriorly beyond the remainder of the neural arch. The postzygapophyseal facets are dorsoventrally elongate, slightly concave, ventrolaterally oriented and connected by a robust intrapostzygapophyseal lamina located dorsal to the neural canal.

The neural spine is vertically oriented and low relative to the size of the centrum. The dorsal end of the spine is wider transversely than long anteroposteriorly and is ‘D-shaped’ in dorsal view, with a straight anterior border. In lateral view, the spine is rectangular and gently concave anteriorly approaching its anterodorsal corner. Short, low spinoprezygapophyseal laminae appear to connect the bases of the prezygapophyses to that of the neural spine. Another, better-developed pair of anterior laminae, seemingly distinct from the spinoprezygapophyseal laminae, extend the length of the neural spine, occupying the anterolateral corners of the spine dorsally but converging ventrally to form a ‘V-shaped’ conformation in anterior view. There is no clear evidence of a prespinal lamina. The posterior surface of the spine is framed by two prominent, posteriorly-projected spinopostzygapophyseal laminae that rapidly diverge from one another dorsally, becoming well separated at the approximate dorsoventral midline of the spine. A sagittally-positioned postspinal lamina spans much of the length of the posterior surface of the neural spine. Though damaged, it appears to expand markedly in transverse dimension dorsally and seemingly does not reach the base of the spine.

The holotypic specimen UNCUYO-LD 301 also preserves the complete right humerus (Fig. 4a; Supplementary Fig. S3). It is 1760 mm in proximodistal length, with a mediolaterally expanded proximal end and a much narrower diaphysis (Table 1; Supplementary Table S1). To our knowledge, it is the longest humerus yet recovered from the Cretaceous, or for any titanosaurian, being 70 mm longer than that of the giant Egyptian titanosaur Paralititan16. Using the length of the Notocolossus humerus in conjunction with the stylopodial proportions of more completely preserved titanosaurs, we estimate the length of the missing femur of UNCUYO-LD 301 at 2166 mm (see Supplementary Information for details). Furthermore, no other sauropod humerus has the anatomical proportions of that of Notocolossus. The Proximal Humeral Robusticity, proposed herein as the ratio of proximal to midshaft mediolateral width, is nearly 2.9, which is substantially greater than that of all other titanosaurians (Table 1). As in Futalognkosaurus17, the proximal end is highly asymmetrical in anterior view, almost straight laterally but markedly proximomedially expanded and rounded medially. In Notocolossus, however, the proximal apex of the humerus is positioned well medial to the medial margin of the humeral midshaft. This greatly enlarged proximomedial expansion is here considered an autapomorphy of the new taxon. By contrast, previous studies9 have recognized that the proximal ends of other titanosauriform humeri are smoothly rounded (as in Ligabuesaurus), straight (e.g., Mendozasaurus, Rapetosaurus) or sigmoidal (e.g., Opisthocoelicaudia, Paralititan, Quetecsaurus, Saltasaurus) in anterior view, without the extraordinary degree of proximomedial expansion seen in Notocolossus.

Table 1 Proximodistal length and proximal and midshaft mediolateral width (mm) of the humerus of the holotype of Notocolossus gonzalezparejasi (UNCUYO-LD 301) compared to those of other titanosaurian sauropods, including other gigantic species. Full size table

Proximally, there is a slight proximolateral process—smaller than that present in Epachthosaurus, Opisthocoelicaudia and Saltasaurus—and a shallow anteromedial fossa, possibly for the insertion of the M. supracoracoideus and M. coracobrachialis brevis, respectively18,19. This ‘coracobrachialis fossa’ is less defined than in some titanosaurs (e.g., Paralititan, pers. obs.) and is bounded distally by a proximolaterally–distomedially oriented crest. The deltopectoral crest is prominent, as in titanosaurs such as Epachthosaurus, Futalognkosaurus17 and Mendozasaurus and extends approximately 41 percent of the total length of the bone. In Neuquensaurus1, Opisthocoelicaudia18, Paralititan16 and Saltasaurus19, by contrast, the crest occupies 50 percent or more of total length. The distal end of the deltopectoral crest of Notocolossus is medially deflected and mediolaterally thicker than the proximal end. Distally, at its anterior apex, the crest possesses a strongly developed, subcircular, centrally concave process for the attachment of the abductor musculature (i.e., M. pectoralis, M. dorsalis scapulae, M. deltoides scapularis). A strong process on the deltopectoral crest is also present in adult and juvenile specimens of Mendozasaurus (pers. obs.). The humeral head is prominent posteriorly, as in Futalognkosaurus. There is a pronounced longitudinal crest near the lateral margin of the posterior surface of the proximal end; it is approximately 300 mm in length and its distal terminus is roughly 600 mm from the proximal margin of the bone.

The diaphysis is elliptical in cross-section, with its long axis oriented mediolaterally and measures 770 mm in minimum circumference. Based on that figure, the consistent relationship between humeral and femoral shaft circumference in associated titanosaurian skeletons that preserve both of these dimensions permits an estimate of the circumference of the missing femur of UNCUYO-LD 301 at 936 mm (see Supplementary Information). (Note, however, that the dataset that is the source of this estimate does not include many gigantic titanosaurs, such as Argentinosaurus5, Paralititan16 and Puertasaurus11, since no specimens that preserve an associated humerus and femur are known for these taxa.) In turn, using a scaling equation proposed by Campione and Evans20, the combined circumferences of the Notocolossus stylopodial elements generate a mean estimated body mass of ~60.4 metric tons, which exceeds the ~59.3 and ~38.1 metric ton masses estimated for the giant titanosaurs Dreadnoughtus and Futalognkosaurus, respectively, using the same equation (see Supplementary Information). It is important to note, however, that subtracting the mean percent prediction error of this equation (25.6% of calculated mass20) yields a substantially lower estimate of ~44.9 metric tons for UNCUYO-LD 301. Furthermore, Bates et al.21 recently used a volumetric method to propose a revised maximum mass of ~38.2 metric tons for Dreadnoughtus, which suggests that the Campione and Evans20 equation may substantially overestimate the masses of large sauropods, particularly giant titanosaurs. Unfortunately, however, the incompleteness of the Notocolossus specimens prohibits the construction of a well-supported volumetric model of this taxon and therefore precludes the application of the Bates et al.21 method. The discrepancies in mass estimation produced by the Campione and Evans20 and Bates et al.21 methods indicate a need to compare the predictions of these methods across a broad range of terrestrial tetrapod taxa21. Nevertheless, even if the body mass of the Notocolossus holotype was closer to 40 than 60 metric tons, this, coupled with the linear dimensions of its skeletal elements, would still suggest that it represents one of the largest land animals yet discovered.

The radial and ulnar condyles on the distal end of the UNCUYO-LD 301 humerus are similarly developed and undivided, with the radial condyle being more poorly defined anteriorly than in some other titanosaurs (e.g., Epachthosaurus, Futalognkosaurus, Paralititan). The anterior face of this condyle is not divided by a notch. The posterior surface of the distal end of the humerus is badly damaged, but it bears an olecranon fossa that is bounded by supracondylar ridges, as in many other titanosaurs (e.g., Mendozasaurus9, Paralititan16).

The articulated right tarsus and pes of the referred specimen (UNCUYO-LD 302) are complete and well preserved (Fig. 4d–f; Supplementary Figs S4–S8). The astragalus is the only ossified element of the tarsus, as in all other unquestionable titanosaurians in which this skeletal region has been preserved in articulation (i.e., Epachthosaurus22, Opisthocoelicaudia18, an unidentified titanosaur from Agua del Padrillo, Argentina [UNCUYO-LD 31323] and another unidentified taxon from La Invernada, Argentina [Museo de la Universidad Nacional del Comahue (MUCPv-)153324]). The astragalus is mediolaterally reduced and has a slightly concave lateral face for the articulation of the fibula. The anterior face has a triangular contour and the anteroposteriorly convex distal surface articulates with the proximal ends of the metatarsals (presumably metatarsals I–IV only; Fig. 4e; Supplementary Fig. S4). The low ascending process would have articulated with a depression in the distal end of the tibia. The distal surface is strongly rugose as in other titanosaurians. The tibial face is not strongly inclined as in Aeolosaurus25. The lateral face of the astragalus exhibits a large foramen (measuring ~40 by 10 mm) near its posteroventral border.

The pes is mediolaterally asymmetrical, though less so than in other neosauropods and includes five short, robust metatarsals that have highly rugose proximal ends. The articulated metatarsus measures approximately 450 mm in mediolateral dimension across its proximal extreme. In contrast to most other titanosauriforms (e.g., Aeolosaurus25, Epachthosaurus22, Gobititan26, Ligabuesaurus27, Rapetosaurus13, the La Invernada titanosaur24, New Mexico Museum of Natural History [NMMNH] P-49967 [a nearly complete distal hind limb tentatively referred to Alamosaurus28] and an unnamed Early Cretaceous titanosauriform from Siberia, Russia [Paleontological Museum, Tomsk State University (PM TGU) 16/029]), metatarsals III and IV are not substantially longer than the others (Table 2; Supplementary Table S3). Indeed, in all other titanosauriforms for which the lengths of metatarsals I and III have been published, metatarsal III is at least 27 percent longer than metatarsal I; typically, it is approximately 40 percent longer (Table 2). In Notocolossus, by contrast, metatarsal III is only 20 percent longer than metatarsal I. The metatarsus of Notocolossus exhibits other distinctive features as well. The minimum mediolateral breadth of metatarsal IV is nearly 70 percent that of metatarsal I (a reversal of character 224 of Wilson30, regarded as a synapomorphy of the eusauropod clade Mamenchisauridae [ = ‘Omeisauridae’] + [Jobaria + Neosauropoda] in that analysis). Moreover, metatarsal IV is slightly longer than the other metatarsals, as in Aeolosaurus, Bonitasaura, Epachthosaurus, Mendozasaurus, the Agua del Padrillo and La Invernada titanosaurs and NMMNH P-49967 (?Alamosaurus). In Antarctosaurus wichmannianus, Euhelopus, Opisthocoelicaudia, Rapetosaurus and Tastavinsaurus, by contrast, metatarsal III is the longest. In Notocolossus, metatarsal V is also relatively long: it is 90 percent the length of metatarsal IV, and, as in all other titanosaurs except Opisthocoelicaudia and Rapetosaurus, it is longer than metatarsal I. Only the even larger pes of ?Alamosaurus has a proportionally longer metatarsal V (Table 2).

Table 2 Proximodistal lengths (mm) of the metatarsals of the referred specimen of Notocolossus gonzalezparejasi (UNCUYO-LD 302) compared to those of other titanosauriform sauropods. Full size table

Metatarsals I and II are twisted about their long axes such that they are dorsoventrally (i.e., anteroposteriorly) deepest proximally and mediolaterally widest distally. Metatarsal I has a ‘D-shaped’ proximal end and its proximal dorsoventral diameter exceeds the proximodistal length of the entire element (Supplementary Table S3). We regard these proportions as autapomorphic of Notocolossus. The medial margin of metatarsal I is convex, whereas the lateral margin is slightly concave for articulation with metatarsal II. The proximal outline of metatarsal II is also ‘D-shaped’, but conversely, the lateral face is slightly convex and the medial face is gently concave. Thus, the proximal contour of the articulated metatarsals I and II is subcircular. The proximal ends of metatarsals III and IV are subquadrangular in shape, whereas that of metatarsal V is slightly semilunar, with the most acute end pointing dorsolaterally. The shafts of all metatarsals are constricted both dorsoventrally and mediolaterally. Their distal ends are mediolaterally broad and range in distal profile from quadrangular in metatarsal I to elliptical in metatarsal V. The latter is proportionally more distally expanded than in most other titanosauriforms (e.g., Bonitasaura, Epachthosaurus, Mendozasaurus, Opisthocoelicaudia, Rapetosaurus, Tastavinsaurus, the La Invernada titanosaur, PM TGU 16/0, Field Museum of Natural History [FMNH] PR 977 [an isolated titanosauriform pes from Texas31 referred to Cedarosaurus by D’Emic32]), with the exception of NMMNH P-49967 (?Alamosaurus).

The pedal phalangeal formula is 2-2-2-2-0, as in Mendozasaurus (pers. obs. of an associated but disarticulated specimen) and the Agua del Padrillo and La Invernada titanosaurs23 (Table 3). Other titanosauriforms, by contrast, differ in the number of phalanges on digits III–V. Notable differences involve digit III, where, as in UNCUYO-LD 302, most taxa (Gobititan, Opisthocoelicaudia, the Padrillo and Invernada taxa) carry two phalanges, but Epachthosaurus has three and FMNH PR 977 may retain four31 (though the pedal phalangeal formula of this specimen has recently been reinterpreted32). Furthermore, Opisthocoelicaudia is reported to possess only a single phalanx on digit IV18, whereas Gobititan apparently retains two phalanges on digit V26. Phalanx I-1 of Notocolossus is considerably proximodistally shorter than the other proximal phalanges, but it remains large and well-developed, unlike in the Invernada titanosaur, where this phalanx is apomorphically reduced24. All proximal phalanges are much mediolaterally wider than dorsoventrally deep, with their widths exceeding half the lengths of their corresponding metatarsals (Supplementary Table S3). Those of digits II–IV are robust, proximodistally elongate and quadrangular in dorsal view. The distal articular surfaces of phalanges II-1 and III-1 are bevelled such that they angle sharply proximolaterally in dorsal view and the medial faces of these phalanges are considerably longer than the lateral. Unlike the other proximal phalanges, phalanx IV-1 is notably ‘waisted’ in dorsal view, such that it is mediolaterally narrowest at midshaft.

Table 3 Pedal phalangeal formulae and total number of pedal phalanges of sauropods for which complete hind feet are known. Full size table

The appearance of the three pedal unguals (phalanges I-2, II-2 and III-2) of UNCUYO-LD 302 is unique within Sauropoda (Fig. 4d,e; Supplementary Figs S4, S5, S8). Their proximal extremes closely resemble those of the pedal unguals of other titanosaurs (e.g., Dreadnoughtus, Epachthosaurus, Mendozasaurus, Rapetosaurus, ?Alamosaurus, the Padrillo and Invernada forms) in being dorsoventrally elongate and elliptical in proximal view; this is especially true for ungual II. Nevertheless, each ungual terminates in a blunt, extremely rugose and irregular distal end. As preserved, unguals II and III are concave distally and longer than ungual I. Whether this condition represents the ‘typical’ morphology of Notocolossus or is pathological is presently unclear (see below). Phalanx IV-2 is an amorphous, proximodistally compressed bony ‘nubbin’ that is flat proximally and convex distally.