Large dinosaur egg accumulations and their significance for understanding nesting behaviour

By: L. Ezquerro, R. Coimbra, B. Bauluz, C. Núñez-Lahuerta, T. Román-Berdiel, M. Moreno-Azanza

Summarized by Angeles Quispe, a fourth-year undergraduate biology student at Binghamton University. She hopes to go into research after graduating college. When she isn’t studying biology, she enjoys going on late night drives, cooking, and going to the gym. 

What was the hypothesis being tested? This study uses egg accumulation sites (nests or clutches) in Paimogo, Portugal to help interpret nesting behaviors, reproductive behaviors, and incubation strategies from theropods in the Late Jurassic Period. These accumulation sites revealed that the eggs were secondary deposits, meaning the eggs were moved from their original locations and the accumulations of them were caused by external forces moving them. 

What data were used? Researchers used eggs and sedimentological data from the site in Paimogo from the Late Jurassic Period to analyze why they were a secondary deposit formation. The Paimogo site contained low meandering streams located near a shoreline that was prone to floods. One way that floodplains are identified in the geologic record is when coarser grained sediments settle at the bottom and finer grained sediments are deposited at the top. The sedimentary facies analysis (i.e. the study of distinct sedimentary characteristics) of this experiment focused on stratigraphic and depositional data in Paimogo. As a result, scientists were able to see how each strata settled and the conditions of soil during the late Jurassic Period. The alternating horizons of sediments proved that Paigomo was a floodplain for the egg accumulations. This meant that there were periodic shifts of energy events; energy levels of the environment were collected by using the deposition of sediment through transportation.  

Methods: The Paimogo study analyzed different stratigraphic profiles of eggs accumulation and the environmental conditions each layer formed. Data was collected from two locations and nearby outcrops to compare the sediment deposition patterns to understand the preservation at the site. The scientists mapped the location of the accumulations using eggshell remains found at outcrop sites. Furthermore, the study analyzed the size and orientation of grains seen in Figure 1, which lead to observations about sedimentary deposits and what was the cause of the orientations. The scientists also identified the types of soils present in the sites, which allowed them to understand more about the processes that lead to their formation.  

Results: The different units give insight to the conditions of the egg accumulations preserved in Paimogo. As seen in Figure 1b, the dinosaurs likely laid their eggs in nesting areas during the transition between soil layers (“Bw-Bwk”, meaning soil horizon B was weathered (Bw) and soil horizon B was weathered and weakly carbonate), which reflect the changes in wet (Bw) and dry (Bwk) periods. Scientists found red sediment and carbonate granules in the sites, which suggests good draining conditions at the time of the egg laying, while the appearance of mottling, also found at the site, indicates short periods of wet conditions. The researchers also found root casts, which are mineralized imprints left by plant roots in the soil, which provides evidence of oxidation-reduction (redox) conditions. The carbonate granules found would have formed in low wet conditions, while the roots would have grown in oxygenated conditions. This contrast resulted in reduced oxygen rates, due to the reaction of water and oxygen present. This dynamic environmental setting influenced the nesting behavior at this site. As a result, drier climates were preferred by the egg-laying dinosaurs. During the breeding season, the eggs were laid in mounds with vegetation during the drier season for stability and to avoid the risk of drowning the eggs, which can be seen in the transitions between the Bw-Bwk soil layers. When floods occurred during wet seasons, the water’s energy levels moved the eggs a short distance and were lightly buried. The mounds and vegetation structures prevented the eggs from becoming entirely buried. All of this evidence suggests that the eggs were a secondary deposit because of the energy exerted by the floods. 

Why is this study important? This study is important because it can give us a broader understanding of dinosaur nesting behavior. A major point seen in this study is the environmental factors that influenced egg-laying behavior. By nesting near finer grained sediments, the eggs could have a layer of protection from rougher energies and the newly born dinosaurs could easily excavate their way out. The light burial would have given an advantage to the eggs: a warm and humid environment to be incubated easier without being actively nested on by parents.Another important implication of this study could be the understanding of why dinosaurs wanted to reproduce in larger areas. This could reduce competition between nesting sites and other dinosaurs.

Broader Implications beyond this study: Now that we understand how these secondary deposits of eggs formed, understanding these theropod dinosaur reproduction behaviors can help us understand other dinosaurs’ nesting behaviors on a broader scale. Laying these eggs before the wetter seasons give us insight into why they were laid there in the first place. We can apply this knowledge to other species of dinosaurs who lived and reproduced in similar conditions.

Citation: L. Ezquerro, Coimbra, R., B. Bauluz, C. Núñez-Lahuerta, T. Román-Berdiel, & M. Moreno-Azanza. (2024). Large dinosaur egg accumulations and their significance for understanding nesting behaviour. Geoscience Frontiers, 15(5), 101872–101872. https://doi.org/10.1016/j.gsf.2024.101872