TAPHONOMIC CHARACTERIZATION OF THE NEOSAUROPOD PALEOHYDROTHERMAL NESTING SITE OF SANAGASTA, LA RIOJA, NW ARGENTINA

LUCAS FIORELLI; GERALD GRELLET-TINNER; ELOISA ARGAÑARAZ

San Juan

Congreso; IV Congreso Latinoamericano Paleontologia de Vertebrados; 2011

The recent discovery of the Sanagasta dinosaur nesting site in the Lower Cretaceous, Los Llanos Formation, La Rioja Province (NW Argentina), has shed unexpected insights on the reproductive behavior of neosauropods. This nesting site documents colonial behavior, site fidelity with possible phylopatry, and an opportunistic environment-dependent-reproduction relationship with a geothermally active paleoenvironment (Grellet-Tinner & Fiorelli, 2010). The taphonomy of this monotypic nesting site is described in order to assess its biostratinomy and fossil diagenesi. The 3D spatial arrangement of the egg-clutches is well preserved and suggests the presence of autochthonous associations (see Martin, 1999) with in-situ ovipositions, yet without any trace of nest structures. The egg-bearing level represents an ?intrinsic? biogenic accumulation(Kidwell et al., 1986) that implies a gregarious behavior for these dinosaurs through their preferential colonization of a specific nesting site (Martin, 1999) that is characterized by a paleohydrothermal environment (Grellet-Tinner & Fiorelli, 2010). The biocenosis is of biogenically deposited eggs, with eggshell mineralization altered by hydrothermal activity, and extremophile microorganisms (stromatolites, cyanobacteria, and diatoms). The thanatocenosis and time-averaging were virtually zero because the eggs buried in the substrate immediately after being laid. In contrast, the taphocenosis (buried associations) represents the most important taphonomic feature. The eggshells display several synchronous taphonomic alterations (e.g., high fragmentation, chemical erosion, and eggshell thinning) produced by acidic hot spring solutions, while re-calcification and silicification (mainly micro-sparry, chalcedony, microchert, opal-A, and epidotes) are the most common eggshell recrystallizations. The presence of ~10 µm Ø opal-A microspheres, epidote aggregates (e.g., clinozoisite through the "saussuritization" process), and fibrous clay minerals (smectites derivates from plagioclase through the "argillization" process) replacing parts of the eggshell and egg membranes indicates extremehydrothermal alteration (see Pirajno, 2009) under exceptional conditions. Subsequently, the eggs were exposed to compression forces leading to fracturing of the shells during eodiagenesis. Considering the association of fossil stromatolites, cyanobacterians, and diatoms, silicified rhizoliths, coupled with sedimentary structures, such as microfacies and hydrothermal microfabrics, and with complete egg clutches, we can infer that theSanagasta nesting site represents aparticular instance of taphocenosis and an exceptional lagerstätte. The Sanagasta dinosaur nesting site is thus the first documentedcase of environment-dependent reproduction amongst neosauropods.