We describe Sarmientosaurus musacchioi gen. et sp. nov., a titanosaurian sauropod dinosaur from the Upper Cretaceous (Cenomanian—Turonian) Lower Member of the Bajo Barreal Formation of southern Chubut Province in central Patagonia, Argentina. The holotypic and only known specimen consists of an articulated, virtually complete skull and part of the cranial and middle cervical series. Sarmientosaurus exhibits the following distinctive features that we interpret as autapomorphies: (1) maximum diameter of orbit nearly 40% rostrocaudal length of cranium; (2) complex maxilla—lacrimal articulation, in which the lacrimal clasps the ascending ramus of the maxilla; (3) medial edge of caudal sector of maxillary ascending ramus bordering bony nasal aperture with low but distinct ridge; (4) ‘tongue-like’ ventral process of quadratojugal that overlaps quadrate caudally; (5) separate foramina for all three branches of the trigeminal nerve; (6) absence of median venous canal connecting infundibular region to ventral part of brainstem; (7) subvertical premaxillary, procumbent maxillary, and recumbent dentary teeth; (8) cervical vertebrae with ‘strut-like’ centroprezygapophyseal laminae; (9) extremely elongate and slender ossified tendon positioned ventrolateral to cervical vertebrae and ribs. The cranial endocast of Sarmientosaurus preserves some of the most complete information obtained to date regarding the brain and sensory systems of sauropods. Phylogenetic analysis recovers the new taxon as a basal member of Lithostrotia, as the most plesiomorphic titanosaurian to be preserved with a complete skull. Sarmientosaurus provides a wealth of new cranial evidence that reaffirms the close relationship of titanosaurs to Brachiosauridae. Moreover, the presence of the relatively derived lithostrotian Tapuiasaurus in Aptian deposits indicates that the new Patagonian genus represents a ‘ghost lineage’ with a comparatively plesiomorphic craniodental form, the evolutionary history of which is missing for at least 13 million years of the Cretaceous. The skull anatomy of Sarmientosaurus suggests that multiple titanosaurian species with dissimilar cranial structures coexisted in the early Late Cretaceous of southern South America. Furthermore, the new taxon possesses a number of distinctive morphologies—such as the ossified cervical tendon, extremely pneumatized cervical vertebrae, and a habitually downward-facing snout—that have rarely, if ever, been documented in other titanosaurs, thus broadening our understanding of the anatomical diversity of this remarkable sauropod clade. The latter two features were convergently acquired by at least one penecontemporaneous diplodocoid, and may represent mutual specializations for consuming low-growing vegetation.

Funding: R.D.F.M. and G.A.C. were supported by the Facultad de Ciencias Naturales of the Universidad Nacional de la Patagonia San Juan Bosco ( http://www.fcn.unp.edu.ar/ ). M.C.L.’s participation in the research was supported by Carnegie Museum of Natural History ( http://www.carnegiemnh.org/ ), the University of Pennsylvania ( http://www.upenn.edu/ ), the National Geographic Society (Grant 6646-99) ( http://www.nationalgeographic.com/ ), the Jurassic Foundation ( http://jurassicfoundation.org/ ), the Evolving Earth Foundation ( http://www.evolvingearth.org/ ), the Delaware Valley Paleontological Society ( http://dvps.essentrix.net/ ), and the Paleontological Society ( http://www.paleosoc.org/ ). L.M.W. and R.C.R. acknowledge funding support from the Ohio University Heritage College of Osteopathic Medicine ( http://www.oucom.ohiou.edu/ ) and the National Science Foundation (IBN-0343744, IOB-0517257, IOS-1050154) ( http://www.nsf.gov/ ). The Ohio Supercomputing Center ( https://www.osc.edu/ ) provided additional support. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

The titanosaur was preserved in a green sandstone horizon that pertains to the upper part of the Lower Member of the Bajo Barreal Formation. This section of the Lower Member is lithologically characterized by these green sandstones, which were deposited in multiepisodic, interlaced fluvial channel systems [ 83 ]. The vast majority of the tetrapod fossils from the Bajo Barreal Formation have been recovered from these sandstones, which exhibit taphonomic and sedimentological properties that were conducive to vertebrate preservation [ 84 ]. The skull and cervical series of the titanosaur were articulated and preserved in a fluvial overflow deposit with a high sedimentary load composed of medium-grained sandstones with abundant pelitic matrix. The degree of articulation and lack of evidence of subaerial weathering of the specimen suggest that it was buried rapidly.

The new titanosaur comes from an exposure of the Lower Member of the Upper Cretaceous Bajo Barreal Formation on the Estancia Laguna Palacios near the village of Buen Pasto in south-central Chubut Province, central Patagonia, Argentina. The Bajo Barreal Formation has produced a diverse continental vertebrate fauna that is among the richest of Patagonian dinosaur-bearing units [ 66 – 75 ]. Its outcrops are widely distributed in southern Chubut and northernmost Santa Cruz provinces. Palynological data from a subsurface equivalent of the Bajo Barreal Formation, the Caleta Olivia Member of the Cañadón Seco Formation, initially suggested a late Albian—Cenomanian age for these deposits [ 76 ]. Subsequently, Bridge et al. [ 77 ] reported Ar—Ar radiometric dates from tuffs of the Bajo Barreal Formation that range in age from 95.8 to 91.0 Ma, corresponding to the middle Cenomanian—middle Turonian of the current Geologic Time Scale [ 65 ]. Most recently, Suárez et al. [ 78 ] obtained radiometric ages from zircons that further support a Cenomanian age for the Lower Member. Clyde et al. [ 79 ] argued for a much younger (Campanian) age for the Bajo Barreal Formation on the basis of magnetostratigraphic and biostratigraphic evidence, but the strata they investigated—exposed in the regions of Lago Colhué Huapi and the Río Chico in southeastern Chubut—have recently been reassigned to a newly identified and significantly younger geologic unit (the Lago Colhué Huapi Formation [ 80 ]). As noted by Canale et al. [ 81 ], the vertebrate fauna of the Bajo Barreal Formation is closely comparable to that of the lower Cenomanian [ 82 ] Candeleros Formation of the Neuquén Basin in northern Patagonia, suggesting that the two units may be correlative.

Here we describe a new and plesiomorphic early Late Cretaceous (Cenomanian—Turonian) titanosaurian sauropod represented by a superbly-preserved adult skull articulated with a partial cervical series. The taxon provides a wealth of new information on the early evolution of Titanosauria and the cranial anatomy of basal members of the clade. The cranium and mandible are only slightly deformed, with most bones fully articulated and all teeth preserved in situ; as such, the new form is one of the very few titanosaurs for which the totality of this anatomical information is available. Furthermore, the unusual anatomy of the cervical series provides novel data on the construction of the neck and tendon system of a Cretaceous sauropod.

In the years since Huene [ 9 ] described the incomplete craniomandibular remains of the Patagonian titanosaur Antarctosaurus wichmannianus (hereafter Antarctosaurus, since A. wichmannianus is the only species of this genus mentioned herein), discoveries of well-preserved titanosaurian skulls have been extraordinarily rare. At present, complete or nearly complete skulls are known only for the following taxa: Nemegtosaurus, from the Upper Cretaceous Nemegt Formation of Mongolia [ 10 , 11 ]; Rapetosaurus, from the Upper Cretaceous Maevarano Formation of Madagascar [ 12 , 13 ]; and Tapuiasaurus, from the Lower Cretaceous Quiricó Formation of Brazil [ 14 ]. Tapuiasaurus was recovered from Aptian strata, whereas Nemegtosaurus and Rapetosaurus come from Maastrichtian deposits. Moreover, several complete or nearly complete but distorted skulls of generically unidentified embryonic titanosaurs are known from the Campanian Anacleto Formation at the Auca Mahuevo locality in northern Patagonia, Argentina [ 15 , 16 , 17 ]. Fragmentary skulls or isolated skull elements are also known for a number of titanosaurs or possible titanosaurs, in addition to Antarctosaurus: Ampelosaurus atacis [ 18 ], Ampelosaurus sp. [ 19 ], Bonatitan [ 20 – 22 ], Bonitasaura [ 23 , 24 ], Brasilotitan [ 25 ], Campylodoniscus [ 9 ], Dreadnoughtus [ 26 ], Isisaurus [ 27 – 30 ], Jainosaurus [ 28 , 30 , 31 ], Karongasaurus [ 32 ], Ligabuesaurus [ 33 ], Lirainosaurus [ 34 , 35 ], Malawisaurus [ 32 ], Maxakalisaurus [ 36 ], Mongolosaurus [ 37 , 38 ], Muyelensaurus [ 39 ], Narambuenatitan [ 40 ], Phuwiangosaurus [ 41 ], Pitekunsaurus [ 42 ], Quaesitosaurus [ 11 , 43 ], Quetecsaurus [ 44 ], Rinconsaurus [ 45 ], Saltasaurus [ 7 , 46 ], Tambatitanis [ 47 ], and Vahiny [ 48 ]. Isolated, generically indeterminate titanosaurian cranial and mandibular elements have also been reported [ 7 , 9 , 21 , 31 , 49 – 64 ]. Nevertheless, complete or even reasonably complete titanosaur skulls remain unknown from the Albian—Santonian (a roughly 30 million year span of the mid- and Late Cretaceous; see Walker et al. [ 65 ]), which represents a significant impediment to understanding of titanosaur cranial anatomy and evolution.

Titanosaurian sauropod dinosaurs were extremely diverse and abundant in Upper Cretaceous continental paleoenvironments in the Gondwanan landmasses, and have been discovered throughout the world [ 1 – 6 ]. Titanosauria currently includes more than 60 genera and is most abundantly represented in South America, particularly in Argentina [ 4 , 5 , 7 , 8 ]. Most currently recognized titanosaurian taxa are represented exclusively or almost exclusively by postcranial bones.

The electronic edition of this article conforms to the requirements of the amended International Code of Zoological Nomenclature (ICZN), and hence the new names contained herein are available under that Code from the electronic edition of this article. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank Life Science Identifiers (LSIDs) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix “ http://zoobank.org/ .” The LSID for this publication is: urn:lsid:zoobank.org:pub:3B8C51B9-C0C2-4562-81D4-0AF58E186B31. The electronic edition of this work was published in a journal with an ISSN, and has been archived and is available from the following digital repositories: PubMed Central and LOCKSS.

a, angular; af, adductor fossa; al, alveolus; aof, antorbital fenestra; arm, ascending ramus of maxilla; awf, apical wear facet; ax, axis; b, bulge; bna, bony nasal aperture; bo, basioccipital; bpt, basipterygoid process; bs, basisphenoid; bt, basal tuber; bwf, beveled wear facet; C, cervical vertebra; c, cochlea; car, canal for cerebral carotid artery; cc, crus communis; cd, condyle; cde, caudal dural expansion; cer, cerebral hemisphere; cor, coronoid; cprs, centroprezygapophyseal ‘strut’; csc, caudal (vertical) semicircular canal; csf, caudal surangular foramen; cts, cerebrotectal (sphenoparietal) venous sinus; d, dentary; df, dental foramen; dwf, distal wear facet; ec, ectopterygoid; ed, endolymphatic duct; f, frontal; fc, fenestra cochleae (fenestra rotunda); fi, fibroblasts; floc, cerebellar flocculus (auricle); fom, foramen magnum; fv, fenestra vestibuli (fenestra ovalis); itf, infratemporal fenestra; j, jugal; l, lacrimal; lab, labial surface; labyr, endosseous labyrinth; lgr, large groove; lin, lingual surface; ls, laterosphenoid; lsc, lateral (horizontal) semicircular canal; lwf, lingual wear facet; m, maxilla; mg, Meckelian groove; mgr, mesial groove; n, nasal; nf, narial fossa; nvf, neurovascular foramen; ns, neural spine; ob, olfactory bulb; occ sin, occipital (dural venous) sinus; ocv, canal for orbitocerebral vein; orb, orbit; os, orbitosphenoid; oto, otoccipital; p, parietal; paof, preantorbital foramen; pf, pneumatic fossa; pfo, pituitary (hypophyseal) fossa; pl, palatine; pm, premaxilla; po, postorbital; podl, postzygodiapophyseal lamina; pof, postorbital foramen; poz, postzygapophysis; pra, prearticular + articular; prdl, prezygodiapophyseal lamina; prf, prefrontal; prz, prezygapophysis; pt, pterygoid; q, quadrate; qf, quadrate fossa; qj, quadratojugal; r, rib; rde, rostral dural expansion; rmf, rostral maxillary foramen; rsc, rostral (vertical) semicircular canal; rsca, ampulla of rostral (vertical) semicircular canal; rsf, rostral surangular foramen; sa, surangular; snf, subnarial foramen; so, supraoccipital; sof, suborbital fenestra; sp, splenial; spha, canal for sphenopalatine artery; sprl, spinoprezygapophyseal lamina; sq, squamosal; stf, supratemporal fenestra; sym, mandibular symphysis; t, tooth; ts, transverse (dural venous) sinus; ttv, canal for transversotrigeminal (rostral middle cerebral) vein; tz, transitional zone; v, vomers; ve, vestibule of inner ear; II, canal for optic nerve; III, canal for oculomotor nerve; IV, canal for trochlear nerve; V 1 , canal for ophthalmic branch of trigeminal nerve; V 1-SO? , canal possibly for the supraorbital nerve (a branch of CN V 1 ); V 2 , canal for maxillary branch of trigeminal nerve; V 3 , canal for mandibular branch of trigeminal nerve; VI, canal for abducens nerve; VII, canal for facial nerve; VIII, canal for vestibulocochlear nerve; IX–XI, shared canal for glossopharyngeal, vagus, and accessory nerves and accompanying vessels; XII, canal for hypoglossal nerve.

AMNH, American Museum of Natural History, New York, New York, United States of America; ANS, Academy of Natural Sciences of Drexel University, Philadelphia, Pennsylvania, United States of America; CCMGE, Chernyshev’s Central Museum of Geological Exploration, Saint Petersburg, Russia; CM, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, United States of America; FGGUB, Facultatea de Geologie şi Geofizică a Universită ii din Bucureşti, Bucharest, Romania; GCP, Grupo Cultural Paleontológico de Elche, Museo Paleontológico de Elche, Elche, Spain; GSI, Geological Survey of India, Kolkata, India; ISI, Indian Statistical Institute, Kolkata, India; MAL, Malawi Department of Antiquities Collection, Lilongwe and Nguludi, Malawi; MB, Museum für Naturkunde der Humboldt-Universität, Berlin, Germany; MCF-PVPH, Museo ‘Carmen Funes,’ Colección de Paleontología de Vertebrados, Plaza Huincul, Neuquén, Argentina; MCSPv, Museo de Cinco Saltos, Cinco Saltos, Río Negro, Argentina; MCZ, Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts, United States of America; MDT-PV, Museo Desiderio Torres-Paleovertebrados, Sarmiento, Chubut, Argentina; MGPIFD-GR, Museo de Geología y Paleontología del Instituto de Formación Docente Continua de General Roca, General Roca, Río Negro, Argentina; MML, Museo Municipal de Lamarque, Lamarque, Río Negro, Argentina; MPCA, Museo Provincial ‘Carlos Ameghino,’ Cipolletti, Río Negro, Argentina; MUCPv, Museo de Geología y Paleontología de la Universidad Nacional del Comahue, Neuquén, Neuquén, Argentina; MZSP-PV, Museu de Zoologia da Universidade de São Paulo, São Paulo, Brazil; TMM, University of Texas Memorial Museum, Austin, Texas, United States of America; UNPSJB-PV, Universidad Nacional de la Patagonia San Juan Bosco, Colección Paleontología de Vertebrados, Comodoro Rivadavia, Chubut, Argentina; USNM, National Museum of Natural History, Washington, District of Columbia, United States of America; ZPAL, Institute of Paleobiology, Polish Academy of Sciences, Warsaw, Poland.

The specimen described in this paper (specimen number MDT-PV 2) is permanently reposited and accessible to all qualified researchers in the fossil vertebrate collection of the Museo Desiderio Torres in Sarmiento, Chubut Province, Argentina. Detailed locality information for the specimen is on file at the Museo Desiderio Torres and is available to qualified researchers upon request. All necessary permits were obtained for the described study, which complied with all relevant regulations.

Results

Systematic Paleontology Saurischia Seeley 1887 [85] Sauropodomorpha Huene 1932 [86] Sauropoda Marsh 1878 [87] Titanosauriformes Salgado, Coria, and Calvo 1997 [88] Titanosauria Bonaparte and Coria 1993 [89] Lithostrotia Upchurch, Barrett, and Dodson 2004 [90] Sarmientosaurus gen. nov. urn:lsid:zoobank.org:act:537DFE26-54EC-4978-AC86-E83A04FA74DE Sarmientosaurus musacchioi sp. nov. urn:lsid:zoobank.org:act:C1090B8D-D051-44F3-B869-8B4A0C802176 Holotype. MDT-PV 2, an originally articulated cranial and cervical skeleton consisting of the nearly complete skull, the partial axis associated with its rib from the right side and articulated with the cranial part of the third cervical vertebra, a fragment of the fifth cervical vertebra, the nearly complete sixth cervical vertebra and its right rib, the partial seventh cervical vertebra, and a section of ossified cervical tendon. Etymology. Sarmiento, for the Patagonian town and the administrative department in which it is located, the latter of which has yielded numerous Cretaceous dinosaur fossils; saurus, Greek, ‘lizard.’ The specific name honors the late Dr. Eduardo Musacchio, a model scientist and educator at the Universidad Nacional de la Patagonia San Juan Bosco in Comodoro Rivadavia, Argentina. Locality and horizon. Estancia Laguna Palacios (44°54'11.6'' S, 69°22'56.7'' W), Sierra Nevada Anticline, Golfo San Jorge Basin, south-central Chubut Province, central Patagonia, Argentina (Fig 1). Uppermost section of the Lower Member of the Upper Cretaceous Bajo Barreal Formation, Chubut Group. The specimen was found in situ in a tuffaceous sandstone that is regarded as Cenomanian—Turonian in age [69,72,76–78,80]. PPT PowerPoint slide

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larger image TIFF original image Download: Fig 1. Map of Chubut Province, central Patagonia, Argentina, showing location of the Estancia Laguna Palacios, the type locality of Sarmientosaurus musacchioi gen. et sp. nov. (modified from Ibiricu et al. [ Map of Chubut Province, central Patagonia, Argentina, showing location of the Estancia Laguna Palacios, the type locality of Sarmientosaurus musacchioi gen. et sp. nov. (modified from Ibiricu et al. [ 232 ]). https://doi.org/10.1371/journal.pone.0151661.g001 Diagnosis. Basal lithostrotian titanosaurian sauropod diagnosed by the following autapomorphies: (1) maximum (rostroventral—caudodorsal) diameter of orbit nearly 40% rostrocaudal length of cranium (as measured from tip of snout to occipital condyle); (2) complex maxilla—lacrimal articulation, with ascending ramus of maxilla embedded in and bordered laterally and medially by lacrimal dorsal process; (3) medial edge of caudal sector of maxillary ascending ramus bordering bony nasal aperture with low but well-defined ridge; (4) ‘tongue-like’ ventral process of quadratojugal that overlaps quadrate caudally; (5) separate foramina for all three branches of the trigeminal nerve; (6) absence of median venous canal connecting infundibular region to ventral part of brainstem; (7) premaxillary teeth subvertical, maxillary teeth procumbent, and dentary teeth recumbent; (8) middle cervical vertebrae with ‘strut-like’ (as opposed to ‘sheet-like’) centroprezygapophyseal laminae; (9) extremely elongate and slender ossified tendon extending along cervical series ventrolateral to vertebrae and ribs. Preservation. The cranium, mandible, and all preserved cervical vertebrae and ribs of the new titanosaur were originally found in articulation (Fig 2). Nevertheless, during the course of laboratory preparation, we were only able to recover the skull, parts of the articulated axis and third cervical vertebra, most of the sixth and seventh cervical vertebrae, and pieces of the fifth cervical vertebra and the second and sixth cervical ribs from the right side. Unfortunately, the remainder of the collected vertebrae (the atlas and cervical four) and ribs were too poorly preserved and damaged by weathering to be salvageable. PPT PowerPoint slide

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larger image TIFF original image Download: Fig 2. Disposition of the type specimen of Sarmientosaurus musacchioi gen. et sp. nov. (MDT-PV 2) upon discovery. (A) Articulated skull in ventral view, showing close association of ossified cervical tendon (arrow) with occipital region of cranium. (B, C) Two views of articulated skull and partial cervical series in ventral view, showing considerable craniocaudal extent and consistently narrow diameter of ossified cervical tendon (arrows). (D) Relationship of a cervical rib (white arrow) with the ossified cervical tendon (black arrow). https://doi.org/10.1371/journal.pone.0151661.g002 In the field, we observed a very slender, dark, cylindrical structure situated adjacent to and oriented parallel to the right ventrolateral area of the articulated cervical vertebrae and ribs (Fig 2). This structure extended from near the right occipital region of the skull and past several vertebrae without changing diameter. Although we observed the structure on only the right side of the specimen, we assume that it was bilaterally symmetrical in the living animal. Therefore, given that the right side of the specimen is generally better preserved than the left, the equivalent structure on the left side presumably eroded away prior to discovery. Given the extraordinary length attained by the cervical ribs of some sauropods (e.g., mamenchisaurids [91,92]), including other titanosauriforms (e.g., Giraffatitan [93], Sauroposeidon [94]), it is conceivable that this structure might represent the caudal end of one of these ribs. Nevertheless, as observed in the field, the structure maintained its same, diminutive diameter alongside several cervical vertebrae, and its cranial extreme was situated immediately caudal to the skull, morphologies that are inconsistent with known sauropod cervical ribs. Furthermore, because the skull, cervical vertebrae, and ribs were all fully articulated, the identification of this structure as a displaced cervical rib shaft seems unlikely. We therefore interpret the structure as an ossified tendon that is distinct from the cervical ribs. Unlike the situation in Nemegtosaurus [10,11] and Tapuiasaurus [14], the skull of Sarmientosaurus was not strongly affected by taphonomic distortion. Instead, the skull is only moderately deformed in its caudodorsal and dorsal areas. Pressure applied to these regions apparently caused the jugal processes of both postorbitals to slide slightly rostrally over the postorbital processes of the corresponding jugals. Nevertheless, these modest alterations demonstrate that the caudal part of the skull was not significantly rostrally displaced relative to more rostral regions. There is no evidence of dorsoventral compression of the snout; indeed, in this area of the skull, only the dorsal parts of the premaxilla and maxilla are damaged, presumably due to pre-diagenetic erosion. The sixth and seventh cervical vertebrae have suffered some lateral deformation, which has mainly affected parts of the neural arches such as the prezygapophyses. During the excavation of the Sarmientosaurus holotype, an abelisaurid tooth was discovered only a few centimeters from the occipital region of the skull, raising the possibility that this titanosaurian specimen was scavenged by this theropod. This is, however, ambiguous, as the Sarmientosaurus bones do not exhibit tooth marks or other feeding traces.

Description and Comparisons Anatomical Terminology. In our description of the dentition of Sarmientosaurus, we employ the terms used by García and Cerda [61].

Cranium The cranium of Sarmientosaurus is 43 cm in length as measured from the rostral tip of the articulated premaxillae to the occipital condyle. It is approximately 24 cm wide across the postorbitals and 24 cm tall from the dorsal margin of the frontal to the ventral end of the quadrate on the right side (see Table 1). Extreme fusion of many cranial bones, as in specimens of Ampelosaurus [18,19] and Saltasaurus [7,46], indicates that the specimen probably corresponds to a skeletally mature (and possibly very old) individual. PPT PowerPoint slide

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larger image TIFF original image Download: Table 1. Measurements (mm) of the skull of MDT-PV 2, the holotype of Sarmientosaurus musacchioi gen. et sp. nov. Abbreviations: L, left; R, right. * = element incomplete, measurement as preserved. https://doi.org/10.1371/journal.pone.0151661.t001

External Cranial Fenestrae In the cranium of Sarmientosaurus, three large openings are clearly visible in lateral view: from rostral to caudal, these are the antorbital fenestra, the orbit, and the infratemporal fenestra. As preserved, the bony nasal apertures (= ‘external nares’ of many paleontological works) open rostrodorsally in a confluent fenestra, as in Rapetosaurus; nevertheless, it appears that, in life, these openings would have been separated by a bony lamina formed by the premaxillae and nasals (the internarial bar). Although this structure has been mostly destroyed by taphonomic processes, the caudally-incomplete narial flange of the premaxillae and a broken rostral projection of the nasals attest to its former existence. Ventral to the rostral end of each antorbital fenestra is a minute, poorly preserved opening that we interpret as the homolog of the preantorbital fenestra; this foramen is discussed further in of our description of the maxilla below. The antorbital fenestra of the new Patagonian taxon is small, and its long axis is aligned obliquely with respect to that of the skull. It is teardrop-shaped, with the wider, rounded terminus situated rostroventrally and the pointed end positioned caudodorsally. The antorbital fenestra of Sarmientosaurus resembles that of the Jurassic brachiosaurid Giraffatitan [95] but differs from those of the basal macronarians Camarasaurus [96] and Europasaurus [97] and the Cretaceous titanosauriforms Abydosaurus [98] and Euhelopus [99–101], which are oriented more vertically. The fenestra of Sarmientosaurus also differs from those of Nemegtosaurus [10,11] and Tapuiasaurus [14], which are larger, and especially that of Rapetosaurus, which is extremely large and rostrocaudally elongate [13]. A greatly enlarged antorbital fenestra also appears to be present in an isolated sauropod (presumably titanosaurian [102]) maxilla from the Maastrichtian Lameta Formation of India (ISI R K 27/528; see Huene and Matley [31]:fig. 19). The shape of the rostral edge of the lacrimal of the Late Cretaceous titanosaur Bonitasaura indicates that the caudodorsal margin of the antorbital fenestra was smoothly rounded in this taxon [24] rather than sharply acute as in Sarmientosaurus. The antorbital fenestra of the new Patagonian taxon is oriented at an angle of approximately 45° relative to the rostrocaudal axis of the skull, comparable to the condition in Giraffatitan and that reconstructed for Bonitasaura. The orbit of Sarmientosaurus is proportionally very large, rostroventrally—caudodorsally elongate, and rounded at its caudodorsal and rostroventral margins, with the caudodorsal end rostrocaudally longer than the rostroventral end. As in many dinosaurs, the orbit is regionally divisible into a dorsal ocular portion (that housed the eyeball and its adnexa) and a ventral non-ocular portion that was occupied by various soft-tissues (e.g., adductor muscles, vessels, nerves). The orbit differs from those of Camarasaurus, Giraffatitan, Nemegtosaurus, Rapetosaurus, and Tapuiasaurus, which are smaller and shaped differently. Although the orbit of Abydosaurus is also proportionally large, it is not as large as in the new Bajo Barreal titanosaur; furthermore, it is subtriangular rather than ovate in contour. The supratemporal fenestra is bordered caudally by a prominent flange (the transverse nuchal crest), and its long axis is oriented mediolaterally, as in Europasaurus, Giraffatitan, and Rapetosaurus. The infratemporal fenestra is rostrocaudally narrow throughout its dorsoventral extent, and its long axis is oriented roughly parallel to that of the orbit, as in Nemegtosaurus and Tapuiasaurus. This contrasts the conditions in Abydosaurus, Camarasaurus, Euhelopus, and Giraffatitan, in which this fenestra is subtriangular and rostrocaudally wide, especially ventrally.

Palatal Complex The palatal region of the Sarmientosaurus holotype was partially damaged by erosion, mainly on its midline. The vomers are incomplete, as is often the case in sauropod skulls [96], and parts of the palatines, ectopterygoids, and pterygoids are also missing. Palatine. Although both palatines are incomplete, the right is better preserved than the left (Figs 3, 5, 6 and 8; S1 Fig; S1, S2, S3 and S4 Movies). The right palatine preserves part of the lateral region, primarily the elongate, rostrolaterally-directed maxillary process. The entire medial section of the bone where it articulates with the pterygoid has been lost, whereas the left palatine preserves most of the pterygoid contact. The body of the maxillary process is dorsomedially inclined and roughly tubular. Its rostrolateral contact with the ectopterygoid is preserved, as is its more caudolateral contact with the rostral end of the pterygoid, although all of these bones are somewhat disarticulated. The rostral end of the maxillary process is fractured into pieces on both sides, but enough is preserved to suggest that its contact with the palatal process of the maxilla is typical for sauropods in that the palatine underlaps the maxilla ventrally. Likewise, the arrangement of the maxillary process of the palatine, the ectopterygoid, and the pterygoid around the suborbital (= postpalatine) fenestra also resembles that of other sauropods in that this fenestra is small and bounded rostrally by the palatine, caudally by the ectopterygoid (with the pterygoid nearby), and laterally by the palatal process of the maxilla. The maxillary process of the palatine narrows and expands again caudomedially as it approaches the pterygoid. As shown on the left side, although somewhat damaged and disarticulated, the pterygoid contact of the palatine is expanded and articulates in the fork between the medial vomerine ramus and the lateral transverse ramus of the pterygoid, as is common in sauropods [95,96,103]. Pterygoid. The pterygoid (Figs 3, 5, 6 and 8; S1 Fig; S1, S2, S3 and S4 Movies) is the largest bone of the palatal complex. Neither pterygoid is complete, but enough is preserved of both to offer a reasonably comprehensive description. In general, the pterygoid of Sarmientosaurus is typical for sauropods in that the bone has a complex shape, with three main processes—the quadrate, vomerine, and transverse rami—arising from the highly arched body. The pterygoid body, which is better preserved on the left side, is expansive, forming a distinct ventral fossa (the postchoanal fossa) that faces rostromedially. The quadrate ramus, also better preserved on the left pterygoid, branches off the caudolateral corner of the body and twists into a more vertical plane as it attaches to the medial aspect of the reciprocal pterygoid ramus of the quadrate. The vomerine ramus passes dorsomedially, contacting its counterpart at the midline to form a thin triangular wedge that approaches the roof of the nasal cavity, where it nearly contacts the subnarial processes of the premaxilla and maxilla. Rostrally, the vomerine rami pass medial to the paired vomers. Near the juncture of the vomerine and quadrate rami, the body of the pterygoid forms a shallow but distinct, caudomedially facing pocket for the articulation of the basipterygoid process. The transverse ramus of the pterygoid is preserved on both sides, but is better preserved on the right side. As is true for most sauropods, the transverse ramus is slender and swept far rostrally, carrying the ectopterygoid with it. The lateral end of the transverse ramus is slender and curves ventrally. The ectopterygoid attaches broadly to its rostral surface, just caudal to the suborbital fenestra. Together, these two bones form the ‘pterygoid flange,’ which is a strong, transverse projection in many other archosaurs, but is a relatively delicate structure in Sarmientosaurus and most other sauropods. As noted above, the palatine articulates with the body of the pterygoid rostrally, between the vomerine and transverse rami of the latter. Vomer. The vomers (Figs 5, 6 and 8; S1 Fig; S1, S2, S3 and S4 Movies) are delicate, paired bones that are damaged and best observed in the CT images. They are thin laminae of bone that attach to the lateral and ventral portions of the vomerine rami of the pterygoids. As noted above, along with the pterygoids, they likely contacted the ventral surfaces of the subnarial processes of the premaxillae and maxillae. This is also the case in Camarasaurus and Diplodocus, and undoubtedly in other sauropods as well, though this part of the cranium is not well understood in most taxa. Ectopterygoid. The ectopterygoids (Figs 5, 6 and 8; S1 Fig; S1, S2, S3 and S4 Movies) are essentially complete but remain largely embedded in matrix. Of the two, the right is the more clearly visible. The ectopterygoid is a relatively simple, slender bone that forms the caudal border of the suborbital fenestra. Its lateral end contacts the medial surface of the maxilla. The rostral end of the ectopterygoid passes ventrally and curves to articulate on the rostral face of the transverse ramus of the pterygoid, such that they collectively form the ‘pterygoid flange,’ as noted above. The lateral, medial, and ventromedial surfaces of the ectopterygoid are smooth. The ectopterygoid of Sarmientosaurus is much shorter rostrocaudally than the bones that Wilson [11] identified as ectopterygoids in Nemegtosaurus and Quaesitosaurus. Quadrate. The right quadrate is, in most regions, better preserved than the left (Figs 3–6 and 8; S1 Fig; S1, S2, S3 and S4 Movies). It articulates with the pterygoid rostromedially, the squamosal caudodorsally, the quadratojugal laterally, and the articular ventrally, forming the jaw joint. Although the rostroventral region of both quadrates is damaged, both preserve at least part of the articulation with the pterygoid, which is better preserved on the left side. On the right quadrate, caudally, the edge of the vertical lamina that articulates laterally with the quadratojugal and ventrally with the articular is eroded. The right quadrate is fairly smooth in areas where its surface is intact. Its long axis is oriented caudodorsally relative to that of the skull. The quadrate is obscured from rostral view by the other bones it contacts. It is best observed in caudal view, where the damaged head that articulates with the squamosal rostrolaterally may be seen at its dorsal end. Other structures of the right quadrate evident in caudal view include the subvertical and plate-like medial edge, the thick expansion that terminates ventrally in the mandibular articulation, and the quadrate fossa, which is delimited medially by the quadrate and laterally by the quadratojugal. The quadrate fossa of Sarmientosaurus faces caudolaterally, as in Nemegtosaurus, Quaesitosaurus, and Rapetosaurus, though it is apparently not as laterally oriented as it is in these taxa. The quadrate fossa is comparable in shape to those of most other titanosaurs (e.g., Narambuenatitan, Nemegtosaurus, Quaesitosaurus, Rapetosaurus), but is seemingly more dorsoventrally elongate than in Phuwiangosaurus [41] and especially Malawisaurus; in the latter African titanosaur, the quadrate fossa appears nearly as wide mediolaterally as it is tall (see Gomani [32]:fig. 5d). The ventral extreme of the right quadrate preserves part of the medial condyle, which is mediolaterally wider than that of Narambuenatitan (see Filippi et al. [40]:fig. 5a). The pterygoid ramus of the right quadrate and its damaged contact with the pterygoid are also evident in caudal view. The pterygoid ramus is mediolaterally thick. The quadrate—squamosal articulation is laterally expanded. The part of the quadrate that is situated more caudally than the squamosal is visible in lateral view. According to Zaher et al. [14], caudolateral exposure of the quadrate is a feature of nemegtosaurids; its occurrence in Sarmientosaurus broadens the distribution of this character to include more basal titanosaurians as well. In overall morphology, the quadrate of the new Patagonian taxon is similar to those of Quaesitosaurus and Rapetosaurus.