Lessons from Einiosaurus

Xenoceratops was a gnarly-looking ceratopsid. There’s no doubt about that. Much like its horned kin, the dinosaur sported a distinctive array of head ornaments from the tip of its nose to the back of its frill. But that’s hardly the entire story behind this newly named dinosaur.

Contrary to many news reports that focused almost entirely on the dinosaur’s appearance, the real importance of Xenoceratops is in its geological and evolutionary context. The dinosaur is the first identifiable ceratopsid from the relatively unexplored Foremost Formation in Canada, and the creature appears to be at the base of a major horned dinosaur subdivision called centrosaurines. While the dinosaur’s name is certainly aesthetically pleasing, Knight Science Journalism Tracker watchdog Charlie Petit rightly pointed out that the ceratopsid isn’t really any more or less fantastic-looking than close cousins such as Styracosaurus, Spinops and Pachyrhinosaurus. The real importance of the dinosaur–a new data point in an ongoing investigation of a little-known part of the Cretaceous–was obscured by a narrowed focus on the dinosaur’s spiky headgear.

Dinosaurs are perpetually struggling to find context in news reports. Indeed, Xenoceratops is just the latest example and not an anomaly. Theropod dinosaurs are often introduced as Tyrannosaurus rex relatives, even when they’re not particularly closely related to the tyrant king, and journalists had such a fun time giggling over calling Kosmoceratops the “horniest dinosaur ever” that the clues the ceratopsid offered about dinosaur evolution in western North America were almost entirely overlooked. Reports on newly discovered dinosaurs usually contain the vital statistics of when the animal lived, where it was found, how large it was and whatever feature strikes our immediate attention, but the tales dinosaurs have to tell about life, death, evolution and extinction are rarely pulled out by journalistic storytellers.

Fossils don’t divulge their stories all at once, though. Paleontologists spend years drawing paleobiological secrets from dinosaur bones–who was related to whom, grand evolutionary patterns and rates of faunal turnover, and how the animals actually lived. These slowly emerging lines of evidence don’t often receive the same degree of attention. The discovery of a new bizarre species immediately garners journalistic attention, but once the dinosaur has been added to the roster, details about the animal’s life are often forgotten unless the creature earns a new superlative or has been found to have some tenuous connection to T. rex.

Rather than just gripe, though, I want to highlight how discovering and naming a dinosaur is only the initial step in paleontology’s effort to reconstruct prehistoric life. Consider Einiosaurus procurvicornis, a dinosaur I’m selecting here for no other reason than I promised a friend that I’d write about the dinosaur soon.

In 1995, paleontologist Scott Sampson named Einiosaurus from remains of multiple individuals strewn through two bonebeds discovered in Montana’s Late Cretaceous Two Medicine Formation. A geologically younger relative of Xenoceratops by about 4 million years, adults of this ceratopsid species are immediately recognizable by a forward-curved nasal horn, a pair of long, straight spikes jutting from the back of the frill and a suite of more subtle cranial ornaments.

Even before Einiosaurus had a name, though, researchers knew that the collected bones of this dinosaur presented a rich fossil database. Five years before Sampson’s paper, paleontologist Raymond Rogers drew on the two ceratopsid bonebeds to argue that multiple individuals of the species had died in prehistoric droughts. Rather than being places where the bodies of solitary animals accumulated over time, Rogers proposed, the rich assemblages recorded mass mortality events which claimed young and old ceratopsids alike.

The bone assemblages and their geological context outline many tragic dinosaur deaths. But clues about dinosaur lives are preserved inside those bones. For her master’s work at Montana State University, paleontologist Julie Reizner examined the bone microstructure of 16 Einiosaurus tibiae from a single bonebed to reconstruct how these dinosaurs grew and outline their population structure.

The research is still awaiting publication in a journal, but according to Reizner’s 2010 thesis and a poster she presented at the annual Society of Vertebrate Paleontology meeting last month, the histological evidence indicates that these horned dinosaurs grew rapidly until about three to five years of age, when their growth significantly slowed. The dinosaurs did not cease growing entirely, but, Reizner hypothesizes, the slowdown might represent the onset of sexual maturity. Additionally, all the dinosaurs in her sample were either juveniles or subadults–there were no infants or adults (or dinosaurs that had reached skeletal maturity and ceased growing). Even among the two groups, there doesn’t seem to be a continuum of sizes but instead a sharper delineation between juveniles and subadults. If this Einiosaurus bonebed really does represent a herd or part of a herd that died at about the same time, the age gap might mean that Einiosaurus had breeding seasons that occurred only during a restricted part of the year, thus creating annual gaps between broods.

Other researchers have drawn from different bony indicators to restore what the faces of Einiosaurus and similar dinosaurs would have looked like. While the underlying ornamental structures are still prominent in ceratopsid skulls, the horns, bosses and spikes would have been covered in tough sheaths. Thus, in 2009, Tobin Hieronymus and colleagues used the relationship between facial integument and bone in living animals to reconstruct the extent of skin and horn on ceratopsids. While the preservation of the Einiosaurus material frustrated their efforts to detect all the skin and horn structures on the skull, Hieronymus and colleagues confirmed that the nasal horn was covered in a tough sheath and that Einiosaurus had large, rounded scales over the eyes. Artists can’t simply stretch skin over the dinosaur’s skull in restorations–the bone itself shows the presence of soft tissue ornamentation that rotted away long ago.

As with most dinosaur species, we still know relatively little about the biology of Einiosaurus. We are limited to what is preserved in the rock, the technologies at our disposal and the state of paleontological theory. All the same, Einiosaurus is much more than a pretty face. The dinosaur was part of a rich, complex Cretaceous ecosystem, and one in a cast of billions in earth’s evolutionary drama. To me, at least, that is the most entrancing aspect of paleontology. We have only barely begun to plumb the depths of dinosaur diversity, and researchers will continue to introduce us to new species at a breakneck pace, but the true wonder and joy of paleontology lies in pursuing questions about the lives of animals we’ll sadly never observe in the flesh.

References:

Hieronymus, T., Witmer, L., Tanke, D., Currie, P. 2009. The facial integument of centrosaurine ceratopsids: Morphological and histological correlates of novel skin structures. The Anatomical Record 292: 1370-1396

Reizner, J. 2010. An ontogenetic series and population histology of the ceratopsid dinosaur Einiosaurus procurvicornis. Montana State University master’s thesis: 1-97

Rogers, R. 1990. Taphonomy of three dinosaur bone beds in the Upper Cretaceous Two Medicine Formation of northwestern Montana: evidence for drought-related mortality. PALAIOS 5 (5): 394–413.

Sampson, S. 1995. Two new horned dinosaurs from the Upper Cretaceous Two Medicine Formation of Montana; with a phylogenetic analysis of the Centrosaurinae (Ornithischia: Ceratopsidae). Journal of Vertebrate Paleontology 15 (4): 743–760.

Riley Black | | READ MORE

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.