Smithsonian.com

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

About 90 million years ago, a flock of teenage Sinornithomimus got stuck in the mud. They didn’t make it out alive. At least 13 of the poor young dinosaurs perished and became preserved in this single bonebed, and a new painting by artist James Gurney offers a look into some of the last moments of these animals.

Gurney’s painting was commissioned by Scientific American as a supplement to their story “Dinosaur Death Trap,” and as detailed in a behind-the-scenes video, the artist put considerable effort into getting everything just right. Using wire outlines and a three-dimensional model, Gurney tried to make his Cretaceous snapshot as lifelike as possible. The results are amazing—dinosaurs brought back to life, just before the moment they perished.

The official 150th Anniversary Archaeopteryx Commemorative 10 Euro Silver Coin issued by the Federal Republic of Germany.

Over the past fifteen years, paleontologists have described more than twenty species of feathered dinosaurs. Even dinosaurs once thought to have dry, scaly skin, such as Velociraptor, have turned out to have feathers. But paleontologists have actually known of at least one feathered dinosaur since the mid-19th century. They just did not know to call it a dinosaur.

In 1861, the German paleontologist Hermann von Meyer described two remarkable fossils preserved in slabs of 150-million-year-old limestone. The first was a single feather—a sure sign that birds have been around for quite a long time—but the second was not as easy to interpret. A partial skeleton surrounded by feathers, the creature seemed to be almost equal parts reptile and bird. Since the skeleton had come from the same type of limestone quarry as the feather, though, von Meyer concluded that both fossils represented the same animal, and he applied the name he had given the feather to the skeleton. Together, these were the first recognized remains of Archaeopteryx lithographica.*

Archaeopteryx immediately became one of the most famous fossil creatures ever discovered. The trouble was that no one could agree on what it was or its relevance to the evolution of other animals. Richard Owen, who purchased the skeleton for what is now London’s Natural History Museum, thought that Archaeopteryx was the earliest known bird, whereas his rival Thomas Henry Huxley thought that it was an evolutionary dead end that did not tell naturalists much about how birds actually evolved. Even though many naturalists recognized that Archaeopteryx was important to questions about how birds evolved from reptiles, there was very little agreement about how that change occurred.

It has only been in the past few decades, with the confirmation that birds are just modified dinosaurs, that Archaeopteryx has been placed in its proper evolutionary context. Although now pre-dated by the feathered dinosaur Anchiornis, Archaeopteryx remains one of the oldest feathered dinosaurs known and is still central to questions about bird origins. (Whether it is actually the earliest bird, though, depends on how we define what a bird is, something that has become increasingly difficult as paleontologists have found more dinosaurs with bird-like characteristics.) The several specimens of Archaeopteryx now known are some of the most exquisite and most important fossils ever found, and so it is fitting that this feathered dinosaur gets a little extra attention for its big 150.

Over at Pick & Scalpel, paleontologist Larry Witmer reports that Germany will be issuing a special 10-Euro commemorative coin imprinted with the famous Berlin specimen of Archaeopteryx (which was discovered in 1877). These will be available on August 11th of this year, just a few days before the 150th anniversary of the first written mention of the fossil. Germany’s Humbolt Museum will also be opening a new exhibit called “Feathered flight—150 years of Archaeopteryx.” For now, that is all that is formally planned to celebrate Archaeopteryx, but Witmer promises that he’ll be adding photos to a Facebook Archaeopteryx gallery throughout the year, and I plan on writing a few posts about this famous fossil as we approach the big August anniversary.

*I say “first recognized” because an Archaeopteryx specimen was discovered in 1855 and misidentified as a small pterosaur by von Meyer in 1875. Its true identity was not discovered until John Ostrom reexamined it in 1970.

The dinosaur track Paravipus didactyloides as seen in a cast (A), a silhouette (B), and a photo of the original (C). From Mudroch et al., 2011.

A little more than a year ago, paleontologists working in Niger announced the discovery of Spinophorosaurus, a sauropod dinosaur with a wicked tail club. Its bones were not the only traces of dinosaurs to be found in the desert area. About three hundred feet from the exceptionally well preserved sauropod skeleton was a trackway containing more than 120 impressions from an unknown predatory dinosaur, and those tracks are the focus of a new study just published in PLoS One.

Described by Alexander Mudroch, Ute Richter and colleagues, the tracks were left by a dinosaur that walked on two toes and strode along the shore of a small lake or river between 176 and 161 million years ago. No body has been found, but since tracks receive their own distinct titles, the scientists have named these tracks Paravipus didactyloides. Their peculiar anatomy indicates that they were left by a raptor.

Among dinosaurs, members of only one group—the deinonychosaurs—are known to have supported themselves on two toes while having a retractable second toe that only barely touched the ground. Given the size of the tracks, the authors of the new study propose that the dinosaur that left them was about the size of Deinonychus from North America.

This is not the first time such tracks have been found. In 2008, a team of scientists described tracks made by a similar kind of dinosaur in the Early Cretaceous rock of China. Designated Dromaeopodus shandongensis, these tracks differed by having a distinct pad where the toe carrying the sickle claw touched the ground. The tracks from Niger have only a small impression in the same position, which indicates that this dinosaur lacked the additional foot pad.

While they can be difficult to interpret, the new tracks may also tell us something about the behavior of this yet-unknown dinosaur. There appear to be at least five different trackways, Mudroch and co-authors state, which were made at three different times. Two sets of early tracks were overlain by another pair of tracks of about the same size. This might indicate that two animals were moving together in one direction and then turned around, stepping on their own tracks. In fact, the pattern of one of these sets appears to indicate that one of the animals abruptly changed speed to avoid running into the other, and if this is true it is further evidence that some of the raptors were social.  Days to weeks after this pair left the area, another individual crossed their tracks and left its own behind. At the moment, though, it is difficult to reconstruct this scene without the discovery of bodies. With any luck, a skeleton of the dinosaur that left the Paravipus tracks will soon turn up.

References:

Li, R., Lockley, M., Makovicky, P., Matsukawa, M., Norell, M., Harris, J., & Liu, M. (2007). Behavioral and faunal implications of Early Cretaceous deinonychosaur trackways from China Naturwissenschaften, 95 (3), 185-191 DOI: 10.1007/s00114-007-0310-7

Mudroch, A., Richter, U., Joger, U., Kosma, R., Idé, O., & Maga, A. (2011). Didactyl Tracks of Paravian Theropods (Maniraptora) from the ?Middle Jurassic of Africa PLoS ONE, 6 (2) DOI: 10.1371/journal.pone.0014642

The skull of Parasaurolophus (a), showing the internal anatomy of the crest, and a cross section of the crest (b). From Weishampel, 1997.

It’s Valentine’s Day, and that means that millions of people will be riffling through their record and CD collections to find the right music to set the proper mood with their special someone. Seventy five million years ago, though, there was no Barry White, and so some deep-voiced dinosaurs made beautiful music together in their own way.

For decades, the crest of the hadrosaur Parasaurolophus puzzled scientists. Such a prominent ornament must have had a function, but what? There were almost as many opinions as there were scientists. Depending on who you asked, the crest was used as a weapon, a foliage deflector, a cranial air tank, or even as a snorkel.

But James Hopson had a different idea. In 1975, he hypothesized that the crests of hadrosaurs like Parasaurolophus were visual display structures that doubled as resonating chambers for vocal communication. (A notion that had also been suggested by Carl Wiman decades before.) The crests were signs of dinosaur sociality. The question was how to test these ideas, but in a landmark 1981 Paleobiology paper David Weishampel looked to the internal anatomy of hadrosaur skulls to see if they could have been using their skulls in the way Hopson had proposed.

Studied from an acoustical perspective, Weishampel found that the crest of Parasaurolophus truly was capable of acting as a resonating chamber for sound. In fact, the internal anatomy of the Parasaurolophus crest was very similar to a woodwind instrument called the crumhorn, and Weishampel proposed that adult Parasaurolophus communicated over long distances through low-frequency sounds. Though not included in this paper itself, Weishampel even created a model of a Parasaurolophus crest using PVC pipe, which sounded something like a tuba when played. Likewise, a recent study of the crested hadrosaurs Lambeosaurus, Corythosaurus and
Hypacrosaurus by David Evans and colleagues found that their nasal passages may have had similar sound-producing capabilities and that their ears were also suited to detecting low-frequency sounds. One can only imagine what an entire hadrosaur symphony—encompassing all the different crest shapes—might have sounded like.

YouTube video of Weishampel playing his hadrosaur horn:

Parasaurolophus did not sound throughout its lifetime, though. By comparing crest shape to the structure of the inner ear, Weishampel suggested that young individuals produced higher-frequency sounds—which traveled shorter distances—whereas adults could produce low-frequency honks that could be heard over much wider areas. (On the basis of potentially different crest shapes for males and females, he also suggested that the different sexes made slightly different sounds, but this difference has not been supported by additional evidence.) During mating season, one could imagine dozens of Parasaurolophus calling to each other, much like living alligators and crocodiles do today. The Late Cretaceous certainly would have been a very noisy place.

For more on dinosaur romance, see my recent Smithsonian article Everything You Wanted to Know About Dinosaur Sex.

References:

Evans, D., Ridgely, R., & Witmer, L. (2009). Endocranial Anatomy of Lambeosaurine Hadrosaurids (Dinosauria: Ornithischia): A Sensorineural Perspective on Cranial Crest Function The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology, 292 (9), 1315-1337 DOI: 10.1002/ar.20984

Hopson, J.A. (1975). The Evolution of Cranial Display Structures in Hadrosaurian Dinosaurs Paleobiology, 1 (1), 21-43

Vergne, A., Pritz, M., & Mathevon, N. (2009). Acoustic communication in crocodilians: from behaviour to brain Biological Reviews, 84 (3), 391-411 DOI: 10.1111/j.1469-185X.2009.00079.x

Weishampel, D.B. (1981). Analyses of Potential Vocalization in Lambeosaurine Dinosaurs (Reptilia: Ornithischia) Paleobiology, 7 (2), 252-261

Weishampel, D.B. (1997). Dinosaurian Cacophony Bioscience, 47 (3), 150-159