December 6, 2012
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.
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.
June 29, 2012
Earlier this month, I wrote a short article for Nature News about 47-million-year-old turtles that died at a very inopportune moment. Several pairs of prehistoric turtle were fossilized in the act of mating—the tragic consequence of sinking to the toxic depths of a prehistoric lake. An unfortunate fate for the reptiles, but a boon for the paleontologists who found the sexy fossils.
The discovery got me thinking about dinosaur sex. I’ve written quite a bit about the topic before—I ran a four-part series on what we know about dinosaur nooky earlier this year—but much of what we know about dinosaur reproduction only outlines the mating habits of Apatosaurus and company. There’s still a lot we don’t know. In fact, some of the most basic questions are the most persistent. What, exactly, “dinosaur style” looked like has been a subject of frequent speculation but very little rigorous research, and no dinosaurs have ever been found fossilized in the act to show us how it was done. But does this mean that we’ll never find dinosaur sex preserved in stone?
Copulation is typically a brief moment in time. For such an intimate snapshot to become part of the fossil record, exceptional circumstances are required. In the case of 320-million-year-old sharks preserved in what may be part of a mating ritual, a quick death and rapid burial in fine-grained sediment locked the fishy forms in rock. We also know a little about how prehistoric insects reproduced thanks to mating pairs trapped in amber. And as for the turtles, the copulating reptiles drifted down to a layer of water that not only killed them, but kept their bodies safe from scavengers as sediment settled on their bodies. For sex to make it into the fossil record, a quick death, rapid burial and high-definition preservation are all required.
Given these conditions, I’m not very hopeful that paleontologists are going to find mating dinosaurs. Even the smallest dinosaurs were too big to be trapped in amber, and as fully-terrestrial animals, dinosaurs did not copulate in the sort of aquatic environment where fast death and burial would have been possible. Dinosaurs just didn’t mate in the kind of habitats where there was a high potential for the amorous pairs to perish and be entombed in sediment. Good news for them, but frustrating for paleontologists.
Still, I shouldn’t be too hasty in saying that we’ll never find mating dinosaurs. I never expected that paleontologists would discover turtles caught in the act, for one thing. And the fossil record is full of surprises, including fossils that detail some aspects of dinosaur behavior. Paleontologists have previously discovered dinosaurs preserved in nesting and sleeping positions, and there’s the fighting dinosaur pair. Maybe someday a fortunate paleontologist will help us solve the prehistoric mating mystery by finding dinosaurs that made love, not war.