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It is not about the chicken and egg dilemma but about a much older mystery. ¿Which came first, soft shell dinosaur eggs or hard shell eggs? A team of Argentine and American paleontologists say they have an answer.
"When describing the clutches belonging to the speciesProtoceratops Mongolia andMussaurus from Argentina, we showed that these two dinosaurs laid eggs with a soft, membranous shell ”, says the paleontologistMatteo fabbri from Yale University and co-author of a paper published today in the magazineNature.
“This changes everything:soft eggshells They are found in lizards, snakes, early amniotes, and pterosaurs, groups considered much more primitive than dinosaurs in their biology. Our discovery shows how the first dinosaurs were more reptilian than we thought ”.
Knowing the evolution of eggs is important to understand the reproductive strategies and behaviors in thetetrapods, that is to say, all those animals with four legs. Modern birds have a wide variety of egg sizes, shapes, and colors. However, not much is known about its evolution.
"Eggs are the only window we have to the reproduction of dinosaurs," says the Argentine paleontologistDiego Pol of the Egidio Feruglio Paleontological Museum in the city of Trelew. “If we want to fully understand their evolution, we cannot ignore their reproductive strategies. They tell us about species survival strategies and how animals go to extraordinary lengths to provide the best chances for their young to survive. "
The results of the new research dispute the prevailing view that dinosaurs always laid hard-shelled eggs and shed some light on the reproductive behavior of early dinosaurs and the rearing of these animals, a subject still under debate.
The study of dinosaur eggs
Paleontologists know that a dinosaur bone is much more common in the field than an egg. However, in the last 30 years several have been found. Dinosaur eggs are known from all over the world and of many species. There are records of dinosaur eggspredators, of dinosaurslong neck and dinosaurs withDuck beak.
But mostly they correspond to Cretaceous period, between 145 and 66 million years ago, that is, to the last 80 million years of the history of the dinosaurs. And all these eggs show a rigid, calcitic eggshell.
The remains of eggs from the first80 million years in which these animals reigned, however, they are few on a world scale. "We basically know the last half of the movie and we're trying to figure out the whole story with what little information we have about the first half," says Pol, known for having found in 2013 the remains of the largest dinosaur ever known, thePatagotitan mayorum.
“That makes the fossilized eggs that we found in Santa Cruz, Argentina, so special that they are approximately 200 million years old. They are a key piece to understand the history of eggs and the reproduction of dinosaurs ”.
This CONICET researcher refers to around 80 eggs that he recently discovered with his team in a formation known asLaguna Colorada in the center of the province ofSanta Cruz, in thePatagonia Argentina.
These eggs belong to a species known since 1976 asMussaurus patagonicus, a primitive herbivorous dinosaur ancestor of thesauropods, that is, dinosaurs with long neck, four legs and large size. The site apparently housed a reproductive colony where these dinosaurs kept their young during their most vulnerable stage.
To study them in detail, Pol turned to the European Synchrotron Radiation Facility (ESRF) in France.
“Along with the eggs of another dinosaur from southern Africa, the eggs ofMussaurus in Santa Cruz they are the oldest known ”, says Argentine paleontologist Claudia A. Marsicano from the University of Buenos Aires, co-author of the research.
First signs of parental care
With the team of the American paleontologist and molecular geneticistMark Norell from the American Museum of Natural History, Pol, Fabbri and Marsicano analyzed these eggs—several of which preserve embryonic and juvenile specimens— next to the remains of an exceptionally preserved clutch of the speciesProtoceratops, discovered in the town of Ukhaa Tolgod in Mongolia. These animals grew to be as big as a sheep and were among the first dinosaursWith horns.
The international team of scientists examined the chemical composition of these fossils with a technique known as Raman spectroscopy. “When we look at the eggshell in cross section, we see acalcitic layer at the top and amembranous layer protein underneath, ”says Fabbri, who recently participated in the research of the first truly semi-aquatic dinosaur,Spinosaurus aegyptiacus.
“These two layers make up the eggshell in any reptile. However, the thickness between the membrane and the calcitic layer varies greatly between groups. It turns out that if the membrane is much thicker than the calcitic layer, then the shell is soft. If the calcitic layer is thicker, then the shell is hard. Our results showed that the first dinosaurs had a soft eggshell and that a hard eggshell evolved independently at three subsequent times. "
This finding has important repercussions. Due to the fact that soft eggshells are more susceptible to the exchange and loss of liquids with the outside, current animals with soft shell eggsthey bury them underground, usually in sandy sediment to limit fluid loss and allow successful incubation.
Also, soft-shelled eggs are usually laid in large numbers, as are somelizards and turtles: many eggs are laid at the same time, buried and then allowed to hatch "We could infer the same for the first dinosaurs," Fabbri speculates.
Sitting on their eggs
As Mark Norell points out: “As far as we know, all dinosaurs laid eggs. In some cases we know that, like modern birds, they had nests. We found a couple of remarkable fossils of animals sitting on top of their nests, just like chickens do on a farm today, so there was probably some form of parental care as well. "
This would also explain why so many fossilized dinosaur eggs from the first 80 million years of their history are not found on Earth. "Something striking has always been the imbalance between the diversity and quantity of records of corporeal dinosaur remains throughout Pangea from the end of the Triassic to the end of the Cretaceous and the record of eggs attributable to this group," warns Marsicano.
"We found a couple fossils of animals sitting on top of their nests, like chickens do on a farm, so there was probably some form of parental care as well," says Mark Norell
“This work shows that this imbalance in the record is due to the fact that dinosaurs in their origin and during much of their history in the Mesozoic putnon-mineralized shell eggs, a soft protein shell that is much more sensitive to degradation and therefore unlikely to become fossilized, ”continues Marsicano.
The acquisition of a shell like the one we see today in birds gives them an advantage when it comes to nesting, since the embryo is protected, and arose in the evolution of dinosaurs much later and independently in different lineages, according to the expert. This evolution from soft to hard shell, scientists think, represents a milestone in their evolutionary history, as it contributed to the reproductive success and therefore the spread and diversification of dinosaurs.
The reasons that led to these transformations, however, are still unknown. "It's one of the biggest questions this job opens up," says Pol.
“If several groups of dinosaurs evolved rigid shells separately then it is possible that there is some common cause, perhaps someenvironmental or climate change, which led everyone to follow this evolutionary path. There are many environmental factors that influence how the rigid shell of eggs forms in today's birds. I think that in the near future we will see various studies trying to address this question ”.
This evolution from soft to hard shell, scientists think, represents a milestone in their evolutionary history, as it contributed to reproductive success.
"All of the main branches that make up the groups of dinosaurs developed hard eggshells, so selection pressure must have been common to all of them," Fabbri speculates. "However, we cannot be sure."
What paleontologists do know is that in recent years there have been advances in the study of the chemistry of fossilization that could lead to more surprising discoveries. “They are revealing data that we did not imagine could be known,” says Pol. “Recently, chemical studies have shown thatsome dinosaur eggs were colored, like that of some birds, because they detected remains of the same pigments that are present in the eggs of bluish birds. Technological advances allow us today to detect these pigments, even when they were chemically altered in the fossilization millions of years ago ”.