When most of us think of dinosaurs, we imagine big, heavy beasts, but these giants shared their ecosystems with much smaller dinosaurs, whose smaller skeletons were generally less likely to be preserved.
The fossilized eggshells of these little dinosaurs can shed light on this lost ecological diversity.
Led by Tsukuba University, the researchers toured an exceptional fossil egg site first discovered in 2015 in Hyogo Prefecture, southwest Japan, and reported their findings in a new study published in Cretaceous investigation.
Found in a layer of red-brown mud from the Ohyamashimo Formation, deposited on an early Cretaceous River floodplain (about 110 million years old), the Kamitaki Egg Quarry was carefully and intensively excavated in the winter of 2019 , and threw more than 1,300 eggs. Fossils Most were isolated fragments, but there were some partial and almost complete eggs.
According to lead author Professor Kohei Tanaka, “Our taphonomic analysis indicated that the nest we found was in situ, not transported and redeposited, because most of the eggshell fragments were placed concave up, not concave down as we see when the eggshells are transported. “
Most of these fossil eggs belong to a new egg genus and species, called Himeoolithus murakamii, and are exceptionally small, with an estimated mass of 9.9 grams, about the size of a modern quail egg. However, the biological classification analysis implies that the eggs did not belong to early birds, but to their cousins, the non-avian theropod dinosaurs (the group that includes carnivores known as Tyrannosaurus and Velociraptor). That places Himeoolithus murakamii among the smallest non-avian theropod eggs reported to date. These small eggs were remarkably elongated in shape, unusual for equally small eggs among Cretaceous birds, but typical among larger non-avian theropod eggs.
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In addition to the abundant Himeoolithus murakamii eggshells, five more ootaxa (different types of egg fossils) were recognized in the town of Kamitaki. All of these ootaxa belonged to small non-avian theropods.
As Professor Tanaka explains, “The great diversity of these small theropod eggs makes this one of the most diverse Early Cretaceous egg locations known. Skeletal fossils of small theropods are quite rare in this area. Therefore, these fossil eggs provide a useful window into the hidden ecological diversity of dinosaurs in the early Cretaceous of southwest Japan, as well as the nesting behavior of small non-avian theropods. “
Header Image Credit: Tsukuba University and Museum of Nature and Human Activities
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