Record of the genus Aeolosaurus ( Sauropoda , Titanosauria ) in the Late Cretaceous of South America : paleogeographic implications Registro del género Aeolosaurus ( Sauropoda , Titanosauria )

The Upper Cretaceous of South America has yielded fossils of the Aeolosaurini titanosaurian Aeolosaurus from Argentina (from the Allen, Los Alamitos, Angostura Colorada, and Bajo Barreal formations) and Brazil (Adamantina and Marília formations). To date, four Aeolosaurus species have been recognized: Aeolosaurus colhuehuapensis, Aeolosaurus rionegrinus, A. rionegrinus? and Aeolosaurus sp. Gondwanatitan faustoi, recently considered a junior synonym of Aeolosaurus, is here demonstrated to be a valid taxon. The occurrence of Aeolosaurus in Turonian-Santonian rocks of central Brazil and in Campanian-Maastrichtian deposits of Argentina suggests that the temporal and geographic distribution of aeolosaurines was greater than previously recognized. The Aeolosaurus records from the Maastrichtian Marília Formation of Brazil demonstrate that this genus persisted after the marine incursion that occurred in northern Patagonia during the Campanian-Maastrichtian. The Late Cretaceous tetrapod assemblages of central Brazil and Patagonia are comparable in age and fossil content.

Several anatomical, taxonomic and systematic studies of Aeolosaurus have been published (Wilson & Sereno, 1998;Salgado & Garcia, 2002;Wilson, 2002;Powell, 2003;Franco-Rosas et al., 2004;Casal et al., 2007;González Riga et al., 2009).Nevertheless, the paleogeographic distribution of this taxon has not yet been studied in detail.Accordingly, the aim of this paper is to document and analyze the distribution of the genus Aeolosaurus and other aeolosaurine taxa in Argentina and Brazil.

Argentina Malargüe Group
The Neuquén Basin (which contains strata belonging to the Malargüe and Neuquén groups) is located in the west-central region of Argentina, and covers Neuquén Province as well as areas of Río Negro, Mendoza, and La Pampa provinces.This basin is important for South American paleontology due to the richness of its Cretaceous dinosaur faunas.It is infilled by intercalated marine, littoral and continental sediments that range in age from the Upper Triassic to the Lower Paleogene (e.g., Digregorio & Uliana, 1980;Leanza, 1999).
Within the Neuquén Basin, fossils of Aeolosaurus have been collected from the Allen Formation, as well as from equivalent units elsewhere in Patagonia (e.g., the Los Alamitos and Angostura Colorada formations; fig.1).
The Allen Formation (which crops out in La Pampa and Neuquén provinces) and the Loncoche Formation (in Mendoza Province) were deposited during the late Campanian-early Maastrichtian (fig.1).They comprise the lower unit of the Malargüe Group and unconformably overlie the Neuquén Group (Legarreta et al., 1989).The beds of the Allen Formation are separated from the Neuquén Group by the lower Campanian Huantraiquican unconformity (Leanza & Hugo, 2001;Leanza et al., 2004).
The recognition and description of the stratigraphic relationships among the various formations of the Bauru Basin has allowed a better understanding of its extensive fossil assemblages in a chronostratigraphic context.

Taxonomy
In order to evaluate the taxonomic status of Aeolosaurus, I use information obtained from Franco-Rosas et al. (2004), Calvo et al. (2007) and González Riga et al. (2009).Data on Aeolosaurus species are derived primarily from the literature as well as from direct observations of specimens.The term Titanosauria is used sensu Wilson and Upchurch (2003), as the clade of Andesaurus delgadoi, Saltasaurus loricatus, their most recent common ancestor and all descendants.The definition of the tribe-level taxon Aeolosaurini employed here follows those proposed by Franco-Rosas et al. (2004), Casal et al. (2007) and González Riga et al. (2009).Aeolosaurini is currently comprised by Aeolosaurus rionegrinus Powell (1987), Aeolosaurus colhuehuapensis Casal et al. (2007) and Gondwanatitan faustoi Kellner & Azevedo (1999).Remane, 2000).

Paleogeographic distribution of Aeolosaurus
The following is a brief review of the geographic, stratigraphic and vertebrate faunal contexts of known specimens of Aeolosaurus (fig.2; table 1).

Argentina Chubut Province
Aeolosaurus colhuehuapensis (holotype: UNPSJB-PV 959) was recently described by Casal et al. (2007) from the Upper Member of the Bajo Barreal Formation as exposed on an ephemeral island in Lago Colhué Huapi in south Chubut Province.According to Casal et al. (2007), the level of the Upper Member from which A. colhuehuapensis was recovered is of Campanian-?Maastrichtian age, in contrast to the Lower Member of the Bajo Barreal Formation which is regarded as Cenomanian-Turonian.These articulated specimens of the A. colhuehuapensis consist of procoelous anterior and middle caudal vertebrae and haemal arches.Associated vertebrate fauna: an indeterminate mesoeucrocodylian and a derived iguanodontian ornithopod.
Aeolosaurus rionegrinus?(Powell, 2003).(According to Salgado et al. (1997), material referred to "Aeolosaurus rionegrinus?" is housed in the MPCA, not in the Museo Argentino de Ciencias Naturales "Bernadino Rivadavia".)Aeolosaurus rionegrinus? is known from strata of the Los Alamitos Formation exposed on the Estancia Los Alamitos, Cona Niyeu, Río Negro Province.According to Powell (2003), the morphology of the caudal centra and neural arches, the position and length of the prezygapophyses, and the orientation and relative position of the postzygapophyseal facets are very similar to the conditions observed in Aeolosaurus rionegrinus from Casa de Piedra, Río Negro Province.Associated vertebrate fauna: same as for Aeolosaurus rionegrinus.
cf. Aeolosaurus (LGP 0001-0010).Two procoelous vertebral centra, a right coracoid and tibia, a possible phalanx and several unidentified fragments from the Maastrichtian Marília Formation, Bauru Group, at Peirópolis, Minas Gerais State were described by Lopes & Buchmann (2008).These specimens were referred to Aeolosaurus because one of the caudal vertebrae (LGP 0005) is heart-shaped in cross-section, has an anteroposteriorly compressed ventral half and has pleurocoellike depressions on its anterolateral portion (Lopes & Buchmann, 2008).Associated vertebrate fauna: same of the CPP 374.
Aeolosaurus sp.(MPMA 477).A caudal vertebra from the upper Maastrichtian Echaporã Member of the Marília Formation is the only record of this genus in this member of the Bauru Group.According to Santucci (2002), this vertebra presents the same conditions as Aeolosaurus sp. from the Adamantina Formation.Associated vertebrate fauna: titanosaurs.
Gondwanatitan faustoi (MN 381).Kellner & Azevedo (1999) described this taxon on the basis of a nearly incomplete skeleton recovered from the Turonian-Santonian Adamantina Formation of Santo Anastácio, São Paulo State.G. faustoi was interpreted as a member of "Titanosauridae" by Kellner & de Azevedo (1999) and Salgado & Garcia (2002), and later as a titanosaur closely related to Austrosaurus and Lirainosaurus (Upchurch et al., 2004).Bertini et al. (2000) and Santucci (2002) considered this species to be synonymous with Aeolosaurus based on the anterodorsally projected neural spine of the anterior caudal vertebrae as well as some appendicular characters.However, Gondwanatitan and Aeolosaurus can be differentiated on the basis of a number of features, such as: 1) centrum of the sixth sacral vertebra with concave posterior articular surface in Gondwanatitan; 2) heartshaped posterior articular surfaces on anterior and middle caudal vertebrae in Gondwanatitan; 3) prominent lateral ridge on the basal portion of the neural arch of the middle caudal vertebrae in Gondwanatitan; 4) large articulation of the prezygapophyses in Aeolosaurus and corresponding articulation absent in Gondwanatitan; 5) lateral tuberosity on ischium of Gondwanatitan; 6) iliac peduncle of ischium less expanded in Gondwanatitan; 7) medial ischial plate less expanded in Aeolosaurus; 8) more slender humerus in Aeolosaurus.On this basis, I refute the proposed synonymy of these two genera and instead consider them distinct.

Discussion
From existing data on the paleogeographic and stratigraphic distribution of the genus, it is clear that Aeolosaurus ranged from at least southern Patagonia to central Brazil.Aeolosaurus colhuehuapensis, Aeolosaurus rionegrinus, Aeolosaurus rionegrinus?, Aeolosaurus sp., and Aeolosaurus-related taxa have all been discovered in Argentina; Aeolosaurus sp. and Aeolosaurus-related taxa are known to occur in Brazil.The following are recorded in the Turonian-Santonian (of Brazil): Aeolosaurus sp. and Aeolosaurus-related taxa.In the Campanian-Maastrichtian (of Argentina) occur Aeolosaurus colhuehuapensis, Aeolosaurus rionegrinus, Aeolosaurus rionegrinus?, Aeolosaurus sp. and Aeolosaurus-related taxa; Aeolosaurus sp. is known from the Maastrichtian (of Brazil) (fig.2, table 1).
The genus Aeolosaurus has so far been recorded at ten Late Cretaceous localities in Argentina and Brazil.Conversely, Aeolosaurus is not presently known to have occurred in other areas of Gondwana.The paleogeographic distribution of Aeolosaurus therefore appears to be the product of Late Cretaceous faunal interchange within South America.The observed distribution of Aeolosaurus in southern South America may be a product of evolutionary change, dispersal or selective preservation.Possible differences in the composition of aeolosaurine species, both in space and time in southern South America, could have originated from paleoecological trends that might be reflected in features such as lithology.However, in Argentina and Brazil, Aeolosaurus is the most commonly reported sauropod genus and consequently this taxon may have been very abundant compared to other South American titanosaurs.Why Aeolosaurus has not yet been found in other areas of South America is unknown.
The earliest occurrence of Aeolosaurus is from the Turonian-Santonian Adamantina Formation (Almeida et al., 2004)   Paulo states, Brazil.The presence of the genus in the Allen, Angostura Colorada, Los Alamitos and Bajo Barreal formations indicates that the taxon was widespread in northern and central Patagonia by Campanian-Maastrichtian time.Fossils of Aeolosaurus are a common occurrence in rocks deposited during this latter time, in Argentina as well as in the Marília Formation of central Brazil.
The record of Aeolosaurus from southern South America is important for several reasons.The fossil assemblage (e.g., Aeolosaurus and the crocodyliform Peirosaurus) in Brazil correlates to the Neuquenian and Allenian faunal assemblages proposed by Leanza et al. (2004) and thus directly cross-correlates the record of Aeolosaurus and Peirosaurus to the standard southern South America Gondwanan chronostratigraphic time scale.These correlations reinforce earlier assessments by Huene (1931) that the Bauru Group and many Patagonian fossil vertebrate-bearing units (e.g., those of the Neuquén Group) are of Late Cretaceous age.
As documented in the present study, the titanosaurian genus Aeolosaurus was widespread during the Late Cretaceous.Although some reports of the genus in central Brazil and northern Patagonia are still preliminary (e.g., González Riga, 1999;Almeida et al., 2004), the fossil record of this titanosaur indicates the existence of Late Cretaceous faunal connections between the areas of southern South America where its fossils are reported (fig.2, table 1).Sauropod dinosaurs were terrestrial animals (Powell, 1992) (2004) ing the Campanian-Maastrichtian reflect a southernwestern in the South America presence.During the Campanian-Maastrichtian, marine ingressions occurred throughout central and northern Patagonia (Pascual et al., 1996).As a result, no dinosaurian faunal interchange could have occurred between central and northern Argentina and Brazil until the Paleocene, because these marine waters would have acted as a barrier to the dispersal of dinosaur faunas from both areas.The genus Aeolosaurus persisted into the Maastrichtian in central Brazil but has not been reported in post-Campanian Argentinean beds.

Conclusions
Upper Cretaceous Argentinean and Brazilian strata preserve an important record of aeolosaurine titanosaurs.Although the fossils of these sauropods are often fragmentary, most of them bear synapomorphies of the genus Aeolosaurus in their caudal vertebrae.The close affinity of the Aeolosaurus specimens from Brazil and Argentina expands both the stratigraphic and geographic ranges of this genus.The early Late Cretaceous (Turonian-Santonian) occurrence of Aeolosaurus extends the stratigraphic range of Aeolosaurus by approximately 28.5 million years.Geological and stratigraphic evidence suggests that central Brazil and Patagonia maintained contact through the early Late Cretaceous but became separated by a marine incursion during the Late Cretaceous (Campanian), as pointed out by Pascual et al. (1996).The occurrence of Aeolosaurus suggests that aeolosaurines were distributed at least in central Brazil and Patagonia during the Late Cretaceous, and that these two areas maintained a faunal connection at this time.This distribution is further supported by the presence of Aeolosaurus at the late Maastrichtian Peirópolis site in Minas Gerais State, where the genus apparently persisted after the marine ingression that occurred in northern Patagonia during the Campanian.Peirosaurid crocodyliforms from the Adamantina  and Marília formations in central Brazil (i.e., Peirosaurus tormini, Uberabasuchus terrificus, Itasuchus jesuinoi) share close affinities with P. tormini and Lomasuchus palpebrosus from coeval rocks in northern Patagonia (Gasparini et al., 1991;Bonaparte, 1996;Gasparini, 1996;Martinelli, 2003;Carvalho et al., 2004).This association further supports an early Late Cretaceous faunal connection between the two areas.The central Brazil assemblage is also broadly comparable with Late Cretaceous tetrapod assemblages from Patagonia, reaffirming the relative homogeneity of the terrestrial faunas of these two areas during this time.Additional work at other Gondwanan localities may eventually extend the ranges of these taxa throughout much of Gondwana during the Late Cretaceous.

Table 1 .-List of the Aeolosaurus records in southern South America
that consequently depended on land or land bridges as migration routes.At present, the fossil record of Aeolosaurus suggests that the taxon originated in central Brazil, then dispersed via south-central Brazil to northern Patagonia and finally to southern Patagonia (fig.3).In contrast, other Turonian faunal elements discovered in northern Patagonia, which are similar to geologically younger South American faunas (e.g., the crocodyliform Peirosaurus tormini found in central Brazil and northern Patagonia), are often proposed to have migrated northward to central Brazil from Patagonia.The Late Cretaceous Argentinean and Brazilian record of Aeolosaurus is singular because paleogeographic reconstructions locate between central-northern Argentina and Brazil dur-Estudios Geol., 66(2), 243-253, julio-diciembre 2010.ISSN: 0367-0449.doi:10.3989/egeol.40338.081Record of the genus Aeolosaurus in the Late Cretaceous of South America 249