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Thanks to a row of huge bony plates, Stegosaurus remains one of the strangest dinosaurs ever found. Photo by the author at the Utah Field House of Natural History in Vernal, Utah.

Undoubtedly familiar to any dinosaur fan, Stegosaurus remains one of the strangest dinosaurs ever discovered. Even among others of its kind, the iconic Jurassic herbivore looks like an oddball. Many other stegosaur species sported long rows of spikes and short plates, but the flashy Stegosaurus had an alternating row of huge bony plates along its back and a relatively modest set of four tail spikes. How could such a strange arrangement of adornments have evolved?

From the arms of tyrannosaurs to the necks of sauropods and the armor of stegosaurs, bizarre dinosaur structures have frequently made paleontologists wonder “What was that for?” There had to be a reason for the deviations in form, and, paleontologists believe, the immediately recognizable plates on the back of Stegosaurus must have had some function. There has been no shortage of hypotheses. Off-the-wall ideas about flying stegosaurs aside, researchers have proposed that the plates along the spine of Stegosaurus protected the dinosaur from attack, were the Jurassic equivalent of solar panels or acted as sexy billboards to attract the attention of potential mates.

Although Stegosaurus certainly had much to fear from the contemporary Morrison Formation predators Allosaurus, Torvosaurus and Ceratosaurus, the dinosaur’s defensive weapons were its tail spikes (called a “thagomizer” by some). If Stegosaurus was anything like its spikier cousin Kentrosaurus, it could swing its tail with deadly force, and a damaged Allosaurus bone suggests that the “roof lizard” did just that. But the keratin-covered plates of Stegosaurus probably didn’t provide the herbivore with much additional protection. The immobile structures jutted upwards, leaving the dinosaur’s flanks exposed to attack. To call the plates “armor” isn’t quite right.

When I was a kid, though, Stegosaurus plates were more often said to help the dinosaur regulate its body temperature. Presuming that Stegosaurus was an ecothermic animal–that is, had a body temperature determined by the surrounding environment–the plates could have helped the dinosaur heat up by turning broadside in the morning and shed heat by turning toward the sun during midday. Using models of plates in wind tunnel experiments, paleontologist James Farlow and colleagues reported in 1976 that the plates could very well have been used to dissipate heat. This doesn’t mean that the plates evolved for that function, though.

In 2010, Farlow and coauthors followed up on the work by comparing the plates of Stegosaurus to the bony armor along the backs of modern crocodylians. While stegosaur plates might have played some passive role in regulating body temperature, they concluded, there was no indication that Stegosaurus plates evolved for that reason, or even were principally used as thermoregulatory equipment. (Not to mention the fact that we now know that dinosaurs were not lizard-like reptiles whose internal physiology was primarily dictated by the temperature outside.) If Stegosaurus plates made any difference in managing body temperature, it was a happy little quirk that rode along with the principal function of the plates.

At present, it appears that the impressive bony fins on the back of Stegosaurus evolved as display structures. A 2005 study by Russell Main and  collaborators, which focused on the microstructure of stegosaur plates, couldn’t find any evidence that the structures were used to radiate heat. Indeed, if stegosaurs truly required such radiators, it’s surprising that Stegosaurus seems unique in its plate arrangement–if plates were really used to regulate body temperature, you’d expect to see the same arrangement in many closely related species. Instead, much like the horns of ceratopsid dinosaurs, the plates and spikes of stegosaurs varied greatly between species. This suggests that visual display was driving the evolution of these structures. Being recognized as a member of a particular species, or displaying an individual’s maturity and vigor during mating season, probably drove the divergence in form among stegosaur ornaments. The question is whether stegosaur plates made any difference in the mating season or they simply served to help species recognize members of their own kind. That debate–over the sexiness of plates, spikes, horns, crests, sails and domes–is just heating up.

References:

Farlow, J., Thompson, C., Rosner, D. 1976. Plates of the dinosaur Stegosaurus: Forced convection heat loss fins? Science. 192,4244: 1123-1125

Farlow, J., Hayashi, S., Tattersall, G. 2010. Internal vascularity of the dermal plates of Stegosaurus (Ornithischia, Thyreophora). Swiss Journal of Geosciences. 103, 2: 173-185

Hayashi, S., Carpenter, K., Watabe, M., McWhinney, L. 2011. Ontogenetic histology of Stegosaurus plates and spikes. Palaeontology. 55, 1: 145-161

Main, R., de Ricqlès, A., Horner, J., Padian, K. 2005. The evolution and function of thyreophoran dinosaur scutes: implications for plate function in stegosaurs. Paleobiology. 31, 2: 291-314

Padian, K., Horner, J. 2010. The evolution of “bizarre structures” in dinosaurs: biomechanics, sexual selection, social selection, or species recognition? Journal of Zoology. 283,1: 3-17

A pair of Stegoceras on display at the Royal Tyrrell Museum, Alberta, Canada. Photo by Sebastian Bergmann, image from Wikipedia.

The history of pachycephalosaurs is mostly a story of domes. Even though some skeletons have been uncovered over the years, the most commonly-found part of these bipedal Cretaceous herbivores is the thickened, decorated skull. As a result, much of what we know about these dinosaurs comes from skull fragments, and this can sometimes seed confusion about which fossils represent new species and which are individuals of already-known dinosaurs.

Take the partial pachycephalosaur skull UCMP 130051, for example. In 1990, paleontologist Mark Goodwin described the skull–discovered in the Judith River Formation of Montana–as an adult of the previously-known dinosaur Stegoceras. The skull was large for a Stegoceras, and lacked the array of nodes commonly seen on the back shelf of the skull but was otherwise matched the anatomy of the common pachycephalosaur. But when paleontologist Robert Sullivan wrote a review of known Stegoceras material in 2003, he thought that UCMP 130051 was distinct enough that it belonged to a new kind of pachycephalosaur he named Hanssuesia sternbergi.

Now the story of UCMP 130051 has taken another turn. In the latest issue of the Journal of Vertebrate Paleontology, Ryan Schott and David Evans argue that the skull is really an adult Stegoceras after all. After reconstructing a Stegoceras growth series with juvenile and subadult specimens, Schott and Evans found that UCMP 130051 more closely resembled younger Stegoceras than other skulls Sullivan attributed to Hanssuesia. UCMP 130051 was just a bit bigger and lacked the nodes on the back of the skull that characterized younger individuals–the rest of the anatomy was “indistinguishable” from Stegoceras.

Exactly why UCMP 130051 was missing the set of bumps seen on younger Stegoceras fits into a wider debate about how much dinosaurs changed as they grew up. The “Toroceratops” controversy is the most prominent example, perhaps matched by the longer debate over “Nanotyrannus“, but pachycephalosaurs also form a facet of discussion. In 2009, Jack Horner and Mark Goodwin proposed that the dome-headed dinosaurs Dracorex and Stygimoloch were really just younger individuals of the contemporary dinosaur Pachycephalosaurus. This proposal required drastic changes to the dinosaur’s skull during its life, including forming a dome, growing long skull spikes, and then resorbing those spikes. The transformation must have been spectacular.

While not quite as drastic as in the transition from the spiky “Stygimoloch” form to adult Pachycephalosaurus, Schott and Evans found that Stegoceras probably went through similar changes. In their study, which focused on the ornamented squamosal bones at the back of the skull, younger individuals had prominent nodes that varied in size and shape. In UCMP 130051, though, those bumps were missing, indicating that they were resorbed when Stegoceras reached adulthood. And while they are tentative about this identification, Schott and Evans point out that some Stegoceras specimens–including UCMP 130051–appear to have resorption pits on the surface of the bone; an indicator that their skull ornaments were changing shape as they dinosaurs reached skeletal maturity. Stegoceras didn’t undergo the same back-and-forth horn growth suggested for Pachycephalosaurus, but the change in those little skull nodes hint that the dinosaur went through a more subdued change as it reached full size.

But the new study by Schott and Evans isn’t just about how young Stegoceras changed into adults. By reconstructing the dinosaur’s growth series, the paleontologists also discovered clues that may help paleontologists parse the ever-growing number of dinosaur species, as well as what all that crazy headgear was for. While young Stegoceras showed a high degree of variation in the shape and number of ornaments on their squamosal bones, for example, the dinosaur’s retained the same general “ornamental pattern” throughout their lives. This means that isolated squamosal bones can be useful in identifying pachycephalosaurs known only from partial skulls (and there are quite a few of them).

Of course, one of the biggest mysteries about pachycephalosaurs is why they had domes and spikes in the first place. Depending on who you ask, the ornaments were used to help the dinosaurs recognize members of their own kind, as sexual signals, as weapons or some combination of these. Schott and Evans prefer a mosaic approach to the problem. The fact that even the youngest Stegoceras specimens had recognizable, diagnostic ornaments on their squamosal bones, the researchers argue, indicates that these bumpy adornments probably acted as species recognition signals. They don’t seem to have any role in defense, and the fact that dinosaurs grew these signals before sexual maturity means that they probably weren’t advertisements for mates. If this is true, though, the question is why adult specimens would lose the display structures so late in life.

Then there’s the dome. Young Stegoceras, Schott and Evans point out, were relatively flat-headed. Thick domes developed as the dinosaurs grew up, and previous studies of Stegoceras skulls hinted that the rounded structures were capable of taking quite a shock. (Some pachycephalosaur fossils may even preserve damage from bouts gone awry.) Paleontologists are not agreed on this point, but it is possible that these dinosaurs really did butt heads. This idea, combined with the fact that domes grew as the dinosaurs approached reproductive and skeletal maturity, might mean that domes were sexual signals, and possibly even used in competitions to garner mates. Frustratingly, though, testing these ideas is extremely difficult. We can’t observe the animals themselves, and can only approach these aspects of their lives indirectly through the detail of fossilized bone. We know more about pachycephalosaurs than ever before, but the evolution of their bizarre features remains contentious.

Reference:

Schott, R., Evans, D. (2012). Squamosal ontogeny and variation in the pachycephalosaurian dinosaur Stegoceras validum Lambe, 1902, from the Dinosaur Park Formation, Alberta. Journal of Vertebrate Paleontology, 32 (4), 903-913 DOI: 10.1080/02724634.2012.679878

Styracosaurus at the American Museum of Natural History. From Brown and Schlaikjer, 1937.

One of my favorite dinosaurs at the American Museum of Natural History is the Styracosaurus. The insanely ornamented creature is presented as if swimming through a wave of plaster, a pose meant to depict the way the dinosaur was found in the field. It is a beautiful mount, but the restored and reconstructed skeleton obscures the fact that the actual specimen is not so complete.

Veteran fossil hunter Barnum Brown discovered the Styracosaurus in 1915. He found the fossil within what is now Canada’s Dinosaur Provincial Park. Most of the dinosaur’s post-cranial skeleton was intact, but as Brown later noted in a 1937 paper he wrote with colleague Erich Schlaikjer, only a few parts of the skull were recovered. That lovely skull on the AMNH mount was mostly reconstructed on the hypothesis that the animal was really a Styracosaurus. Brown believed that the few parts which were collected were enough to name a distinct species of this dinosaur: Styracosaurus parksi.

Brown’s new species was the third flavor of Styracosaurus to be named. Paleontologist Lawrence Lambe named the first species, Styracosaurus albertensis, in 1913, and Charles Gilmore followed with Styracosaurus ovatus in 1930. Both were very spiky dinosaurs distinguished by the prominent spikes jutting out of the parietal bones on their frills. But Brown considered his dinosaur to be a separate species on the basis of slight differences in the few skull elements he had collected. The squamosal bone—another frill element—seemed to be longer and different in shape than the animal Lambe had named Styracosaurus albertensis.

Early 20th century paleontologists had a tendency to over-split dinosaurs on the basis of very slight differences. Naming a new genus or species was easy to justify during the early bone rushes. There were so few specimens, and researchers understood so little about how dinosaurs grew up, that variations among individuals or differences attributable to age were often taken as the hallmarks of distinct species. And traits thought to distinguish between dinosaur genera turned out to be less diagnostic than originally thought. Styracosaurus once seemed to be unique in having spiky parietals, for example, but similar features have since been found in closely related centrosaurine dinosaurs such as Achelousaurus, Einiosaurus, Centrosaurus brinkmani, Pachyrhinosaurus and, the new kid on the block, Spinops. In order to sort out Styracosaurus, in 2007 paleontologists Michael Ryan, Robert Holmes and A.P. Russell reviewed the material attributed to this dinosaur.

Ryan, Holmes and Russell counted only two Styracosaurus species as valid: S. albertensis and S. ovatus. Brown’s specimen, while incomplete, fell within the variation documented for S. albertensis, and so S. parksi was sunk. And at the genus level, Ryan and co-authors distinguished Styracosaurus from similar dinosaurs by the anatomy of the ornaments at each slot on the parietal part of the frill. The first ornament is typically a tiny nub, the second either appears as a small tab or hook, the third is a large spike and the fourth is also a large spike.  (The remaining ornaments at positions five through seven vary in size and shape between individuals.)

A restoration of Rubeosaurus ovatus by Lukas Panzarin. Image from Wikipedia.

But the Styracosaurus genus was recently winnowed down even further. Most Styracosaurus specimens belonged to the northern species S. albertensis, but the species S. ovatus was represented by a single specimen found in Montana. This significantly extended the range of Styracosaurus, at least until paleontologists Andrew McDonald and Jack Horner suggested in 2010 that the Montana dinosaur really represented a different genus. On the basis of the partial frill and other skull fragments, they named the dinosaur Rubeosaurus. It was another weird horned dinosaur with a huge nasal horn, and the third parietal horns were directed inward, towards each other, rather than outward like in Styracosaurus. Within just a few years, three species of Styracosaurus were cut down to just one.

References:

Brown, B., Schlaikjer, E. 1937. The skeleton of Styracosaurus with the description of a new species. American Museum Novitates. 955, 1-12

Andrew T. McDonald & John R. Horner, (2010). “New Material of “Styracosaurus” ovatus from the Two Medicine Formation of Montana”. Pages 156–168 in: Michael J. Ryan, Brenda J. Chinnery-Allgeier, and David A. Eberth (eds), New Perspectives on Horned Dinosaurs: The Royal Tyrrell Museum Ceratopsian Symposium, Indiana University Press, Bloomington and Indianapolis, IN.

Ryan, M., Holmes, R., Russell, A. (2007). A revision of the late campanian centrosaurine ceratopsid genus Styracosaurus from the Western Interior of North America
Journal of Vertebrate Paleontology, 27 (4), 944-962 DOI: 10.1671/0272-4634(2007)27[944:AROTLC]2.0.CO;2

Triceratops (left) and Torosaurus (right). Art by Nicholas Longrich.

More than 120 years ago, the Yale paleontologist Othniel Charles Marsh described two of the most spectacular horned dinosaurs of all time. The first, named Triceratops in 1889, had three impressive horns jutting out of its face and a solid, curved frill. Two years later, Marsh named Torosaurus, another great, three-horned dinosaur, but with a longer frill perforated by two round holes. Although the two overlapped in space and time, they seemed distinct enough that paleontologists considered them to be separate dinosaur genera. That is, until Museum of the Rockies paleontologists John Scannella and Jack Horner suggested that these two dinosaurs were really one in the same.

Scannella and Horner presented their “Toroceratops” hypothesis at the 2009 Society of Vertebrate Paleontology meeting in Bristol, England, and the following summer their paper came out. Based on skull anatomy, bone microstructure and other lines of evidence, the paleontologists proposed that Marsh’s Torosaurus was really the skeletally mature form of Triceratops. As Triceratops grew, the dinosaur’s frill would have changed size and shape, and those trademark Torosaurus holes would have opened up. An enigmatic fossil named Nedoceratops seemed to show this intermediate anatomy and was cited by Scannella and Horner as a dinosaur caught in the act of changing. Poor reporting on the research sent the public into a tizzy—Triceratops fans wept, wailed and gnashed their teeth at the suggestion that paleontologists were taking away one of their favorite dinosaurs, but only those with an affinity for Torosaurus had anything to fear. Since Triceratops was named first, the name had priority and Torosaurus would therefore be sunk. (No one seemed to care a whit that poor, neglected Nedoceratops would suffer the same fate.)

But should we sink Torosaurus? In the two years since Scannella and Horner’s paper came out, paleontologists have gone back and forth about whether such a radical, late-life transformation in Triceratops was even possible. Early last year, ceratopsian expert Andrew Farke of the Raymond M. Alf Museum of Paleontology criticized the Triceratops transformation hypothesis and pointed out that Nedoceratops did not actually fit neatly into the sequence of changes Scannella and Horner had proposed. Naturally, the Museum of the Rockies paleontologists disagreed, and in a response published in December of 2011, Scannella and Horner reaffirmed the relevance of Nedoceratops to the extreme changes Triceratops might have undergone as it grew up.

Now another set of challengers has appeared. In a paper published last night in PLoS One, Yale University paleontologists Nicholas Longrich and Daniel Field concluded that Triceratops and Torosaurus truly were distinct dinosaurs, after all.

Most of what we know about Triceratops and Torosaurus has been extracted from skulls. Post-cranial skeletons are rare and, in the case of Torosaurus, incompletely known, and so the current argument is centered on how the skulls of these horned dinosaurs changed. In the new study, Longrich and Field coded twenty four different characteristics—relating to bone surface texture, fusion between skull bones, and other features—in a swath of Triceratops and Torosaurus skulls. The paleontologists then used this data to sort the different specimens into growth stages based on their cranial development. If Torosaurus truly represented the mature form of Triceratops, then all the Torosaurus should have come out as adults.

The skulls of Torosaurus YPM 1831 and Triceratops YPM 1822 compared. Image courtesy Nicholas Longrich.

Of the six Torosaurus examined, five fell into a range between young and old adults. But there was one particularly large individual that seemed to be significantly younger. When Andrew Farke issued his critique of the “Toroceratops” hypothesis last year, he noted that a skull designated YPM 1831 was a possible candidate for a young Torosaurus. The paper by Longrich and Field supported this idea—YPM 1831 grouped with the subadult dinosaurs. “It’s a little surprising considering how damn big the skull is—probably about nine feet long—but it’s not fully mature,” Longrich said. “It’s like a teenager,” he noted, “a physically big animal but not all that mature yet.” The development of ornaments on the skull, the fact that some bones are not fused, and a bone texture associated with rapidly growing bone are possible signs that this dinosaur was not yet an adult.

If YPM 1831 really was a subadult Torosaurus, then it is probable that Triceratops and Torosaurus were distinct dinosaurs. Indeed, if Torosaurus truly was the fully mature form of Triceratops, then we should not find any juvenile or subadult Torosaurus specimens. “[B]oth Torosaurus and Triceratops,” Longrich and Field concluded, “span a range of ontogenetic stages,” and the features which distinguished each dinosaur appear to have developed before full maturity.

But Scannella disagrees. “Nothing in this paper falsifies the synonymy of ‘Torosaurus‘ and Triceratops,” he says. In particular, Scannella notes that the new study relies on comparative anatomical techniques, but does not employ studies of dinosaur bone microstructure which shows how individual skull bones were changing. Scannella explained:

Comparative morphology is useful in examining dinosaur ontogeny, however it shouldn’t be considered in a vacuum. There are other factors which provide a wealth of information on dinosaur growth. For example, by examining histology, the microstructure of the bones, we can actually see how the thick, solid frill of Triceratops expanded, became thinner, and developed the characteristic holes of the ‘Torosaurus‘ morph. You can look at a Triceratops squamosal under a microscope and see how it was transforming. We are also finding that the stratigraphic position of specimens is critical to understanding morphological trends.

Other subtle skull modifications are also in contention, such as how fusion between bones in the skull relates to maturity. Among other features, Longrich and Field looked at the fusion of skull bones to help determine which age bracket particular specimens fell within. “We think that what the fusions are telling you is that growth has slowed,” Longrich explained, “because you can no longer deposit new bone between those bones. This seems to be a fairly reliable indicator of maturity in relatively fast-growing animals like lizards, mammals, and birds.” In the case of Triceratops and Torosaurus, skull fusion seemed to occur in a particular sequence. “First the skull roof is fused, next the hornlets on the frill and cheeks fuse, then the beak and the nose fuse on. It’s a very regular pattern which suggests we can use this as a reliable way of getting at roughly where the animals fit in the developmental series,” Longrich said.

Yet Scannella and Horner have previously argued that the timing and degree of skull bone fusion aren’t as clear. Recently discovered specimens are contributing to the picture of how variable skull fusion might be. “The Museum of the Rockies has collected over a hundred new Triceratops from the Hell Creek Formation of Montana in the last decade,” Scannella said, and these specimens indicate that the details of skull fusion varies between individuals. “We have some huge, fairly mature Triceratops in which much of the skeleton is unfused; and there are also smaller, less mature specimens with many skeletal elements fused,” Scannella explained.

How the skulls of dinosaurs like Triceratops fused is not yet entirely clear, but, according to Andrew Farke, the degree of fusion between skull bones might be reliable for getting a general idea of how old an animal was. “There is little argument that the individual bones of the braincase tend to be unfused in young animals, and fused in old animals,” Farke pointed out, and further explained that “The same goes for the hornlets (epinasals and epijugals) on the face of ceratopsian dinosaurs,” he said, since “young animals tend to have unfused hornlets and old animals have fused hornlets.” Such features are what made the YPM 1831 Torosaurus stand out as a possible subadult to Farke’s eye.

Exactly which dinosaur YPM 1831 represents remains uncertain. The skull is the best candidate so far for a teenage Torosaurus, but this ambiguous specimen alone cannot end the debate. In fact, we have so much left to learn about Triceratops and Torosaurus—particularly about how their post-cranial skeletons changed as they aged—that a great deal of exploration and description remains to be done before this debate can be resolved. And this isn’t the only dinosaur name game in progress. The tiny tyrant “Raptorex” may have been a juvenile Tarbosaurus, the huge Anatotitan likely represents a mature Edmontosaurus, Titanoceratops was probably a big Pentaceratops, and the thick-skulled Dracorex and Stygimoloch might represent early growth stages of Pachycephalosaurus. Some of these changes sting—both Torosaurus and Anatotitan were childhood favorites of mine, and I’d hate to see them go—but, ultimately, these debates will help us better understand how dinosaurs grew up.

References:

Longrich, N., & Field, D. (2012). Torosaurus Is Not Triceratops: Ontogeny in Chasmosaurine Ceratopsids as a Case Study in Dinosaur Taxonomy PLoS ONE, 7 (2) DOI: 10.1371/journal.pone.0032623

Gorgosaurus as envisioned by Lambe. Clockwise from the upper left - standing, sitting, in repose, and feeding. From Lambe, 1917.

There just doesn’t seem to be any way around it—almost any time a new study about the feeding habits of Tyrannosaurus comes out, there is at least one news story that frames the research with the question of whether the great Cretaceous carnivore was exclusively a predator or a scavenger. There’s no reason for journalists to keep going back to the well for the same opener. The overhyped argument made a splash during the mid- to late 1990s thanks to Jack Horner and Don Lessem’s book The Complete T. rex and a number of cable documentaries, but the debate has been over for years. As articulated by tyrannosaur specialists such as Thomas Holtz, Tyrannosaurus was an active predator but was not above scavenging if there was an easy meal to be had. In this way, Tyrannosaurus may have been something akin to a modern day spotted hyena—an adept hunter, but one also capable of crushing through bone and making the most of any Triceratops carcasses that might be around.

Part of the reason why the idea of Tyrannosaurus as an obligate scavenger took off was because it was presented as a novel and heterodox idea championed by a famous paleontologist. In television documentaries, especially, the argument was framed as a rebuttal to the classic idea of Tyrannosaurus as a powerful and nigh-unstoppable predator. But, as Horner himself pointed out in The Complete T. rex, “T. rex as a scavenger isn’t a new idea.”  About a century ago, when the tyrannosaurs were strange and new, the Canadian paleontologist Lawrence Lambe hypothesized that the huge carnivores relied upon rotting carcasses to survive.

Lambe named and initially described Gorgosaurus in 1914. The skeleton of the giant, carnivorous dinosaur was mostly complete, and Lambe focused on the basic description of the dinosaur in his first paper on the specimen. How Gorgosaurus made a living, however, Lambe saved for a more comprehensive 1917 paper. The picture that emerged was of an imposing flesh-eater, but one that was incredibly lazy.

While pointing out that the reconstruction of a prehistoric animal’s appearance and lifestyle relied on some amount of conjecture, Lambe felt that certain anatomical portions of the Gorgosaurus skeleton could be taken as tell-tale signs of the animal’s habits. For one thing, the large boot at the lower edge of the pubic bone indicated to Lambe that this part of the hip bore most of the dinosaur’s weight while it was taking a break from bumbling around the landscape. “This position of rest, and particularly the recumbent one of repose at full length,” Lambe wrote, “were probably those most frequently assumed by a reptile having the form, and the supposed sluggish disposition of Gorgosaurus.” This creature was the antithesis of the dynamic, active theropod dinosaurs in the paintings of Charles R. Knight. As Lambe put it:

Was this reptile agile, alert, and quick of movement? Was it capable of capturing its prey by a sudden rush from some place of concealment, or by overtaking it after a pursuit possibly of some length? Was its victim eaten when killed? Did it engage in spirited encounters with its own kind as depicted in Knight’s well known restoration of Dryptosaurus in accordance with Cope’s views subsequently modified? The writer believes that Gorgosaurus was sluggish and not a quick mover, and that it fed, not on the fresh flesh of animals necessarily of its own killing but rather on carcasses found or stumbled across during its hunger impelled wanderings.

Gorgosaurus must not have been a picky eater, either. Lambe suggested that anything from dead hadrosaurs to decaying turtles would have “enticed” the dinosaur’s appetite. A giant dinosaur that feeds entirely on carrion can’t afford to pass up any meals.

Of course, part of Lambe’s reconstruction relied on the idea that dinosaurs were simply big reptiles. The notion that they were active, more bird-like animals—which was entertained by naturalists such as Richard Owen, E.D. Cope, and O.C. Marsh during the late 19th century—was falling out of fashion. But Lambe cited another line of evidence. The teeth of Gorgosaurus showed almost no signs of wear.

Lambe’s reasoning went like this. Even though we now know that dinosaurs replaced their teeth throughout their lives, Lambe thought Gorgosaurus had only one set of adult teeth (as in us mammals). Since the teeth of Gorgosaurus showed almost no signs of damage or abrasion, then, Lambe proposed two hypotheses: either his large Gorgosaurus specimen was a juvenile animal that had not eaten enough food to have conspicuous marks on its teeth, or the dinosaur ate only soft flesh. Lambe tossed the first idea. The Gorgosaurus skeleton was far too large to belong to a juvenile. Instead, Lambe proposed that the dinosaur let decay soften up its meals before digging in:

It is believed, therefore, that Gorgosaurus confined itself to feeding upon carcasses of animals that had not been freshly killed, that it was not as an intrepid hunter but as a scavenger that it played its useful part in nature, and no doubt its services were fully required when we consider the immense numbers of trachodonts, ceratopsians, stegosaurs, and other dinosaurs and reptiles that lived and died at this particular time of the Cretaceous period.

This same line of reasoning could be applied to other tyrannosaurs as they were understood at the beginning of the 20th century. All were big, had short arms and were often reconstructed as being on the more rotund and slow-moving side of scale. Tyrannosaurus may have had enough star power to regularly overcome this type of imagery, but the point is that the idea that tyrannosaurs were obligate scavengers was not a brand new idea that came out of nowhere in the 1990s. Paleontologists had considered the possibility and, eventually, opinions changed. Horner’s hypothesis stirred the pot again using different lines of evidence. But the anatomy of tyrannosaurs and healed bite wounds on herbivorous dinosaurs are consistent with the idea that tyrannosaurs hunted, and toothmarks on other bones indicate that tyrannosaurs could make the most of carcasses and probably scavenged. Choosing one extreme or the other doesn’t make sense. Tyrannosaurus was a complex animal in a complex ecosystem; the Cretaceous was not a world of black-and-white.

References:

Horner, J.; Lessem, D. 1993. The Complete T. rex. New York: Simon & Schuster. pp. 203-217

Lambe, L. 1914. A new genus and species of carnivorous dinosaur from the Belly River Formation of Alberta, with a description of the skull of Stephanosaurus marginatus from the same horizon. The Ottawa Naturalist XXVIII, 13-20

Lambe, L. 1917. The Cretaceous theropodus dinosaur Gorgosaurus, Memoirs of the Geological Survey of Canada. 100, pp. 1–84.