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A restoration of the Cretaceous snake Sanajeh about to gulp down a baby sauropod. Model by Tyler Keillor, photographed by Ximena Erickson.

In 1925, when Yale University paleontologist George Wieland published a paper titled “Dinosaur Extinction,” no one knew why the great archosaurs had disappeared. The fact that the extinction of the dinosaurs was even worth explaining was a new idea. From the time dinosaurs were initially described in the early 19th century through the beginning of the 20th, their existence and disappearance simply seemed to be part of a grand progression of life that required no special attention or explanation. Even when paleontologists began to puzzle over why the dinosaurs vanished, many thought that dinosaurs were inevitably doomed by strange, internal growth factors that made them so large, stupid and ornate that they could not possibly adapt to a changing world.

But Wieland took a slightly different view. While his paper was more opinion than science—there was nothing measured, quantified or tested in the article—Wieland believed that he had perhaps identified some of the “invisible influences” that triggered the demise of the dinosaurs. Egg-eaters were of primary concern.

Wieland was not the first to suggest that the destruction of dinosaur eggs led to the group’s extinction. As pointed out by Wieland himself, paleontologists Charles Immanuel Forsyth Major and Edward Drinker Cope had previously speculated that small mammals may have raided dinosaur nests so frequently that Triceratops and its Mesozoic ilk were incapable of reproducing successfully. This hypothesis seemed plausible in general, but Wieland disagreed about mammals being the primary culprits. Small Mesozoic mammals seemed too weak to break open tough dinosaur eggs, and the most voracious modern-day nest thieves seemed to be those reptiles capable of swallowing eggs whole. “The potent feeders on dinosaur eggs and young must be sought for amongst the dinosaurians themselves,” Wieland remarked, “and perchance, amongst the earliest varanids [monitor lizards] and boids [boa snakes].”

Wieland believed that egg-eating must have been rampant during the age of the dinosaurs. In fact, he thought that a diet of eggs may have even led to the evolution of some of the largest of all predatory dinosaurs. Considering the giant Tyrannosaurus, Wieland wrote, “What more likely than the immediate ancestors of this dinosaur got their first impulse toward gigantism on a diet of sauropod eggs, and that, aside from the varanids, the theropod dinosaurs were the great egg-eaters of all time?” The cruel irony of this idea was that the immense predatory dinosaurs also reproduced by laying eggs, and Wieland considered it “quite inferable” that their nests, in turn, would have been raided by smaller monitor lizards and snakes.

Dinosaurs were not entirely defenseless against such attacks. Though dinosaurs were often thought in the 1920s to be reptiles write large, Wieland speculated that dinosaurs would have provided some parental care, were probably more active than living lizards and crocodiles and, among the egg-eating varieties, may have even sought out unprotected nests in coordinated “droves.” “With such active and powerful beasts at the jungle-edge,” Wieland wrote, “life was varied and sanguinary, be it within scientific dignity to say so.” Unfortunately, an active and varied existence could not save the dinosaurs. Both ecological factors and the supposed inability of dinosaurs to change sealed the fate of the dinosaurs, Wieland concluded; the great loss of eggs and the “racial senility” of dinosaurs ultimately ushered the group into extinction.

When Wieland wrote his paper, he could only speculate about predation on dinosaur eggs and babies. In the decades since, however, paleontologists have turned up rare fossil evidence that small predators truly did snap up young dinosaurs in various stages of development. In 2010, paleontologists announced the discovery of Sanajeh, a late-Cretaceous snake that may have fed on the eggs of sauropod dinosaurs. Several years before that, a different team of paleontologists found several baby Psittacosaurus skeletons in the fossilized stomach contents of the opossum-sized mammal Repenomamus, and in a 1994 paper, paleontologist James Kirkland suggested that small crocodyliforms like the slender Fruitachampsa may have also gobbled up eggs and little dinosaurs since their bones are sometimes found in association with dinosaur nests.

Despite these recent discoveries and hypotheses, however, there is no indication whatsoever that dinosaurs were driven to extinction by egg-eaters, reptilian or otherwise. Perhaps such a view was tenable when only a few dinosaur genera were known and we understood very little about their ecology, but not now. We have a greatly revised understanding of what happened at the end of the Cretaceous—a mass extinction that wiped out not only the dinosaurs, but a vast swath of life forms on land and sea. There is no hint of a run on dinosaur eggs in the fossil record, and the various types of supposed egg predators lived alongside dinosaurs for millions of years without killing off their egg-laying food supply. Dinosaur eggs certainly were a nutritious resource that were undoubtedly exploited by carnivores and omnivores, but such depredations were not the keys to dinosaur extinction.

References:

Wieland, G. 1925. Dinosaur Extinction. American Naturalist. 59 (665): 557-565

The partial skeleton of an oviraptorid dinosaur brooding over a nest. From Clark et al., 1999.

What caused the end-Cretaceous mass extinction is one of the greatest mysteries of all time. Paleontologists have racked up a long list of victims—including the non-avian dinosaurs—and geologists have confirmed that a massive asteroid that struck the earth near the modern-day Yucatan peninsula was probably the extinction trigger, but just how that impact translated into a global extinction crisis is still being figured out. Of course, dinosaurs were the most charismatic creatures to perish during the event, and for years Sherman Silber has been forwarding his own peculiar proposal.

An infertility specialist at St. Luke’s Hospital in St. Louis, Missouri, Silber believes that dinosaurs died out because there were not enough females. In a 2004 paper written with David Miller and Jonathan Summers, Silber suggested that dinosaurs had a crocodile-like reproductive strategy in which temperature determined the sex of developing embryos inside their eggs. In American alligators, for instance, lower nest temperatures produce more females, whereas nests with higher temperatures produce more males. If dinosaurs development was also temperature-dependent, Silber and co-authors proposed, then the climatic changes created in the wake of the asteroid strike may have caused many more male dinosaurs to be born than females. Incapable of finding enough mates to ensure their survival, the dinosaurs were wiped out.

Of course, this idea was entirely based on inference. Most birds—the direct descendants of small, feathered dinosaurs—have their sex determined genetically, and there is no clear indication that the sex of developing dinosaurs was determined by variations in temperature. (In fact, the recent discovery that some sauropod dinosaurs repeatedly laid eggs in nurseries heated by geothermal activity would appear to argue against this point.) Furthermore, this hypothesis did not make sense of the fact that turtles and crocodylians—groups in which the sex of offspring is dependent on temperature—survived the extinction and all the subsequent temperature fluctuations the planet has undergone over the past 65 million years. Paleontologists panned Silber’s hypothesis, particularly since it had been previously proposed by other scientists and set aside, but news services ate up the story. “Fried eggs may have wiped out dinosaurs” announced Discovery News, and the BBC reported “Fewer females wiped out dinosaurs.”

Not to be discouraged, Silber has an article in-press with the Middle East Fertility Society Journal which retreads the argument he made in 2004. In it, Silber does not provide details about what scientists have learned about the reproductive habits of dinosaurs, what might have happened after the asteroid impact, or even the range of animals wiped out in the extinction event. Instead, Silber assumes that the sex of dinosaurs was temperature-dependent and that some sort of climate shift would have resulted in an over-abundance of males. As before, he provides no satisfying explanation as to why species with known temperature-dependent sexes survived, merely stating “Crocodilians and other TSD species (but not dinosaurs) survived because they could adapt successfully to the changing environment.” This statement tells us nothing about why these animals did not follow Silber’s predictions.

In fact, Silber was a co-author on another analysis which directly contradicted what he expected. Printed last year in Biology Letters with co-authors Jonathan Geisler and Minjin Bolortsetseg, the study looked at the survival of vertebrates found in Montana’s Hell Creek Formation, representing the very end of the Cretaceous just before the mass extinction, and those in the Tullock Formation, representing environments just after the catastrophe. Since sex determination in dinosaurs is unknown, they were left out, but the scientists found that the performance of Silber’s hypothesis among the other animals was “dismal.”

Of 32 extinctions and 30 survivals, the overwhelming majority of cases were inconsistent with what was expected under Silber’s hypothesis. In fact, most of the species with temperature-determined sex survived whereas 61 percent of species with genetically-determined sex went extinct, with small mammals being the hardest hit. The authors could not explain why this was the case—why turtles and crocodiles survived unscathed while so many other species perished remains an open question—but it was clear that Silber’s hypothesis failed. Being that he was listed as the first author on this paper, it is strange that he has returned to his original hypothesis in the forthcoming Middle East Fertility Society Journal paper.

Whether the sex of dinosaurs was determined by temperature or genetics remains unknown, but we cannot assume that they were just like alligators and crocodiles. After all, birds are modern dinosaurs and most have genetically determined sexes, and recent discoveries have shown dinosaurs to be extraordinarily bird-like. Perhaps dinosaurs were also like birds in having genetically determined sexes. Further research is required to figure this out.

Even then, though, reproduction is just one small part of the global extinction puzzle. Following the intense volcanic eruptions of India’s Deccan Traps, the asteroid impact 65.5 million years ago initiated intense ecological changes on a global scale. Scientists are still trying to find ways to detect how this major event forever changed life on earth.

References:

Clark, J.M., Norell, M.A., & Chiappe, L.M. (1999). An oviraptorid skeleton from the Late Cretaceous of Ukhaa Tolgod, Mongolia, preserved in an avianlike brooding position over an oviraptorid nest American Museum Novitates, 3265, 1-36

Miller, D. (2004). Environmental versus genetic sex determination: a possible factor in dinosaur extinction? Fertility and Sterility, 81 (4), 954-964 DOI: 10.1016/j.fertnstert.2003.09.051

Silber, S. (2011). Human male infertility, the Y chromosome, and dinosaur extinction Middle East Fertility Society Journal DOI: 10.1016/j.mefs.2011.01.001

Silber, S., Geisler, J., & Bolortsetseg, M. (2010). Unexpected resilience of species with temperature-dependent sex determination at the Cretaceous-Palaeogene boundary Biology Letters DOI: 10.1098/rsbl.2010.0882

One of many dinosaur eggs discovered at the Sanagasta site. From the Nature Communications paper.

Even though they grew to be some of the largest animals ever to walk the earth, sauropod dinosaurs started off small. From numerous nesting sites found all over the world it appears that gravid female sauropods, rather than putting all their effort into laying a few enormous eggs, created large nests of numerous, relatively small eggs. But why they selected particular nesting sites has long been a mystery. Now, in the journal Nature Communications, paleontologists Gerald Grellet-Tinner and Lucas Fiorelli provide evidence that nesting female sauropods picked at least one site based upon its natural heat.

In northwestern Argentina’s La Rioja Province lies a bed of white Cretaceous rock called the Los Llanos Formation. Within that formation, paleontologists have found numerous clutches of eggs at Sanagasta. The eggs are very similar to those of sauropod dinosaurs found elsewhere in Argentina, but the focus of the new study is not so much the eggs as the environment they were deposited in. In one particular area, designated sub-site E, the egg clutches are found dispersed three to ten feet away from geysers, vents, and other hydrothermal features which were active between 134 and 110 million years ago—that is, the eggs were laid in a naturally-heated nursery incubated between 140 and 212 degrees Fahrenheit. During the time the dinosaurs occupied this site, it must have looked somewhat reminiscent of some areas of Yellowstone National Park, but with sauropods wandering among the hot springs instead of elk and bison.

Although this is a wonderful discovery, the fact that these dinosaurs came back to the hydrothermally-active site again and again is not unusual. Some ground-nesting birds, such as the Polynesian megapode, seek out sites warmed by volcanic activity to create their nests, and so it seems that sauropod dinosaurs, too, were very selective about where they created their nests. With this in mind, paleontologists can take a closer look at other nesting sites around the world for clues as to why certain sites were “hot spots” for dinosaur nests.

For more on this discovery, see Not Exactly Rocket Science and Nature News.

Gerald Grellet-Tinner & Lucas E. Fiorelli (2010). A new Argentinean nesting site showing neosauropod dinosaur reproduction in a Cretaceous hydrothermal environment. Nature Communications, 1-8 : 10.1038/ncomms1031

A clutch of fossilized dinosaur eggs on display at the Royal Tyrrell Museum in Canada. From Flickr user Traumador.

Last month a group of nine students from Montana State University, Dawson Community College and Rocky Mountain College left for China to study dinosaur eggs, and they have been chronicling their experiences on the new blog MSU China Paleontology Expedition. About two weeks into their six-week stay, most of the posts so far cover “culture shock” (both on the part of the Montana students and the Chinese citizens they encounter), though some of the more recent posts have begun to share the scientific work being undertaken by the team. Among things the students have been studying: the sediment encasing some of the eggs to figure out what kind of environment they were preserved in, and I look forward to reading more updates from the students during the rest of their stay in China.

The remains of the fossil snake Sanajeh (left) associated with sauropod eggs (upper right) and a partial baby sauropod skeleton (lower right). From the PLoS Biology paper.

When discussing dinosaurs, the topic of what they ate often comes up, but what about the creatures that ate them? Obviously some dinosaurs ate other dinosaurs, but the famous prehistoric archosaurs were not immune to predation from other kinds of hunters, especially when the archosaurs were babies. In 2005, for example, paleontologists described a specimen of the 130-million-year-old mammal Repenomanus giganticus with the remains of baby dinosaurs preserved inside it, and now a new study in the journal PLoS Biology adds a prehistoric snake to the list of dinosaur predators.

About 67.5 million years ago, in what is now western India, sauropod dinosaurs laid nests of up to 12 eggs. Unlike their parents, the developing and newborn sauropods were not large enough to defend themselves from most predators, and so it is not surprising that the bones of the newly-described snake Sanajeh indicus seem to be common among the preserved dinosaur nests. At an estimated 3.5 meters long, this boa-like snake would certainly have been large enough to eat baby dinosaurs for breakfast, and one peculiar specimen suggests that it did so.

Even though there are many associations between sauropod nests and snake bones, one particular fossil appears to show a Sanajeh caught in the act of nest robbing. The remains of the snake are coiled around a crushed sauropod egg, with an additional two eggs and the partial skeleton of a baby sauropod found right next to it. By all appearances this snake died in a sauropod nest when a storm caused a mudslide, burying predator and prey together.

Interestingly, though, it appears that the snake did not simply swallow the eggs. According to the authors of the new study, Sanajeh had a relatively limited gape, so instead they propose that the snake crushed dinosaur eggs with its body and then ate the contents of the eggs. Baby sauropod dinosaurs probably outgrew the threat of predation from these snakes by time they were about one year old, the authors estimate, but in the egg and as newborns they were very vulnerable.

Jeffrey A. Wilson, Dhananjay M. Mohabey, Shanan E. Peters, Jason J. Head (2010). Predation upon Hatchling Dinosaurs by a New Snake
from the Late Cretaceous of India PLoS Biology, 8 (3) : 10.1371/journal.pbio.1000322