What the world's oldest eggs reveal about dinosaur evolution

March 14, 2019, University of Toronto Mississauga
What the world's oldest eggs reveal about dinosaur evolution
Illustration of Massospondylus eggs and young dinosaurs. Credit: Julius Csotonyi

A study of the world's earliest known dinosaur eggs reveals new information about the evolution of dinosaur reproduction.

An international team of researchers led by Robert Reisz of the Department of Biology at the University of Toronto Mississauga studied the fossilized remains of eggs and eggshells discovered at sites in Argentina, China and South Africa—widely separated regions of the supercontinent Pangea. At 195 million years old, they are the earliest known eggs in the , and they were all laid by a group of stem sauropods—long-necked herbivores that ranged in size from four to eight metres in length and were the most common and widely spread dinosaurs of their time.

Reisz is puzzled by the fact that "reptile and mammal precursors appear as skeletons in the fossil record starting 316 million years ago, yet we know nothing of their eggs and eggshells until 120 million years later. It's a great mystery that eggs suddenly show up at this point, but not earlier."

According to Koen Stein, a post-doctoral researcher at Universiteit Gent and lead author of the project, the eggs represent a significant step in the evolution of dinosaur reproduction. Spherical, and about the size of a goose egg, these dinosaur eggshells were paper-thin and brittle, much thinner than similar-sized eggs of living birds."We know that these early eggs had hard shells because during fossilization they cracked and broke, but the shell pieces retained their original curvature."

What the world's oldest eggs reveal about dinosaur evolution
Credit: University of Toronto Mississauga

Members of the team, including Edina Prondvai and Jean-Marc Baele, analyzed shell thickness, membrane, and distribution of pores, looking for clues about why these early eggs might have developed hard shells. The results of the study show that hard-shelled eggs evolved early in dinosaur evolution with thickening occurring independently in several groups, but a few million years later other reptiles also developed hard-shelled eggs. One possibility is that hard and eventually thicker shells may have evolved to shield fetal dinosaurs and other reptiles from predators. "The hard shells would protect the embryos from invertebrates that could burrow into the buried egg nests and destroy them," says Reisz.

Reisz adds that the study raises interesting questions for future investigation. "For example, we would like to understand why and their avian descendants never developed viviparity () and continued to rely on egg laying, while non dinosaurian reptiles and mammals, including ancient aquatic reptiles succeeded in evolving this more advanced reproductive strategy."

The study is co-authored by Koen Stein and Edina Prondvai (Universiteit Gent), Timothy Huang (Jilin University), Jean-Marc Baele (Université de Mons) and P. Martin Sander (Universität Bonn) and is published in the journal Scientific Reports. It follows up on earlier research by Reisz, published in 2012, that examined nests of Massospondylus embryos in discovered at nesting sites in South Africa, and a 2013 publication on dinosaur embryology in Lufengosaurus from China.

Explore further: Ancient dinosaur nursery oldest nesting site yet found

More information: Koen Stein et al. Structure and evolutionary implications of the earliest (Sinemurian, Early Jurassic) dinosaur eggs and eggshells, Scientific Reports (2019). DOI: 10.1038/s41598-019-40604-8

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FredJose
1 / 5 (9) Mar 15, 2019
, including ancient aquatic reptiles succeeded in evolving this more advanced reproductive strategy."

As if the evolutionary process is able to think, reason and design more advanced strategies just like an intelligent being. Please explain HOW it was able to figure out the more advanced strategy if :"it" doesn't exist as a sentient being, capable of perceiving problems and devising solutions to them.
"The fool said in his heart there is no God" Psalm 14:1
FredJose
1.4 / 5 (10) Mar 15, 2019
, including ancient aquatic reptiles succeeded in evolving this more advanced reproductive strategy."

As if the evolutionary process is able to think, reason and design more advanced strategies just like an intelligent being. Please explain HOW it was able to figure out the more advanced strategy if :"it" doesn't exist as a sentient being, capable of perceiving problems and devising solutions to them.
"The fool said in his heart there is no God" Psalm 14:1
torbjorn_b_g_larsson
5 / 5 (2) Mar 16, 2019
Have the effrontery to address others using risible magic incantations*, but not the energy to do a simple web search?

Here is how: https://en.wikipe...volution .

* Also, yes, we now know from observation that religion is the type of confidence trick that astrology is.
Pearlman_CTA
not rated yet Mar 18, 2019
per the Moshe Emes series and YeC framework for understanding science, it is the old chicken and egg q.
The One designer/creator aka G-d of Abraham created the proto-dino kinds mature / in full stature, not as eggs.
Then the gap between these created and the eggs was not millions of years but about 1k years max, perhaps just days, and strata represent placement and sediment layers all from the 1656 anno mundi 'Mabul' mass extinction impacts epoch aka global flood year by Noa(c)h.

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