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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.

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

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

The upper and lower jaws of Duriavenator, illustrated when they were thought to belong to Megalosaurus, in A History of British Fossil Reptiles Vol. II. Image from Wikipedia.

If you have been following the Dinosaur Alphabet series so far, you may have noticed a pattern among the first four entries. At one time or another, all the dinosaurs I’ve selected so far were thought to be different animals. The horned Agujaceratops was originally named as a species of Chasmosaurus, the distinctive high-spines of Becklespinax gave Richard Owen’s dopey Megalosaurus its hump, the sauropod Cetiosaurus was originally envisioned as a giant crocodile, and the armored Dyoplosaurus was lumped in with its cousin Euoplocephalus before being split back out again as a distinct genus. I didn’t intend this trend, but it struck me when I came across  one of the rejected candidate for yesterday’s entry for the letter D. Had it not shared much of its story with Becklespinax, I would have picked Duriavenator:

Megalosaurus was a mess. Even though this Jurassic carnivore has been a prehistoric icon ever since it was named by William Buckland in 1824, it has been one of the most confounding of all dinosaurs. That’s because generations of researchers attributed dozens of fragments and isolated bones to the dinosaur, creating a monstrous composite of animals from different places and times. Dinosaurs were unfamiliar animals–the name itself only coined in 1842–and 19th-century naturalists didn’t have the kind of geologic resolution their intellectual descendants rely on to properly constrain when particular species lived. Sometimes researchers named too many species on the basis of scrappy, non-overlapping material, and other times they applied the same name ad infinitum to roughly similar fossils.

Eventually, though, it became apparent that Megalosaurus was unstable. No one could say what the dinosaur really looked like or what bones could accurately be attributed to the predator.  The situation was so bad that, in 2008, paleontologist Roger Benson and colleagues stripped the name Megalosaurus from everything save for the fragment of jaw originally used to name the animal. Whether the rest of the fossils really belonged to Megalosaurus remained to be seen, and, as Benson demonstrated later the same year, at least one other theropod had been improperly obscured behind the famous name.

In 1883, anatomist Richard Owen described a partial theropod skull found on Dorset, England, as another piece of Megalosaurus “bucklandi.” The sharp-toothed dinosaur was only represented by parts of the upper and lower jaws, but, given how little was known about Megalosaurus to start with, Owen’s assignment was reasonable. Nearly a century later, paleontologist Michael Waldman proposed that these fossils represented a previously unknown species of the dinosaur he called Megalosaurus hesperis. Other researchers weren’t sure that the bones really belonged to Megalosaurus, but it wasn’t until Benson’s reexamination that the fossils were split out as a different dinosaur. While the dinosaur was a close cousin of Megalosaurus bucklandii, Benson was able to pick out subtle anatomical characteristics that distinguished the fragmentary skull. In Benson’s analysis, what once was Megalosaurus took on a new life as Duriavenator hesperis.

Benson’s reconstruction of Megalosaurus, with known elements in white and reconstructed portions in grey. While Duriavenator was older and anatomically distinct, the dinosaur would have been similar in form to Megalosaurus. From Benson, 2010.

Unfortunately, we don’t know very much at all about Duriavenator. The dinosaur lived about 170 million years ago in Jurassic England and was a large carnivore of comparable size to the 20-foot-plus Megalosaurus, but that’s where the evidence gives out. Perhaps other Duriavenator specimens are resting in museum collections, but until the discovery of a nearly complete skeleton allows paleontologists to connect the jaws to a body, the dinosaur will be an enigma. But here Megalosaurus itself gives us reason to hope. The Duriavenator paper was just part of Benson’s effort to rehabilitate Megalosaurus, and in 2010 he published a refined, revised reconstruction of the dinosaur’s skeleton based on material collected from Stonesfield, Oxfordshire–the locality where the original jaw came from. Perhaps, with a little detective work in the lab and in the field, paleontologists might also be able to fill out the form of Duriavenator and other Middle Jurassic mysteries.

References:

Benson, R., Barrett, P., Powell, H., Norman, D. 2008.  The taxonomic status of Megalosaurus bucklandii (Dinosauria, Theropoda) from the Middle Jurassic of Oxfordshire, UK. Palaeontology, 51, 2: 419-424.

Benson, R. 2008.  A redescription of “Megalosaurus” hesperis (Dinosauria, Theropoda) from the Inferior Oolite (Bajocian, Middle Jurassic) of Dorset, United Kingdom. Zootaxa 1931: 57-67

Benson, R. 2010. A description of Megalosaurus bucklandii (Dinosauria: Theropoda) from the Bathonian of the UK and the relationships of Middle Jurassic theropods. Zoological Journal of the Linnean Society 158: 882. doi:10.1111/j.1096-3642.2009.00569.x.

Waldman, M. 1974. Megalosaurids from the Bajocian (Middle Jurassic) of Dorset. Palaeontology 17, 2:325-339.

Fossil teeth, found by Ferdinand Hayden in Montana, which Joseph Leidy attributed to the dinosaur “Trachodon.” From Leidy, 1860, image from Wikipedia.

More than 150 years ago, a young naturalist picked up a collection of isolated teeth and bones weathering out of the ground in what is now northern Montana. These weren’t the remains of any living animals but vestiges of Cretaceous life that naturalists had only just begun to recognize and categorize. Even the young explorer who picked them up, Ferdinand Hayden, didn’t know what they were, and so he sent them back east for identification. As the Philadelphia-based polymath Joseph Leidy later determined, some of Hayden’s scrappy finds were dinosaurs–among the earliest recorded dinosaur discoveries in the American West.

Hayden wasn’t the first person to discover fossils in North America. First Nations peoples were familiar enough with strange fossil bones that the prehistoric remnants inspired their legends, and naturalists such as Thomas Jefferson puzzled over what was left of Ice Age mammals such as mastodons and giant ground sloths. Dinosaurs got a relatively early start, too, although naturalists didn’t always realize what they had found. Even though he misidentified the fossil as part of a giant fish, explorer Meriwether Lewis found part of a dinosaur rib in the vicinity of what is now Billings, Montana, when he passed through the area in 1806 on his famous expedition with William Clark. And starting in the 1830s, the Amherst geologist Edward Hitchcok described scores of Early Jurassic dinosaur tracks, which he attributed to prehistoric birds.

All the same, the bits and pieces Hayden found showed that the wilds of the western territories harbored dinosaurs and were a portent of the “Bone Wars” that would later unfold among the badlands of Montana, Wyoming and Colorado. Now, the Great Falls Tribune reports, paleontologist Kristi Curry Rogers and her geologist husband Ray Rogers believe that they have located the place where Hayden stumbled across the Cretaceous tidbits.

Even though Hayden did not keep detailed field notes, a brief mention in a technical paper of the area in which he found the fossils helped the Rogers team narrow down their search area. From there, they followed game trails and looked for sites that would have produced the kinds of fossils Hayden picked up. They can’t be entirely certain that their site is the very same Hayden sampled, and they are wary of divulging the exact location given how often fossil sites are vandalized, but the Rogers have placed Hayden’s stop somewhere in Montana’s Missouri River Breaks north of Winifred. With assistance from the Bureau of Land Management, they want the area to be placed in the National Register of Historic Places–a testament to Hayden’s lasting contribution to American paleontology.