Paleobiology of South American titanosaurs - CONICET Mendoza

Paleobiology of South American titanosaurs González Riga, Bernardo J.1,2

Introduction The sauropod clade Titanosauria is one of the most diverse and interesting dinosaur groups. Their fossils show an amazing size range, and some of them, were the largest animals ever to walk the Earth. Titanosaurs like Argentinosaurus, Futalognkosaurus, and Puertasaurus surpassed lengths of 30m and masses of 70 tons (Mazzetta et al., 2004). Titanosaurs exhibit a worldwide distribution useful for paleobiogeographic studies. They were important terrestrial herbivores during the Jurassic and specially the Cretaceous periods, and their remains have been recovered from all continents except Antarctica. Moreover, their fossil record is important for studying the composition of dinosaur faunas near the mass extinction at the K-T boundary. Another interesting aspect of titanosaurs is their extraordinary diversity, similar to that of the Late Cretaceous North American hadrosaurids. At the time of this writing, more than 48 titanosaur species have been discovered, around one third of all known sauropods. In this context, phylogenetic studies of titanosaurs are important as a basis for further evolutionary and paleobiogeographic interpretations. Likewise, locomotion and growth strategies are topics of great interest. Unfortunately, these research topics have been limited by the fossil record, since most titanosaur species, like other sauropods, are represented by incomplete and mostly disarticulated skeletal elements, such as dorsal and caudal vertebrae, and appendicular bones. However, this situation is changing with the discovery of well preserved specimens, such as the titanosaur Rapetosaurus collected in Madagascar (Curry Rogers and Forster, 2001). Recently, several new genera, such as Gobititan (You et al. 2003) and Sonidosaurus (Xu et al., 2006), were discovered in Asia. In Europe, the titanosaurs Aragosaurus (Sanz et al., 1987; Canudo et al., 2001) and Lirainosaurus (Sanz et al., 1999) and the basal somphospondilyan Tastavinsaurus (Canudo et al., 2008) were described. Likewise, during the last decade, 18 new titanosaur genera were discovered in South America; in Argentina and Brazil the record now reaches 30 genera (see table 1).

1

Departamento de Paleontología, IANIGLA, CCT-CONICET Mendoza, Avda. Ruiz Leal s/n, Parque Gral. San Martín, 5500, Mendoza, Argentina. [email protected]. 2

Instituto de Ciencias Básicas, Universidad Nacional de Cuyo. 125

Paleontología y dinosarios desde América Latina Species

Author

Lithostratigraphic unit

Country

Age

Adamantisaurus mezzalirai

Santucci and Bertini 2006

Adamantina Formation

Brazil

Campanian-?Maastrichtian

Powell, 1987b

Angostura Colorada Formation

Argentina

late CampanianMaastrichtian

Bajo Barreal Formation

Argentina

late Cenomanian-early Turonian

Lohan Cura Formation La Amarga Formation

Argentina Argentina

late Apitan-Albian Barremian-early Aptian

Candeleros Formation

Argentina

early Cenomanian

Aeolosaurus rionegrinus Aeolosaurus colhuehapensis Agustinia ligabuei Amargatitanis macni Andesaurus delgadoi

Casal, Martínez, Luna, Sciutto and Lamanna 2007 Bonaparte 1999 Apesteguía 2007 Calvo and Bonaparte 1991

Antarctosaurus wichmanianus Argentinosaurus huinculensis

Huene 1929

?Anacleto Formation

Argentina

early Campanian

Bonaparte and Coria 1993

Huincul Formation

Argentina

late Cenomanian

Argyrosaurus superbus

Lydekker 1893


Argentina

late Cenomanian-early Turonian

Baurutitan britoi

Kellner, Campos and Trotta 2005

Marília Formation

Brazil

Maastrichtian

Barrosasaurus casamiquelai

Salgado y Coria 2009

Anacleto Formation

Argentina

early Campanian

Bonatitan reigi Bonitasaura salgadoi Chubutisaurus insignis Epachthosaurus sciuttoi Futalonkosaurus dukei Gondwanatitan faustoi Laplatasaurus araukanicus Ligabuesaurus leanzai Malarguesaurus florenciae Maxakalisaurus topai Mendozasaurus neguyelap Muyelensaurus pecheni Neuquensaurus australis Pellegrinisaurus powelli Pitekunsaurus macayai

126

Martinelli and Forasiepi 2004 Apesteguía 2004 Del Corro 1975

Allen Formation

Argentina

Bajo de la Carpa Formation Cerro Barcino Formation

Argentina Argentina

Powell, 1990


Argentina

Portezuelo Formation

Argentina

Bauru Group

Brazil

Santonian-Maastrichthian

?Anacleto Formation

Argentina

early Campanian

Lohan Cura Formation

Argentina

late Aptian-Albian


Argentina

late Turonian-early Coniacian

Adamantina Formation

Brazil

Campanian-?Maastrichtian

Río Neuquén Subgroup (Portezuelo-Plottier formations)

Argentina

late Turonian- Coniacian


Argentina

late Turonian-early Coniacian

?Anacleto Formation

Argentina

early Campanian

?Anacleto Formation Anacleto

Argentina

early Campanian

Argentina

early Campanian

Calvo, Porfiri, González Riga and Kellner 2007 Kellner and Azevedo 1999 Huene 1929 Bonaparte, González Riga, and Apesteguía 2006 González Riga, Previtera, and Pirrone 2009 Kellner, Campos, Azevedo, Trotta, Henriques, Craik, and Silva 2006 González Riga 2003 Calvo, González Riga and Porfiri, 2007 (Lydekker, 1893) Powell, 1986 Salgado 1996 Filippi and Garrido 2008

Formation

late Campanian-early Maastrichtian Santonian Aptian-Albian late Cenomanian-early Turonian late Turonian-early Coniacian

Paleobiology of South American titanosaurs Puertasaurus reuili

Novas, Salgado, Calvo, and Agnolin 2005

Pari Aike Formation

Argentina

early Maastrichtian

Rinconsaurus caudamirus

Calvo and González Riga 2003

Río Neuquén Subgroup (Portezuelo-Plottier formations)

Argentina

late Turonian-Coniacian

Allen Formation

Argentina

late Campanian-early Maastrichtian

Lecho Formation

Argentina

Campanian-Maastrichtian

Marília Formation

Brazil

Maastrichtian

Marília Formation

Brazil

Maastrichtian

Rocasaurus muniozi Saltasaurus loricatus Trigonosaurus pricei Uberabatitan ribeiroi

Salgado and Azpilicueta 2000 Bonaparte and Powell 1980 Campos, Kellner, Bertini, and Santucci 2005 Salgado and Souza Carvalho 2008

Table 1. South American titanosaur species.

Moreover, in Patagonia, extraordinary embryos and eggs (Chiappe et al., 1998, 2001; Salgado et al., 2005a) and exceptionally articulated specimens (Martínez et al., 2004; Calvo et al., 2007a; González Riga et al., 2008) were found. These discoveries show the importance of the South American record for understanding both the phylogeny and paleobiology (ontogenetic stages, behavior and locomotion) of titanosaurs. The objective of this paper is to present a brief overview of some recently discovered South American titanosaurs. Nomenclature: In this paper I follow traditional or “Romerian” anatomical nomenclature (see Wilson, 2006b). Institutional Abbreviations: IANIGLA-PV, Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales, Colección Paleovertebrados, Mendoza, Argentina; MUCPv, Museo de Paleontología de la Universidad Nacional del Comahue, Neuquén, Argentina; SMU, Southern Methodist University, Dallas, United States of America.

Morphological diversity Until recently, the anatomy of the titanosaur skull was a mystery. In the fossil record, sauropod skull bones are extremely scarce due to the taphonomic processes of disarticulation, dispersion, and fragmentation (González Riga et al., 2009c). The most abundant skull elements are teeth and braincases (figure 1.B), such as preserved in Saltasaurus (Powell, 1992, 2003), Bonatitan (Martinelli and Forasiepi, 2004), Muyelensaurus (Calvo et al., 2007b), and Pitekunsaurus (Filippi and Garrido, 2008), as well as in unnamed specimens (e.g., Tidwell and Carpenter, 2003; Calvo and Kellner, 2006; Paulina-Carabajal and Salgado, 2007). Almost complete skulls are known in the Mongolian genera Nemegtosaurus (Nowinski, 1971; Wilson, 2005) and Quaesitosaurus (Kurzanov and Bannikov, 1983), and in Rapetosaurus (Curry Rogers and Forster, 2001, 2004). The skulls of Nemegtosaurus and Quaesitosaurus are distorted, 127

Paleontología y dinosarios desde América Latina

and these taxa were originally included within Dicraeosaurinae (Nowinski, 1971; McIntosh, 1990). Later, Salgado and Calvo (1997) were the first to consider these taxa as titanosaurs. In contrast, Upchurch (1999) nested these two genera within Diplodocoidea. Most recently, Wilson (2005) redescribed the skull of Nemegtosaurus and confirmed the hypothesis of Salgado and Calvo (1997), identifying key features that link this taxon to titanosaurs. In Argentina, two extraordinary titanosaur skulls have been discovered in recent years. One of them was found in Rincón de los Sauces (Neuquén Province, northern Patagonia) (Calvo et al., 1997) and is articulated with an almost complete specimen, perhaps the most complete titanosaur skeleton yet discovered. The other is a skull, articulated with a partial cervical series that was recovered in central Patagonia (Chubut Province) (Martínez, 1998; Martínez et al., 2006). Both skulls are under study, and represent key specimens for understanding South American titanosaur evolution.

Figure 1. Anatomy of the South American titanosaurs. A-C, skull elements; D-F, cervical vertebrae. A, Hypothetical ontogenetic development of the titanosaur skull: embryonic skull from Auca Mahuevo (left) and adult skull of Nemegtosaurus (right) in left lateral view; B, Muyelensaurus, braincase in posterior view; C, Bonitasaura, right dentary in ventral and dorsal views; D, Futalognkosaurus, posterior cervical vertebra in anterior view; E-F, Mendozasaurus, mid-posterior cervical vertebra in anterior and right lateral views. Abbreviations: (Bo) basioccipital, (Bt) basaltuber, (Dp) diapophysis, (Fm) foramen magnum, (Le) lateral expansion of neural spine, (So) supraoccipital, (Sprl) spinoprezygapophyseal lamina, (Sdc), supradiapophyseal cavity. (A, from Salgado et al., 2005a; B, from Calvo et al., 2007b; C, from Apesteguía, 2004; D, redrawn from Calvo et al., 2007c; E-F, from González Riga, 2005). 128

Paleobiology of South American titanosaurs

Another important discovery is Bonitasaura from the Bajo de la Carpa Formation of northern Patagonia. This species, represented by skull and postcranial bones, offers new information about the feeding mechanism of derived titanosaurs (figure 1.C). This genus exhibits a non-dentigerous zone in the dentary that is developed into a cutting surface that would have permitted effective slicing of tougher vegetation and minimized tooth wear (Apesteguía, 2004). In summary, the discovery of titanosaur skulls is extremely rare. In most species the skull is unknown, and this problem affects cladistic analyses, where phylogenetic relationships are consequently mostly determined using postcranial elements. In the axial skeleton, South American titanosaurs exhibit great morphological variation. For example, the huge cervical vertebrae of Mendozasaurus (IANIGLA-PV 076-1, late TuronianConiacian of Mendoza Province, Argentina) are characterized by very short centra, large and deep supradiapophyseal fossae, and laminar and transversely expanded mid-posterior neural spines (figure 1. E-F). This architecture suggests the development of relatively wide, robust, and short necks in some titanosaurs (González Riga, 2005). Some of these features are also present in Futalognkosaurus (figure 1.D), an articulated and exceptionally preserved giant titanosaur from late Turonian-early Coniacian strata of Neuquén Province (Calvo et al., 2007a, 2007c). Ligabuesaurus, from the Aptian-Albian of the same province, also has laterally expanded neural spines on its posterior cervical vertebrae. However, these structures differ from those of Mendozasaurus and Futalognkosaurus, since the lateral borders of the neural spines are formed by splayed lateral spinoprezygapophyseal laminae (Bonaparte et al., 2006). A comparison of the cervical vertebrae of Rinconsaurus, Trigonosaurus, Saltasaurus, and Mendozasaurus (figures 1.E-F and 2.A-C.) shows differences in overall proportions, the size and shape of the neural spines, the position and morphology of pre- and postzygapophyses, and the development of the supradiapophyseal cavity. It is probable that these contrasting neck structures are related to particular ecological adaptations. Titanosaurs exhibit interesting features in the presacral vertebrae, such as the presence of spongy bone. This character was recognized by Powell (1992) and later by Wilson and Sereno (1998), who considered it a synapomorphy of a new clade that they named Somphospondyli (literally “spongy vertebra”). This character was studied in detail in several sauropods by Wedel (2003a, 2003b, 2005) using computed tomography. This author recognized different morphological categories based on pneumatic characters (acamerate, procamerate, camerate, polycamerate, semicamerate, camellate, and somphospondylous). Wedel (2003b) identified a camerate structure , and a fully camerate condition in an unnamed taxon from the Cedar Mountain Formation of Utah; in contrast, he described a fully camellate (or somphospondylous) structure in Saltasaurus. Wedel’s studies indicate that the pneumatic structure within Titanosauriformes are complex, and evolved either via several independent origins or numerous reversals. To test this variation, further studies of the cervical anatomy of additional titanosaur taxa using computed tomography are necessary.

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Figure 2. Anatomy of the South American titanosaurs. A-C, cervical vertebrae; D-F, caudal vertebrae. A, Rinconsaurus, posterior cervical vertebra in right lateral view; B, Saltasaurus, middle cervical vertebra in right lateral view; C, Titanosaur DGM “Serie A” from Brazil, posterior cervical vertebra in right lateral view; D, Malarguesaurus, anterior and posterior caudal vertebrae in left lateral view; E, Mendozasaurus, anterior and posterior caudal vertebrae in left lateral view; F, Saltasaurus, anterior and posterior caudal vertebrae in left lateral view (A, redrawn from Calvo and González Riga, 2003; B, from Powell, 1992; C, redrawn from Powell, 1987; D, from González Riga et al., 2009a; E, from González Riga, 2003; F, from Powell, 2003).

The caudal vertebrae of titanosaurs are another region of the skeleton that shows great variation. Traditionally, caudal procoely is an important feature of derived titanosaurs, that show a typical “ball and socket” articulation (McIntosh, 1990; Salgado et al., 1997a), such as in Saltasaurus (figure 2.F). However, this procoely is not a consistent and uniform character. For example, Rinconsaurus possesses a sequence of typical strongly procoelous anterior caudals followed by a series of amphicoelous, opisthocoelous, and biconvex centra (Calvo and González Riga, 2003). Likewise, these caudal variations are more complex in non-titanosaurian somphospondyls and basal titanosaurs, that exhibit a transition between the ‘‘amphicoelous type’’ of brachiosaurs and the ‘‘strongly procoelous type’’ of derived titanosaurs. For example, basal somphospondyls like Phuwiangosaurus from the Lower Cretaceous of Thailand (Martin et al., 1994, 1999) and Tangvayosaurus from the Aptian-Albian of Laos (Allain et al., 1999), show amphicoelous or amphiplatyan caudal centra. Andesaurus from the early Cenomanian of Neuquén Province, Argentina (Calvo and Bonaparte, 1991), has slightly procoelous anterior caudal vertebrae 130


associated with platycoelous middle and posterior caudals. Malawisaurus, from the Lower Cretaceous of Africa (Jacobs et al., 1993), has strongly procoelous anterior caudal vertebrae with gently amphicoelous or platycoelous middle and posterior ones. Mendozasaurus (figure 2.E) has slightly procoelous middle caudal centra with reduced condyles, associated with typical strongly procoelous anterior caudals. Finally, Malarguesaurus (figure 2.D), from the late Turonian-early Coniacian of Mendoza Province (Argentina), shows a different pattern characterized by a welldeveloped procoely only in the posterior section of the tail; in contrast, anterior and mid-caudals are procoelous-opisthoplatyan (González Riga et al., 2009a). Other titanosauriforms that share with Malarguesaurus the presence of procoelous-opisthoplatyan (or “procoelous-distoplatyan” after Tidwell et al. [2001]) anterior caudals are a “Pleurocoelous” specimen (SMU 61732) from the Lower Cretaceous of Texas and the brachiosaurid Cedarosaurus from the Barremian of Utah (Tidwell et al., 1999). However, these two taxa differ from Malarguesaurus in having amphiplatyan posterior caudal centra.

Figure 3. Anatomy of the South American titanosaurs. A-C, A, articulated left hindlimb of the La Invernada titanosaur (MUCPv-1533) in dorsolateral view, B, articulated left pes of La Invernada titanosaur (MUCPv-1533) in dorsal view; C, articulated pes of Epachthosaurus in dorsal view. (A-B, from González Riga et al., 2008; C, from Martínez et al., 2004).

Study of the appendicular skeleton is essential for understanding the locomotion, evolutionary trends, and behavior of titanosaurs. Numerous changes in the limb skeleton are related to the acquisition of a wide-gauge limb posture: the shoulder girdle is broader in derived titanosaurs than in other sauropods, owing to the combined effects of the large coracoids and the enlarged, crescentic sternal plates (Wilson, 2006a). In titanosaurs the manual phalanges were strongly reduced, unossified, or absent (Salgado et al., 1997a; Martínez et al., 2004). The femur is specialized, with the proximal one-third of the shaft deflected medially (Salgado et al., 1997a), the distal femoral condyles beveled 10 degrees dorsomedially, and a highly eccentric midshaft cross-section (Wilson, 2006a). These synapomorphies suggest that titanosaurs are the markers of wide-gauge sauropod trackways (Wilson and Carrano, 1999). 131


In the titanosaurian record, articulated pedes are scarce, although disarticulated phalanges are present in many quarries. Three complete and articulated titanosaur pedes have been described: one from Mongolia, that of Opisthocoelicaudia Borsuk-Bialynicka 1977, and two from Argentina: those of Epachthosaurus Powell, 1990 (Martínez et al., 2004) and an unnamed titanosaur (MUCPv-1533) from the La Invernada locality (figure 2.A-B) (González Riga et al., 2008). The pes of the La Invernada titanosaur (figure 3.B) has five metatarsals and a phalangeal formula of 2-2-2-2-0. The first three digits have sickle-shaped claws with articular faces that suggest mobility in the horizontal and vertical planes, as in other sauropods (Bonnan, 2005). In particular the first phalanx of digit I is a very reduced structure. In contrast, Epachthosaurus (figure 3.C) and Opisthocoelicaudia have phalangeal formulae of 2-2-3-2-0 and 2-2-2-1-0, respectively. A preliminary study of this record suggests that titanosaurs underwent a progressive reduction in the size and number of pedal phalanges in digits III and IV toward the end of the Cretaceous (González Riga et al., 2008).

Comments about taxonomic studies A critical review of titanosaur cladistic studies will be presented elsewhere. However, it is important to note that: (1) The fragmentary preservation of most titanosaurs and the use of different methodological criteria have produced controversial aspects in the cladistic analyses. For example, Titanosauria, as a taxon that includes different titanosaur groups, was proposed by Bonaparte and Coria (1993), in the paper that described the giant Argentinosaurus, perhaps the largest sauropod known. Salgado et al. (1997a) defined Titanosauria as “the most recent common ancestor of Andesaurus delgadoi and Titanosauridae, and all of its descendants”. Subsequently, this node-based clade was re-defined using different criteria. First, Sereno (1998) proposed to define Titanosauria through a stem-based definition, as “all somphospondyls closer to Saltasaurus than to Euhelopus”. Second, Salgado (2003a) followed the definition of Sereno. Third, Wilson and Upchurch (2003) adopted the nodebased definition originally proposed by Salgado et al. (1997a). Finally, Upchurch et al. (2004) defined Titanosauria more inclusively as “a stem-based taxon defined as Titanosauriformes more closely related to Saltasaurus than to Brachiosaurus”. This changing panorama has important consequences for the study of basal titanosaurs. For instance, if we follow the original definition, Phuwiangosaurus, Ligabuesaurus, Malarguesaurus, and Chubutisaurus are placed outside Titanosauria. In a second alternative, following the definition of Sereno (1998) or Upchurch et al. (2004) these taxa could be considered titanosaurs, such as were emphasized by González Riga et al. (2009a). In this context, a careful revision of the phylogenetic definitions of titanosaur clades used during the last decade is necessary. (2) The phylogenetic study the taxonomic descriptions of cladistic analysis or, at least, a the 30 titanosaur genera known 132

of titanosaurian sauropods is in its initial stages. For example, the most South American titanosaur taxa do not include a preliminary phylogenetic study based on previous papers. Of from South America, only six (Mendozasaurus, Rinconsaurus,


Ligabuesaurus, Muyelensaurus, Futalongkosaurus, and Malarguesaurus) include cladistic analyses in their descriptions (González Riga, 2003; Calvo and González Riga, 2003; Bonaparte et al., 2006; Calvo et al., 2007a; Calvo et al., 2007b; González Riga et al., 2009a). This indicates that most papers emphasize diagnosis, description, and comparison without further considerations. Likewise, detailed taphonomic studies of South American titanosaur quarries are scarce. Detailed taphonomic descriptions are published only for Mendozasaurus (González Riga and Astini, 2007), Futalognkosaurus (Calvo et al., 2007a), Epachthosaurus (Rodriguez, 1993; Martínez et al., 2004), and nests from Auca Mahuevo (Chiappe et al., 2004). The paucity of such studies reduces the possibility of accurate comparisons between different taxa and vertebrate associations, since the original positions of bones and specimens are often unknown or undocumented (González Riga et al., 2009c). (3) Most cladistic studies of sauropods include few titanosaur taxa. However, in recent years, the analyses of Salgado et al. (1997a), Sanz et al. (1999), Curry Rogers and Forster (2001), Smith et al. (2001), Wilson (2002), González Riga (2003), Calvo and González Riga (2003), Upchurch et al. (2004), Curry Rogers (2005), Bonaparte et al. (2006), Calvo et al. (2007a, 2007b), and González Riga et al. (2009a) have increased the number of titanosaur genera analyzed and show a progressive inclusion of characters useful for resolving relationships within the clade. Salgado et al. (1997a) recognized, for the first time, the close relationship between Brachiosaurus and titanosaurs, and proposed to link these taxa in the clade Titanosauriformes. Curry Rogers (2005) included in her analysis several African titanosaurs and recognized a new unnamed clade uniting Rapetosaurus and Nemegtosaurus. Preliminary analyses of some new titanosaur taxa have recognized new clades: Lognkosauria, which unites grouping Mendozasaurus and Futalognkosaurus (Calvo et al., 2007a), and Rinconsauria, grouping Rinconsaurus and Muyelensaurus (Calvo et al., 2007b). Other South American titanosaur clades include Saltasaurinae and Aeolosaurini. Saltasaurinae was phylogenetically defined by Salgado et al. (1997a) as a node-based group, and redefined by Sereno (1998) as a stem-based group. It includes Saltasaurus (Bonaparte and Powell, 1980; Powell, 2003), Rocasaurus (Salgado and Azpilicueta, 2000), and Neuquensaurus (Powell, 2003; Salgado et al., 2005b). Aeolosaurini was phylogenetically proposed by Franco-Rosas et al. (2004) and comprises Aeolosaurus (Salgado and Coria, 1993; Salgado et al., 1997b) and Gondwanatitan (Kellner and Azevedo, 1999).

Conclusions and perspectives South American titanosaur studies are shedding light on to diverse phylogenetic and paleobiological topics. From a cladistic viewpoint, different clades have been recognized (Lognkosauria, Rinconsauria, Aeolosaurini, Saltasaurinae), revealing a greater diversity of Late Cretaceous herbivorous dinosaur faunas. At present, to test these interpretations, the author is working on more detailed cladistic analyses that include all new titanosaur species recently 133


discovered in South America (Brazil, Argentina), Africa, and Asia. In most titanosaur taxa skull characters are unknown; this aspect must be carefully considered in phylogenetic analyses. From a paleobiological viewpoint, several new discoveries are important. Ontogenetic topics have been studied through embryos and osteohistology. First, the discovery of exceptionally preserved embryos, eggs, and nests at Auca Mahuevo (Neuquén Province, Patagonia) has produced new ontogenetic and phylogenetic information (Chiappe et al., 1998, 2001). Specifically, these embryos provide an opportunity to test previous hypotheses regarding neosauropod phylogeny, for instance, the proposed correlation between different characters expressed in adult skulls (figure 1.A.) (Salgado et al., 2005a). Second, new osteohistological studies promise to provide insight into the growth strategies of titanosaurs (Salgado, 2003b; González Riga and Curry Rogers, 2006), as in other sauropod taxa (Curry Rogers and Erickson, 2005). These analyses will also be important for understanding evolutionary aspects, when osteohistological sections of most relevant sauropod species become available. Third, detailed sedimentological and taphonomic analyses of bone assemblages have supported systematic studies, indicating the association of different specimens and the in situ position of articulated bones (González Riga et al., 2009c). Moreover, such analyses yield information about the environments where titanosaurs lived. Finally, the ichnological record offers valuable information about different strategies of titanosaur locomotion and behavior. For example, in Argentina two sauropod ichnotaxa have been named: Sauropodichnus giganteus Calvo 1991, from the early Cenomanian of Neuquén Province, and Titanopodus mendozensis González Riga and Calvo 2009, from the late Campanian-early Maastrichtian of Mendoza Province. Sauropodichnus footprints show a wide gauge trackway, and were probably produced by basal titanosaurs (Calvo, 1999; Calvo and Mazzetta, 2004). They are preserved in lower levels of the Candeleros Formation, a unit that represents poorly channeled ephemeral flows and playa lake deposits. Titanopodus footprints were discovered at the Agua del Choique track site and offer an excellent example of the wide-gauge style of locomotion. This track site comprises more than 250 tracks produced by Late Cretaceous sauropods, probably medium-sized saltasaurine or aeolosaurine titanosaurs. Tracks are preserved on a calcareous sandstone bed of the Loncoche Formation (González Riga and Calvo, 2007; 2009). Most of the trackways are parallel and show the same direction of travel suggesting that titanosaurs moved in social groups (González Riga et al., 2009b). This discovery constitutes the second piece of evidence of gregarious sauropod behavior recorded in South America; the first was documented in Late Cretaceous strata of Bolivia (at the Humaca and Toro Toro track sites, see Lockley et al., 2002). From a paleoenvironmental point of view, the ocurrence of sauropod tracks in tidal facies of the Loncoche Formation is not direct evidence of a permanent habitat for these huge vertebrates. Instead, this record indicates that some titanosaurs had the capacity to walk across marginal marine environments, in this case related to the Late Cretaceous Atlantic transgression that covered central-northern Patagonia during the Maastrichtian-Paleogene.

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Acknowledgements Our studies have been financed by projects of CONICET PIP 0713/09 (to B. González Riga), ANPCYT-PICT 2005-33984 (to J. Calvo), Universidad Nacional de Cuyo (2009-2011 to B. González Riga), CONICET PIP 5222 (to W. Volkheimer) and CONICET PIP 5132 (to M. Prámparo), and supported by the IANIGLA (Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales, CONICET, CCT-Mendoza). I am particularly grateful to Matthew Lamanna, Mathew Wedel and Michael Taylor for their critical review and constructive comments.

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