Cathartesaura anaerobica gen. et sp. nov., a new rebbachisaurid (Dinosauria, Sauropoda) from the Huincul Formation (Upper Cretaceous), Río Negro, Argentina

A new mid-sized sauropod from ‘La Buitrera’, Huincul Formation (Turonian-Coniacian), Río Negro Province, Cathartesaura anaerobica gen. et sp. nov. is described. It is known from an incomplete, but associated skeleton that includes diagnostic postcranial elements such as a posterior cervical, an anterior caudal, and a mid caudal vertebra, a left scapula, a left ilium and a right femur. Cathartesaura anaerobica gen. et sp. nov. exhibits several autapomorphies, such as a posterior cervical vertebra with an accessory lamina that arises from the mid length of the prezygodiapophysial lamina and reaches the centrum, a thin, wing-like transverse processes on the anterior caudal vertebrae mostly supported by a ventral bony bar that frames a deep triangular fossa, and each anterior caudal neural spine with the lateral laminae composed of the spinoprezygapophyseal lamina, the lateral spinopostzygapophyseal lamina and the spinodiapophyseal lamina. The new taxon is related to Rebbachisaurus and Limaysaurus because of the presence of several synapomorphies, which support its inclusion in a separate clade of basal Diplodocoidea, the Rebbachisauridae. Aspects of rebbachisaurid vertebral anatomy including musculature and pneumaticity are briefly commented upon. A cladistic analysis is presented in an attempt to partially resolve basal diplodocoid phylogenetic relationships.

The fossil record of the Cretaceous tetrapod fauna from Río Negro Province, North Patagonia, shows a high diversity of forms (Bonaparte, 1996;Apesteguía, 2002). A good example of this diversity is the locality known as 'La Buitrera', in which the upper member of the Candeleros Formation (Cenomanian-Turonian) as well as the basal member of the Huincul Formation (Turonian-Coniacian) crop out. This locality has provided rich faunas that include sphenodontians, snakes, araripesuchid crocodyliforms, theropod and sauropod dinosaurs, mammals, and ceratodontiform dipnoans (Apesteguía et al., 2001). A new sauropod from the basal Huincul Formation of 'La Buitrera' is described here. It belongs to a monophyletic group of basal Diplodocoidea, known as rebbachisaurids (Bonaparte, 1997).
The presence of a monophyletic subgroup of diplodocoids in the Cretaceous of Africa and South America was first recognized by Bonaparte (1996Bonaparte ( , 1997, who coined the family name Rebbachisauridae for this clade. That name was subsequently used by several authors (e.g. Wilson, 1999;Sereno et al., 1999;Pereda-Suberbiola et al., 2001Carvalho et al., 2003;Harris & Dodson, 2004), without a formal diagnosis or valid definition until very recently Upchurch et al., 2004;Wilson, in press).
This group of moderately sized sauropods survived until the Upper Cretaceous, sharing environments with other sauropod lineages such as basal titanosaurs. They are mostly known from rather fragmentary remains, and many have not been described in detail yet. Most specimens remain unpublished or are only partially described (e.g. Nigersaurus taqueti), and only a few, such as Limaysaurus tessonei (Calvo & Salgado, 1995) are nearly complete. Therefore, the in-group relationships of the Rebbachisauridae remain unclear.
Despite the dorsal vertebra described by Nopcsa (1902) from Neuquén Province and the materials from Morocco described by Lavocat (1954), most materials referred to Rebbachi-sauridae were only recognized as such in the last decade. A growing number of genera were assigned to this family, including Rayososaurus agrioensis Bonaparte 1996, from the Aptian of Neuquén, Argentina; Rebbachisaurus garasbae Lavocat 1954, from the Aptian-Albian of Morocco, Africa; Nigersaurus taqueti Sereno et al., 1999, from the Aptian-Albian of Niger, Africa; Rebbachisauridae indet. MPS-RV II Pereda- Suberbiola et al. 2003, from the Lower Cretaceous of Spain; and Limaysaurus tessonei (Calvo & Salgado, 1995) from the Cenomanian of Neuquén, Argentina.
The fossil record of the group was also expanded by additional undescribed materials from Cenomanian-Coniacian beds of Chubut Province, Central Patagonia (Lamanna et al., 2001). Although the higher-level systematic placement of the group is complex, most authors interpret them as basal diplodocoids (Calvo & Salgado, 1995;Wilson & Sereno, 1998;Wilson, 2002;Upchurch et al., 2004). On the other hand, the in-group relationships are still unclear. Furthermore, the still poor knowledge of their anatomy invokes the risk of including other species that were part of the wide radiation experienced by basal diplocoids between the Late Jurassic and Early Cretaceous among Rebbachisauridae, thus rendering them a taxonomic 'waste-basket'. A cladistic analysis is presented here as an attempt to partially resolve their phylogenetic relationships.
Institutional Diagnosis. Mid-sized sauropod dinosaur characterized by the following derived features: posterior cervical vertebra with an accessory lamina that arises from the middle of the prezygodiapophysial lamina and reaches the centrum; thin wing-like transverse processes of anterior caudal mostly supported by the ventral bar forming a deep triangular fossa, also framed by the prezygodiapophyseal and centroprezygapophyseal laminae; anterior caudal neural spine with the lateral lamina composed of the spinoprezygapophyseal lamina, the lateral spinopostzygapophyseal lamina and the spinodiapophyseal lamina.
Etymology. Cathartes, for the extant vultur genera abundant in the quarry area; saura (Greek), fem. lizard, reptile. Additionally, it implies the combination of the generic and specifical names of those vultures (Cathartes aura Linnaeus, 1758).

Cathartesaura anaerobica, nov. sp.
Diagnosis. Same as for genus. Etymology. Fem. for ANAEROBICOS S. A., an Argentine company of adhesives that provided fieldwork and lab support in the extraction and protection of the fossil materials.
Holotype. MPCA-232, consisting of the following associated material from one quarry and presumably from a single specimen: a posterior cervical vertebra, a dorsal vertebra, an anterior caudal, a mid caudal vertebra, left scapula, a left? ilium, and a right femur. A dorsal vertebra, a humerus and one metatarsal were also collected, but they are poorly preserved.

COMPARATIVE DESCRIPTION
Posterior cervical vertebra ( Fig. 2A, B). A complete posterior cervical vertebra (possibly the eighth or ninth) was recovered. This vertebra is twice as tall as it is long. The neural arch makes up approximately 2/3 of the total height. The centrum is opisthocoelous, elongate and dorsoventrally compressed, its length being three times its height. The cervical system of pneumatic cavities consists of two independent and successive foramina that connect to their respective internal camerae (Wedel et al., 2000). They are oval in shape with an oblique long axis, and are located on the lateral wall of the centrum occupying at least 3/4 of its length. Both "pleurocoels" (foramina and camerae) are separated by a thin oblique septum ("pleurocentral" lamina sensu Calvo & Salgado, 1995) that runs parallel to the posterior centrodiapophyseal lamina.
The first camera is slightly smaller than the second and is placed above the parapophysis and rostral to the level of the diapophysis. In contrast to the second camera, the first camera is only a shallow concavity with no internal divisions. The second and larger camera is more complex, excavating the interior of the centrum up to its posterior end. Actually, this camera is separated from the concave surface of the posterior articulation of the centrum by a thin lamina of bone (Fig. 2B).
The neural canal is large with a horizontal floor, and is developed as a wide arch that occupies most of the neural arch base. In lateral view, there are four main laminae that converge on the diapophysis around the middle of the vertebral length as in Haplocanthosaurus and diplodocoids. A deep, posterior, and triangular fossa is framed by the postzygodiapophysial and posterior centrodiapophysial laminae. The centropostzygapophysial lamina, which narrows upwards, closes the triangle posteriorly. A similar, but shallower and inverse triangle is framed by the prezygodiapophysial and anterior centrodiapophysial laminae.
Two weaker accessory laminae are visible on the lateral aspect of the neural arch. The first connects the postzygodiapophysial and spinoprezygapophysial laminae at the middle of their spans (AL1). Originally proposed as an autapomorphy for Limaysaurus tessonei, it certainly represents a synapomorphy of a more inclusive group (Gallina et al., 2002). The second accessory lamina arises from the mid-length of the prezygodiapophysial lamina and reaches the centrum (AL2). It is here considered as an autapomorphy of Cathartesaura. In posterior view, two weak laminae run parallel to the spinopostzygapophysial laminae. Their presence in other rebbachisaurids is unknown.
The neural spine is square in cross-section at its base, and becomes a thin lamina at the summit. The spine is obliquely inclined and oriented forward, its origin being in the posterior half of the centrum. It is buttressed anteriorly by the spinoprezygapophysial laminae and posteriorly by the spinopostzygapophysial laminae. There is also a lateral column-like, robust lamina that reaches two cm in width (probably the SPZAL of Calvo & Salgado, 1995).
Anatomical comments. Most researchers (e.g. McIntosh, 1990) consider that the system of cavi- ties in the sauropod axial skeleton is profoundly associated with the lightening of the skeleton. Furthermore, Wedel et al. (2000) and Wedel (2003a, b) proposed that the cavities are the result of pneumatic invasion by the pulmonary air sac system. On the other hand, Bonaparte (1999) suggested that the cavities and laminae in presacral vertebrae mostly served as attachment for hyposomatic musculature which, for Wedel & Sanders (2002), were developed in other anatomical structures.
In Cathartesaura, the development of tall, distally widened neural spines, as well as strong spinopostzygapophyseal and accesory laminae on their posterior sides, indicates a strong development of the epaxial musculature, most probably the M. longus colli dorsalis, Mm. intercristales, and Mm. interspinales of birds (see Wedel & Sander, 2002). This important development of dorsal structures also avoided an excessive flexure of the neck. On the other hand, the lack of the ansa costotransversia (bony bridge formed by cervical rib + diapophysis + parapophysis) due to taphonomy, precludes an analysis of the relative development of the hypaxial musculature.
The pneumaticity of the cervical vertebrae of Cathartesaura anaerobica is well-developed and provides an important source of anatomical information. Based on the morphological types of Wedel et al. (2000), Cathartesaura bears camerate vertebrae, with lateral foramina that open into a large central camera that bifurcates into smaller camerae as in Limaysaurus and Diplodocus.
Anterior caudal vertebra (Fig. 3A, B, C). One complete anterior caudal vertebra has been recovered, probably the second or third of the caudal series. This vertebra is more than three times taller than long. The centrum is amphyplatian and subcircular in shape, with a length / height ratio of about 1. 5. There is no sign of pneumatic cavities in the centrum.
The tall neural arch is at least twice the centrum height (a synapomorphy of Diplodocoidea, reverted in Diplodocidae) as in Haplocanthosaurus, Losillasaurus, and the indeterminate rebbachisaurid MPS-RV II. In the latter, as is the case in the Titanosauriformes , the neural arch is located toward the anterior half of the centrum. Two deep fossae are located laterally on the most anterior part of the base of the neural arch. The neural canal is semicircular and well defined. The development of the zygapophyses shows a significant reduction of both prezygapophysis and postzygapophysis, a condition also present in Dicraeosaurus, Amazonsaurus and the isolated rebbachisaurid neural arch MCF-PVPH-633. Moreover, the re-duced prezygapophyses fit in narrow slits where each postzygapophyses are dorsomedially located. As in most diplodocoids, the transverse processes are wing-like, but in Cathartesaura they are thin laminae of bone mostly supported by the ventral bar. They border a deep triangular fossa, also bounded by the diapoprezygapophyseal and centroprezygapophyseal laminae, which occupies the entire lateral face of the neural arch, and is here considered an autapomorphy of Cathartesaura.
The neural spine is S-shaped in lateral view as in Rebbachisauridae indet. MPS-RV-II, and it is positioned at the most anterior part of the neural arch, its base being oriented backwards and its summit forward. It is composed of the intersection of four conspicuous laminae, one each at the anterior, posterior and lateral sides. These laminae increase in thickness distally to end in bulky borders. Both prespinal and postspinal laminae reach the base of the neural arch.
Mid caudal vertebra (Fig. 3F). One mid-caudal vertebra has been recovered. The centrum is amphyplatian and elongate. The lateral sides of the centrum are concave and the ventral surface is flat. In anterior view the centrum is triangular (wide base) as in Limaysaurus and Amazonsaurus.
The neural arch makes up approximately half the total height and is located at the middle of the centrum. Neither the prezygapophysis nor the postzygapophysis exceed the centrum length, and the prezygapophysis is oriented horizontally. Transverse processes are not developed. The neural spine is transversely compressed and directed posteriorly. Scapula (Fig. 4A). A left scapula was collected, but its dorsal edge is poorly preserved. The bone, 68 cm in length, is considerably expanded distally, and the minimal width of the post-acromial region reaches only 16 cm. Despite the poor preservation, it can be said that it was racquetshaped as in other rebbachisaurids. The hooklike acromial process is well defined, as in Rayososaurus.
In ventral view the shaft exhibits a strong curvature, developed medially with respect to the level of the acromion. The glenoid border forms a low angle with respect to the direction of the shaft. The scapula-coracoid contact seems to be rounded.
Ilium. The left ilium was recovered, but remains mostly unprepared. The iliac blade is slightly concave in lateral view, and the caudal part is curved outwards. The dorsal edge of the ilium is convex. The postacetabular process and the ischial peduncle are poorly preserved. Although incomplete, the pubic peduncle is devel- oped perpendicular to the blade. This kind of acetabular orientation is also present in titanosauriforms and Amanzonsaurus maranhensis.
Femur (Fig. 4B). A right femur was recovered. This bone is 1, 38 m long, similar in size to that of Limaysaurus tessonei (1,41m long). This femur is robust and anteroposteriorly compressed. The distal end is poorly preserved so the distal condyles are not observable. In posterior view, the fourth trochanter is located on the upper mid-section and is developed as a lateral short bulge in anterior view; this condition is also observable in Camarasaurus grandis, Haplocanthosaurus priscus and Dicraeosaurus hansemanni.
The lateral margin of the femur is straight and a lateral bulge (as present in titanosauriforms, see  is not developed.
The data matrix was run using the program NONA, version 2.0 (Goloboff, 1993), and the character polarity was determined by comparison with the outgroup. Multi-state characters were treated as unordered. Five most parsimonious cladograms were obtained, with relatively high indices (140 steps, consistency index: 0.66, retention index: 0.73) (Fig. 5A). The basal nodes 1, 2, 3, 4, and 5 (Neosauropoda, Macronaria, Diplodocoidea, Rebbachisauridae and Flagellicaudata) and the more derived taxa (nodes 6 and 7, Dicraeosauridae and Diplodocidae) are relatively well supported. In contrast, the other nodes are weaker due to missing data.
The cladogram shows some similarities with previous studies (Wilson, 2002;Harris & Dodson, 2004), although it also shows some novel groups, which add to our knowledge of rebbachisaurid evolution and the phylogenetic position of controversial taxa (i.e. Losillasaurus, Haplocanthosaurus, Suuwassea). The strict consensus cladogram shows an unresolved polytomy at the base of Rebbachisauridae with the exception of a node shared by L. tessonei and Nigersaurus taqueti (Fig. 5C). Rebbachisauridae is here sup-   (25.2); presence of anterior caudal neural spines with the lateral border initially flat but ending in a distally thickened bulky summit (53.1); wide-based, triangular mid-caudal centra (60.2); racquet-shaped scapular blade (63.2); and presence of a hook-like acromion process (69.1).
Although the strict consensus of our analysis retrieves Limaysaurus tessonei and Nigersaurus taqueti as a monophyletic subclade, there is no strong support for it.
In our analysis, Diplodocoidea is supported by twenty unambiguous synapomorphies. The recently described Suuwasea, considered as a basal diplodocoid, is here considered a basal diplodocid. Losillasaurus, which varies widely in previous analyses from being a basal neosauropod ) to a basal eusauropod (Harris & Dodson, 2004), is here considered a basal macronarian or closely related to them. Additionally, Haplocanthosaurus is again found to be a paraphyletic taxon (Calvo & Salgado, 1995), but more closely related to basal macronarians (as in Wilson & Sereno, 1998;Casanovas et al., 2001) than to diplodocoids.

DISCUSSION AND CONCLUSIONS
Diplodocoids were globally distributed sauropods that evolved some lineages restricted to northern landmasses (diplodocids) and southern landmasses (dicraeosaurids). Their basal forms, however, show a more ambiguous distribution.
Although several Early Cretaceous dinosaurs (e.g. basal neosauropods; basal neotetanurans; basal neoceratosaurians; eurypodans and basal iguanodontians) show a rather worldwide distribution since Late Pangean times, the incipient Laurasian -Gondwanan provincialism initiated during the Late Jurassic (Bonaparte, 1979;Rauhut, 2002)  The Uppermost Cretaceous strata of the Gondwanan landmasses are marked by the dominance of abelisaurian theropods and titanosaurian sauropods. However, their faunal make-up is more complicated in the earlier stages of the Upper Cretaceous.
Southern Neuquén basin is, perhaps, the richest fossiliferous area for Upper Cretaceous continental tetrapods. Its faunal content was recently characterized in detail by Leanza et al. (2004), and separated into four recognizable faunal assemblages, in concordance with their stratigraphic distribution: Limayan, Neuquenian, Coloradoan and Allenian assemblages.
The occurrence of rebbachisaurids in the early Upper Cretaceous of Patagonia (Limayan assemblage) is noteworthy, because they represent the only diplodocoids worldwide recorded during this interval Calvo & Salgado, 1995;Calvo & Salgado, 1998;Calvo, 1999;Apesteguía et al., 2001;Gallina et al., 2002). Furthermore, their absence in rocks younger than the "Mid" Cretaceous suggested to several authors the presence of an extinction event that involved these diplodocoids (Salgado, 2001;Apesteguía, 2002;Leanza et al., 2004). Salgado (2001) correlated this event with that responsible for the complete extinction of the Laurasian sauropods, and the diversification of the narrowcrowned titanosaurs, which Gallina et al. (2002) related directly to the rise of saltasaurine titanosaurs. Additionally, Apesteguía (2002) correlated the diplodocoid extinction to that of carcharodontosaurid theropods, the rise of coelurosaurian and abelisaurian theropods, and the change in dominance from araripesuchid to notosuchid mesoeucrocodylians, and from chelid to podocnemidoid turtles. This occurred during his Gondwanan Evolutionary Stage, between the Early Gondwanan and the South American Domain for tetrapod faunas.
The youngest record of rebbachisaurids is considered by most authors as those from the Huincul Formation (Neuquén basin) and the Bajo Barreal Formation (San Jorge basin). Although the square-shaped mandible of the Coloradoan titanosaur Antarctosaurus wichmannianus (Huene, 1929) suggested some authors to consider diplodocoids as present in Campanian times (Jacobs et al. 1993;Upchurch 1999;Wilson & Sereno, 1998), recent findings (Apesteguía, 2004) demonstrated that titanosaurs also had square jaws. Therefore, there is no compelling evidence to support the presence of rebbachisaurids after Coniacian times. This is apparently true for Neuquén, San Jorge and Austral basins, from North, Central and Southern Patagonia respectively.
Late Cretaceous sauropod lineages are considered convergent Wilson, 2002) in their 'horse-like' skulls, restriction of the cylindrical, narrow-crowned teeth to the anterior part of the snout, comb-like dentition (Coria & Chiappe, 2001), square mandibular symphyses, and nostrils retracted to the top of the head. Both main lineages (titanosaurs and rebbachisaurids) are convergent in the loss of accessory articulations (i.e. hyposphenehypantrum complex). This trait would have provided a higher mobility and greater flexibility to the vertebral column (Wilson & Carrano, 1999). In this way, the rebbachisaurid anterior caudal vertebrae show a marked reduction of the bony elements related to the intervertebral contact (zygapophyses), as well as the amphyplatian nature of their vertebral centra. On the other hand, these anterior caudal vertebrae show well-developed, wing-like transverse processes and tall neural spines (at least in Cathartesaura). All these features together suggest an important development of the musculature and elastic ligaments, related to the pelvic girdle, and to the mid and posterior caudal vertebrae respectively, the latter further evolving in relation to "whiplash" mechanics of flagellicaudate diplodocoids.
The diversity, biogeography and ecological importance of the Rebbachisauridae in the Late Cretaceous terrestrial ecosystems is still far from understood (Gallina et al., 2002). However, the discovery of a new rebbachisaurid taxon has broad implications in our comprehension of the sauropod evolution in the Cretaceous of Gondwana.