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Will we ever find mating dinosaurs? No one knows, but the discovery of fossils showing dinosaur behavior—such as this nesting oviraptorosaur—hint that we may someday find dinosaur sex set in stone. Photo by Steve Starer, image from Wikipedia.

Earlier this month, I wrote a short article for Nature News about 47-million-year-old turtles that died at a very inopportune moment. Several pairs of prehistoric turtle were fossilized in the act of mating—the tragic consequence of sinking to the toxic depths of a prehistoric lake. An unfortunate fate for the reptiles, but a boon for the paleontologists who found the sexy fossils.

The discovery got me thinking about dinosaur sex. I’ve written quite a bit about the topic before—I ran a four-part series on what we know about dinosaur nooky earlier this year—but much of what we know about dinosaur reproduction only outlines the mating habits of Apatosaurus and company. There’s still a lot we don’t know. In fact, some of the most basic questions are the most persistent. What, exactly, “dinosaur style” looked like has been a subject of frequent speculation but very little rigorous research, and no dinosaurs have ever been found fossilized in the act to show us how it was done. But does this mean that we’ll never find dinosaur sex preserved in stone?

Copulation is typically a brief moment in time. For such an intimate snapshot to become part of the fossil record, exceptional circumstances are required. In the case of 320-million-year-old sharks preserved in what may be part of a mating ritual, a quick death and rapid burial in fine-grained sediment locked the fishy forms in rock. We also know a little about how prehistoric insects reproduced thanks to mating pairs trapped in amber. And as for the turtles, the copulating reptiles drifted down to a layer of water that not only killed them, but kept their bodies safe from scavengers as sediment settled on their bodies. For sex to make it into the fossil record, a quick death, rapid burial and high-definition preservation are all required.

Given these conditions, I’m not very hopeful that paleontologists are going to find mating dinosaurs. Even the smallest dinosaurs were too big to be trapped in amber, and as fully-terrestrial animals, dinosaurs did not copulate in the sort of aquatic environment where fast death and burial would have been possible. Dinosaurs just didn’t mate in the kind of habitats where there was a high potential for the amorous pairs to perish and be entombed in sediment. Good news for them, but frustrating for paleontologists.

Still, I shouldn’t be too hasty in saying that we’ll never find mating dinosaurs. I never expected that paleontologists would discover turtles caught in the act, for one thing. And the fossil record is full of surprises, including fossils that detail some aspects of dinosaur behavior. Paleontologists have previously discovered dinosaurs preserved in nesting and sleeping positions, and there’s the fighting dinosaur pair. Maybe someday a fortunate paleontologist will help us solve the prehistoric mating mystery by finding dinosaurs that made love, not war.

A pair of Tyrannosaurus restored in the act at Spain’s Jurassic Museum of Asturias. Photo by Mario Modesto, from Wikipedia.

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:

How did the biggest dinosaurs get it on?

Sex and Dinosaur Necks

Intimate Secrets of Dinosaur Lives

References:

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

A restoration of Lambeosaurus magnicristatus, a dinosaur once thought to represent the male form of Lambeosaurus lambei, but now known to be a distinct species. Art by Богданов, via Wikipedia.

Figuring out how dinosaurs mated is a frustrating task. There is relatively little that can be gleaned from the fossil record, and much of what paleontologists suspect about behavior and soft tissue anatomy comes from comparisons to birds (specialized, living dinosaurs) and crocodylians (the closest living relatives to the dinosauria). Even worse, exactly how to tell male and female dinosaurs apart from one another has puzzled scientists have decades. If we even can’t sort the females and the males, how can we accurately envision dinosaurian sex?

For a time, it seemed like the skeletal construction of dinosaurs might hold the answer. The clue paleontologists were looking for was sexual dimorphism. This is a difference between males and females of the same species as expressed in secondary characteristics—not the fiddly bits actually used during mating, but traits like size, bizarre ornamentation, coloration and similar features. Detecting such differences in dinosaurs requires a large sample of individuals of the same species which are about the same age and come from the same time and place (the more closely a paleontologist can approximate a population in a sample, the better). If such a group can be separated out into two distinct types—say, with one being larger than the other and with a larger crest—then there is a possibility that those two forms represent females and males.

Paleontologists have hypothesized sexual dimorphism for multiple dinosaur species, from Protoceratops to Tyrannosaurus. None of the proposed cases is especially well supported. What might seem to be a split between robust and gracile forms of a species—often taken as males and females, respectively—might actually represent different growth stages of the same dinosaur, different species of dinosaur, or individual variation in a small sample size.

The case of Lambeosaurus is a good example of the difficulties involved in distinguishing the dinosaur sexes. In 1975, paleontologist Peter Dodson undertook a review of the many, many species of hadrosaur described from the roughly 77-million-year-old strata of Alberta, Canada’s Oldman Formation. Paleontologists had named three genera and twelve species of crested hadrosaurs from this area, but after comparing the skulls of these dinosaurs, Dodson concluded that only the dinosaurs Corythosaurus casuarius, Lambeosaurus lambei and Lambeosaurus magnicristatus were present. More than that, Dodson proposed that he had discovered sexual dimorphism in each of these species, with the anatomy of these dinosaur’s crests being the primary way to tell females from males.

But paleontologists David Evans and Robert Reisz found a different pattern when they re-examined the sample of Lambeosaurus from Alberta. In the case of Lambeosaurus magnicristatus, in particular, a combination of a small sample size and an incomplete fossil had caused the confusion. Dodson included only two individuals of the hadrosaur species in the study, and since the crest of one individual was larger than that of the other, concluded that the two skulls represented the two sexes. As Evans and Reisz pointed out, the crest of the specimen Dodson regarded as female had been broken and so seemed superficially smaller. If the missing part had been in place, the difference between the two individuals would have disappeared.

Other paleontologists suggested different dimorphic schemes. James Hopson proposed that individuals grouped under the species Lambeosaurus lambei were females, and the species Lambeosaurus magnicristatus were males, and Kenneth Carpenter advocated a similar lumping. The problem with such a scenario is that the two species are not found at the same stratigraphic level. Evans and Reisz pointed out that Lambeosaurus lambei is found in greater numbers at a lower geologic level than the much rarer Lambeosaurus magnicristatus. The species did not overlap and so cannot represent different sexes of the same species.

Other attempts to set sex differences for dinosaurs have met similar frustrations. For a time, it was thought that male and female Tyrannosaurus could be distinguished on the basis of a tiny bone at the base of the tail. A row of small, spike-like bones called chevrons runs beneath much of the tail in dinosaurs, and it was thought that the first chevron in female Tyrannosaurus—the one closest to the hips—was reduced in size so that eggs could more easily pass out of the body. A similar observation had been reported before in crocodylians, and the fact that the trait seemed to be associated with larger specimens of Tyrannosaurus appeared to indicate that female tyrants were more robust than males of the same age. But this turned out to be a mistake. Crocodylian expert Gregory Erickson and colleagues discovered that the report on the reduced chevron in crocodylians was in error, and the fact that a complete chevron was found in the huge Tyrannosaurus “Sue” further eliminated the connection between the bone and sex identification.

As paleontologists Kevin Padian and Jack Horner pointed out in a Journal of Zoology paper published last year, sexual dimorphism “has never been conclusively established in dinosaurs.” Yet there is a way to identify at least one of the dinosaur sexes. The clues can’t be seen in the gross anatomy of skeletons or flashy ornaments, but in the structure of dinosaur bones.

In 2005, researchers Mary Schweitzer, Jennifer Wittmeyer and Jack Horner reported that they had found “gender-specific reproductive tissue” in a Tyrannosaurus specimen given the name “B-rex.” The specific type of tissue, called medullary bone, indicated that the particular dinosaur was female. Comparison to modern birds provided the key to this puzzle. Medullary tissue forms inside the long bones as a source of calcium when female birds are laying eggs. The same tissue is not naturally found in males. While there is no methodology to identify male dinosaurs in a similar way, the presence of medullary tissue inside dinosaur limb bones can be used to identify egg-laying females.

Paleontologists Andrew Lee and Sarah Werning ran with this finding to investigate how dinosaurs reached sexual maturity. Not only did Lee and Werning find medullary bone in two other dinosaurs—the ornithischian herbivore Tenontosaurus and the theropod Allosaurus—but, by combining these findings with evidence of dinosaur growth, they found that dinosaurs began reproducing when they were still actively growing. Tenontosaurus, Allosaurus and Tyrannosaurus had the dinosaurian equivalents of teen pregnancies, and this finding fit with the idea that dinosaurs lived fast and died young. Dinosaurs started having sex before they were skeletally mature, which corresponds to a lifestyle of rapid growth and a high likelihood of death before reaching maximum body size.

With any luck, future discoveries and studies of medullary bone will help us better understand when and how dinosaurs reproduced. Perhaps, paired with analyses of dinosaur skeletal anatomy, this peculiar type of bone may even help test ideas about sexual dimorphism in dinosaurs. If you can identify at least some female dinosaurs in a sample, you can then look to see if that subgroup contains any particular skeletal features that set them apart. The trouble is that medullary bone only works for identifying egg-laying females—males or females that are not reproducing cannot be distinguished this way. Still, the fact that paleontologists are able to pick out even a few female dinosaurs is a wonderful discovery that has the potential to show us previously unknown aspects of dinosaur biology. We are only just beginning to learn the more intimate secrets of dinosaur lives.

This post is the third in a short series of articles on dinosaur reproduction that will run through Valentine’s Day. Because nothing spells romance like dinosaur sex.

Previous Installments:

How did the biggest dinosaurs get it on?

Sex and Dinosaur Necks

References:

Dodson, P. 1975. Taxonomic implications of relative growth in lambeosaurine hadrosaurs. Systematic Zoology, 24 (1), 37-54

Erickson, G., Kristopher Lappin, A., & Larson, P. (2005). Androgynous rex – The utility of chevrons for determining the sex of crocodilians and non-avian dinosaurs Zoology, 108 (4), 277-286 DOI: 10.1016/j.zool.2005.08.001

Evans, D., & Reisz, R. (2007). Anatomy and Relationships of Lambeosaurus magnicristatus, a crested hadrosaurid dinosaur (Ornithischia) from the Dinosaur Park Formation, Alberta Journal of Vertebrate Paleontology, 27 (2), 373-393 DOI: 10.1671/0272-4634(2007)27[373:AAROLM]2.0.CO;2

Lee, A., & Werning, S. (2008). From the Cover: Sexual maturity in growing dinosaurs does not fit reptilian growth models Proceedings of the National Academy of Sciences, 105 (2), 582-587 DOI: 10.1073/pnas.0708903105

Padian, K., & Horner, J. (2011). The evolution of ‘bizarre structures’ in dinosaurs: biomechanics, sexual selection, social selection or species recognition? Journal of Zoology, 283 (1), 3-17 DOI: 10.1111/j.1469-7998.2010.00719.x

Schweitzer, M., Wittemeyer, J., Horner, J. (2005). Gender-Specific Reproductive Tissue in Ratites and Tyrannosaurus rex Science, 308 (5727), 1456-1460 DOI: 10.1126/science.1112158

The backside of Diplodocus, photographed at the Utah Field House of Natural History. Photo by the author.

In 1991, the American Museum of Natural History unveiled one of the most fantastic fossil displays ever created. Placed at the center of the renovated Theodore Roosevelt Rotunda, an adult Barosaurus rears back to protect its offspring from an oncoming Allosaurus. The defending sauropod’s head is 50 feet up in the air, although whether or not such an immense, long-necked dinosaur could have pulled off such a feat has been a continuing point of contention. Even in a typical posture, Barosaurus must have had a powerful heart to pump blood along its 25-foot neck, and who knows how hard the dinosaur’s heart would have to work to continue bloodflow to the animal’s head if it reared up? Some paleontologists consider such a feat physically impossible, but as paleontologist William Gallagher pointed out while teaching my Paleontology 101 class at Rutgers University, male Barosaurus had a good reason to rear up. How else would the huge dinosaurs have positioned themselves to mate?

Exactly how dinosaurs got it on has inspired no small amount of speculation. The largest dinosaurs of all, the sauropods, have been especially perplexing. We often say that these dinosaurs “shook the earth” with their footsteps, but did they also make the bed rock with their lovemaking? (I apologize for that joke, and will keep the geology puns to a minimum. Promise.) Paleontologist Beverly Halstead famously wondered about dinosaur sex in public lectures and articles, and he suggested that standard “dinosaur style” was for a male to come alongside the female and throw its leg over the female’s back as she lifted her rump into the air to move her tail out of the way. In the case of sauropods such as Diplodocus, Halstead even imagined that the amorous dinosaurs might intertwine their tails. While other paleontologists have considered the tail-twisting aspect unlikely—sauropod tails were balancing organs and were too stiff to intimately coil around each other—the basic dinosaur position Halstead promoted has remained a prominent possibility for the dinosaur kama sutra.

But not everyone agreed that giants such as Apatosaurus and Brachiosaurus were capable of such nupital acrobatics. These animals were so immense—Apatosaurus, not even the largest sauropod, is estimated to have weighed more than 23 tons—that some researchers thought the kind of positions Halstead was promoting would give the dinosaurs fractured legs and broken spines. At a symposium of vertebrate morphologists held at the University of Chicago in 1994, biologist Stuart Landry, Jr. gave a short presentation entitled “Love’s Labors Lost: Mating in Large Dinosaurs.” He did not see how sauropods could have mated on land. A large, rearing sauropod, he told his audience, “would have to support 10 to 20 tons in a precarious position two or three meters off the ground.” A male Apatosaurus would be liable to tip over and possible take the female with him. Instead, Landry suggested that the largest dinosaurs looked for muholes or bodies of water to buoy themselves up. When a conference attendee asked if he was proposing that all dinosaurs mated in water, Landry responded, “I would say the very large ones must have.” Of course, this hypothesis required a large number of Jurassic and Cretaceous hot tubs of just the right depth for sauropods to reproduce, and scientific models of sauropods have suggested that these dinosaurs were actually quite buoyant and unstable in water. Sauropods were diverse, disparate and widespread animals that roamed in terrestrial habitats all over the world—there’s no reason to presume that the largest dinosaurs had to seek out the nearest deep lake when they got the itch.

Biomechanics expert R. McNeill Alexander also considered the weight problem in his 1989 book Dynamics of Dinosaurs & Other Extinct Giants, but came to a different conclusion. Even though a male sauropod would have rested a great deal of weight on the back of the female during mating, Alexander pointed out that the stresses and strains would not have been any more severe than those caused while the female dinosaur was walking. (After all, walking requires shifts in weight as the dinosaur balances and goes through each step cycle, and so a dinosaur’s skeleton had to be strong enough to cope with these shifts.) “If dinosaurs were strong enough to walk they were strong enough to copulate,” Alexander wrote. “They were presumably strong enough to do both.”

Without living specimens to observe, we will never know all the intimate details of sauropod sex. Still, there are only a limited number of positions that could have worked for the dinosaurs. For reasons that I’ll write about later this week, the consensus among paleontologists is that male dinosaurs probably had relatively small penises. (Shhh! Don’t tell Tyrannosaurus. He’s already upset about all those “useless forelimbs” jokes.)  An amorous male would have to position his cloaca—the orifice used for both expelling waste and mating in crocodylians, birds and probably dinosaurs—right up to the cloaca of a female, and the female’s tail would have undoubtedly presented an obstacle. Rather than simply leaning straight against the top of a female like an elephant or rhinoceros does, a male sauropod would probably have to rear up at a relatively oblique angle, and the female would have to assist by moving her tail (which is also a way in which female dinosaurs could have exerted mate choice and confounded any hot-under-the-collar males they would rather not mate with). Perhaps some museum will look into the problem and try to mount a pair of coupling sauropod skeletons—much like the mating Tyrannosaurus at Spain’s Jurassic Museum of Asturias—but even then we are limited by what we can imagine. Whether we want to imagine a pair of Brachiosaurus in flagrante delicto is another matter altogether.

This post is the first in a short series of articles on dinosaur reproduction that will run through Valentine’s Day. Because nothing spells romance like dinosaur sex.

References:

Alexander, R. M. 1989. Dynamics of Dinosaurs & Other Extinct Giants. New York: Columbia University Press. pp. 57-58

Anderson, J. The Perplexing Puzzle of Maladroit Mating. Chicago Tribune. August 30, 1994.

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