Scientists have studied the morphology and structure of Cretaceous-period dinosaur eggshells collected from the El Gallo Formation of Baja California, Mexico.
Dinosaurs were one of the most enigmatic and interesting species of animals that have existed on Earth. They lived in a variety of environments that ranged from forest to arid land, including the coast around the oceans.
Non-avian dinosaurs faced a period of extinction along the Earth, with the fossils found in different parts of the world being the only evidences of their existence.
Since the sensational discovery of dinosaur eggs in Gobi Desert in the early 1920s, the remains of fossil eggshells, eggs, and nests have been found in the Mesozoic deposits — mostly from the Late Cretaceous epoch — on all continents.
Just in North America alone, there exist at least 40 paleontological sites with fossilized dinosaur eggs and eggshells in Canada, the United States, and Mexico.
“Most of the works on the classification of fossil eggshells are based on their different characteristics such as the general morphology, pore shape, and thickness, but mainly the crystalline arrangement,” said lead author Dr. Abel Moreno from the Universidad Nacional Autónoma de México and colleagues.
“A few works focus on the element and the isotopic analysis that we carried out.”
“These were done to find information about the possible body temperature of some of the dinosaur groups, but above all, we wanted to infer their paleoenvironmental conditions.”
In the study, the researchers examined five dinosaur eggshells found in the coastal area of El Rosario, Baja California.
“The three of the five eggs were laid by ornithopods of the hadrosaur family (duck-billed dinosaurs) and one by a theropod of the troodontidae family (small, bird-like dinosaurs),” they said.
“The remaining sample was too damaged to classify by the naked eye.”
Using high-resolution scanning electron microscopy, the scientists analyzed the external and internal surfaces and a cross-section of each eggshell.
“In contrast to the smooth outer surface of the theropod shell, the shells from the ornithopods and the unknown sample had nodes at different distances across the shell,” they said.
“Images of shell cross-sections from the ornithopods revealed that mammillary cones — calcite crystals on the inner surface of the shell — formed thin, elongated columns arranged in parallel, with irregular pores.”
“In contrast, the eggshell from the theropod showed thicker, shorter cones arranged in a bilayer, with wider pores.”
The unknown sample more closely resembled the ornithopod eggshells, leading the authors to hypothesize that it was probably also from the hadrosaur family.
“In addition, we conducted an elemental composition analysis, which is the first such analysis on dinosaur eggshells collected in Mexico,” they said.
“Our findings might help reveal how the fossilization process varied among species and localities.”
The results were published in the journal ACS Omega.
Nerith R. Elejalde-Cadena et al. 2020. Searching for a Clue to Characterize a Crystalline Dinosaur’s Eggshell of Baja California, Mexico. ACS Omega 5 (40): 25936-25946; doi: 10.1021/acsomega.0c03334