Dinosaur extinction factor may have been how long eggs took to hatch

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A surprising factor in the extinction of the dinosaurs may have been how long their eggs took to hatch

Approximately 66 million years ago, a massive asteroid or comet smashed into the Earth near what we now think of as the Yucatan peninsula of Mexico. On the other side of the world, in India, at a place called the Deccan Traps, a period of intense volcanic eruption began — one that would last tens of thousands of years.

These catastrophic and powerful events are often considered the primary causes of the mass extinction event at the end of the Cretaceous period that wiped out most of the dinosaurs along with 75% of life on the globe.

But brand-new research reveals another factor that may have played a role in ending the era of the most massive creatures to ever walk the surface of the planet. It seems dinosaur eggs took a particularly long time to hatch. That means that when they had to compete for sparse resources in a post-extinction event world with the more efficient amphibians, reptiles, birds, and mammals that made it through that era into the next one, dinosaurs may have lost out.

Compared to reptiles, birds lay few eggs, and they are particularly large. This could hamper their competitiveness, since it exposes them to destructive risks. But bird eggs hatch about twice as fast as reptiles (their behavior keeps eggs warm and stable), which researchers think helps enough survive to hatch. Dinosaurs still exist in the form of birds — avian dinosaurs — and so researchers thought that the eggs of the non-avian varieties would still hatch at about the same fast rate as bird eggs do. After all, from what we can tell, non-avian dinosaur and bird eggs have similar structures and birds are the only remaining dinosaurs for us to base these hypotheses on.

But the new study, published January 2 in the journal Proceedings of the National Academy of Sciences, reports that dinosaur eggs took far longer to hatch. For one species studied, researchers estimate that a comparable bird egg would take between 40 and 82 days to hatch. The dinosaur egg, it seems, would have incubated between 83 and 171 days before it was ready, more like a reptile.

And that changes a lot of what we know about dinosaurs.

It's all about the teeth
The amount of time it takes for young to be born has a significant impact on how a species lives. It can define mating season, migratory behavior, and other characteristics.

Dinosaurs had large eggs and, in general, adults expended more energy than comparably sized reptiles or amphibians, putting a limit on their competitiveness.

By studying the growth of embryonic teeth in other species, researchers have been able to determine how long it took for the infants of those species to develop. So the team behind this study, consisting of researchers from Florida State, the University of Calgary, and the American Museum of Natural History, decided to try to calculate embryonic tooth growth in two dinosaur species, Hypacrosaurus stebingeri (a sort of "duck-billed" dinosaur) and Protoceratops andrewsi (a less-famous relative of the Triceratops).

The researchers saw that a certain measure that can be used to calculate embryonic tooth development in both human and crocodilian species exists in dinosaur species as well. So they evaluated fossil teeth from the above species.

Their calculations showed that the Protoceratops egg would have taken more than twice as long to incubate as a comparable bird egg, and would have been just a bit quicker to develop than a similar reptile. The Hypacrosaurus egg would have incubated even longer, needing more time than a similar reptile.

As the study authors write, this means that many hypotheses of dinosaur behavior may need to be re-evaluated. It was thought that perhaps these species made long migrations back and forth from the Arctic between seasons, but long egg incubation periods may have made this impossible. And while these new findings are just based on evaluations of fossils from two species of dinosaur, the authors say they think these long incubation periods would most likely be found in all toothed dinosaurs — though further research could always change that conclusion.

The other big effect this may have had is on the extinction of these creatures. We already believe dinosaurs expended more energy and needed more resources than reptiles or amphibians. They took a long time to mature, unlike many mammals and birds. When the resources of the world were devastated by a changed climate after the asteroid struck and during the period of volcanic activity, it became hard for any large species to survive. Slow hatching rates would have been just another blow to the non-avian dinosaurs. And that may help further explain why none made it through that time.

Some Dinosaur Eggs Took Six Months or More to Hatch

For decades now, the drumbeat of dinosaur news has been their similarity to birds. They were warmblooded! They had feathers! And they’re still around, because birds are actually dinosaurs.

All true, but those that were nonavian dinosaurs, as they are now called, were not all beak and tweet. They were closely related to other living reptiles like crocodiles, and new findings about how long their eggs took to hatch bring that point home.

Scientists reported on Monday that by using a new technique on exceedingly rare fossils of unhatched dinosaur embryos, they determined that those embryos took twice as long to hatch as bird eggs of a similar size. The embryo of a large duck-billed dinosaur took at least six months to hatch, and the eggs of larger dinosaurs may have taken even longer.

The long incubation times complicate thinking about dinosaur behavior. While some kinds of dinosaurs may have tended their eggs and young, for others the difficulty of hanging around for most of a year to watch buried eggs would have been too much. And long incubation times mean slow reproduction, a definite disadvantage when a comet or asteroid slams into the planet, as happened 65 million years ago, leading to the extinction of dinosaurs and many other species. But not birds.

Gregory M. Erickson of Florida State University, the lead author of the study, in Proceedings of the National Academy of Sciences, said the study was undertaken because fossil dinosaurs in the egg are so rare that “virtually nothing is known about their embryology.”

David J. Varricchio at Montana State University, who has studied fossilized dinosaur eggs and was one of the scientists who reviewed the paper for the journal, said the research took a new line of evidence — embryonic tooth age — and the technique could prove valuable in future studies.

Scientific opinions have varied on incubation times. Dr. Varricchio and other scientists had studied how porous fossilized eggshells were, which led them to conclude that the vast majority of dinosaur eggs were buried. And that behavior, similar to living reptiles, suggested long incubation times, he said.

But, Dr. Erickson said, most researchers thought that because dinosaurs were closely related to modern birds their incubation rates were birdlike.

Dr. Erickson used teeth from rare fossil embryos found in fossilized eggs that were about to hatch. He and his colleagues counted daily growth markers in the teeth, calculating that tooth growth accounted for about 40 percent of incubation time.

He worked with Mark A. Norell, the head of paleontology at the American Museum of Natural History, a co-author of the paper, to study a sample of teeth from 71- to 75-million-year-old embryos of Protoceratops, a sheep-sized dinosaur found in Mongolia. They came up with an incubation time of at least 83 days, almost three months.

Dr. Erickson said the information from embryo teeth was the first direct evidence of how many days nonavian dinosaurs took to hatch.

Darla K. Zelenitsky, a co-author and a paleontologist at the University of Calgary, gave him access to the embryo of an Hypacrosaurus, a duck-billed dinosaur that was about 30 feet long and laid eggs the size of soccer balls. The dinosaurs lived about 76 million years ago and laid their eggs in what is now Alberta, Canada. The incubation period for those eggs was at least 171 days, almost six months, the team concluded. David Ian Kay, a graduate student at Florida State, also contributed to the research.

Many dinosaurs were bigger than Hypacrosaurus, Dr. Erickson said, and because incubation increases with the size of the egg in both birds and reptiles, some eggs must have taken the greater part of a year just to hatch.

Such long incubation periods raise all sorts of questions, Dr. Erickson said. How could dinosaurs have migrated, as some have suggested, if they spent most of a year with their eggs?

“Do we expect them to have parental care?” Dr. Varricchio asked. He said that although there is evidence for parental care among some dinosaurs, those species had smaller eggs and most likely shorter incubation times. The very long incubation times would have meant dinosaur parents staying in one place for a whole year defending eggs and young.

Long incubation periods also meant that the dinosaurs had to pick nesting sites that would be protected for many months from floods, drought and predation.

They were also not reproducing as fast as other animals at the time of the mass extinction 65 million years ago, which may have contributed to their disappearance. Birds, for example, had already appeared and their incubation times were probably shorter.

The incubation times would have been only one strike against the dinosaurs surviving a planetwide catastrophe, however. Dr. Erickson said they had other disadvantages. “These animals were profligate wasters of energy,” he said. They were big and warmblooded and “even the smallest dinosaurs took over a year to mature,” including time after hatching.

“The dinosaurs found themselves holding some bad cards,” he said. “They had a dead man’s hand.”

Dinosaur 'Baby Teeth' Reveal That Dino Eggs Hatched Slowly

What to expect when you're expecting a baby dinosaur? Expect to wait.

That's the conclusion of a study by researchers at Florida State University who determined how long it took dinosaurs to hatch from their eggs by studying their teeth. Much like tree rings, teeth have growth lines called lines of von Ebner that can be used to estimate the age of an animal.

Researchers had expected dinosaurs might take the same time to hatch as bird, between a week and a half and three months. But in fact, they stayed in the shell far longer — between three and six months.

The leader of the study, Florida State University professor of anatomy and vertebrate paleontology Greg Erickson, says you can think of it like layers of paint. Every day, a liquid layer of dentine fills in the inner portion of the tooth and mineralizes, leaving distinct growth lines on the tooth that scientists can measure.

The researchers studied two types of dinosaurs, Protoceratops andrewsi and Hypacrosaurus stebingeri. These two dinosaurs produced eggs that range from the smallest known dinosaur eggs, to some of the largest. "The Hypacrosaurus had a four kilogram egg — imagine that as four times larger than the egg of an ostrich," says Erickson. "They look like volleyballs."

The eggs revealed that dinosaurs probably spent about three to six months inside the egg before hatching, depending on the size of the dinosaur.

The long incubation time of the eggs could have played a role in the extinction of dinosaurs after the K-Pg extinction event. "You can imagine after the asteroid hit all of a sudden the resources went to nothing," says Erickson "Even when they (dinosaurs) did reproduce, they had extremely long incubation periods on top of it." Unfortunately for the dinosaurs, animals that reproduce quickly are better equipped to adapt to challenges and are more likely to survive extinction events.

The biggest limitations to the study are the number of specimens the researchers were able to analyze. While dinosaur eggs are fairly common fossils, intact eggs containing a skeleton are very rare. Erickson and his colleagues hope to look at more dinosaurs, including carnivorous dinosaurs, to see if their speculations about incubation times are true for all types of dinosaurs.

The new research published in the journal PNAS on Monday.

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