A restoration of Nyasasaurus in its Middle Triassic habitat, based on the known bones and comparisons to closely related forms. The description of Nyasasaurus was one of the year’s most important dinosaur stories. Art by Mark Witton.

There’s always something new to learn about dinosaurs. Whether it’s the description of a previously-unknown species or a twist in what we thought we knew about their lives, our understanding of the evolution, biology, and extinction is shifting on a near-daily basis. Even now, paleontologists are pushing new dinosaurs to publication and debating the natural history of these wonderful animals, but the end of the year is as good a time as any to take a brief look back at what we learned in 2012.

For one thing, there was an exceptional amount of dino-hype this year. A retracted paper that mused on the nature of hypothetical space dinosaurs, a credulous report on an amateur scientist who said he had evidence that all dinosaurs were aquatic, and overblown nonsense about dinosaurs farting themselves into extinction all hit the headlines. (And the less said about the Ancient Aliens dinosaur episode, the better.) Dinosaurs are amazing enough without such sensationalist dreck, or, for that matter, being transformed into abominable human-raptor hybrids by Hollywood.

Not all the dinosaurs to wander into the media spotlight were atrocious, though. The glossy book Dinosaur Art collected some of the best prehistoric illustrations ever created, and the recently-released All Yesterdays presented dinosaurs in unfamiliar scenes as a way to push artists to break from severely-constrained traditions. Dinosaurs were probably much more unusual than we have ever imagined.

Indeed, new discoveries this year extended the range of fluff and feathers among dinosaurs and raised the question of whether “enfluffledness” was an ancient, common dinosaur trait. Paleontologists confirmed that the ostrich-like Ornithomimus–long suspected to have plumage–sported different arrangements of feathers as it aged. New insight on the 30-foot-long carnivore Yutyrannus affirmed that even big tyrannosaurs were covered in dinofuzz. And while both Ornithomimus and Yutyrannus belonged to the feathery subset of the dinosaur family tree that includes birds, the discovery of fluff on a much more distantly related theropod–Sciurumimus–hints that feathers were a much older, more widespread dinosaur feature than previously expected. Paired with previous finds, Sciurumimus suggests that protofeathers either evolved multiple times in dinosaurian history, or that the simple structures are a common inheritance at the base of the dinosaur family tree that was later lost in some groups and modified in others.

While some traditionalists might prefer scaly dinosaurs over fuzzy ones, feathers and their antecedents are important clues that can help paleontologists explore other aspects of paleobiology. This year, for example, researchers reconstructed dark, iridescent plumage on Microraptor on the basis of fossil feathers, and, as display structures, feathery decorations will undoubtedly have a role to play in the ongoing debate about how sexual selection influenced dinosaur forms.  Feathers can also be frustrating–a new look at the plumage of Anchiornis and Archaeopteryx will undoubtedly alter our expectations of how aerially capable these bird-like dinosaurs were and how they might have escaped predatory dinosaurs that dined on the prehistoric fowl. Such lines of inquiry are where the past and present meet–after all, birds are modern dinosaurs.

Feathers aren’t the only dinosaur body coverings we know about. Skin impressions, such as those found with the ankylosaur Tarchia, have also helped paleontologists discern what dinosaurs actually looked like. Pebbly patterns in Saurolophus skin can even be used to differentiate species, although paleontologists are still puzzled as to why hadrosaurs seem to be found with fossil skin traces more often than other varieties of dinosaur.

And, speaking of ornamentation, a damaged Pachycephalosaurus skull dome might provide evidence that these dinosaurs really did butt heads. How the adornments of such dinosaurs changed as they aged, though, is still a point of controversy. One of this year’s papers threw support to the idea that Torosaurus really is a distinct dinosaur, rather than a mature Triceratops, but that debate is far from over.

Other studies provided new insights into how some dinosaurs slept, the evolutionary pattern of dinosaur succession, what dinosaur diversity was like at the end of the Cretaceous, and how dinosaurs grew up, but, of course, how dinosaurs fed is a favorite place that lies at the intersection of science and imagination. A poster at the annual Society of Vertebrate Paleontology meeting deconstructed how Tyrannosaurus rex–suggested to have the most powerful bite of any terrestrial animal ever–tore the heads off of deceased Triceratops. The herbivorous Diplodocus, by contrast, munched soft plants and stripped branches of vegetation rather than gnawing on tree bark, and the tiny, omnivorous Fruitadens probably mixed insects with its Jurassic salads. Studying dinosaur leftovers also explained why paleontologists didn’t find more of the mysterious Deinocheirus, which thus far has been identified by only one incomplete fossil–the long-armed ornithomimosaur was eaten by a Tarbosaurus.

We also met a slew of new dinosaurs this year, including the many-horned Xenoceratops, the archaic coelurosaur Bicentenaria, the sail-backed Ichthyovenator, the stubby-armed Eoabelisaurus, and the early tyrannosaur Juratyrant. This is just a short list of species I wrote about–a few that add to the ever-increasing list.

To properly study dinosaurs and learn their secrets, though, we must protect them. One of the most important dinosaur stories this year wasn’t about science, but about theft. An illicit Tarbosaurus skeleton – pieced together from multiple specimens smuggled out of Mongolia–has brought wide attention to the fossil black market, as well as the poachers and commercial dealers who fuel it. The fate of this dinosaur remains to be resolved, but I’m hopeful that the dinosaur will be returned home and will set a precedent for more vigorously going after fossil thieves and their accomplices.

Out of all the 2012 dinosaur stories, though, I’m especially excited about Nyasasaurus. The creature’s skeleton is as yet too fragmentary to know whether it was true dinosaur or the closest relative to the Dinosauria as a whole, but, at approximately 243 million years old, this creature extends the range of dinosaurs back in time at least 10 million years. That’s another vast swath of time for paleontologists to examine as they search for where dinosaurs came from, and those discoveries will help us better understand the opening chapters in the dinosaurian saga. That’s the wonderful thing about paleontology–new discoveries open new questions, and those mysteries keep us going back into the rock record.

And with that, I must say goodbye to Dinosaur Tracking. On Tuesday I’m starting my new gig at National Geographic’s Phenomena. I’ve had a blast during my time here at Smithsonian, and I bid all my editors a fond farewell as I and my favorite dinosaurs head off to our new home.

Editor’s Note: Best wishes to Brian on his future travels and we all thank him for his hard work over the past 4 (!) years, writing every day about something new on dinosaurs. It’s not nearly as easy as he makes it look. – BW

Dinosaurs are much more than real monsters that fire our imaginations, but, let’s face it, part of their persistent appeal is that many were enormous prehistoric oddities. And it’s just that aspect of dinosaurian nature that is raising ire in a historically-rich California town and on an Australian golf course.

San Juan Capistrano, California is famous for the local cliff swallows and the historic Spanish architecture, but the town has recently been in the news because of an unwelcome dinosaur. According to the LA Times, a huge sauropod statue erected in the town’s petting zoo has drawn the ire of those who seek to retain some semblance of southern California’s past. Where kids and the zoo’s owner sees the dinosaur as a fanciful distraction, local historians argue that the dinosaur is totally out of place with the rest of the town’s decor. The dinosaur is staying put for now, but may yet be removed if the city decides that there’s just no place for a dinosaur in a place where Californian history and modern life already mix.

A different dinosaur is frustrating Australia’s professional golfers. The wealthy owner of the Palmer Coolum Resort has installed a 26 foot long, animatronic Tyrannosaurus rex in the middle of the course. Along with other recent installations, ESPN reports, the dinosaur is expected to adversely affect the games of Australian PGA Championship golfers set to play there. With the resort’s owner promising more dinosaurs on the way, the sports group has decided to move the tournament elsewhere after this year. Whether a sauropod looks out of place is one thing–having a T. rex get in the way of your shot is another.

Not everyone is so bothered by giant dinosaurs, though. A Best Western hotel in Colorado is taking on an entirely prehistoric theme, including fossil casts and dinosaur sculptures. In addition to attracting tourists, the hotel’s owner says he wants to draw attention to Colorado’s exceptional fossil sites, such as the nearby track site at Dinosaur Ridge. Dinosaur sculptures are frustrating eyesores to some and paleo-vacation essentials to others.

The “Morphotype 1″ tunnel complex: points marked “a” represent tunnels, and points marked “b” signify vertical shafts. From Colombi et al., 2012.

Dinosaurs never cease to surprise. Even though documentaries and paleoart regularly restore these creatures in lifelike poses, the fact is that ongoing investigations into dinosaur lives have revealed behaviors that we never could have expected from bones alone. Among the most recent finds is that dinosaurs were capable of digging into the ground for shelter. Burrows found in Australia and Montana show that some small, herbivorous dinosaurs dug out cozy little resting places in the cool earth.

But when did dinosaurs develop burrowing behavior? The distinctive trace fossils found so far are Cretaceous in age, over 100 million years after the first dinosaurs evolved. That’s why a new PLoS One paper by paleontologist Carina Colombi caught my eye. In the Triassic rock of Argentina’s Ischigualasto Basin, Columbi and coauthors report, there are large-diameter burrows created by vertebrates that lived approximately 230 million years ago. Archaic dinosaurs such as Eoraptor and Herrerasaurus roamed these habitats–could dinosaurs be responsible for the burrows?

Colombi and colleagues recognized three different burrow forms in the Triassic rock. Two distinct types–differentiated by their diameter and general shape–were “networks of tunnels and shafts” that the authors attributed to vertebrates. The third type showed a different pattern of “straight branches that intersect at oblique angles” created by the burrowing organism and the plant life. The geology and shapes of the burrows indicate that they were created by living organisms. The trick is figuring out what made the distinct tunnel types.

In the case of the first burrow type, Colombi and collaborators propose that the structures were made by small, carnivorous cynodonts–squat, hairy protomammals. In the other two cases, the identities of the burrow makers isn’t clear. The second type included vertical shafts that hint at a vertebrate culprit. Dinosaurs would have been too big, but, Colombi and coauthors suggest, other cynodonts or the bizarre, ancient cousins of crocodiles–such as aetosaurs or protosuchids–could have created the burrows. Unless remains of these animals are found associated with the burrows, it is impossible to be sure. Likewise, the third type of trace might represent the activities of animals that burrowed around plant roots, but there is no clear candidate for the trace-maker.

As far as we know now, Triassic dinosaurs didn’t burrow. Even though they were not giants, they were still too large to have made fossils reported in the new research. Still, I have to wonder if predatory dinosaurs such as Herrerasaurus, or omnivores like Eoraptor, dug poor little cynodonts out of their burrows much like the later deinonychosaurs scratched after hiding mammals. There’s no direct evidence for such interactions, but, if small animals often sheltered from heat and drought in cool tunnels, perhaps predators tried to nab prey resting in their hiding places. One thing is for sure, though: we’ve only just started to dig beyond the surface of Triassic life.

References:

Colombi, C., Fernández, E., Currie, B., Alcober, O., Martínez, R., Correa, G. 2012. Large-Diameter Burrows of the Triassic Ischigualasto Basin, NW Argentina: Paleoecological and Paleoenvironmental Implications. PLoS ONE 7,12: e50662. doi:10.1371/journal.pone.0050662

Dinosaurs are often thought of as kid’s stuff. In America, at least, going through a “dinosaur phase” is just another part of childhood, and somewhere along the way we’re expected to stop acting like walking encyclopedias to Mesozoic life. Yet this narrow view of dinosaurs as nothing more than pre-teen kitsch obscures the essential truths these animals can share with us about evolution, extinction, and survival.

As paleontologist Michael Novacek argues in the video above, the history of dinosaurs is also our history–our mammalian ancestors and relatives snuffled and scurried through a dinosaur-dominated world for more than 150 million years. We can’t understand where we came from without considering dinosaurs. And, says paleontologist Matt Bonnan, “Dinosaurs put our place in the world into perspective.” By asking questions about dinosaurs–when did they live and what was the world like then?–the history of life on Earth comes into focus, and the answers to these queries help us better understand the pervasive forces of evolution and extinction through time.

These critical aspects of nature can be difficult to detect on the timescales of our lives, but become much more apparent when we can peek into deep time by sifting through the remains of creatures that roamed the Earth long ago. An individual dinosaur discovery might not have any practical use or even significantly change our understanding of the past, but when considered together with the ever-growing body of research about dinosaurs, it can help us understand how we came to be on this planet and may even give us some clues about the future–how species emerge and decline, how creatures adapt, and how life evolves after catastrophic extinction events.

What do you think is the best case for the importance of studying dinosaurs?

A reconstruction of Irritator. Photo by Kabacchi, image from Wikipedia.

Spinosaurs are often called “fish-eating dinosaurs.” Their long, shallow snouts recall the jaws of crocodiles, and, based on gut contents and fossil geochemistry, it seems that these dinosaurs truly were piscivores. Yet spinosaurs weren’t on a strict fish diet. In 2004, Eric Buffetaut and colleagues described a spinosaur tooth embedded in the fossilized neck vertebrae of an Early Cretaceous pterosaur found in Brazil’s roughly 110-million-year-old Santana Formation. The paleontologists couldn’t say whether the dinosaur caught its prey on the wing or scavenged a fresh carcass, but, based on fossils previously found in the same geologic formation, one spinosaur stood out as the probable culprit–Irritator challengeri.

The spinosaur’s quirky name symbolizes its unconventional back story. As explained in the 1996 description of the dinosaur by David Martill and colleagues, the mostly complete skull of Irritator had been artificially modified by a commercial fossil dealer prior to being purchased and making its way into the collection of Germany’s Stuttgart State Museum of the Natural Sciences. The tip of the snout was made up of bone from elsewhere on the skull, “concealed by blocks of matrix removed from other parts of the specimen and a thick layer of Isopon car body filler.” The fabrication not only deceived the buyers, but was especially difficult to remove from the authentic fossil. Martill and colleagues named the dinosaur Irritator as a tribute to “the feeling the authors felt (understated here) when discovering that the snout had been artificially elongated.”

Martill and collaborators originally proposed that Irritator was a maniraptoran dinosaur–a relative of the feathery deinonychosaurs, oviraptorosaurs, and their kin. That same year, however, paleontologist Andrew Kellner recognized that Irritator was actually a spinosaur–one of the croc-snouted, and often sail-backed, predatory dinosaurs. Kellner also named what he suspected was another spinosaur found in the same geologic formation–”Angaturama limai“–but many researchers suspect that this animal is the same as Irritator, and the so-called “Angaturama” remains may even complete the missing parts of the Irritator skeleton.

But even after Irritator was properly identified, there was still work to be done. Diane Scott undertook the painstaking work of fully cleaning the skull of the encasing matrix, which led to a new description by Hans-Dieter Sues and coauthors in 2002. Irritator is represented by the most complete skull yet known for any spinosaur. Among other new aspects, it was apparent that the back of the skull was significantly deeper among spinosaurs than had previously been thought. And even though Martill and co-authors originally described a prominent crest on the top of the spinosaur’s skull, the fully-prepped fossil showed that this bone did not actually belong to the Irritator skull.

There’s still much we have to learn about spinosaurs. Most of these dinosaurs are only known from bits and pieces. And despite starring in Jurassic Park III, Spinosaurus itself is among the most poorly known dinosaurs of all, and the fragmentary nature of so many of these dinosaurs makes it possible that paleontologists have named too many genera. In their study, Sues and coauthors argue that Suchomimus is really just a different species of Baryonx, and even Irritator might be a distinct species of Spinosaurus. Researchers have only just begun to track the record of these long-snouted dinosaurs, although, hopefully, future finds will not be quite so aggravating as Irritator.

This is the latest post in the Dinosaur Alphabet series.

References:

Buffetaut, E., Martill, D., Escuillie, F. 2004. Pterosaurs as part of a spinosaur diet. Nature. 430: 33

Martill, D., Cruickshank, A., Frey, E., Small, P., Clarke, M. 1996. A new crested maniraptoran dinosaur from the Santana Formation (Lower Cretaceous) of Brazil. Journal of the Geological Society 153: 5-8.

Sues, H., Frey, E., Martill, D., Scott, D. 2002. Irritator challengeri, a spinosaurid (Dinosauria: Theropoda) from the Lower Cretaceous of Brazil. Journal of Vertebrate Paleontology. 22, 3: 535-547

Did Deinonychus and other “raptors” use their foot claws to restrain prey? Art by Emily Willoughby, image from Wikipedia.

When paleontologist John Ostrom named Deinonychus in 1969, he provided the spark for our long-running fascination with the “raptors.” Similar dinosaurs had been named before–Velociraptor and Dromaeosaurus were named four decades earlier–but the skeleton of Ostrom’s animal preserved a frightening aspect of the dinosaur that had not yet been seen among the earlier finds. The assembled remains of Deinonychus included the dinosaur’s eponymous “terrible claw”–a wicked, recurved weapon held off the ground on the animal’s hyperextendable second toe. Combined with the rest of the dinosaur’s anatomy, Ostrom argued, the frightening claw indicated that Deinonychus must have been a active, athletic predator.

But how did Deinonychus and its similarly-equipped relatives use that awful toe claw? The appendage looks fearsome, but paleontologists have not been able to agree on whether the claw was using for slashing, gripping, pinning, or even climbing prey. Some researchers, such as Phil Manning and collaborators, have even argued that the claws of Velociraptor and related dinosaurs were best suited to scaling tree trunks–a conclusion consistent with the contentious hypothesis that the ancestors of birds were tree-climbing dinosaurs.

Left hind foot of Deinonychus antirrhopus. Image from Wikipedia.

All this assumes that the claws of deinonychosaurs correspond to a special behavior, but can foot claw shapes really give away the habits of dinosaurs? That’s the question posed by a new PLoS One study by zoologist Aleksandra Birn-Jeffery and colleagues.

Based on observations of living animals, researchers have often tied particular claw shapes to certain behaviors–relatively straight, stubby claws likely belong to an animal that runs on the ground, while tree-climbing species have thin claws with small, sharp points. But nature isn’t quite so neat as to have a single, tell-tale claw shape for perchers, ground-runners, climbers, and predators. Even then, researchers don’t always interpret claw shapes the same way–depending on who you ask, the foot claws of the early bird Archaeopteryx either indicate that it was a climber or could only run on the ground.

To parse this problem, Birn-Jeffery and co-authors studied the geometry of the third toe claw–on dinosaurs, the middle toe claw–in 832 specimens of 331 species, together representing different lifestyles of birds, lizards, and extinct dinosaurs. The claw shapes didn’t strictly conform to particular behaviors. In the climber category, for example, the frill-necked lizard has lower claw curvature than expected, and, among predatory birds, the common buzzard, secretary bird, and greater sooty owl has less sharply recurved claws that anticipated for their lifestyle.

When the dinosaur data was dropped into the mix, the deinonychosaurs didn’t seem to fit in any single category. The sickle-clawed carnivores fell into the range shared by climbers, perchers, predators, and ground dwellers–these dinosaurs could be said to be anything from wholly terrestrial runners to perchers. And even though the researchers identified a general claw shape that corresponded to walking on the ground–deeper claws with less curvature–the dinosaurs did not strictly fit into this category alone.

Some dinosaurs, such as Microraptor, had claws that might have been suited to climbing. However, dinosaurs that we might regard as behaviorally similar showed differences–Velociraptor seemed to best fit the ground-dweller category, while the larger Deinonychus seemed to have claws more akin to those of predatory birds. This doesn’t mean that Microraptor was definitely a climber, or that Velociraptor wasn’t a predator. As the authors show, the different behavioral categories are not so easily distinguishable as previously thought, and saying that an animal definitely engaged in a particular behavior because of claw shape alone tempts oversimplification.

No wonder there has been such a range of interpretation about dinosaur foot claws! While the new study focused on the third toe claw rather than the famous, second deinonychosaur toe claw, the point of the analysis still applies. Claw geometry alone is not a reliable indicator of behavior. That’s to be expected–as the authors point out, claws are multi-functional, are are unlikely to represent just one type of behavior or habitat. Birds that use their claws to perch may also use them to kill prey, or birds that primarily live in the trees may also forage on the ground. Claw shape is constrained by different aspects of natural history, and reflect flexibility rather than strict adherence to a particular lifestyle. Deinonychosaur claws definitely hold clues to the natural history of dinosaurs, but drawing out those clues is a difficult, convoluted process.

Reference:

Birn-Jeffery, A., Miller, C., Naish, D., Rayfield, E., Hone, D. 2012. Pedal Claw Curvature in Birds, Lizards and Mesozoic Dinosaurs – Complicated Categories and Compensating for Mass-Specific and Phylogenetic Control. PLoS ONE. 7,12: e50555. doi:10.1371/journal.pone.0050555

The arms of Therizinosaurus–as yet, the rest of the dinosaur is missing. Photo by FunkMonk, image from Wikipedia.

The most famous set of arms in the history of dinosaurs belong to Deinocheirus–eight foot long appendages from a huge ornithomimosaur that roamed Mongolia around 70 million years ago. But the immense ostrich-mimic wasn’t the only giant omnivore of its time, nor the only one made famous by its imposing arms. About 20 years before the discovery of Deinocheirus, a joint Soviet-Mongolian expedition found extremely long, tapering claws and a few other bones from a gigantic reptile. The identity of this animal took decades to untangle.

Paleontologist Evgeny Maleev described the paltry remains in a 1954 paper. Based on rib fragments, a bone from the hand, and three claws, Maleev believed that he was looking a gargantuan turtle. He named the creature Therizinosaurus cheloniformis–roughly, the “turtle-like scythe lizard.”

The animal’s claws played a key role in the identification. No terrestrial animal had such claws, he argued. Such armaments “may have been originally used by the animal for cutting aquatic vegetation or for another function, constrained by movement and acquiring food.” And even though Maleev only had pieces to work with, he proposed that Therizinosaurus was about 15 feet long with claws at least three feet long. This aquatic, apparently armor-less turtle lived in a time of hadrosaurs, tyrannosaurs, and sauropods.

Therizinosaurus wasn’t recognized as a dinosaur until 1970. In that year, paleontologist Anatoly Konstantinovich Rozhdestvensky published a re-evaluation of Maleev’s fossils that found the rib to be from a sauropod dinosaur, but the hand bone and the claws to be from some as-yet-unknown theropod. This recognition only spawned a new mystery–what sort of theropod dinosaur was Therizinosaurus, and what was the creature doing with such fearsome claws?

More complete forelimb and shoulder material described by Rinchen Barsbold in 1976 showed that Therizinosaurus had extraordinarily robust arms–quite a departure from the trend seen in large carnivorous dinosaurs, in which the arms seemed to become smaller as skulls became more heavily-built. At a time when theropod was generally considered to be synonymous with “carnivorous dinosaur”, it’s not surprising that experts speculated that Therizinosaurus was a monstrous predator who used claws, rather than teeth, to slice up the hadrosaurs and sauropods of its time. That’s the way I encountered the dinosaur in the books I read as a kid–a little-known, Cretaceous hadrosaur-shredder.

What researchers didn’t recognize was that Therizinosaurus represented an entirely new variety of theropod dinosaur. More complete skeletons of related forms such as Segnosaurus, Erlikosaurus, Alxasaurus, and Beipiaosaurus revealed the presence of a previously-unknown group of dinosaurs with long necks, beaked mouths, fat bodies, and stout arms tipped with ludicrously-long claws. These were omnivorous or herbivorous dinosaurs, not carnivores, although paleontologists didn’t immediately agree on what lineage they belonged to. Some thought they might be aberrant ornithischians–on the opposite side of the dinosaur family tree from theropods–or strange variations on the sauropod theme. By the mid-90s, however, paleontologists recognized that these truly were theropods, and ones belonging to the maniraptoran group that also encompasses the strange alvarezsaurs, beaked and crested oviraptorosaurs, the sickle-clawed deinonychosaurs, and birds. This group of tubby, feathery dinosaurs became known as the therizinosaurs.

Although Maleev didn’t recognize it when he named Therizinosaurus, he had found one of the most spectacular dinosaurs of all time–a giant, fluffy, omnivorous dinosaur that challenged what we thought we knew about theropods. Still, our image of Theriziniosaurus relies on the skeletons of more complete, closely-related dinosaurs. So far, we only really know what the arms of this dinosaur looked like, and the hindlimb elements described in the 1980s may or may not belong to another creature. We’re still waiting for the true nature of this undoubtedly bizarre dinosaur to come into focus.

References:

Barsbold, R. 1976. New data on Therizinosaurus (Therizinosauridae, Theropoda) [translated]. In Devâtkin, E.V. and N.M. Ânovskaâ (eds.), Paleontologiâ i biostratigrafiâ Mongolii. Trudy, Sovmestnaâ Sovetsko−Mongol’skaâ paleontologičeskaâ kspediciâ, 3: 76–92.

Maleev, E.A. 1954. “New turtle−like reptile in Mongolia [translated].” Priroda, 1954, 3: 106–108.

Zanno, L. 2010. A taxonomic and phylogenetic re-evaluation of Therizinosauria (Dinosauria: Maniraptora). Journal of Systematic Palaeontology. 8, 4: 503–543.

A restoration of Nyasasaurus in its Middle Triassic habitat, based on the known bones and comparisons to closely related forms. Art by Mark Witton.

For the past twenty years, Eoraptor has represented the beginning of the Age of Dinosaurs. This controversial little creature–found in the roughly 231-million-year-old rock of Argentina–has often been cited as the earliest known dinosaur. But Eoraptor has either just been stripped of that title, or soon will be. A newly-described fossil found decades ago in Tanzania extends the dawn of the dinosaurs more than 10 million years further back in time.

Named Nyasasaurus parringtoni, the roughly 243-million-year-old fossils represent either the oldest known dinosaur or the closest known relative to the earliest dinosaurs. The find was announced by University of Washington paleontologist Sterling Nesbitt and colleagues in Biology Letters, and I wrote a short news item about the discovery for Nature News. The paper presents a significant find that is also a tribute to the work of Alan Charig–who studied and named the animal, but never formally published a description–but it isn’t just that. The recognition of Nyasasaurus right near the base of the dinosaur family tree adds to a growing body of evidence that the ancestors of dinosaurs proliferated in the wake of a catastrophic mass extinction.

In March of 2010, Nesbitt and a team of collaborators named a leggy, long-necked creature from the same Triassic rock unit in Tanzania they named Asilisaurus kongwe. This creature was a dinosauriform–a member of the group from which the first true dinosaurs emerged–and, even better, appeared to to be the closest known relative to the Dinosauria as a whole. The find hinted that the dinosaur lineage had probably split off from a common ancestor by this time, meaning that the most archaic dinosaurs may have already existed by 243 million years ago. Roughly 249-million-year-old footprints of dinosauriforms found among Poland’s Holy Cross Mountains, described by different researchers later the same year, added evidence that the dinosauriforms were diversifying right from the beginning of the Triassic–not long after the catastrophe that decimated life on earth at the end of the Permian, around 252 million years ago.

Nyasasaurus is another step closer to the first true dinosaurs, and is just as old as Asilisaurus. To find an animal with such distinctive, dinosaur-like traits in the Middle Triassic indicates that dinosaurs already existed, or their ancestral stem was already established. Either way, Eoraptor and kin from South America can no longer be considered as the first dinosaurs, but rather a later radiation of forms. Even though our knowledge of Nyasasaurus is only fragmentary–the dinosaur is represented by a right humerus and a collection of vertebrae from two specimens–the dinosauriform nonetheless marks an additional 12 million years of dinosaur time that paleontologists are only just starting to explore.

Whether or not we ever achieve a more complete view of Nyasasaurus depends on the luck and the caprices of the fossil record. In the new paper, Nesbitt and coauthors point out that the rare, fragmentary nature of the remains found so far reflects that dinosauriforms–and early dinosaurs–were marginal parts of the ecosystems they inhabited. Dinosaurs did not dominate from the very start. They were  relatively meek, small animals that lived in a world ruled by archosaurs more closely related to crocodiles. It was only in the Late Triassic and Early Jurassic, when their archosaurian competition was diminished, that dinosaurs became dominant. That means the earliest dinosaurs and their ancestors are few and far between in the Triassic record.

Still, when I asked Nesbitt what Nyasasaurus might have looked like, he cited other dinosauriforms and early dinosaurs as templates to constrain our expectations. Nyasasaurus may have looked quite like Asilisaurus–a leggy animal with an elongated neck–although Nyasasaurus may have been bipedal. Future finds will test this idea, but the fact remains that paleontologists are closing in on what the very first dinosaurs were like. As paleontologists uncover more early dinosaurs and dinosauriforms, the dividing line between the two disappears–scientists are starting to smooth out the evolutionary transition between the first dinosaurs and their ancestors. What role Nyasasaurus played in that transformation isn’t yet clear, but the creature is a signal that over 10 million years more of uncharted dinosaur history remains in the rock.

References:

Nesbitt, S., Sidor, C., Irmis, R., Angielczyk, K., Smith, R., Tsuji, L. 2010. Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira. Nature 464, 7285: 95–98. doi:10.1038/nature08718

Nesbitt, N., Barrett, P., Werning, S., Sidor, C., Charig, A. 2012. The oldest dinosaur? A middle Triassic dinosauriform from Tanzania. Biology Letters. http://dx.doi.org/10.1098/rsbl.2012/0949

The articulated, almost-complete hand of Hagryphus giganteus. From Zanno and Sampson, 2005.

When I think of oviraptorosaurs – feathered, beaked, omnivorous theropods–my mind immediately jumps to Mongolia’s famous brooding dinosaurs and other forms extracted from Asia’s Cretaceous rock. But these weird dinosaurs were present in North America, too. Among the latest to come to the attention of paleontologists is Hagryphus giganteus–a large oviraptorosaur known from little more than a hand and pieces of foot.

Paleontologists started to report on the oviraptorosaurs of North America’s Late Cretaceous in the 1930s. They just didn’t immediately recognize the dinosaurs for what they were. Scrappy remains of these dinosaurs were attributed to the ostrich-like ornithomimosaurs and Cretaceous birds. It was only in the 80s and 90s that researchers began to untangle the identities of these dinosaurs. Based on specimens found in Canada, Montana, and the Dakotas, there may have been at least three different genera present–Caenagnathus, Chirostenotes, and Elmisaurus–around 75 million years ago. That depends on who you ask, though. Researchers disagree about which genera are valid. The material from these dinosaurs is so fragmentary that it’s difficult to tell just how many different forms we’re looking at.

But Hagryphus, described by paleontologists Lindsay Zanno and Scott Sampson in 2005, was different. Represented by a nearly-complete left hand, part of the left radius, and fragments of the foot, this theropod lived further to the south in the 75-million-year-old swampy environment preserved in Utah’s Grand Staircase-Escalante National Monument. Much like other dinosaurs found in the same formation, and other southern species from roughly contemporaneous deposits, the known remains of Hagryphus are distinct from the equivalent bones known from the northern species. Not only was Hagryphys bigger–Zanno and Sampson estimated that the dinosaur was about 10 feet long, quite large for an oviraptorosaur–but bones in the dinosaur’s hand were much more robust.

Zanno and Sampson considered that the unique nature of Hagryphus might be because the individual was an older specimen of one of the northern oviraptorosaurs. They rejected this hypothesis, arguing the the dinosaur’s distinctive hand proportions were more consistent with being it a different taxon than changes due to growth. If they’re right, this fits the general pattern of Utah’s Kaiparowits Formation in preserving dinosaurs that were related to those found in Montana and Alberta but were unique genera and species.

So how many oviraptorosaurs were there in North America around 75 million years ago? We probably haven’t found traces of all of them, but based on what has been described so far there were probably at least two and as many as four. We need more complete skeletons to be sure.

The same problem affects other small-bodied theropod dinosaurs from the Late Cretaceous. Based on teeth and fragmentary remains, paleontologists used to think that the deinonychosaur Troodon had a range from southern Utah to Alaska. As parts of additional specimens come out of the ground, paleontologists are starting to realize that what seemed to be just one dinosaur is really a collection of different genera or species spread across the latitudes. And regardless of what Hagryphys is, the existence of an oviraptorosaur in Utah greatly extends the range of these dinosaurs during the 75-million-year-old time frame. Exposures between southern Utah and Montana may very well hold additional oviraptorosaur specimens–individuals that will be critical to understanding how these dinosaurs evolved.

This is the latest post in the Dinosaur Alphabet series.

Reference:

Zanno, L., Sampson, S. 2005. A new oviraptorosaur (Theropoda, Maniraptora) from the Late Cretaceous (Campanian) of Utah. Journal of Vertebrate Paleontology. 35:4, 897-904

A cautious Camptosaurus approaches a resting Allosaurus. Even though the carnivore undoubtedly hunted the herbivore at times, the two weren’t constantly at war with each other. Art by John Conway, from All Yesterdays.

The dinosaurs I grew up with were both intensely exciting and incredibly dull. They were creatures unlike anything I had ever seen, but their drab, scaly flesh was always fit snugly to their bones with little embellishment. For decades, this has been the paradox of prehistoric restorations. Reconstructed skeletons are gloriously magnificent and introduce us to strange creatures that we never could imagined if we did not already know they existed. Yet the art of reviving these organisms has often been incredibly conservative. Dinosaurs, in particular, have often been “shrink-wrapped”–their skin tightly pulled around a minimalist layer of muscle distributed over the skeleton. This may be part of why dinosaur restorations look so weird. As John Conway, C.M. Kosemen, Darren Naish and Scott Harman argue in their new book All Yesterdays, no living lizard, fish, bird or mammal adheres to such a limited “skin on the bones” fashion. Dinosaurs were not only skeletally distinctive, but they undoubtedly looked stranger and behaved more bizarrely than we have ever imagined. The recently-published Dinosaur Art started to realize these possibilities, but All Yesterdays goes even further in melding science and speculation about dinosaur biology.

On a superficial level, All Yesterdays is a gorgeous collection of speculative artwork. Divided into two sections–the first featuring Mesozoic life in new or little-seen vignettes, and the second imagining how we would restore modern animals if we only had partial skeletons to work from–the book features some of the most wonderful paleoart I’ve ever seen. Scott Hartman’s crisp skeletal reconstructions form the framework from which Conway and Kosemen play with muscle, fat and flesh, and, following Naish’s introductory comments, Kosemen provides scientific commentary about how each illustration is not quite so outlandish as it seems. A curious Camptosaurus approaching an Allosaurus at rest is a reminder that, much like modern animals, prey and predators were not constantly grappling with each other, just as a snoozing rendition of the Tyrannosaurus “Stan” shows that even the scariest dinosaurs had to snooze. The gallery’s feathered dinosaurs are especially effective at demonstrating the fluffy weirdness of the Mesozoic. Conway’s peaceful scene of feather draped Therizinosaurus browsing in a tree grove is the best rendition of the giant herbivore I’ve ever seen, and his fluffy, snowbound Leaellynasaura are unabashedly adorable.

The second half of the book continues the same theme, but in reverse. How would artists draw a cat, an elephant or a baboon if we only had skeletons or bone fragments? And what would those scraps suggest about the biology of long-lost animals? If there are paleontologists in the future, and they have no other source of information about our world, how will they restore the animals alive today? They might have no knowledge of the fur, fat, feathers and other structures that flesh out modern species, creating demonic visions of reptilian cats, eel-like whales and vampire hummingbirds.

Working in concert, the two sections will give casual readers and paleoartists a jolt. While some might gripe about Todd Marshall adding too many spikes and dewlaps to his dinosaurs, or Luis Rey envisioning deinonychosaurs at play, the fact of the matter is that dinosaurs probably had an array of soft tissue structures that made them look far stranger than the toned-down restorations we’re used to. As All Yesterdays presents in various scenes, maybe sauropods liked to play in the mud, perhaps hadrosaurs were chubbier than we ever imagined and, as depicted in one nightmare-inducing panel, Stegosaurus could have had monstrous genitals. None of these scenarios are supported by direct evidence, but they are all within the realm of possibility.

The cover of All Yesterdays, a visual celebration of speculative paleontology.

More than a gallery of speculative art, All Yesterdays is an essential, inspirational guide to any aspiring paleoartist. Those who restore prehistoric life are limited by the evidence at hand, this is true, but “more conservative” does not mean “more accurate.” Using comparisons with modern animals, artists have far more leeway than they have ever exercised in imagining what prehistoric life was like. We’ve seen enough Deinonychus packs tearing apart Tenontosaurus, and far too many malnourished dinosaurs. We need more fat, feathers, accessory adornments and scenes from quieter moments in dinosaur lives that do not involve blood and spilled viscera. Professional paleoartists are beginning to embrace these ideas–Jason Brougham’s recent restoration of Microraptor is an appropriately fluffy, bird-like animal rather than the flying monster Naish and collaborators decry–but All Yesterdays is a concentrated dose of prehistoric possibilities that are being artistically explored.

Some of the book’s restorations may turn out to look quite silly. As lovely as Conway’s rendition is, I still don’t buy the “bison-back” idea for high-spined dinosaurs such as Ouranosaurus. Then again, depending on what we discover in the future, some of the illustrations might seem quite prescient. The important thing is that All Yesterdays demonstrates how to push the boundaries of what we imagine while still drawing on scientific evidence. The book is a rare treat in that each section explicitly lays the inspiration for each speculative vision, providing references for those who want to dig deeper.

If anything, All Yesterdays shows that we should not be afraid of imagination in science. Even though we know far more about dinosaur biology and anatomy than ever before, there are still substantial gaps in our understanding. In these places, where bones might not have much to tell us, science meets speculation. The result is not anything-goes garishness, but an exploration of possibilities. Somewhere within that murky range of alternatives, we may start to approach what dinosaurs were truly like.

You can purchase All Yesterdays in any of its various formats here.

The arms of the huge ornithomimosaur Deinocheirus. How did such herbivorous theropods get to be so big? Photo by Eduard Solà, image from Wikipedia.

When I was first becoming acquainted with dinosaurs in the mid 1980s, “theropod” was synonymous with “carnivorous dinosaur.” Large or small, from Tyrannosaurus to Compsognathus, every theropod I knew of sustained itself on the flesh of other organisms. But it was just about that time that new discoveries and analyses revealed that many theropod dinosaurs were omnivores, or even herbivores. The ostrich-like ornithomimosaurs, beaked oviraptorosaurs and utterly bizarre therizinosaurs, in particular, embodied a switch from an ancestral meat-filled diet to one more reliant of fruit and foliage. Not only that, but these herbivorous theropods grew almost as large as the biggest carnivores–the ornithomimosaur Deinocheirus, the ovriraptorosaur Gigantoraptor and Therizinosaurus were all enormous Cretaceous dinosaurs. But why did these plant-chomping dinosaurs become giants?

In the latest of a spate of papers considering herbivorous theropods, paleontologists Lindsay Zanno and Peter Makovicky paired evolutionary trees with mass estimates derived from femora lengths and a bit of number crunching to see if there was any distinct evolutionary pattern that might explain why Deinocheirus and similar herbivorous theropods grew to such large sizes. Were these Late Cretaceous dinosaurs just the culmination of an evolutionary trend towards ever-larger body size–called Cope’s Rule–or was something else at work?

Zanno and Makovicky didn’t find any sign of directional selection for larger body size. Even though the earliest representatives of the ornithomimosaurs, oviraptorosaurs and therizinosaurs in Asia were much smaller than their Late Cretaceous relatives, the paleontologists point out that this signal has probably been biased by preservation. The 125-million-year-old deposits that contain small members of these groups seem to be skewed towards “mid-sized vertebrates,” the authors point out, and don’t seem to preserve larger dinosaurs that might belong to the same lineages. Indeed, therizinosaurs of about the same age from North America, such as Falcarius, were larger than species in Asia, meaning that herbivorous dinosaurs might have occupied a range of body sizes and evolved larger body sizes at multiple intervals. There was no simple, straight-line trend of bigger and bigger bodies through time.

Nor did a herbivorous lifestyle alone seem to account for gigantism among these dinosaurs. Even though big herbivores gain particular benefits from their size in terms of breaking down tough, low-quality foods more efficiently, Zanno and Makovicky doubt that this relationship drove the evolution of increased body size in the dinosaurs. Instead, they favor “passive processes” that might be tied to ecology and whether these dinosaurs were omnivores more than herbivores. And, as the paleontologists stress, the pattern relies on how complete we think the dinosaur record is. Some ecosystems might be preferentially preserving larger or smaller dinosaurs, which has the potential to skew the big picture. While Zanno and Makovicky ruled out some possibilities, we still don’t really know what accounts for the multiple herbivorous theropod growth spurts.

Post-Script: After four years working with Smithsonian magazine’s wonderful crew, and over 1,000 posts about various aspects of dinosauriana, it’s time for me to move on. I’ll be leaving Dinosaur Tracking next month. Don’t fret, I’ll still be digging into dinosaur science, but I’ll be at a new blog elsewhere on the web (stay tuned for details). I am deeply indebted to my editors Brian Wolly, Sarah Zielinski and, of course, Laura Helmuth (now doing a great job at Slate), as well as the rest of the Smithsonian staff for inviting me to come here and geek out about dinosaurs every day. And many thanks to all of you–the readers and commenters who have helped make this blog a success. You have all made blogging for Dinosaur Tracking an absolute pleasure.

Reference:

Zanno, L., Makovicky, P. 2012. No evidence for directional evolution of body mass in herbivorous theropod dinosaurs. Proceedings of the Royal Society B. 280. doi: 10.1098/rspb.2012.2526

Archaeopteryx had a wing that was different from that of modern birds, and, as seen here, might have been a glider more than a powered flyer. Art by Carl Buell, courtesy of Nicholas Longrich.

How did feathered dinosaurs take to the air? Paleontologists have been investigating and debating this essential aspect of avian evolution for over a century. Indeed, there have been almost as many ideas as they have been experts, envisioning scenarios of dinosaurs gliding through trees, theropods trapping insects with their feathery wings and even aquatic Iguanodon flapping primitive flippers as flight precursors (I didn’t say that all the ideas were good ones). The biomechanical abilities of bird ancestors and their natural history has always been at the center of the debate, and a new Current Biology paper adds more fuel to the long-running discussion.

At present, hypotheses for the origin of avian flight typically fall into one of two categories. Either bird ancestors accrued the adaptations necessary for flight on the ground and, through evolutionary happenstance, were eventually able to take off, or small tree-dwelling dinosaurs used their feathery coats to glide between trees and, eventually, flapped their way into a flying lifestyle. There are variations on both themes, but feathers and the characteristic avian flight stroke are at the core of any such scenario. In the case of the new paper, Yale University paleontologist Nicholas Longrich and colleagues draw from the plumage of early bird Archaeopteryx and the troodontid Anchiornis to examine how feathers changed as dinosaurs started to fly.

In modern flying birds, Longrich and coauthors point out, the wing arrangement typically consists of “long, asymmetrical flight feathers overlain by short covert feathers.” This pattern creates a stable airfoil but also lets the flight feathers separate a little during the upstroke of a wing beat, therefore reducing drag. When the paleontologists examined the fossilized wings of Archaeopteryx and Anchiornis, they found different feather arrangements that would have constrained the flight abilities of the Jurassic dinosaurs.

Both prehistoric creatures had long covert feathers layered on top of the flight feathers. Anchiornis, in particular, appeared to have an archaic wing form characterized by layers of short, symmetrical flight feathers and similarly shaped coverts. Archaeopteryx showed more specialization between the flight feathers and the coverts but still did not have a wing just like that of a modern bird. As a result, Longrich and collaborators hypothesize, both arrangements would have stabilized the wing at the cost of increased drag at low speeds, making it especially difficult for Anchiornis and Archaeopteryx to take off. As an alternative, the researchers suggest that these dinosaurs might have been parachuters who jumped into the air from trees, which might hint that “powered flight was preceded by arboreal parachuting and gliding.”

The trick is determining whether Anchiornis and Archaeopteryx actually represent the form of bird ancestors, or whether the dinosaurs, like Microraptor, were independent experiments in flight evolution. At the Society of Vertebrate Paleontology conference in Raleigh, North Carolina last month, flight expert Michael Habib quipped that all that was needed to make dromaeosaurs aerially competent was the addition of feathers. If Habib is right, and I think he is, then there could have been multiple evolutionary experiments in flying, gliding, wing-assisted-incline-running and other such activities. There’s no reason to think that flight evolved only once in a neat, clean march of ever-increasing aerodynamic perfection. Evolution is messy, and who knows how many ultimately failed variations there were among flight-capable dinosaurs?

The three-step Anchiornis-Archaeopteryx-modern bird scenario of wing evolution fits our expectations of what a stepwise evolutionary pattern would look like, but, as the authors of the new paper point out, shifting evolutionary trees currently confound our ability to know what represents the ancestral bird condition and what characterized a more distant branch of the feathered dinosaur family tree. We need more feathery fossils to further investigate and test this hypothesis, as well as additional biomechanical and paleoecological information to determine whether such dinosaurs really took off from trees. We must take great care in distinguishing between what an organism could do and what it actually did, and with so much up in the air, the debate on the origin of flight will undoubtedly continue for decades to come.

Reference:

Longrich, N., Vinther, J., Meng, Q., Li, Q., Russell, A. 2012. Primitive wing feather arrangement in Archaeopteryx lithographica and Anchiornis huxleyi. Current Biology DOI: 10.1016/j.cub.2012.09.052

An illustration showing the only known bones from Genyodectes. Art in Woodward, 1901, image from Wikipedia.

Paleontologists are naming new dinosaurs at an astonishing rate. In fact, they’re only just begun to skim the diversity of dinosaurs preserved in the world’s Mesozoic formations–hundreds of unknown dinosaur species are undoubtedly hiding in stone. But even among dinosaurs that have a formalized identity, there are many that we know relatively little about. Among them is Genyodectes serus, a carnivorous dinosaur known from the tip of its fearsome jaws and little else.

Though it’s far from being a household name, Genyodectes holds a significant place in the history of South American paleontology. Aside from a tooth found a few years before, the incomplete fossil snout of a Genyodectes was the first definitive non-avian theropod dinosaur found on the continent. As described by paleontologist A.S. Woodward in 1901, the remains of Genyodectes mostly consisted of pieces from the lower jaw, as well as the premaxillary bones and fragments of the maxillary bones in the upper jaw, all of which sported frighteningly long, curved teeth.

There was never any question that Genyodectes was a theropod dinosaur. All the principally carnivorous dinosaurs that we know of fell among various branches of this group. But what sort of theropod dinosaur was it? During the 20th century, different paleontologists proposed that it was a megalosaurid (then a generalized term for big predatory dinosaurs), a tyrannosaur or, after additional theropod remains started to come out of South America, one of the stubby-armed abelisaurids.

After the specimen was given a fresh cleaning, paleontologist Oliver Rauhut reexamined Genyodectes with an eye towards what the dinosaur was and where it came from. Based on notes and geological details, Rauhut proposed that the dinosaur was found in Cañadón Grande in Argentina’s Chubut province in a Cretaceous deposit that probably dates to around 113 million years old. And, based on the limited remains, Rauhut hypothesized that Genyodectes was a later, southern cousin of North America’s Ceratosaurus. While the only known specimen of Genyodectes was cracked and damaged by erosion, the size and the anatomy of the dinosaur’s teeth most closely resembled that of Ceratosaurus–especially in having extremely long teeth in the maxilla. Given this relationship, we might expect that Genyodectes had some kind of skull ornamentation like the nasal and eye horns of its cousin, but we need more fossils to be sure.

Reference:

Rauhut, O. 2004. Provenance and anatomy of Genyodectes serues, a large-toothed ceratosaur (Dinosauria: Theropods) from Patagonia. Journal of Vertebrate Paleontology. 24, 4: 894-902

A restoration of Gigantspinosaurus. Art by Conty, image from Wikipedia.

Stegosaurus was a weird dinosaur. We’ve known that for well over a century, but, as Darren Naish has often pointed out, Stegosaurus was strange even compared to its Jurassic relatives. The dinosaur’s arrangement of broad, alternating plates is a departure from the arrangements of smaller plates, back spikes and accessory spines seen on many other stegosaurs, including the perplexingly well-armed Gigantspinosaurus sichuanensis.

Ornamented with a double row of short, narrow plates along its back, the roughly 160-million-year-old Gigantspinosaurus generally resembled other stegosaurs from Late Jurassic Asia, such as Tuojiangosaurus. But, as you might be able to guess from the dinosaur’s name, the feature that immediately sets Gigantspinosaurus apart from similar species is a enormous hooked spine that jutted out from behind the shoulder blade. These striking spikes were found close to their life position on the first skeleton of this dinosaur to be found–erroneously attributed to Tuojiangosaurus, before being redescribed as Gigantspinosaurus in 1992–although their exact orientation isn’t entirely clear. Did the shoulder spikes curve straight backward, or were they tiled slightly upwards? And, more significantly, how did such prominent ornaments evolve? No one knows.

As yet, we know relatively little about the natural history of Gigantspinosaurus. The dinosaur has a name, and skin impressions have helped researchers restore what the stegosaur looked like, but many aspects of the spiky herbivore’s biology remain mysterious. In the grand scheme of stegosaur evolution, though, the ornamentation of Gigantspinosaurus has sometimes been taken as evidence that similar forms had shoulder spikes. In addition to paired spikes along its tail, the Late Jurassic stegosaur Kentrosaurus possessed an extra pair of spikes along its side. These were originally placed over the hips, but, due to the discovery of Gigantspinosaurus, some researchers have argued that the spikes truly belong at the shoulders.

Frustratingly, paleontologists have yet to find a Kentrosaurus skeleton with side spikes in place. But the discovery of Gigantspinosaurus doesn’t necessarily mean that its cousin Kentrosaurus had the same arrangement. Among stegosaurs, the two genera were relatively distantly related, and it’s entirely possible that more than one side spike arrangement evolved. As paleontologist Heinrich Mallison has argued, the hips of Kentrosaurus seem to possess areas where the spikes could have articulated, and this arrangement would be consistent with the dinosaur’s ornamentation pattern–small plates at the front give way to spikes along the stegosaur’s back and tail. Indeed, the side spikes on Kentrosaurus more closely resemble the same structures along the dinosaur’s back and tail and the shoulder spike of Gigantspinosaurus. If Kentrosaurus had plates up front and serially homologous spikes along the back, then why shouldn’t the hip spikes remain a reasonable hypothesis? Together, Gigantspinosaurus and Kentrosaurus might represent different alternatives in the stegosaur armory.

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