Did egg-sitting failure doom the dinosaurs?

<body leftmargin="0" bgcolor="#000060" text="#FFFFFF" link="#FFFF00" vlink="#FF9900"> <b> <p class="MsoNormal"><font face="Arial" size="5">Did egg-sitting failure doom the dinosaurs?</font></p> </b> <p class="MsoNormal"><font face="Arial" size="1">June 22, 2005<br> Special to World Science<br> <br> </font><font size="2" face="Arial"><b>In China and some other places over the past century, the ground has yielded up what some scientists say is a mystery: huge numbers of unhatched dinosaur eggs.</b></font> </p> <p class="MsoNormal"><font size="2" face="Arial"><b>The quantity of eggs “is abnormal,” said Hong-Yu Zhang of Shandong University of Technology, Zibo, China, in a recent email. For instance, there are far more dinosaur eggs than fossilized eggs of other reptiles, he added.<br> <br> Zhang and a group of colleagues say they have now found an explanation—in a new theory on why the dinosaurs died out. The theory holds that the dinosaurs couldn’t or didn’t keep their eggs warm enough when prolonged cold spells hit. <br> <br> This idea has been proposed before, Zhang added, but his theory also gives an account of why it would occur, based on chemistry. Moreover, Zhang’s team claims their proposal explains an additional puzzle: why many other animals, including non-dinosaur reptiles, survived the low temperatures.<br> <br> Many scientists today believe a global temperature plunge, possibly brought on by an asteroid hitting Earth, finished off the dinosaurs. The new theory incorporates this idea, but goes further. It proposes that dinosaurs were particularly sensitive to low temperatures partly because of the amounts and types of amino acids—a type of chemicals—that their bodies contained. <br> <br> The proposal took shape after the researchers found that reptiles alive today are very unusual among animals in the content of their amino acids. Zhang and three colleagues described the findings in a paper in the May 23 Web issue of the research journal Biochemical and Biophysical Research Communications.<br> <br> Amino acids are one of the most ubiquitous substances in all living creatures. They make up the molecules, called proteins, of which the bulk of our bodies are made. Twenty different amino acids account for most of the content of these molecules in all life forms.<br> <br> Proteins containing certain amino acids are more prone than others to bend or distort under high temperatures. This can cause animals to malfunction or die in excessive heat. Similarly, proteins consisting of other amino acids tend more than others do to effectively freeze or jam up in the cold, with similar results.<br> <br> According to Zhang's group, dinosaurs had an amino acid makeup that left them vulnerable to this second scenario. <br> <br> Modern reptiles, the researchers found, have a strikingly different amino acid content from every other class of animals: a much higher than normal concentration of the type that tends to distort proteins in the heat. This makes them somewhat vulnerable to excessive heat, but improves their cold-tolerance, because they have fewer of the amino acids that fail in the cold.<br> <br> The other living dinosaur relatives—birds—have roughly the same amino acid distribution as all the other non-reptile animals, the researchers said. (Most scientists believe birds descend directly from dinosaurs, which would mean that technically the dinosaurs didn’t completely die out).<br> <br> The amino acid findings suggest an explanation for the dinosaurs' demise, Zhang and colleagues continued. Dinosaurs, they reason, were most likely like the birds and other animals in their amino acid content. This would make them and almost all other animals relatively vulnerable to cold. <br> <br> But among all these animals, dinosaurs may been least able to keep their young warm, the researchers added. This is because unlike mammals, they didn't keep their young inside their bodies—they laid them as eggs. And unlike birds, they perhaps didn't incubate their eggs effectively.<br> <br> This may also explain why the living reptiles, the turtles, crocodiles and others, survived, Zhang and colleagues explained. Favorable mutations gave these dinosaur relatives an amino acid distribution that protected them against cold, so they survived, along with the birds, when the other dinosaurs died.<br> <br> In fact, according to Zhang’s group, almost every other class of animals had something to protect them from the cold. Fish and amphibians could lay their eggs in the water, which tends to be insulated against quick temperature swings, for example.<br> <br> If Zhang and colleagues are right, dinosaur eggs might have been alone in the cold. This is how the situation looks based on the thousands of dinosaur eggs tha have turned up in China’s Henan and Guangdong provinces alone, “which is incomprehensible” otherwise, Zhang insisted.<br> <br> His assertion that dinosaurs were uniquely vulnerable to the cold echoes a debate that has gone on for decades: whether dinosaurs were cold-blooded, or unable control their own temperature from within.<br> <br> The jury is still out on this. But the new findings suggest they were either cold-blooded or, in any case, not as good as birds are at maintaining proper body temperature, Zhang explained. Birds are warm-blooded.<br> <br> If dinosaurs incubated their eggs, their incubation was probably "not as efficient as that of birds," Zhang wrote in an email. This could not "help them survive the disaster."<br> <br> Another open question that would bear on Zhang’s theory is to what extent dinosaurs incubated their eggs, and how. <br> <br> Experts say the fossil record indicates at least some dinosaurs built nests, but that some dinosaurs must have been too big to sit on their eggs. These dinosaurs might have covered the eggs with vegetation or sand.<br> <br> Did feathers help birds to incubate their eggs effectively, whereas dinosaurs couldn’t? Zhang wrote that his field of expertise doesn’t cover that question, although “it seems reasonable that feathers [would] do good to birds to survive the disaster.”</b></font> </p> </body>