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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 Parasaurolophus at the Natural History Museum of Utah. Photo by the author.

Of all the crested hadrosaurs, Parasaurolophus is one of my favorites. The long, slightly-curved tube that projects from the back of the dinosaur’s head is a wonderful ornament. But why did this peculiar dinosaur decoration evolve?

Parasaurolophus was initially described by paleontologist William Parks  in 1922 on the basis of a skeleton found in the vicinity of Alberta’s Red Deer River. This dinosaur was clearly different from other ornamented hadrosaurs–such as Corythosaurus and Saurolophus–that had been found before, and especially perplexing was the makeup of the dinosaur’s crest. The structure was not solid–a break in this part of the skull revealed a series of internal tubes separated by thin walls of bone.

No one was exactly sure why Parasaurolophus had a hollow crest, but the supposed hadrosaur lifestyle generated a number of speculative answers. Hadrosaurs were supposed to be amphibious dinosaurs who acted like giant, dabbling ducks. After all, their broadened snouts gave them the popular moniker “duckbill dinosaurs.” Paleontologists therefore considered the dinosaur’s crest in reference to a life spent foraging for soft plants in Cretaceous swamps.

Paleontologist James Hopson reviewed these ideas in a 1975 Paleobiology paper about the role hadrosaur crests may have played in display. In 1933 Alfred Sherwood Romer speculated that the crest might have been used as a snorkel or an air storage chamber. While there was no hole in the crest to allow air to come in–the snorkel idea was scuttled–the air tank hypothesis was popular. As a young dinosaur fan, I remember encountering an image of a submerged Parasaurolophus in Edwin Colbert’s The Dinosaur Book with a solid black line running through the crest to indicate the amount of stored air. Another book, Rudolph Zallinger’s Dinosaurs and Other Prehistoric Reptiles, featured an even more detailed vision of Corythosaurus and Parasaurolophus paddling around beneath the surface of a prehistoric lake. But this notion didn’t last either. The anatomy of hadrosaurs has undeniably cast them as terrestrial animals, not expert swimmers, and the amount of air these dinosaurs were able to store in their crests would have been miniscule compared to their lung volume–the supposed air tanks would not have done them much good.

Charles Mortram Sternberg, son of the celebrated dinosaur collector Charles H. Sternberg, proposed a different variation of the aquatic feeding theme. In 1935 Sternberg wrote a paper on the “hooded” hadrosaurs from the Late Cretaceous of Canada and proposed that a U-shaped bend in the tubular crest passage prevented water from entering the respiratory system while the dinosaur was feeding underwater. Again, this idea is based on the notion that hadrosaurs frequently dipped their heads underwater to feed, and paleontologist John Ostrom later pointed out that, in such a scenario, the water pressure would have overcome the air pressure inside the crest and flooded the passage. Whatever the function of the Parasaurolophus crest, the structure was certainly ill-suited to underwater feeding.

Paleontologists kicked around a few other ideas. In a series of papers published in the late 30s and 40s, Martin Wilfarth suggested that elaborate hadrosaur crests were attachment areas for long, fleshy snouts. No evidence was found to support this. Likewise, Ostrom’s later suggestion that the nasal passages were extended to give the dinosaurs a better sense of smell was refuted–there was no indication that  the convoluted passageways had anything to do with a better sense of smell.

Hopson himself considered the crests to primarily be visual display structures, and hadrosaurs with hollow crests, such as Parasaurolophus, may have also used their crests as resonating chambers to send low-frequency sounds over long distances. This is the view generally taken now, but settling on particular functions for the crests does not necessarily illustrate how those structures evolved. Perhaps the origin of the various hadrosaur crest shapes was driven by pressures associated with species recognition–the need to identify members of one’s own kind, be they parents, rivals, mates, etc. Then again, perhaps some aspect of sexual selection was at play. Exactly what evolutionary factors led to the origin of such strange skull shapes is difficult to ascertain. Much remains unknown about the evolution and social significance of fantastic ornaments in dinosaurs.

References:

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

Naish, D. 2009. The Great Dinosaur Discoveries. Berkeley: University of California Press. pp. 72-73

Line drawings of the skeleton of Carnotaurus (top) and the forelimb of Aucasaurus (bottom). Notice the small size and odd proportions of the arms. From Senter, 2009.

As mighty as Tyrannosaurus rex was, its tiny forelimbs have also made it one of the most mocked dinosaurs of all time. The stubby arms of this predator once seemed mismatched to its enormous frame, and some of the hypotheses put forward to explain their function just made the “tyrant king” seem sillier. The ideas that Tyrannosaurus used their arms to tickle mates during nuptial encounters or to help push themselves off the ground after sleeping were comic gold.

When scientists stopped looking at size alone and studied what Tyrannosaurus arms could tell us about the dinosaur’s muscular anatomy, however, it was immediately apparent that its forelimbs were not useless vestiges after all. Though small, the forelimbs of Tyrannosaurus were actually quite beefy and probably acted like meathooks in securing live prey. As reconstructed by paleontologist Ken Carpenter, Tyrannosaurus was a “clutcher” that held struggling prey close with its claws while its enormous head took care of the dirty work. It’s about time that we cut Tyrannosaurus a break. But there is another group of dinosaurs that truly did have amusingly stunted arms.

Last year paleontologist Phil Senter conducted a review of vestigial structures in dinosaurs. Such structures were scattered across a wide array of genera, but Senter stressed that “vestigial” does not mean the same as “useless.” Instead he looked for structures that became so reduced in size that they could no longer carry out their original function even if they still retained some other secondary function. For example, despite having only two fingers, Tyrannosaurus rex retained a single bone from its third finger—the metacarpal—which was enclosed within its hand. Since many other theropods, including some early tyrannosauroids, had three fingers, this splint of bone in Tyrannosaurus rex fits the definition of a vestigial structure.

With this framework in place, Senter recognized that the entire group of predatory dinosaurs called the abelisaurids had partially vestigial arms. Represented by dinosaurs such as Carnotaurus and the recently described Skorpiovenator, the abelisaurids had stout upper arm bones followed by much shorter lower arm bones (the radius and ulna) held together by an immobile elbow joint. They also had a reduced number of stubby, fused fingers, which could not grasp and lacked claws, making their arms useless for prey capture. Whereas Tyrannosaurus had functional forelimbs that played a role in stabilizing struggling prey, Carnotaurus and its kin had only tiny forelimbs that probably just hung there.

Why abelisaurids like Carnotaurus, Aucasaurus and Majungasaurus had vestigial forelimbs is unclear. It is difficult to imagine what these dinosaurs could have been doing with their arms, and it is possible that their forelimbs did not have any function at all. (As we learned from the debates about the arms of Tyrannosaurus, it is easy to come up with stories about the potential function of a trait but difficult to test those ideas.) In terms of how their arms got that way, however, in 2002 Alexander Vargas proposed that changes during the early development of these dinosaurs may have been involved. According to Vargas, the stumpy, fused, vestigial forelimbs of the abelisaurids may have been caused by a loss of function in two genes that regulate the development of the forelimb, HOXA11 and HOXD11. This is a plausible explanation, but it only gives us the proximal trigger for the change in these dinosaurs. Determining why such stubby arms were widespread among these dinosaurs—and how they hunted without the use of their forelimbs—is another matter.

References:

Agnolin, F., & Chiarelli, P. (2009). The position of the claws in Noasauridae (Dinosauria: Abelisauroidea) and its implications for abelisauroid manus evolution Paläontologische Zeitschrift, 84 (2), 293-300 DOI: 10.1007/s12542-009-0044-2

Senter, P. (2010). Vestigial skeletal structures in dinosaurs Journal of Zoology, 280 (1), 60-71 DOI: 10.1111/j.1469-7998.2009.00640.x