Etymology: In reference to the Sierra de la Demanda, the mountain chain where the type specimen was found and from Greek sauros, lizard, reptile.

Type species: Demandasaurus darwini sp. nov.; see below.

Diagnosis.—As for the type and only known species.

Etymology: In honour of the naturalist Charles R. Darwin (1809–1882).

Type locality and age: Upper section of the Castrillo de la Reina Formation (Tenadas de los Vallejos II locality), regarded as Late Barremian to Early Aptian in age (Martín-Closas and Alonso Millán 1998).

Holotype: A partial skeleton represented by the right (MDS-RVII,275) and left (MDS-RVII,332) premaxillae, left dentary (MDS-RVII,443), six isolated teeth (MDS-RVII,340; MDS-RVII,436; MDS-RVII,437; MDS-RVII,438; MDS-RVII,440; MDS-RVII,441), three cervical vertebrae (MDS-RVII,589; MDS-RVII,606 [axis]; MDS-RVII,806), five cervical ribs (MDS-RVII,379; MDS-RVII,458; MDS-RVII,466; MDS-RVII,587; MDS-RVII,811), two dorsal vertebrae (MDS-RVII,242; MDS-RVII,798), nine dorsal ribs (MDS-RVII, 14; MDSRVII,301; MDS-RVII,364; MDS-RVII,365; MDS-RVII,368; MDS-RVII,382; MDS-RVII,592; MDS-RVII,600; MDS-RVII,611), nineteen caudal vertebrae (MDS-RVII,2; MDS-RVII,3; MDS-RVII,4; MDS-RVII,9; MDS-RVII,10; MDS-RVII,11; MDS-RVII,12; MDSRVII,15; MDS-RVII,101; MDS-RVII,216; MDS-RVII,217; MDSRVII,218; MDS-RVII,470; MDS-RVII,602; MDS-RVII,605; MDSRVII,609; MDS-RVII,610; MDS-RVII,629 and MDS-RVII,805), nine haemal arches (MDS-RVII,23; MDS-RVII,99; MDS-RVII,231; MDS-RVII,232; MDS-RVII,590; MDS-RVII,591; MDS-RVII,594; MDS-RVII,796; MDS-RVII,797), left (MDS-RVII,18) and right (MDS-RVII,19) ischia, and left femur (MDS-RVII,16).

Diagnosis.—Demandasaurus darwini gen. et sp. nov. is characterised by 9 autapomorphic characters: (1) the teeth are ornamented with longitudinal crests on the labial and lingual faces of the crown, and bear mesial and distal carinae; (2) the posterior cervical vertebrae have an infraprezygapophyseal chamber with a forked vertical accessory lamina; (3) the posterior cervical vertebrae have a rhombic accessory structure where the centroprezygapophyseal (cprl), prezygodiapophyseal (prdl) and spinoprezygapophyseal (sprl) laminae are connected, dorsally to the prezygapophyses; (4) the centroprezygapophyseal laminae (cprl) are divided in the cervical and dorsal vertebrae; (5) presence in the mid dorsals of two large neural arch pneumatic foramina that pass all the way through the neural arch anteroposterioriy; (6) presence of two large, deep pneumatic cavities, divided by accessory laminae, in the transverse processes of the anterior caudals; (7) in the anterior caudal vertebrae the anterior centroparapophyseal (acpl), posterior centroparapophyseal (pcpl) and posterior centrodiapophyseal laminae (pcdl) are very wide and make contact posteriorly and ventrally with the diapophysis; (8) presence of two parallel laminae running in an anteroposterior direction, an upper one from the prezygapophysis to the base of the centropostzygapophyseal (cpol), and a lower one from the base of the prezygapophysis to the dorsal surface of the anterior caudal centra; (9) presence of two parallel crests running anteroposterioriy on the lateral faces of the middle-posterior caudal vertebral centra.

Description

Premaxillae (Fig. 2).—Two premaxillae have been preserved (MDS-RVII,275, right premaxilla, and MDS-RVII,332, left premaxilla). Both lack the nasal and maxillary processes. The general shape is subrectangular (taller than wide), similar to that of Nigersaurus (Sereno et al. 2007). In medial view they are thickest in their tooth-bearing portion. The anterior surface is somewhat weathered, but shows a rugose ornamentation comprising irregularly alternating crests and valleys (Fig. 2A, D). The anterior side is gently convex, and the posterior side is convex-concave. Medially the premaxillae present a flat and smooth surface for the interpremaxillary symphysis (Fig. 2C). The contact with the maxilla is sinuous. The surface for the articulation with the maxilla is smaller than that of the symphyseal area. The anterior premaxillary margin is non-stepped (almost straight), similar to that displayed by the other diplodocoids (Wilson 2002; Rauhut et al. 2005). The premaxilla has four dental positions. The right premaxilla presents the most mesial teeth inside (Fig. 2A, C). The disposition of these teeth and the alveoli indicates that the functional teeth of the premaxillae are procumbent, a character present in other diplodocoids, such as Diplodocus (see Wilson and Sereno 1998: fig. 6).

Dentary (Fig. 3).—A fragment of the left dentary (MDSRVII,443), lacking its distal end, the dentary symphysis and the functional teeth, is preserved (Fig. 3A, B). Anteriorly, the dentary becomes more robust and dorsoventraly expanded (Fig. 3A). The anteroventral margin is rounded, and is well distinguished from the more derived diplodocoids, which have a sharply projecting triangular process (Wilson 2002; Harris 2006). The preserved part lacks a mandibular fenestra. Dorsally it has an elongated depressed area which runs along the dorsolateral part of the mandibular ramus. This structure could be equivalent to the vascular canal displayed by Nigersaurus (Sereno et al. 2007), though much more developed. Distally this depressed area becomes a groove that ends up as a continuation of the alveoli. In dorsal view, it is U-shaped (Fig. 3B); the mandibular ramus turns medially to form an angle of 57° with its anteroposterior axis. Six alveoli are preserved, and one more is sectioned. This reduced number of dental positions distinguishes Demandasaurus from other rebbachisaurids such as Nigersaurus, which have more than 30 teeth in the dentary (Sereno et al. 1999). The more mesial alveoli are rectangular and larger than the posterior ones, which are subsquare. The teeth are situated in the most mesial part of the main mandibular body and in the most mesial part of the mandibular ramus.

Teeth (Fig. 4).—The preserved teeth (MDS-RVII,340; MDSRVII,436; MDS-RVII,437; MDS-RVII,438; MDS-RVII,440; MDS-RVII,441) are not complete (the roots are lacking from all specimens). In addition to these individual teeth, there are others situated within the premaxillae (MDS-RVII,275 and MDS-RVII,332), and the tooth MDS-RVII,436 is attached to the rostral face of the premaxilla MDS-RVII,332 (Fig. 2D); all these show the same morphological characters. The teeth of Demandasaurus are fairly unique and morphologically welldifferentiated from those of other rebbachisaurids (Calvo and Salgado 1995; Salgado et al. 2004; Apesteguía 2007; Sereno et al. 2007). They are elongated and slender (pencil-type), almost straight, with a slight curve in a lingual direction, labiolingually compressed (Fig. 4A); they have an elliptical cross-sectional shape at mid-crown (Fig. 4B), and are somewhat more circular toward the base of the crown. The teeth that preserve the apex lack a wear facet, their end is narrow and sharp, and as such they were possibly not functional (Fig. 4A). In teeth attributed to Nigersaurus two wear facets have been described on opposite faces of the crown, forming very different angles with the surface of these faces (Sereno and Wilson 2005). The mesial and distal edges of the teeth of Demandasaurus display carinae without denticles (Fig. 4A, C) that gradually disappear towards the base. The presence of carinae in pencil-type teeth has also been described in derived South American titanosaurs such as Rinconsaurus caudamirus (Calvo and Gonzalez Riga 2003) and Muyelensaurus pecheni (Calvo et al. 2007). The Demandasaurus teeth exhibit enamel thickness differentiation (as in Nigersaurus; Sereno et al. 1999), the enamel being thicker on the labial face (Fig. 4B), unlike Limaysaurus, which has undifferentiated enamel (Salgado et al. 2004). The enamel of Demandasaurus is practically smooth to the naked eye. The labial side usually has 4–5 very scarcely developed longitudinal crests, which fade away toward the apex; on the lingual face there are up to 3 crests, which are also scarcely marked (Fig. 4A, C; Table 1). On the tooth MACN PV N101 (Apesteguía 2007) and in Nigersaurus (Sereno and Wilson 2005) there are faint crests and valleys, developed only on one face of the tooth, and Limaysaurus has smooth enamel (Calvo and Salgado 1995). No attrition surfaces are visible on the teeth of Demandasaurus, suggesting that there was no contact between them. The combination of teeth ornamented with longitudinal crests on the labial and lingual faces of the crown, and the presence of mesial and distal carinae constitutes an autapomorphy of Demandasaurus.

Cervical region.—The axis (MDS-RVII,606), one anterior to middle cervical vertebra (MDS-RVII,589) and one posterior cervical vertebra (MDS-RVII,806) have been preserved (Table 2). The vertebrae lack the camellate pneumatic structure that is characteristic of titanosauriform sauropods (Wilson and Sereno 1998; Wedel et al. 2000). Eleven cervical ribs are also known.

Axis (Fig. 5).—MDS-RVII,606 is complete, though poorly preserved. It is lacking part of the laminae on both sides, part of the anterior articular face, the postzygapophyses, and part of the neural spine. Moreover, attached to it is a fragment of the prezygapophysis of the third vertebra. The axis exhibits an opisthocoelous vertebral centrum, longer than wide. The ventral side is spindle-shaped, with anterior and posterior widening and a keel that connects the two parts. The anterior articular face is divided into two parts; the upper portion corresponds to the odontoid process. The posterior articular face is oval. The lateral face is taken up by a deep pleurocoel, the anterior part of which displays a subcircular fossa. The diapophysis is best preserved on the right side and forms a process positioned just dorsal to the neurocentral junction. The diapophyses project laterally. The prezygapophyses are short, rounded in outline, and with articular facets oriented laterodorsally. The laminae observed are the centroprezygapophyseal, centropostzygapophyseal, prezygodiapophyseal, postzygodiapophyseal, and anterior and posterior centrodiapophyseal. The neural spine has a V-shaped section and is inclined roughly 50° posterodorsally (Fig. 5).

Anterior-middle vertebra (Fig. 6).—MDS-RVII,589 is almost complete, lacking only the end of the neural spine. The left side is flattened, which has caused a displacement of some of the vertebral laminae. The height of the cervical vertebra is less than the length of the centrum (Table 2). This is opisthocoelous, with a well-developed sub-hemispherical anterior articulation and a concave posterior articulation (Fig. 6A, D). The anterior articulation is asymmetrical in lateral view, with its apex positioned dorsally. The ventral side has a prominent sagittal keel. The anteroventral part of the centrum is concave, and the posterior part is convex. The centrum is short, and the anteroposterior length / height ratio of the posterior face is 2.84. The articular faces are slightly taller than they are wide (Fig. 6D). A large, deep oval pleurocoel takes up the lateral face of the centrum. This pleurocoel is complex, being more deeply excavated in its anterior and posterior parts, which are separated by a pleurocentral lamina (Fig. 6B). Furthermore, there are small laminae delimiting smaller depressions in the posterior part of the pleurocoel. The parapophyses are short, situated in the ventrolateral half of the centrum (Fig. 6B), and lack a pneumatic cavity in their dorsal surface. The prezygapophyses are long and directed dorsally and anteriorly. The postzygapophyses are fractured, and the articular facets are not preserved. The neural spine is broken and only its base is preserved, yet from this it can be observed to be simple. The vertebra MDS-RVII,589 possesses a well-developed system of laminae, as is characteristic of Eusauropoda (Wilson 2002). These include: centroprezygapophyseal (divided), centropostzygapophyseal, anterior centrodiapophyseal, posterior centrodiapophyseal, spinoprezygapophyseal, spinopostzygapophyseal, prezygodiapophyseal and postzygodiapophyseal. There is also an accessory lamina situated between the postzygodiapophyseal and spinoprezygapophyseal laminae (Fig. 6B), which Sereno et al. (2007) call the epipophyseal-prezygapophyseal lamina. This accessory lamina is present, more or less developed, in rebbachisaurids such as Limaysaurus (Calvo and Salgado 1995), Cathartesaura (Gallina and Apesteguia 2005), Zapalasaurus, and Nigersaurus (Sereno et al. 2007). A similar lamina can be seen in macronarians such as Galvesaurus, Camarasaurus, and Euhelopus (Barco et al. 2006; Wilson and Upchurch 2009), or in dicraeosaurids such as Amargasaurus. The homology of this structure is difficult to establish at present.

Posterior cervical vertebra (Fig. 7).—The posterior cervical vertebra (MDS-RVII,806) is incomplete but well preserved. It is different from the anterior-middle cervical and represents the transition to the dorsal vertebrae, which have a high neural arch. Unlike other sauropods, the parapophysis and diapophysis are located posteriorly.

The centrum is opisthocoelous, but it is proportionally shorter and lower than the centrum of the anterior-middle cervical MDS-RVII,589. The lateral faces of the centrum possess a large, oval pleurocoel that is not divided by laminae; in its interior there are vertical and horizontal sheets of bone. The neural spine is not preserved. In anterior view, above the neural canal there is a deep infraprezygapophyseal chamber (Fig. 7A), which is delimited by the centroprezygapophyseal lamina and the prezygapophysis; inside this chamber there is a forked vertical accessory lamina; this structure is absent in other sauropods, and we consider it to be an autapomorphy of Demandasaurus. The diapophysis, as well as the whole of the neural arch, is positioned quite high in relation to the centrum. The postzygodiapophyseal lamina and two accessory laminae delimit a subtriangular surface that extends dorsally (Fig. 7B). A forked centroprezygapophyseal lamina reaches the prezygapophysis. The prezygapophyses are developed vertically and extend far from the vertebral centrum. Dorsal to the prezygapophyses, the vertebra has conspicuous rhombic structures where the centroprezygapophyseal, prezygodiapophyseal, and spinoprezygapophyseal laminae are connected (Fig. 7A); this character constitutes another autapomorphy of Demandasaurus. The postzygapophyses are not preserved. Part of the accessory lamina that connects the spinoprezygapophyseal and postzygodiapophyseal laminae is preserved. Between these three laminae a subtriangular pneumatic depression is located medially.

Cervical ribs (Fig. 8).—Six cervical ribs are preserved, three of which are fragmentary. The most complete cervical rib is MDS-RVII,811, which is shorter than the centrum in length, a feature displayed by diplodocoids (Sereno et al. 2007). The ribs are gracile with pneumatic cavities that vary in position between the capitulum and the tuberculum. On the anterior ramus, they display pneumatic cavities in a dorsal (MDSRVII,379) or in a medial position (MDS-RVII,562). The angle between the tuberculum and the capitulum is less than 90°, except for the most posterior rib (MDS-RVII,811), where the angle is near 90°. MDS-RVII,811 was found in association with the posterior cervical MDS-RVII,806. The tuberculum and the capitulum are well flattened mediolaterally, giving them a laminar appearance. MDS-RVII,379 presents an anterior process (spine, sp of Sereno et al. 2007). This anterior process is strongly developed, even in MDS-RVII,562, more so than the rib. The shape of this anterior process varies from mediolaterally flattened (MDS-RVII,562) to dorsoventrally flattened in the most posterior, MDS-RVII,811 (Fig. 8A, C). The general shape of the rib varies from subtriangular in MDSRVII,811 (Fig. 8A, B) to oval in MDS-RVII,562.

Dorsal region.—The dorsal region is represented by two incomplete dorsal vertebrae and ten ribs.

Dorsal vertebrae (Fig. 9).—Two mid-posterior, probably consecutive dorsal vertebrae (MDS-RVII,242; MDSRVII,798) are partially preserved. The preserved parts are in excellent condition, but they both lack the neural spine and a good part of the transverse processes (Fig. 9). The interior of the vertebrae exhibits normal spongy osseous tissue, without pneumatic cavities. The vertebral centra are opisthocoelous, with an anterior articular face that is slightly convex and a posterior one that is slightly concave. By contrast, Rebbachisaurus has amphicoelous dorsal centra (Wilson 2002). The vertebral centrum of Demandasaurus is subcircular in section. The articular faces are subhexagonal and slightly higher than wide (Fig. 9A, C). The centrum is scarcely elongated anteroposteriorly (the Elongation Index EI sensu Upchurch 1998, is less than 1). In ventral view, it has a broad, gentle ventral groove. The dorsal centra have large, deep, clearlyedged pleurocoels (Fig. 9B). These pleurocoels are oval and dorsoventrally asymmetrical, and taper to points on the anterior and posterior side. The inner part of the pleurocoels lacks internal divisions, as in diplodocoids and other rebbachisaurids, such as Rebbachisaurus garasbae, Limaysaurus, and Amazonsaurus (Lavocat 1954; Calvo and Salgado 1995; Carvalho et al. 2003).

The neural arch is seemingly vertical and is in a slightly posterior position in relation to the centrum (Fig. 9B). It is deeply excavated anteriorly, in the sense of Upchurch et al. (2004a) and Harris (2006). The transverse processes run dorsolaterally at approximately 45° to the horizontal, as in other rebbachisaurids, such as Limaysaurus, Nigersaurus, and Histriasaurus (Dalla Vecchia 1999; Salgado et al. 2004; Sereno et al. 2007). The neural spine is simple. The parapophysis and the diapophysis are positioned very high in relation to the vertebral centrum, such that the parapophysis is above the prezygapophysis. The transverse processes lie posterior to the parapophyses. The prezygapophyses and postzygapophyses have large articular surfaces that are subtriangular and subrectangular respectively. They incline at roughly 45°, joining at their medial part. The dorsal vertebrae display very deep, semi-oval infraprezygapophyseal and infrapostzygapophyseal cavities, with two large pneumatic foramina that pass all the way through the neural arch anteroposteriorly (Fig. 9A, C). These pneumatic foramina are delimited by the ceiling of the neural canal (ventrally) and by the centroprezygapophyseal and centropostzygapophyseal laminae (laterally). In the dorsal part of the infraprezygapophyseal lamina cavity there is also an opening laterally. This type of Pneumatisation is present in theropods, such as Majungatholus and Aves (O'Connor and Claessens 2005), but has not been described in sauropods and is an autapomorphic character of Demandasaurus. There is a reduced, laminar hyposphene, the medial edges of which join the postzygapophyses (Fig. 9C). There are only two other rebbachisaurids in which the hyposphene-hypanthrum articulation is developed: Histriasaurus and Nopcsaspondylus (Apesteguía 2007).

The pedicle is distinctively wide mediolaterally in anterior view, due to the fact that it is composed of the centroprezygapophyseal lamina and the anterior centroparapophyseal lamina. The centroprezygapophyseal laminae are forked, as occurs in the cervical vertebrae, which is considered an autapomorphy of Demandasaurus. There is no prezygodiapophyseal lamina, an absence shared with Haplocanthosaurus and Dicraeosaurus (Upchurch 1998). There are some very narrow spinoprezygapophyseal laminae, which run very close to one another, separated by a rugose area. Close to the contact with the prezygapophyses, they finally fuse. The prezygoparapophyseal laminae are very well developed, and together with the spinoprezygapophyseal laminae (sprl) they delimit deep subtriangular cavities (Fig. 9A). Through these cavities an accessory lamina runs parallel to the prezygoparapophyseal laminae, yet without making contact with the spinopostzygapophyseal laminae. The presence of the prezygoparapophyseal laminae distinguishes our specimen from other rebbachisaurids such as Limaysaurus and Nigersaurus, which lack this lamina (Salgado et al. 2004; Sereno et al. 2007). The spinopostzygapophyseal laminae are well developed, forked at their contact with the postzygapophyses, fusing at the base of the neural spine. The centropostzygapophyseal laminae are well developed, forking at their dorsal end to form a medial centropostzygapophyseal lamina.

The postzygodiapophyseal lamina is well-developed and exhibits an accessory lamina at its contact with the postzygapophyses. In lateral view there is an undivided, narrow, vertical posterior centrodiapophyseal lamina. The anterior centroparapophyseal lamina and the posterior centroparapophyseal laminae join to form a gentle depression. The lateral lamina would be formed by a spinodiapophyseal lamina, which distinguishes our specimen from more derived diplodocoids, where the lateral lamina is a result of the union of the lateral spinopostzygapophyseal laminae and the spinodiapophyseal lamina (Wilson 2002).

Dorsal ribs.—Ten dorsal ribs are preserved, one of which (MDS-RVII,364) is complete. The ribs are gracile. The capitulum is oval and has a concave articular surface. The tuberculum is more robust, with a sub-oval section and a concave articular surface. The tuberculum and the capitulum are joined by a lamina of fine bone that presents an excavated pneumatic area. The ribs lack pneumatopores, and the interior of the bone displays a normal spongy structure.

Caudal region.—There are eight anterior caudals (MDSRVII,15; MDS-RVII,470; MDS-RVII,602; MDS-RVII,605; MDS-RVII,609; MDS-RVII,610; MDS-RVII,629 and MDS-RVII,805), four middle caudals (MDS-RVII,2; MDS-RVII,3; MDS-RVII,9 and MDS-RVII,11), and seven middle-posterior caudals (MDS-RVII,4; MDS-RVII,10; MDSRVII,12; MDS-RVII,101; MDS-RVII,216; MDS-RVII,217 and MDS-RVII,218) preserved. The vertebrae MDS-RVII, 605 and MDS-RVII,609 lack the articular facets for the haemal arches, and thus they could be regarded as the first two caudals. The distal caudal vertebrae are not represented. Nine disarticulated haemal arches were found.

First caudal vertebra (Fig. 10).—MDS-RVII,605 is nearly complete, lacking part of the vertebral centrum, the diapophyses, the lateral ends of the transverse processes, and the greater part of the neural laminae. The inside of the bone exhibits normal spongy tissue. The vertebral centrum is slightly opisthocoelous. The anterior articular face is flat, somewhat oval, its major axis running in a dorsoventral direction, and its surface is irregular because it could have been fused to the last sacral vertebra (Fig. 10A). The posterior articular face is concave and probably subcircular (it lacks the left half; Fig. 10B). The vertebral centrum is slightly higher than long, and longer anteroposteriorly than wide mediolaterally (Table 2). It lacks pleurocoels. The neural spine is very high: 2.7 times the height of the centrum (Table 2). It is curved posteriorly, the curvature being greater in the dorsal half. The neural canal is large, elliptical, with a greater dorsoventral development anteriorly, and subcircular in posterior view.

MDS-RVII,605 presents a complex system of laminae. The probable position of the diapophyses has been inferred on the basis of the confluence of the anterior centrodiapophyseal and the prezygodiapophyseal laminae. They would be in an anterior position with respect to the parapophyses. Ventral to the diapophyses and confluent with the anterior centrodiapophyseal lamina (in left lateral view) is the paradiapophyseal lamina. The spinodiapophyseal lamina is very prominent. The postzygodiapophyseal lamina is subhorizontal, and runs anteriorly to join the spinodiapophyseal lamina, in such a way that it would not have made contact with the diapophyses. Between the anterior centrodiapophyseal lamina and the prezygodiapophyseal lamina there is a triangular depressed area. In lateral view the anterior centrodiapophyseal lamina can be seen to fork at its dorsal end.

The bases of the prezygapophyses are preserved, which project in front of the vertebral centrum. In contact with the prezygapophyses are the spinoprezygapophyseal and centroprezygapophyseal laminae and an intraprezygapophyseal lamina. The postzygapophyses are situated at the base of the neural spine; they make contact with one another medially. Their articular surfaces are subelliptical and directed lateroventrally. They project far enough to reach the plane of the posterior articular face of the vertebral centrum. The postzygapophyses connect ventrally with thick centropostzygapophyseal laminae, which delimit a triangular area (scarcely depressed) between the neural canal and the postzygapophyses. Laterally our specimen presents postzygodiapophyseal laminae and dorsally the well-developed spinopostzygapophyseal laminae.

In lateral view, in the dorsal and middle-posterior parts of the vertebral centrum are the poorly-preserved transverse processes. They are very tall dorsoventrally and seem to display an anterior pneumatic cavity similar to that exhibited by the rest of the anterior vertebrae of the caudal series of Demandasaurus. Between the anterior part of the transverse process and the edge of the anterior articular face of MDSRVII,605 there is (right lateral) a broad and deep pneumatic cavity, oval in shape (with its major axis running in an anteroposterior direction) and with an accessory lamina that divides it into two subequal halves. The anterior side of the transverse process is reached by a lamina from the anterodorsal edge of the vertebral centrum, which could be the anterior centroparapophyseal lamina.

The neural spine is rectangular in lateral view and posteriorly recumbent, above all in the dorsal half. The dorsal end of the neural spine is very wide in anterior view, with two lateral hanging processes (Fig. 10A); this is where the spinoprezygapophyseal laminae end. This dorsal end has the shape of a helmet, with pneumatic cavities situated in a ventrolateral position. In cross section the neural spine is cruciform. It is formed by the spinodiapophyseal lamina and by very prominent prespinal and postspinal laminae (Fig. 10). These robust laminae comprise the spinoprezygapophyseal laminae (anterior) and the spinopostzygapophyseal laminae (posterior), which run very close together (though separately) and between which there is a rugose area that links them. The prespinal structure becomes wider at the dorsal end. Between the spinoprezygapophyseal laminae it presents a scarcely marked prespinal lamina, which dorsally ends up joining the right spinoprezygapophyseal lamina. There is also an accessory lamina that dorsally links the prezygapophyses with the ventrolateral part of the neural spine (it is scarcely developed, and visible on the right side of the spine). The spinodiapophyseal lamina is somewhat sinusoidal in outline. In posterior view the postspinal structure is wider at the dorsal end; in the lateral surfaces of the posterior part of the neural spine there are shallow pneumatic depressions.

Anterior and middle caudal vertebrae (Figs. 11, 12).—Internally, the caudal vertebrae are spongy without large internal cells. The anterior caudals possess amphicoelous vertebral centra (Fig. 11), which distinguish them from the weakly procoelous vertebrae of derived diplodocoids such as Barosaurus, Diplodocus, Dicraeosaurus, and Amargasaurus (Salgado et al. 2004), with articular faces that are subhexagonalsubsquare in outline, as displayed with variations in all the caudals. The anterior caudal centra are flattened anteroposteriorly, and the middle-posterior centra are flattened dorsoventrally (Figs. 11, 12). All the caudals except the anteriormost present a very deep ventral groove. The articular facets for the haemal arches are very well developed in the posterior part of the centrum in all the vertebrae, except in the first ones, where the articular facets are absent. The ventral side is concave in lateral view. The centra of the anterior caudals are relatively short and vary little in length anteroposteriorly, whereas the middle-posterior ones are longer (Table 2). The vertebral centra lack pleurocoels. The middle and middle-posterior caudals (Fig. 12) exhibit two parallel laminar crests running in an anteroposterior direction, an upper one from the prezygapophysis to the base of the centropostzygapophyseal, and a lower one from the base of the prezygapophysis and the dorsal surface of the anterior caudal centra (Fig. 12A, C). Further, they present two crests running anteroposteriorly on their lateral face (in Limaysaurus there is a similar crest; Salgado et al. 2004). This combination of laminar crests on the middle and posterior caudal vertebrae is unique among the sauropods, and we interpret it as autapomorphic in Demandasaurus. In all the caudals the neural canal is well developed and is excavated in the vertebral centrum, as a result of which the dorsal edge of the centrum is concave.

In the anterior caudals, the neural spine is on the middle of the centrum, whereas in the middle-posterior caudals it is in an anterior position. The anterior position of the neural arch is convergent in Titanosauriformes (Salgado et al. 1997) and in some rebbachisaurids (Salgado et al. 2004). The neural spine of the anterior caudals is more than twice as high as the vertebral centrum, as in Diplodocoidea (Calvo and Salgado 1995; Harris 2006). The neural spine is distinctly inclined posteriorly in the anterior caudals (Fig. 11A₂), becoming practically vertical in the middle ones (Pereda Suberbiola et al. 2003). In the posterior caudals the neural spine is straight and is located on the posterior part of the vertebral centrum. The neural spine of the anterior caudals exhibits triangular lateral projections near its dorsal end (Pereda Suberbiola et al. 2003), very similar to those observed in Nigersaurus (Sereno et al. 2007).

The anterior caudals are morphologically similar to the dorsals, with a broad and complex system of laminae, some of which are present on the middle caudals. The lateral face of the neural spine in the anterior caudals runs along the spinodiapophyseal laminae; on the anterior face of the neural spine there is a slanting accessory lamina that links the spinoprezygapophyseal lamina with the spinodiapophyseal lamina (spdl). The neural spine has a petal-shaped transverse section, a synapomorphy of Rebbachisauridae (Wilson 2002). The anterior caudals have a robust and complex prespinal lamina to which the spinoprezygapophyseal laminae contribute and which contains a much-reduced “prespinal lamina”; a postspinal structure is formed in a similar way by the spinopostzygapophyseal lamina. There is a lamina parallel to the spinopostzygapophyseal lamina only on the right side, which runs next to the postzygapophyses (accessory to the spinopostzygapophyseal lamina). The course of the spinoprezygapophyseal laminae is “festooned” as in Nigersaurus (Sereno et al. 2007), in such a way that in some parts they almost touch one another, while in other parts they diverge, as happens at the dorsal end. The first caudals have anterior centrodiapophyseal and posterior centrodiapophyseal laminae that are very wide and undivided. The anterior caudals have a transverse process (actp of Gallina and Otero 2009) that is rectangular in shape and occupies part of the vertebral centrum (Fig. 11). These transverse processes are complex and are formed by the spinodiapophyseal and prezygodiapophyseal laminae (dorsally), by anterior centroparapophyseal plus posterior centroparapophyseal laminae (ventrally) and the posterior centrodiapophyseal lamina (posteriorly). These laminae delimit two large, oval pneumatic depressions (one on each side), inside which there are accessory laminae distributed irregularly (Fig. 11A). The transverse processes are deeply excavated below the prezygodiapophyseal lamina in the medial margin. In posterior view, the transverse processes display small depressions separated by a thick ridge. This transverse process combines two autapomorphies: the laminar complex that is associated with the diapophysis and the deep pneumatic cavities with accessory laminae in their interior. The middle-posterior caudals are not pneumatized.

The anterior caudals have a small hyposphene in the form of a crest. The prezygapophyses are small and project clearly anteriorly, in such a way that they go beyond the anterior face of the vertebral centrum in the anterior caudals. The postzygapophyses are also small and present an almost vertical articular face.

Haemal arches (Fig. 13).—Nine haemal arches are preserved (MDS-RVII,23; MDS-RVII,99; MDS-RVII,231; MDSRVII,232; MDS-RVII,590; MDS-RVII,591; MDS-RVII,594; MDS-RVII,796; MDS-RVII,797). These have a Y shape in anterior view. The most anterior ones in the series have a haemal canal that is closed (“cross-bridged”, Fig. 13A–D), but this becomes open in the posterior ones (MDS-RVII,23, MDS-RVII,232, and MDS-RVII,594, see Pereda Suberbiola et al. 2003: fig. 2F), as occurs in Diplodocus and Apatosaurus (Marsh 1896; Osborn 1899). The closed haemal canal is the primitive stage of the character displayed by basal sauropods and by Flagellicaudata (Wilson and Sereno 1998; Wilson 2002; Harris 2006). The presence of a closed haemal canal in the anterior caudals distinguishes Demandasaurus from all other rebbachisaurids, in which it is open (Calvo and Salgado 1995; Sereno et al. 1999; Carvalho et al. 2003; Salgado et al. 2004). The chevron haemal canal is short (23–27% chevron length) in the anterior ones (Table 3). These values are similar to those of diplodocoids and basal sauropods, differentiating it from the representatives of Titanosauriformes, which display values greater than 30%.

The ventral ramus in the anterior chevrons is fairly straight, forming an angle of 150° with respect to the proximal end (Fig. 13). The ventral ramus is flattened mediolaterally. Its anterior and posterior edges run parallel to one another, lacking the distal expansion (in lateral view) presented by Tastavinsaurus (Canudo et al. 2008). The distal end of the ventral ramus in lateral view is sharply pointed. This distal end is slightly widened mediolaterally in the anterior haemal arches (MDS-RVII,591). The most posterior haemal arch (MDSRVII,594) lacks distal fusion of the ventral ramus (Fig. 13E), as well as lacking the anterior projection displayed by more derived diplodocoids such as Dicraeosaurus, Diplodocus, and Barosaurus (Upchurch et al. 2004a).

Appendicular skeleton.—The appendicular skeleton of Demandasaurus darwini gen. et sp. nov. is only known for the two ischia and the left femur.

Ischium.—The two ischia (MDS-RVII,18, left; MDSRVII,19, right) are preserved and were figured in Pereda Suberbiola et al. (2003). The three constituent parts are clearly made out: the pubic peduncle, the iliac peduncle and the ischial branch. The ischium makes a significant contribution to the acetabulum. The iliac peduncle is well developed and is more prominent than the pubic one. The outline of the articulation with the ilium is trapezoidal. The posteroventral end of the iliac peduncle displays a small notch similar to the one shown by Haplocanthosaurus priscus Hatcher, 1903. The pubic peduncle has a well-marked neck. The outline of the articulation with the pubis is triangular and is very short in comparison with the total length of the ischium (15%), which makes it one of the shortest in the sauropod record. There is an elongated and well-developed tuberosity on the lateral surface of the proximal end. The right ischium displays a large nutritive foramen in a medial position on the pubic peduncle.

The ischial branch is distinctively straight, elongated (approximately 80% of the length of the ischium) and gracile, like that of Haplocanthosaurus priscus. The ischial branch is perpendicular to the iliac peduncle, which means that it projects posteriorly, as occurs in Camarasaurus (Ostrom and Mcintosh 1966). The section of the ischial branch at its midpoint is subtriangular. The ischiadic shaft is slightly twisted and much smaller than in sauropods such as Camarasaurus or Tastavinsaurus (Canudo et al. 2008). The distal end of the ischium shows little lateral expansion in relation to the main shaft. The section of the distal end is blade-like. The dorsodistal end of the ischia presents an elongated-triangular, rugose, symphyseal area, which occupies a sixth of the total length (condition described as emarginate distal to pubic peduncle by Wilson 2002). In their anatomical position, the ischia form a nearly coplanar structure in cross-section (Pereda Suberbiola et al. 2003).

The absence of a significant distal expansion and the narrow ischiadic shaft clearly distinguish the ischium of Demandasaurus from that of more derived diplodocoids such as Apatosaurus (Upchurch et al. 2004b). Its overall gracility and its well-developed iliac peduncle distinguish it from basal macronarians such as Camarasaurus (Ostrom and Mcintosh 1966) and derived ones such as titanosaurs (Salgado et al. 1997). Among the rebbachisaurids, the ischium of Limaysaurus tessonei differs from that of Demandasaurus in having a somewhat curved shaft (Calvo and Salgado 1995) and in lacking the marked neck of the iliac peduncle as seen in Demandasaurus.

Femur.—MDS-RVII,16 is the left femur, preserved virtually complete (Table 4). The femur is well figured in Pereda Suberbiola et al. (2003). It has a diaphysis that is slightly curved and distinctively gracile (slenderness index: 0.12); the section of the diaphysis is subelliptic in outline, with the major axis in a mediolateral direction. The proximal and distal ends are aligned medially and display the same mediolateral expansion. The distal end is twisted roughly 30° medially with respect to the lateromedial axis of the diaphysis. The femoral head is well individualised and is located almost perpendicular to the longitudinal axis of the diaphysis, with a small elevation above the greater trochanter. The articular head is oval in proximal view. It lacks an anterior trochanter, a character it shares with other eusauropods (Wilson 2002). It displays a well-developed pilaster on the posterolateral side of the proximal third, beneath the area of the greater trochanter.

The diapohysis is curved on its medial side and practically straight on the lateral side. Its interior presents a central cavity described in basal sauropods such as Shunosaurus (Zhang et al. 1984). The diaphysis is compressed anteroposteriorly with an eccentricity close to 0.5 (Table 4). It displays a scarcely developed proximolateral bulge below the greater trochanter, similar to that of other rebbachisaurids (Calvo and Salgado 1995) and much less developed than that shown by Titanosauriformes (Salgado et al. 1997). The lateral bulge is located in a rather low position, and does not reach the proximolateral corner of the femur. The fourth trochanter is situated on the posterior face of the femur, without occupying any part of the medial side of the diaphysis. It is closer to the proximal end (one third of the total way along). It consists of a small bulge, similar in its development to that of other rebbachisaurids (Salgado et al. 2004). The scarce development of the fourth trochanter is convergent in certain titanosaurians (Sanz et al. 1999). Situated medially in relation to the fourth trochanter is a depressed area parallel to it. The lateromedial width of the diaphysis decreases distally.

The distal end is expanded both mediolaterally and anteroposteriorly. The condyles are well developed; the tibial is larger than the fibular, and they are separated from one another by a deep intercondylar groove. The distal surface of the distal end of the femur presents a bulge coinciding with the fibular condyle. The epicondyle is well developed and is separated from the fibular condyle by a groove. The tibial condyle is situated in the middle part of the distal end of the femur; it has a proximal projection at its posterior end, and is slightly projecting on its anterior side.