An international team of scientists unearthed a remarkable diversity of fossil eggshells in Utah's Cedar Mountain Formation, challenging long-held assumptions about the prehistoric ecosystems of Cretaceous North America. The discoveries, made within the Mussentuchit Member of the formation, revealed that at least six different fossil egg species, known as ootaxa, existed there around 100 million years ago.
For 50 years, the region was known for just one type of fossil eggshell, scientifically referred to as Macroelongatoolithus carlylei, which is a large fossil egg type believed to have been laid by large oviraptorosaurs, a group of bird-like dinosaurs. However, recent efforts have dramatically expanded this record.
Led by the North Carolina Museum of Natural Sciences, the team collected over 4,000 fossil eggshell fragments from 20 locations within the Mussentuchit Member. These fragments, which looked like ordinary stones, were actually remnants of ancient eggshells.
The Mussentuchit Member is a sedimentary rock layer that is part of Utah's Cedar Mountain Formation, where the first fossilized egg from the Cenomanian age was described 50 years ago. Phys.org notes that this egg was previously thought to be the only representative from that period in North America.
Through detailed microscopic analyses, the researchers assessed shell thickness, internal microstructure, and pore systems. They used scanning electron microscopy to spot finer details of the eggshell fragments, according to IFLScience. The team categorized the fragments based on their external surface features, allowing them to distinguish between different ootaxa.
The findings confirmed that at least six different fossil egg species existed in the area. Three of the egg-laying species were laid by oviraptorosaur dinosaurs, with analysis suggesting at least two or three different sizes co-existing in the ecosystem. Two of the identified ootaxa belonged to Spheroolithus, which are eggs laid by ornithopods, a group of herbivorous dinosaurs. The final eggshell type was identified as Mycomorphoolithus kohringi, a crocodylomorph reptile. Several of these findings mark the first time that these eggshells have been identified in North America, including Mycomorphoolithus, previously only known from Europe.
"It is tricky based on eggshells alone. We are reliant on finding associated bones, shells, whole eggs, nests, and embryos together to make the best estimates," said Dr. Josh Hedge, Visiting Assistant Professor of Biology at Lake Forest College, according to IFLScience. "At this point, we have a pretty good idea of which types of eggshells were used by larger dinosaur groups, but it is certainly very difficult to assign them to individual species."
The study provides the earliest evidence of multiple oviraptorosaurs living in close range in North America. The findings support the idea that Cretaceous North American ecosystems were more complex than previously realized. The findings indicate a greater variety of nesting behaviors and species interactions during that period.
Shell fragments are useful indicators of biodiversity when the body fossil record is limited, and they were vital pieces of evidence for understanding the area's ancient history. Even though they are less dramatic than huge fossilized skulls, eggshell fragments are important for understanding ancient biodiversity, particularly when larger fossils are lacking.
The researchers attempted to categorize the eggshell fragments based on their features and data found at the dig site. Data found at the dig site included fossilized bones that might have been associated with the eggs and their shells. Ootaxa classify eggs based on their structural characteristics rather than directly linking them to specific species, allowing scientists to study the diversity of ancient life through eggshell remains.
The study by the researchers is published in the open-access journal PLOS ONE.
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