Scientists Discover Earliest Dinosaurs Laid Soft-Shelled Eggs

This fossilized egg was laid by Mussaurus, a long-necked, plant-eating dinosaur that grew to 20 feet in length and lived between 227 and 208.5 million years ago in what is now Argentina. (Photo courtesy of research team)

(CN) — New research released Wednesday suggests that the earliest dinosaur eggs were soft and leathery instead of hard, a conclusion that challenges beliefs that have been widely held for years. 

Of the numerous groups that make up most vertebrates, such as birds and lizards, one of the largest are amniotes — creatures that lay eggs equipped with a special kind of membrane, or amnion, that helps to protect the embryo of the egg from drying out. 

Most amniotes are split into two groups: those that lay soft, leathery eggs like turtles and lizards, and those that lay hard eggs protected by well-calcified shells, such as birds.

It has been widely believed that hard-shell eggs were a tremendous evolutionary jump for amniotes, because they are better protected from the environment and potential predators compared to soft-shell eggs. This leap, researchers say, likely helped amniote animal species diversify and thrive throughout countless generations.

Hard-shell eggs do have a particular advantage over soft-shell eggs, however: They tend to last longer for researchers to study. Due to their more vulnerable state, soft-shell eggs do not typically survive long enough to become a part of the fossil record, presenting a steep challenge to scientists seeking to examine them.

This has resulted in some speculation among the scientific community regarding soft-shell and hard-shell eggs, and how they relate to dinosaurs. Researchers have long suspected that because our modern crocodilians and birds — the closest living thing to dinosaurs — lay hard-shell eggs, it follows that most non-flying dinosaurs did the same.

But a new research effort led by the American Museum of Natural History and Yale University, according to a study published Wednesday in the journal Nature, revealed that data taken from fossilized eggs show that the eggs had much in common with eggs laid by today’s turtles — a species that lays soft-shelled eggs.

Mark Norell, chair and Macaulay Curator in the museum’s division of paleontology and lead author of the study, said that the research effort began when experts noticed that the discoveries of dinosaur eggs in the past few decades were linked with only certain groups. 

“The assumption has always been that the ancestral dinosaur egg was hard-shelled,” Norell said with the release of the study. “Over the last 20 years, we’ve found dinosaur eggs around the world. But for the most part, they only represent three groups — theropod dinosaurs, which includes modern birds, advanced hadrosaurs like the duck-bill dinosaurs, and advanced sauropods, the long-necked dinosaurs.”

This led researchers, Norell said, to wonder what had happened to other types of dinosaur eggs. For example, the bones of ceratopsian dinosaurs, such as the triceratops, are frequently uncovered but researchers very rarely find their eggs.

“So why weren’t their eggs preserved? My guess — and what we ended up proving through this study–is that they were soft-shelled.”

Researchers made this discovery by studying the fossilized eggs left behind by two dinosaur species: the long-necked Mussaurus and the sheep-sized Protoceratops. 

Using advanced geochemical research methods, scientists were able to study the composition and biomineral makeup of the eggs’ embryos and found that a unique residue was inside the inner layer of the eggs. After comparing the characteristics of the dinosaur eggs with the eggs of today’s birds and reptiles, researchers were able to confidently conclude that the eggs — and those of other early dinosaurs — were soft and leathery.

Scientists suggest that early dinosaurs likely cared for their eggs much in the same way that many modern creatures do. For example, to protect their offspring from the elements, researchers say dinosaurs that laid soft-shell eggs likely buried them in soft soil or sand and kept them warm with decomposing plants, just like many turtles and reptiles. 

Another study, also released Wednesday in Nature, reveals that an ancient fossil discovered nearly ten years ago is actually a soft-shelled egg, one that possibly belonged to a massive and now-extinct sea lizard.

The egg, dubbed “The Thing” when it was first discovered in 2011 by Chilean scientists in Antarctica, has been housed for years at Chile’s National Museum of Natural History where it has sat without a label or proper identification.

That changed when a team of researchers from the University of Texas at Austin, led by UT Austin’s Jackson School of Geosciences postdoctoral researcher Lucas Legendre, examined the fossil and determined that it was an egg from roughly 66 million years ago.

Shaped somewhat like a deflated football and measuring at 11-by-7 inches, the egg is the second-largest egg of any known animal and the largest soft-shell egg ever discovered.

This revelation is significant for two potentially crucial reasons.

The Antarctic egg puts into question the preconceived notions researchers had on the growth limits of soft-shelled eggs. And researchers suspect that the egg was left behind by a giant sea creature. 

Researchers think that the egg was laid by a large marine reptile that was at least 20-feet long, not unlike the great and extinct sea creatures that belonged to the mosasaur family.

If this theory is correct, the fact that the sea creatures laid the egg in the first place is astounding, as scientists have generally believed that those creatures did not lay eggs at all. 

Researchers have not figured out exactly how creatures like this would have laid their eggs but suspect they maneuvered their long tails onto nearby beaches and laid them in the sand, all while keeping the bulk of their massive bodies submerged in water, or laid them in open water like some modern fish species do.

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