A new Cretaceous terrestrial ecosystem from Gondwana with the description of a new sauropod dinosaur

A unique site at the northern area of Patagonia (Neuquén, Argentina) reveals a terrestrial ecosystem preserved in a detail never reported before in a Late Cretaceous deposit. An extraordinary diversity and abundance of fossils was found concentrated in a 0.5 m horizon in the same quarry, including a new titanosaur sauropod, Futalognkosaurus dukei n.gen., n.sp, which is the most complete giant dinosaur known so far. Several plant leaves, showing a predominance of angiosperms over gymnosperms that likely constituted the diet of F. dukei were found too. Other dinosaurs (sauropods, theropods, ornithopods), crocodylomorphs, pterosaurs, and fishes were also discovered, allowing a partial reconstruction of this Gondwanan continental ecosystem.


INTRODUCTION
In March 2000 a very large sauropod cervical vertebra was found in the northern shore of Lake Barreales, some 90 km northwest of Neuquén city, Northwestern Argentina (Fig. 1A). Since then, over 1000 specimens (including 240 fossil plants and 300 teeth) have been collected in an area of about 400 m 2 that is called the Futalognko site. The importance of this region led to the establishment of a study center -the Centro Paleontológico Lago Barreales  Neurapophysis of the atlas laminar and rectangular, and posteriorly directed; neural spine of the axis high and triangular; posterior border of the neural spine on middle cervical elements concave; ventral depression between parapophyses on middle cervical centra; anterior dorsal vertebrae with horizontal and aliform diapophyses; preand postzygapophyses of anterior dorsal vertebrae horizontal; first caudal vertebra with prespinal lamina bifurcated on its base forming two small infraprespinal laminae; supraspinal cavity in first caudal vertebra bordered by the prespinal and lateral laminae; 2 nd and 3 rd sacral ribs fused; wide and well developed iliac peduncle on ischia.

DESCRIPTION AND COMPARISON
Futalognkosaurus dukei is the most complete giant sauropod ever found, with an estimated length between 32 to 34 meters (Calvo 2006). The atlas has a laminar, rectangular neurapophysis that expands upward and curves medially. The axis has a high and robust neural spine with triangular shape. The centrum lacks pleurocoels differing in this respect from Saltasaurus (Powell 2003) and Alamosaurus (Lehman and Coulson 2002). All cervicals have a triangular shaped neural spine that is compressed lateromedially and elongated anteroposteriorly. Like in Malawisaurus dixeyi (Jacobs et al. 1993) and Gondwanatitan faustoi (Kellner and Azevedo 1999), the cervicals lack pleurocoels. Middle cervicals bear high and sail-shaped neural spines as in Malawisaurus and Rapetosaurus (Rogers and Forster 2001). Futalognkosaurus shares with Rapetosaurus neural arches three times higher than the centra in anterior and middle cervicals. In lateral view, the spinoprezygapophyseal border is straight and the spinopostzygapophyseal margin is concave, a feature not generally observed in other members of the Titanosauria. Posterior cervicals have neural arches at least three times higher than the centrum and neural spines compressed proximodistally and expanded laterally as in Mendozasaurus neguyelap (González Riga 2003Riga , 2005  Some anatomical features such as the procoelic condition of the caudal vertebra, allows the allocation of Futalognkosaurus dukei in the Titanosauria. In order to assess the phylogenetic position of Futalognkosaurus dukei relative other titanosaurs, we have used a data matrix (65 characters) proposed by Calvo et al. (in press). In this study we analyze the relation of Futalognkosaurus with 18 taxa using the program PAUP, version 3.0 (Swof-ford 1989). Camarasaurus grandis Cope 1877 was assigned as outgroup (Fig. 3) and all multi-state characters were considered unordered (see appendices). The application of the heuristic method with delayed transformation optimization produced one most parsimonious tree ( Besides Futalognkosaurus, other sauropod dinosaur remains that represent two additional yet undescribed taxa were also found in the Futalognko site, but none was articulated or nearly as complete. Theropod specimens were also unearthed and indicate the presence of several species, two of which were formally described. One represents Megaraptor namunhuaiquii, previously recovered in the Sierra del Portezuelo, around 80 km South from the Futalognko site, in levels of the Portezuelo Formation (Novas 1998). The material of Megaraptor collected at the Futalognko site consists of a complete and articulated arm (Fig. 4E), showing that the bones originally interpreted as a foot of this taxon (Novas 1998) are, in fact, the hand (Calvo et al. 2004b). The new specimen demonstrates that Megaraptor is not a member of the Coelurosauria, but a basal tetanuran that developed a unique elongated hand with very large sickle-shaped claws, more so than the ones of spinosaurids (Charig and Milner 1997).
The second theropod belongs to the genus Unenlagia that was also first described from the Sierra del Portezuelo (U. comahuensis). This genus has been regarded as the sister group of Aviale (Archaeopteryx + modern birds) (Novas and Puerta 1997) while others suggest that it was a more basal dromaeosaurid (Norell et al. 2001). The new species recovered from the Futalognko quarry, named Unenlagia paynemili, belongs to a more slender species that has a small anterior process in the pubis (not present in U. comahuensis) and indicates that those enigmatic Unenlagia-type maniraptoran dinosaurs were more common (Calvo et al. 2004a, Fig. 4F). Additional theropod material consists of isolated bones and over 250 teeth that show several morphotypes, suggesting the presence of a rather diverse theropod fauna in this quarry (study in progress). Most are very similar to those reported in dromaeosaurids (  (Fig. 4C).
Remains of ornithischian dinosaurs were also collected in the quarry and consist of isolated bones (dorsal vertebra, femur, and ilium) that are referable to Ornithopoda, which are rare in South America (Fig. 4G). Based on the separated distal condyles of the femur, they are referable to the Iguanodontia.
Other fossil reptiles are crocodylomorph teeth, some referable to Peirosauridae (Fig. 4D), which were found in the Neuquén Group (Digregorio 1972), Morocco (Buffetaut 1994) and Brazil (Price 1955). Recently a lower jaw of a tiny individual was also recovered (Fig. 4L) and belongs to a new species closely related to Comahuesuchus. The new specimen shows that those rare and unusual notosuchid crocodylomorphs have a large posterior tooth that shows coarse serrations similar to theropod dinosaurs, increasing the groups of crocodylomorphs with "theropod-like" teeth (Campos et al. 2001). Among the pterosaur bones recovered is the proximal part of an ulna referable to the Azhdarchoidea sensu Kellner (2003), with an estimated wing span of around 6 m (Kellner et al. 2007, Fig. 4H). Histological sections show the presence of reticular fibro-lamellar bone, with numerous vascular canals, indicating that it was still growing at time of death and therefore can be considered the largest pterosaur recovered from the former Gondwana. Remains of pelomedusoid turtles and undetermined reptilian eggshells were also found.
Unexpectedly, the Futalognko quarry has also yielded delicate fish specimens; the first from the Neuquén Group reported so far (Fig. 4I). There are six specimens representing three different taxa: a small Euteleostei and two clupeomorphs (Gallo et al. 2003). Despite not being complete, all preserved elements are articulated. Ganoid scales of semionotid fishes and freshwater bivalves were also collected.
Lastly, about 240 plant specimens were recovered. Leaves are well preserved, with the majority corresponding to dicotyledonian angiosperms that are shown to have been abundant in this area (Fig. 4J,K). Some gymnosperm leaves, fructifications and remains of conifers were found too. In this wide diversity of foliar structures there are indications of herbaceous habit for some specimens as well as signs of caducity. Therefore, in this complex association, angiosperms seem to constitute the main element of the ancient flora in this region, and this material consists the first record of those derived plants in the Upper Cretaceous of the Neuquén Basin (Passalia et al., in press).
The fossil content in this region is not limited to the Futalognko quarry. In a range of 1500 m, there are three more quarries at the same level, that have also yielded a variety of specimens, including an articulated partial skeleton of a new theropod (Fig. 4M) and isolated remains of ornithopods, including jaws (Fig. 4N). Very well preserved pleurodiran turtles (including skull) were also found. Two other sites have been discovered in the Plottier Formation that overlies the Portezuelo layers, only 25 m above the Futalognko quarry and also contain large quantities of dinosaur bones, particularly sauropods, bivalves and plant remains, albeit in lower diversity.
Other been reconstructed based on fossils collected in large areas and from different layers whose outcrops sometimes are hundreds of meters apart and possible represent a time span of a few million years. The excavated part of the Futalognko site is restricted to a small area (400 m 2 ) and due to a controlled collecting program in which all specimens are plotted on a quarry map, it can be demon-strated that the fossils were found in a bed about 0.5 m thick, representing a time span in the scale of years. We were also able to reconstruct the taphonomic history of this unusual deposit.
Based on geological data, the Portezuelo Formation was deposited in a flat landscape as most of the stratigraphic units of the Neuquén Group. A detailed sedi- shows that an active and wide meandering river system was installed in this area during the late Cretaceous (Turonian-Coniacian). Moreover, the cyclic deposition of different fluvial facies suggests that the climate was warm and humid, intercalated with short dry seasons.
Three different sedimentological facies deposits that are associated to this meandering system could be recognized: channel, point bar and small lake deposits, the later corresponding to an abandoned meander. Our sedimentological and paleontological studies show that all three facies are present at the Futalognko site (Fig.  1C). The point bar deposits were formed first. During the dry season, fine-grained sediments (that latter one become sandstones and show well preserved fossils) were deposited latter. The wet seasons were responsible for flooding events that carried the animal remains downstream the river channel and deposited them in fine conglomerates. Those fossil remains usually are crushed with signs of erosion indicating a moderate transportation.
Regarding the remains of Futalognkosaurus dukei we hypothesize that it died of unknown causes on a floodplain, near the margins of a river. The carcass was possibly partially dismembered by theropod dinosaurs present in this area such as Megaraptor and small dromaeosaurids (and, perhaps also some crocodylomorphs like peirosaurids). A short time after its death, a single flooding event of great intensity washed the carcass of this giant sauropod dinosaur into the river channel. Due to its giant size the carcass likely acted as a barrier, reducing the competency of the currents generated by subsequent smaller flooding events that are very common in the wet seasons. This resulted in the deposition of additional organic remains around this huge carcass. At some point, the river changed its course laterally, leaving an oxbow lake behind. Surprisingly some tiny fish skeletons were found together with the dinosaur elements. Since they are complete, the only explanation for their presence along with the large bones is that they were trapped when this lake was formed. The few bivalves preserved in the same horizon were trapped too.
Subsequently wind and rain carried sediments, leaves and branches from the nearby vegetation inside the small lake. Most of the plant material is carbonized indicating that a reducing environment was installed in this lake and that the original connection with the river was broken. These fossil plants are preserved in laminated and rippled siltstone around the dinosaur bones resulting in this very unusual combination of fossils. The events that lead to this accumulation must have happened in a comparatively short time otherwise, the bones of Futalognkosaurus would have been decomposed.
All this sedimentological and paleontological data make us confident to regard the plants and animals found and preserved in the Futalognko site as part of the same terrestrial ecosystem which can be reconstructed as follows. During the Late Cretaceous (Turonian-Coniancian), Patagonia had a warm and humid climate with dry seasons, and a meandering river system crossed a forest formed mainly by angiosperms, with few subordinated gymnosperms. This region was home to small ornithopods, small and large theropods (e.g., Megaraptor, Unenlagia), and giant sauropod dinosaurs such as Futalognkosaurus. Other reptiles present in this ecosystem were peirosaurid and notosuchid crocodylomorphs, pleurodiran turtles and large azhdarchoid pterosaurs. This large diversity of specimens with different potential preservation (e.g., leaves, invertebrates, delicate fish skeleton and large dinosaur bones) associated in one quarry at the same horizon was never reported in Gondwana. This unique site consists a rare occasion where a Late Cretaceous terrestrial ecosystem can be confidently reconstructed.

ACKNOWLEDGMENTS
The research activities at the Futalognko site could not have been done without the help of students and volunteers -too many to be listed here -that have worked there since 2000. We would also like to thank Marcelo N.F. Trotta (Museu Nacional/UFRJ), Valéria Gallo (Universidade do Estado do Rio de Janeiro/UERJ) and Rodrigo Santucci (Departamento Nacional de Produção Mineral/Brasília) for several suggestions that improved the manuscript. We specially wish to thank Duke Energy Argentina, Duke Foundation and United Way International for developing and supporting the Proyecto Dino and the research at the new Centro Paleontológico Lago Barreales (CePaLB). This project was also partially funded by the National University of Comahue and Chevron-Texaco for the projects T-021 and I-122 An Acad Bras Cienc (2007)