October 23, 2012
The annual Society of Vertebrate Paleontology meeting is a test of endurance. The science comes fast and furious in presentations, posters, hallway conversations and shouted exchanges over the din of the bar, with no consideration for how dehydrated, weary or hungover you might be. (Paleontologists study hard and party harder.) By the last day, my brain ached with details of flying Microraptor, bounding crocodiles, marsupial bone microstructure and dozens of other topics. When my friends at the conference asked “What did you like best?” after the technical sessions finally concluded, I was only capable of grunts and indelicate gestures.
I’ve had a day to settle down and process what I saw. And I know this–at SVP, dinosaurs rule. This isn’t to say that the conference is all about the Mesozoic celebrities. I saw many excellent talks on prehistoric fish, mammals, amphibians and other forms of ancient life. But, for a dinosaur fan, SVP offers a glut of dinosaur science from new discoveries about the beloved Tyrannosaurus rex to brand-new species that have only just come out of the ground. Since this blog is called Dinosaur Tracking, I’m going to focus on some of the stand-out dinosaur science I saw during the meeting.
Montana State University graduate student Jade Simon’s presentation focused on giant Cretaceous dinosaur eggs found in Idaho, but the implications of the discovery were what really grabbed by attention. According to Simon and her collaborators, the pair of elongated, oblong eggs most closely match those found in the nests of oviraptorosaurs–beaked, feathered theropods like Citipati and eponymous Oviraptor. Yet the two eggs were so large that they suggested a dinosaur of prodigious size, on the scale of the 25-foot-long Gigantoraptor recently found in China. If Simon and coauthors are correct, then an enormous, as-yet-undiscovered oviraptorosaur strutted around Idaho around 100 million years ago. The next step–finding this fantastic creature’s bones.
Simon wasn’t the only researcher showing off dinosaur eggs. Just prior to her presentation, meeting attendees were treated to a pair of talks about dinosaur embryos found in the Late Jurassic rock of Portugal. These deposits are similar in age to those of the famous Morrison Formation of the American west and share many of the same types of dinosaurs. An embryo studied by Ricardo Araújo and coauthors appears to be a nascent Torvosaurus–a giant Jurassic carnivore that topped Allosaurus in bulk–and paleontologist Octávio Mateus followed with a skeletal embryo of Lourinhanosaurus, a mid-size theropod dinosaur found in the same formation. The embryo described by Mateus stood out because it was found by his parents–amateur paleontologists–in a nest of 100 eggs, including crocodile eggs mixed in with those of dinosaurs. Was this nest a communal site used by many mothers? The embryo and the nest it was found in will definitely help us better understand how some baby dinosaurs entered the world.
The SVP crowd also got treated to previews of various dinosaurs that are slowly making their way to press. Researcher Corwin Sullivan presented some scrappy evidence that a second giant tyrannosaur might have lived alongside the recently named Zhuchengtyrannus, and Nathan Smith showed off some new material from what may be two new species of sauropodomorph dinosaurs collected from Antarctica. Oliver Rauhut added to the list with a new theropod from Argentina that looks like a more archaic version of Allosaurus, and visitors to the poster session got to check out what might be a new species of Diabloceratops that Eric Lund and his colleagues have been working on. Most of the new dinosaur presentations followed the same format–where the fossils were found, how much of the skeleton was found, what sort of dinosaur the species is–but, in time, we should get fuller details of these dinosaurs in progress.
But not all the presentations at the conference were on new field discoveries. Increasingly, paleontologists are scanning, slicing and otherwise studying fossils in new ways, drawing ever more data about dinosaur biology from old bones. The first talk I walked into, by Eric Snively, reconstructed the neck musculature of Allosaurus for insights into the feeding behavior of this Jurassic hypercarnivore. As it turned out, Allosaurus probably had quite a strong neck and used this power to stabilized its flexed head while ripping flesh from prey–think of a giant, toothy falcon. In another session, Jason Bourke created virtual models to examine whether sauropod dinosaurs such as Camarasaurus and Diplodocus had their nasal openings on the tops of their heads–as was shown when I was a kid–or had nostrils further down the snout. The airflow models better fit the nose-at-end-of-snout model, although, as Bourke pointed out, there’s still quite a bit we don’t know about sauropod soft tissues.
Unsurprisingly, Tyrannosaurus got some love, too. Sara Burch reexamined the shoulders and forelimbs of old T. rex in an attempt to reconstruct the dinosaur’s musculature. Among other things, Burch found that the dinosaur’s arms underwent significant functional changes over time. The arms of the tyrant weren’t fading away, but modified for different uses than that of earlier relatives. What exactly the dinosaur was doing with its infamously small arms, though, we still don’t know.
Within the various new areas of research, though, dinosaur histology has been providing paleontologists with some of the most tantalizing details of prehistoric biology. My friend Carolyn Levitt presented her new research on the microstructure of Kosmoceratops and Utahceratops bones. These horned dinosaurs didn’t show any lines of arrested growth (LAGs) in their bones–rings thought to mark annual slowdowns in bone growth and often used to roughly age dinosaurs–while previously studied dinosaurs from more northern sites in North America do show these markers. This might mean that, like mammals, dinosaurs maintained high-running metabolisms but their growth was still influenced by environmental pressures, such as cold or dry seasons, in their surrounding environment. In a time of scarce resources, dinosaurs in highly seasonal habitats probably slowed their growth while those in lusher environments did not face the same pressures. Indeed, the dinosaurs with the most LAGs were the northernmost, while Utahceratops and Kosmoceratops were the southernmost sampled.
In a similar vein, a poster by Julie Reizner looked at the histology of the horned dinosaur Einiosaurus and what the microstructure details might say about the ceratopsid’s biology. The sampled dinosaurs, found in a rich bonebed, suggest that growth in Einiosaurus slowed at about three to five years of age, which might mean that these dinosaurs made a dash for reproductive maturity before their growth slowed. The fact that Reizner’s animals were predominately young and perished long before full skeletal maturity–or, in other words, still had some growing to do–is consistent with the idea that dinosaurs generally lived fast and died young.
And I would be remiss if I didn’t mention that there was an entire session devoted to Appalachia–a Late Cretaceous subcontinent formed when a shallow sea split North America in two, of which my former New Jersey home was a part. Paleontologists have made fascinating discoveries on the sister continent, Laramidia, but Appalachia has often been ignored given that we as yet knew little of the dinosaurs that lived there. Still, there is much to be learned by going back to the fragmentary and rare dinosaurs of that early eastern landmass. In addition to featuring Dryptosaurus, New Jersey’s fearsome tyrannosauroid, Stephen Brusatte reexamined the few remains of “Ornithomimus” antiquus. This ostrich-like dinosaur probably belonged to a different genus and was not as primitive as previously thought. Shortly after Brusatte’s talk, Matthew Vavrek spoke about dinosaurs found in the high Arctic of Appalachia. Hadrosaurs, deinonychosaurs, tyrannosaurs and others lived along the northwestern coast of the continent and may help use better understand the differences between Appalachia and Laramidia. The most frustrating aspect of all of this is that the eastern dinosaurs are so poorly known–we need more dinosaurs.
The findings I mention here are just a scattered sampling of SVP, based upon the talks and posters I personally encountered. With three sessions going at the same time, it was utterly impossible to see everything. (Please chime in about your own favorite presentations in the comments.) Nevertheless, it was amazing to see paleontologists showing off new finds and going back to fossil collections for new information. We’re learning more, at a faster rate, than ever before. As multiple experts said to me during this conference, it’s a great time to be a paleontologist. The SVP dinosaur sessions left no doubt of that, and I can hardly wait for next year.
Thankfully, many other paleontologists have been sharing their thoughts about the conference through the #2012SVP Twitter hashtag and on their blogs. For an outsider’s perspective on the conference, see Bora Zivkovic’s rundown of the meeting, as well as Victoria Arbour’s summary of SVP silliness. Out of everything, though, I think this year’s attendees will all remember the conference center’s whoopee cushion chairs–caught on video by Casey Holliday’s lab. I hope that next year’s conference in Los Angeles is just as exhausting, and just as fun.
July 20, 2012
In 1994, paleontologists made a discovery that turned one dinosaur’s name into an irony. That dinosaur was Oviraptor – the so-called “egg thief” discovered several decades before, but that turned out to be a caring mother.
The story starts in 1923. In that year, an expedition from the American Museum of Natural History discovered dinosaur eggs in the Cretaceous rock of Mongolia’s Gobi Desert. At the time, the paleontologists thought that the eggs had been laid by Protoceratops – a small horned dinosaur that commonly found in these deposits – but there was another dinosaur associated with one nest. The AMNH team also discovered the skull of a toothless theropod dinosaur on top of a clutch of eggs. When paleontologist Henry Fairfield Osborn described the dinosaur in 1924, he presumed that the theropod’s jaws were well-suited to crushing eggs, and that this dinosaur was killed in the act of robbing another dinosaur’s nest. Oviraptor seemed like a fitting name for the Cretaceous looter.
Only, that Oviraptor was probably brooding over the nest. In 1993, fieldwork at another Gobi Desert site uncovered similar eggs, and, fortuitously, same of the eggs preserved the delicate skeletons of near-term embryos. The most spectacular baby was the little skeleton of an Oviraptor-like dinosaur, curled up inside its egg. Even better, the shape of this egg matched the supposed Protoceratops eggs discovered years before. Osborn’s Oviraptor wasn’t stealing eggs, but watching over them, and this conclusion was sooner supported by beautiful skeletons of oviraptorosaur skeletons preserved on their nests, their arms spread to encompass the eggs.
But there was something else very curious about the embryo described by Mark Norell and colleagues in 1994. In the same nest, the paleontologists discovered the partial skulls of two little dromaeosaurids – sickle-clawed dinosaurs such as Velociraptor. These two tiny dinosaurs were either embryos or hatchlings, but why should they be preserved in the same nest with a totally different species?
Norell and co-authors suggested several possibilities. The baby dromaeosaurids could have been the prey of adult oviraptorosaurs, might have been trying to prey on oviraptorosaur eggs, or, after death, could have been transported a short distance into in oviraptorosaur nest. The most tantalizing possibility, though, is that one of the two dinosaur taxa was a nest parasite. Perhaps, when no one was looking, a mother Velociraptor – or similar dinosaur – added a few eggs to an oviraptorosaur’s nest, passing off her parenting duties. Then again, the scenario could have played out the other way around (although I would not envy a baby oviraptorosaur born into a family of vicious raptors).
Frustratingly, we may never know why these two species of dinosaurs were preserved together in the same nest. But I have to wonder if some non-avian dinosaurs were brood parasites. After all, some species of birds – the one lineage of living dinosaurs – sneak their eggs into the nests of other birds, so it’s not inconceivable that this behavior has much deeper, Mesozoic roots. Perhaps, as paleontologists continue to collect and study dinosaur eggs, someone will find more direct evidence of sneaky oviraptorosaurs, raptors, or other dinosaurs.
Norell MA, Clark JM, Demberelyin D, Rhinchen B, Chiappe LM, Davidson AR, McKenna MC, Altangerel P, & Novacek MJ (1994). A theropod dinosaur embryo and the affinities of the flaming cliffs dinosaur eggs. Science (New York, N.Y.), 266 (5186), 779-82 PMID: 17730398
April 18, 2012
How did dinosaurs come to rule the Mesozoic world? No one knows for sure, but the way dinosaurs reproduced probably had something to do with it. Dinosaurs grew fast, started mating before they hit skeletal maturity, and laid clutches of multiple eggs—a life history that may have allowed dinosaurs to rapidly proliferate and diversify. And egg laying itself may have been critical to why many dinosaurs were able to attain gigantic sizes. By laying clutches of small eggs, dinosaurs may have been able to sidestep biological constraints that have limited the size of mammals.
But there was a catch. Consider a large dinosaur, such as Diplodocus. Infant Diplodocus hatched out of eggs roughly the size of a large grapefruit, and if they were lucky, the dinosaurs grew to be more than 80 feet long as adults. And the little sauropods were not just small copies of adults. Like many other dinosaurs, individual Diplodocus changed drastically during their lives, and young dinosaurs may have preferred different habitats and food sources from those of more mature individuals. As outlined by Daryl Codron and co-authors in a new Biology Letters paper, this peculiar life history may have been a consequence of laying eggs.
Codron’s group created a virtual dinosaur assemblage to see how intensely dinosaurs might have competed with one another as they grew. If all dinosaurs started off relatively small, then the largest species had to pass through a series of size classes and change their ecological role as they matured. This ramped up the pressure on young dinosaurs. Juvenile dinosaurs had to contend with other juveniles as well as dinosaurs that topped out at smaller sizes. In a diverse Late Jurassic ecosystem, for example, young Allosaurus, Torvosaurus and Ceratosaurus not only had to compete with one another, but also with smaller carnivores like Ornitholestes, Coelurus, Marshosaurus and Stokesosaurus. Dinosaurs would have faced the most competition at small size classes, and this may have driven some dinosaur lineages to become large.
The new paper also suggests that dinosaur life history may have played a role in the demise of the non-avian species. Competition at smaller size classes, Codron and colleagues suggest, drove dinosaurs to become bigger and bigger, and this created a lack of species that were small at maturity. Mammals and avian dinosaurs occupied those niches. This could have made dinosaurs more vulnerable to the intense pressures of the end-Cretaceous extinction. If the catastrophe targeted large animals, but was less severe among small animals, then non-avian dinosaurs would have been doomed. The big dinosaurs disappeared, and there were no small non-avian dinosaurs left to quickly proliferate in the aftermath.
As John Hutchinson pointed out in a Nature news story about this research, however, we’re going to need a lot more testing to see if this hypothesis holds up. The conclusion is based on a virtual model of ecosystems that we can’t study directly, and mass extinctions are frustratingly complicated phenomena.
Of course, a new dinosaur extinction scenario is irresistible journalist bait. Various news sources picked up the extinction hook (promoted in the paper’s press release) and pointed to the fact that dinosaurs laid eggs as the seeds of their undoing. But this isn’t quite right. After all, turtles, crocodylians and birds all laid eggs, too, and they survived. And mammals did not survive the end-Cretaceous extinction unscathed—several mammalian lineages disappeared or took major hits during the catastrophe. Likewise, not all dinosaurs alive during the final days of the Cretaceous were huge. Titans like Tyrannosaurus, Triceratops and Edmontosaurus are the most famous end-Cretaceous dinosaurs, but in western North America alone, there were also relatively small ceratopians, oviraptorosaurs and troodontid dinosaurs that topped out at about six feet in length. Were these dinosaurs still too big to survive? Was the threshold even lower? If it was, then the reason why medium-sized animals such as crocodylians survived, and why some mammals disappeared, becomes even more complicated. Why non-avian dinosaurs perished, and why so many other lineages survived, remains a mystery.
Codron, D., Carbone, C., Muller, D., & Clauss, M. (2012). Ontogenetic niche shifts in dinosaurs influenced size, diversity and extinction in terrestrial vertebrates Biology Letters DOI: 10.1098/rsbl.2012.0240
April 6, 2012
Every kid knows how Easter eggs wind up in their yard. According to the canonical weirdness that is the holiday tradition, the Easter bunny delivers the colorful eggs overnight. But the origin of the eggs themselves is hardly ever mentioned. According to a well-timed press release from the University of Leicester, non-avian dinosaurs are the best candidates for some of the candy eggs hidden away on lawns.
There is some real science behind the silliness. In the latest issue of Palaeontology, researchers Nieves López-Martínez and Enric Vicens described a new type of dinosaur egg discovered in the Cretaceous strata of northeastern Spain. The roughly 70-million-year-old eggs, given the name Sankofa pyrenaica to distinguish them from other egg forms previously found, exhibited a strange combination of features.
Many non-avian dinosaur eggs are more or less symmetrical in shape, regardless of whether they are spherical or elongated. Bird eggs, such as those from hens, often have an asymmetrical, tear-drop shape in which one end of the egg is more pointed and the other more rounded. There is some overlap in form, though. Eggs attributed to the small, sickle-clawed dinosaur Troodon and to the beaked oviraptorosaur Citipati are both elongated and taper to one end, and so seem similar to bird eggs.
The profile of the newly described egg type is most similar to that of archaic birds. But, according to López-Martínez and Vicens, the microscopic structure of the Sankofa eggs shares more in common with those laid by non-avian dinosaurs such as Troodon than with birds. The Sankofa eggs exhibit a mix of characteristics seen in both non-avian dinosaurs and archaic birds. Without fossils of the chicks developing inside the eggs, or even associated bones of adult animals, exactly what sort of creature laid this egg is ambiguous.
As part of the study, López-Martínez and Vicens created a chart of non-avian and avian dinosaur egg shapes. And, while it didn’t resolve the true identity of Sankofa, University of Leicester paleontologist Mark Purnell employed the comparative diagram in an attempt to track what sorts of dinosaur—avian or otherwise—could have laid the colorful eggs offered in England’s shopping districts. While many treats had the traditional hen egg shape, Purnell found, at least one resembled the Sankofa egg shape. I have to wonder if other Easter egg variations fit within the non-avian dinosaur range. Perhaps an expedition to the supermarket is in order.
LÓPEZ-MARTÍNEZ, N., & VICENS, E. (2012). A new peculiar dinosaur egg, Sankofa pyrenaica oogen. nov. oosp. nov. from the Upper Cretaceous coastal deposits of the Aren Formation, south-central Pyrenees, Lleida, Catalonia, Spain Palaeontology, 55 (2), 325-339 DOI: 10.1111/j.1475-4983.2011.01114.x
February 14, 2012
Over the past few days I have written about the dinosaurian Kama Sutra, the idea that sauropods had sexy necks, and how to sex a Tyrannosaurus rex (Answer: very carefully). But there is one topic that I have saved for last: what the Tab A, Slot B reproductive anatomy of dinosaurs actually looked like.
Whenever I bring up dinosaur sex in conversation—which is probably far too often—questions about the anatomy of the dinosaurian penis arise almost immediately. I am not sure why this is. Maybe it’s because we expect such impressive, terrifying creatures to have equally scary gonads. Few things would be better nightmare fuel. Whatever the reason for this interest, though, the sad truth is that we don’t know very much about the reproductive organs of male dinosaurs. No one has yet found a fossilized impression or other vestige of a non-avian dinosaur’s penis, a discovery that would have a good shot at the cover of Nature or Science. Instead, restoring a dinosaur’s delicate bits requires some evolutionary context.
Male dinosaurs must have had the equipment for internal fertilization. This was a mode of reproduction passed on by their ancient ancestors. Around 375 million years ago, the first vertebrates with limbs, the early tetrapods, began to crawl along the water’s edge. These amphibious creatures had to stay wet to survive, and like their fish ancestors, they reproduced in the water. Females probably laid soft eggs in aquatic cradles and males squirted sperm over the egg clusters to fertilize them. By about 315 million years ago, however, the early radiation of amphibious vertebrates had produced a lineage of creatures capable of reproducing away from the water. These lizard-like animals, akin to Hylonomus from the Carboniferous strata of Nova Scotia, laid eggs that encompassed an internal pond surrounded by membranes and a tough outer shell. This was the amniotic egg—one of the most important evolutionary innovations of all time. But males could no longer fertilize eggs by excreting sperm over egg clusters in the water. Egg-laying on land required internal fertilization before the female deposited her eggs. All descendants of these creatures, from the dinosaurs to creatures that carry offspring inside the body (placental mammals like humans), continued this tradition.
A different set of evolutionary brackets is needed to narrow down what a dinosaurian penis might have looked like. Birds are living dinosaur descendants, and crocodylians are the closest living relatives to dinosaurs as a group, and so we can expect that features shared between birds and crocodylians were also present in dinosaurs. One such trait is a cloaca. This charming-sounding orifice, from the Latin word for “sewer”, is the common opening for the reproductive, urinary and intestinal tracts in birds and crocodylians of both sexes. Dinosaurs almost certainly had cloacae, too, and this means that the genitals of Stegosaurus, Deinonychus, Argentinosaurus and all other dinosaurs were hidden away internally. You wouldn’t be able to watch Allosaurus walk by and see anything swinging around.
And that brings us to the thrilling details of size and shape. The difficulty is that, according to a 2006 estimate by Steve Wang and Peter Dodson, there may have been more than 1,850 genera of dinosaurs during a span of more than 150 million years. Almost any generalization about dinosaur sex organs is going to be wrong in some respect, and looking for modern analogs is a complicated task. If we look to modern avian dinosaurs for hints, we are met with a bizarre array of reproductive organs and strategies. Males of most bird species don’t have a penis at all and pass genetic material to females through a brief encounter given the cringe-inducing term “cloacal kiss.” Then again, the Argentine lake duck Oxyura vittata has the longest penis in relation to body length of any known vertebrate, and ducks in general have become infamous for having bizarre sex organs that have a lock-and-key arrangement. In general, though, it seems that the presence of a penis in male birds is the ancestral state, and that the loss of a penis is an evolutionary specialization.
Things are not so varied on the other branch of our evolutionary bracket. Male crocodylians have relatively small penises. This condition, combined with the fact that a penis seems to be the archaic state for male birds, means that male dinosaurs probably had penises as well. As paleontologist Kenneth Carpenter colorfully described, “[A]ssuming you were stupid enough to sneak up under a T. rex and pull the cloaca open, the last thing you would ever see during the last moments of your life would be a penis if it was a male, probably similar to that seen in a crocodile.” The organ probably would have had a single head and a runnel along the top for sperm to travel down, as seen in the closet living relatives of dinosaurs.
We will probably never know the full range of dinosaurian penis variation. I doubt that such diverse and disparate creatures would have had a one-size-fits-all anatomy, although I also doubt the horrifying idea—which comes up often in internet comment threads—that male dinosaurs might have had long, prehensile organs which allowed them to inseminate at a distance. No matter what their gonads looked like, though, male dinosaurs probably had to get very close to their female partners during sex. There were only a limited number of positions which would have worked for dinosaurs.
But we actually know a little more about the reproductive anatomy of female dinosaurs than male dinosaurs. Dinosaur penis anatomy is constrained by what we know about the evolutionary relationships of dinosaurs and what we are willing to imagine, but a few significant fossils have given paleontologists a general idea of the female dinosaur reproductive tract. The most fantastic of them is a pelvis of an oviraptorosaur—one of the feather-covered, beaked dinosaurs that were relatively close cousins of dinosaurs like Velociraptor—with two eggs preserved inside. Described in 2005 by Tamaki Sato and colleagues, the hips show that the female oviraptorosaur had died just before laying those eggs. This fortuitous discovery illustrated that at least some dinosaurs had a mix of bird- and crocodylian-like reproductive features.
While female birds have only one oviduct—thought to be an adaptation related to becoming light enough to fly—the presence of two eggs in the dinosaur suggested the presence of two oviducts, as in crocodylians. But the fact that there were only two eggs indicated the dinosaur laid a small number of eggs at a time. Instead of producing a large clutch of eggs and laying them all at once, like a crocodylian, the dinosaur only laid two eggs each round and arranged those pairs around the nest. (Oviraptorosaurs have famously been found preserved on top of nests which seem to show a ring of paired eggs.) The female dinosaur did not have a reproductive system just like that of a bird or a crocodile, but a combination of traits seen in the modern lineages.
Other eggs hint that some of the largest dinosaurs might have been more crocodylian-like. No one has yet found a Diplodocus with eggs preserved in the hip region, but paleontologists have found numerous eggs referred to sauropod dinosaurs. Some of these show a pathological condition in which eggs are coated with a second shell layer. According to Kenneth Carpenter, there are two possible ways for this to happen. One possibility is that the egg stalled while going through the shell gland and received a second covering because of the delay. But the other explanation is that some dinosaurs might have produced a larger number of eggs relatively rapidly, and sometimes so many eggs filled the reproductive tract of a mother dinosaur prior to laying that an egg might be pushed back up the oviduct where it would be coated in another shell coating. This pathology is often seen among crocodylians and other reptiles, but is rarer among birds, and the idea that sauropods laid eggs in large clutches seems to fit the nests attributed to these dinosaurs. Dinosaurs like Brachiosaurus and Mamenchisaurus laid nests of multiple eggs which were relatively small compared to their body size, so it is possible that they deposited entire clutches, while smaller dinosaurs such as oviraptorosaurs could lay a limited number of eggs at a time.
There is much we don’t know about dinosaur sex. From possible positions to anatomy, mysteries abound. But the subject has moved beyond silly speculation. A better understanding of dinosaur evolutionary relationships has given paleontologists a framework from which to hypothesize about different aspects of dinosaur reproduction, and those ideas have been tested by discoveries in the fossil record. Future finds and analyses will undoubtedly flesh out some of the remaining unknowns. We are only just beginning to discover some of the most intimate secrets of dinosaur lives.
This is the final installment of the dinosaur sex series. For more, please see my Smithsonian article “Everything you wanted to know about dinosaur sex” and the previous entries in the series:
Brennan, P., Birkhead, T., Zyskowski, K., van der Waag, J., & Prum, R. (2008). Independent evolutionary reductions of the phallus in basal birds Journal of Avian Biology, 39 (5), 487-492 DOI: 10.1111/j.0908-8857.2008.04610.x
Brennan, P., Prum, R., McCracken, K., Sorenson, M., Wilson, R., & Birkhead, T. (2007). Coevolution of Male and Female Genital Morphology in Waterfowl PLoS ONE, 2 (5) DOI: 10.1371/journal.pone.0000418
Carpenter, K. 1999. Eggs, Nests, and Baby Dinosaurs. Bloomington: Indiana University Press. pp. 78-81
McCracken, K. (2000). The 20-cm Spiny Penis of the Argentine Lake Duck (Oxyura vittata) The Auk, 117 (3) DOI: 10.1642/0004-8038(2000)117[0820:TCSPOT]2.0.CO;2
Sato, T., Cheng, Y., Wu, X., Zelenitsky, D.K., Hsaiao, Y (2005). A Pair of Shelled Eggs Inside A Female Dinosaur Science, 308 (5720), 375-375 DOI: 10.1126/science.1110578
Wang, S., & Dodson, P. (2006). Estimating the diversity of dinosaurs Proceedings of the National Academy of Sciences, 103 (37), 13601-13605 DOI: 10.1073/pnas.0606028103