Gerald Grellet-Tinner is a graduate student at USC who is also a dinosaur egg specialist. He has also participated in a couple of expeditions to Auca Mahuevo.

During the fall of 1998, I was at the American Museum of Natural History working on dinosaur eggs and eggshells with one of the museum's curators, Mark Norell. I was studying the eggs of non-avian theropods (large meat-eating dinosaurs) recently found in the Gobi Desert of Mongolia. A friend of ours, Luis Chiappe, who at the time was working at this museum showed me a paper that he co-authored on sauropod dinosaur eggs from Argentina. These eggs had just been found by himself, Lowell Dingus, and Rodolfo Coria in Auca Mahuevo, Patagonia. Based on the paper they were unique because they contained casts of the fossilized integument (skin), skeletal remains and teeth of embryos. The same dayas I was working at the scanning electron microscope (SEM) of the AMNH, Luis brought me some of these specimens. The eggshell structure was marvelously preserved with no or little diagenesis (alteration of original material during burial and fossilization). During the following months, Luis invited me to come along the next expedition in March 1999. My first order of business was to study the eggshell structure of the eggs that were truly associated with embryonic remains in order to associate this type of eggshell with a dinosaur species. However, little did I know when we arrived at the field site after an interesting journey by car from Buenos Aires, that my original plans were going to be shattered.

The first morning I was introduced to the field site, I immediately realized that we were walking on eggs. The mudstone layer on which we were trekking was littered with eggs and even with what we now acknowledge as egg clutches partially protruding from the ground. Confronted with so many eggs, I revised my initial plans to sample whole eggs, to study eggshell variation, sample clutches, to evaluate if they were laid by the same kind of dinosaur, and try to estimate the number of eggs originally laid in a clutch. This type of research has already been conducted by French paleontologists who have studied the Late Cretaceous egg sites from sauropod dinosaurs in southern France. Comparing our results with those already published could prove very interesting. I therefore spent most of my time during this field season working at the egg quarry and collecting specimens on the four different levels of egg-bearing layers. The view of the badlands from the quarry was phenomenal. Sedimentary layers extending to the horizon were changing colors as the day progressed. The vastness of this parched landscape contrasted highly with the alpine settings I was used to from Swizerland. It reminded me more than anything else of the unusual beauty of the Navaho and Kayenta Formations I worked on in the northeastern part of Arizona.

The only exception to the "egg" routine was when Rodolfo Coria asked me to help him for one week to devise armored plaster jackets and bracings to assure the safe transportation from the field of the new complete and articulated theropod that we found and quarried.

Back from the field as I was working at the SEM I was amazed to observe that a great proportion of the eggshells had a layer adhering to the innermost surface of the shell, which I immediately suspected to be a cast of the membrana testacea. The membrana testacea is the organic membrane made of protein fibers found adhering to the inner side of an egg. This membrane serves as another barrier to separate the growing chick from the outside world. For instance when a common chicken egg is hard boiled the white membrane sticking to the calcitic eggshell is the modern version of the membrana testacea. After corresponding with R. Kohring who published several papers on this topic, I concluded that to date we had the oldest preserved membrana testacea in the world. This became the subject of a poster I presented at the Society of Vertebrate Paleontology meeting in Mexico City in 2000, that brought me the first prize in the poster competition. Along with the description of the membrana testacea, I related my observations on other eggshell characters, which combined together, led me to interpret that these Late Cretaceous sauropod dinosaurs were laying their eggs in deep excavations dug in clastic sediments with grains larger than 1.7mm, or in shallower nests covered with decaying plant matter. After our discovery of eggs in shallow nests surrounded by a rim during our 2000 field season, it is now possible to narrow my original interpretation based on eggshell structure down to a shallow nest with a rim covered with plant matter as nesting strategy for these dinosaurs. Further research on the acicular (needle-like) rhombohedric crystalline structure of these eggshells allowed me to ascertain that this type of calcite crystals found in saurischian (sauropod and theropod dinosaurs) eggshells is a primitive character, these were the type of eggs laid by the common ancestors of these groups. This conclusion is further supported by our observations of eggshell formation in living birds.

I would like to acknowledge that the countless hours spent at the SEM, the preparation and presentation of my results would have not been possible without the financial support of the Infoquest Foundation, the Jurassic Foundation, USC Earth Sciences travel grants and Luis Chiappe.