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housands of sauropod egg clutches, some containing eggs with exquisitely preserved embryonic bone and integument , have been discovered in the Late Cretaceous nesting site of Auca Mahuevo (Chiappe et al. 1998, 2000, 2001, 2004; Dingus et al. 2000; Chiappe and Dingus, 2001; Coria et al. 2002) and adjacent localities in northwestern Patagonia, Argentina (fig. 10.1). Five expeditions to this extraordinary area (1997, 1999, 2000, 2001, and 2002) have yielded a wealth of information for understanding the prehatching development, the nesting structure, the egg morphology and malformation, and the reproductive behavior of these dinosaurs. Cranial characters of the in ovo embryos allowed the identification of the eggs as those of titanosaurs (Chiappe et al. 1998, 2001; Salgado et al. 2005). Textural differences in the sediments containing some clutches have illuminated aspects of the nest structure of these animals (Garrido et al. 2001; Chiappe et al. 2004). Microstructural studies have expanded our understanding of the eggshell variation (Grellet-Tinner et al. 2004) and incidence of egg malformation (Jackson et al. 2001, 2004) within a titanosaur population. Detailed mapping of clutch spatial distribution and egg-bed stratigraphic position, together with studies of their sedimentary context, have provided the basis for inferring aspects of the nesting behavior of these dinosaurs (Chiappe et al. 2000; Chiappe and Dingus 2001). Combining taxonomic constraint with extensive sampling, research at Auca Mahuevo and its adjacent localities offers the clearest picture to date of sauropod reproduction and embryonic development. In this chapter, we summarize the major developments of this research program and discuss their significance in light of previous interpretations of the reproductive biology of these colossal dinosaurs. GEOLOGICAL SETTING AND ASSOCIATED DINOSAUR FAUNA Auca Mahuevo lies approximately 120 km northwest of the city of Neuquén in the homonymous Argentine province (fig. 10.1). Two adjacent nesting sites, Barreales Norte and Barreales Escondido , are 15 and 22 km south of Auca Mahuevo, respectively. These three sites occur within an 85-m-thick sequence of sandstone, 285 TEN Nesting Titanosaurs from Auca Mahuevo and Adjacent Sites UNDERSTANDING SAUROPOD REPRODUCTIVE BEHAVIOR AND EMBRYONIC DEVELOPMENT Luis M. Chiappe, Frankie Jackson, Rodolfo A. Coria, and Lowell Dingus T siltstone, and mudstone of the Anacleto Formation (fig. 10.3), one of the lithostratigraphic units of the fossiliferous Cenomanian–Campanian Neuquén Group (Ramos 1981; Legarretta and Gulisano 1989; Ardolino and Franchi 1996; Leanza 1999; Dingus et al. 2000). Recent paleomagnetic analysis of rocks from the lower portion of the Auca Mahuevo section containing egg-beds 1–3 established the presence of a Reversed magnetozone in the Anacleto Formation (Dingus et al. 2000). In conjunction with earlier biochronologic correlations, this magnetozone was tentatively correlated with C33R, in the early–middle Campanian, between 83.5 and 79.5 million years ago (Dingus et al. 2000). Exposures at Auca Mahuevo include at least four distinct egg-bearing layers (figs. 10.2, 10.3; egg-beds 1–4), which occur in uniform mudstones representing overbank deposits on a fluvial plain (Chiappe et al. 2000). Two of these layers (egg-beds 2 and 3) can be subdivided into two horizons of eggs, separated by a few centimeters of sediments in the case of egg-bed 3 and about 1 m in the case of egg-bed 2. Egg-beds 3 and 4 are laterally continuous for at least several kilometers (Chiappe et al. 2000; fig. 10.2). Most egg clutches exhibit no discernible evidence of nest structure. However, thin sandstones representing abandoned channel and crevasse splay deposits occur within the Auca Mahuevo section, and in egg-bed 4 they occasionally preserve nesting trace fossils (Chiappe et al. 2004). Less than 1 m above the nesting structures in egg-bed 4 is a sandy red mudstone that contains a great number of sauropod tracks (Loope et al. 2000). The sauropod tracks are recognizable as thin (1-cm-thick), laterally discontinuous limy deposits that measure up to 80 cm in diameter and are oval to circular in shape. These platter-shaped features are interpreted to contain precipitates of calcium carbonate within the track depression, possibly from evaporation of standing water (Loope et al. 2000). Traceable over several kilometers, the track horizon provides an index layer useful for verifying that the underlying nesting traces occur on the upper surface of a single sandstone stratum . Additional layers of calcium carbonate precipitates, also interpreted as footprints, occur elsewhere in the Auca Mahuevo section. Clutches from egg-bed 3 occur in paleovertisols (Chiappe and Dingus 2001), recognizable by the abundance and widely varying...

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