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A restoration of Saurolophus angustirostris based upon skeletal and soft-tissue fossils. Art by L. Xing and Y. Liu, from Bell, 2012.

We love to bring dinosaurs back to life. From museum displays and academic papers to big-budget movies, we have an obsession with putting flesh on old bones. How much anatomical conjecture and artistic license is required to do so varies from dinosaur to dinosaur.

Some dinosaurs are known from a paltry collection of fragments and require a considerable among of reconstruction and restoration on the basis of better-known specimens of related species. Other dinosaurs are known from complete skeletons and require less osteological wrangling, but they still present the challenge of filling in the soft tissue anatomy that the skeleton supported in life. Every now and then, though, paleontologists discover skin impressions associated with the bones of dinosaurs. These rare fossils can give us a better idea of what the outside of some dinosaurs looked like.

Skin impressions are found most often with hadrosaurs. These herbivores, such as Edmontosaurus and the crested Corythosaurus, were plentiful and seemed to dwell in habitats where deceased dinosaurs could be buried rapidly by sediment, a key to the preservation of soft-tissue anatomy. In the roughly 68-million-year-old strata of Canada and Mongolia, for example, skeletons of two different species of the hadrosaur Saurolophus have been found associated with skin impressions. But these fossils can do more than help use restore the outer appearance. According to a new paper by University of Alberta paleontologist Phil Bell, subtle differences in Saurolophus skin traces can help paleontologists distinguish one species of dinosaur from on another on the basis of soft tissue anatomy alone.

In 1912, professional dinosaur hunter Barnum Brown named the hadrosaur Saurolophus osborni from skeletons found in Alberta’s Horseshoe Canyon Formation. Although not mentioned at the time, three skeletons of this species were associated with skin impressions from various parts of the body, including the jaw, hips, foot and tail. Forty years later, from skeletons found in a huge bonebed called the “Dragon’s Tomb” in Mongolia’s Nemegt Formation, paleontologist Anatoly Konstantinovich Rozhdestvensky named a second species, Saurolophus angustirostris. Numerous skin impressions were found with skeletons of this species, too. The fact that two Saurolophus species had been found with intact skin impressions provided Bell with a unique opportunity to compare the outer anatomy of two closely related dinosaurs.

Both Saurolophus species had pebbly skin. Like other hadrosaurs, the skin of these dinosaurs was primarily composed of non-overlapping scales or tubercles of varying shape. In detail, though, Bell ascertained that the skin of the two species differed enough that one species can be readily distinguished from the other.

Along the base of the tail, the North American species (S. osborni) had mosaic-like clusters of scales, while the species from Mongolia (S. angustirostris) seemed to have vertical bands of specialized scales interspersed with larger, rounded scales Bell terms “feature scales.” This pattern in S. angustirostris remained consistent in young and old individuals—evidence that this was a real pattern peculiar to this species and not just a matter of variation among individuals.

Frustratingly, the skin impressions from the North American species cover less of the body and come from fewer specimens than those from the Dragon’s Tomb. That limits the possible comparisons between the species. Still, based on the consistent differences between the Saurolophus species in the skin at the base of the tail, it appears that paleontologists might be able to use soft-tissue anatomy to identify and diagnose particular dinosaur species. This could be especially useful for the study of hadrosaurs. These dinosaurs are notoriously difficult to tell apart on the basis of their post-cranial skeleton, but Bell’s study hints that skin impressions might show prominent differences. Judging a dinosaur by its cover might not be such a bad idea.

References:

Bell, P. (2012). Standardized Terminology and Potential Taxonomic Utility for Hadrosaurid Skin Impressions: A Case Study for Saurolophus from Canada and Mongolia PLoS ONE, 7 (2) DOI: 10.1371/journal.pone.0031295

A reconstruction of Velociraptor, complete with a scleral ring in the eye, at the Wyoming Dinosaur Center in Thermopolis, WY. Photo by the author.

What’s scarier than a Velociraptor? A Velociraptor at night. That’s the hook I used last spring when a study published in Science used the fossilized bony rings that once supported dinosaur eyes to discern which species might have run around during the day and which stalked the night. (In truth, you wouldn’t have much to fear from Velociraptor at either time—the feathered dinosaur was about the size of a turkey and probably specialized in prey smaller than themselves.) Since the time that study was published, however, other researchers have raised questions about whether or not we can really use remnants of dinosaur eyes to study their behavior.

The idea behind the 2011 Science study by paleontologists Lars Schmitz and Ryosuke Motani was relatively simple. In dinosaurs, as in many other vertebrates except mammals and crocodylians, a ring of small bones helped support the pupil and iris of the eye. The structure is technically known as a scleral ring and acts not only as a proxy for eye size. A wider hole in the middle of the ring would indicate an ability to take in more light, and thus would be consistent with noctural habits, while a relatively smaller window would be more consistent with daytime behavior. Applied to dinosaurs, the study seemed to show that many predators hunted at night while large herbivores were most active during the mornings and evenings.

In a comment published in December last year, however, researchers Margaret Hall, Christopher Kirk, Jason Kamilar and Matthew Carrano pointed out that this correspondence may not be so simple. In addition to questioning the statistical analysis used by Schmitz and Motani, Hall and co-authors noted that there is a considerable degree of overlap in scleral ring anatomy between animals active at night and those active during the day. Among birds and lizards, for example, the scleral rings of some day-dwelling species are very much like those of nocturnal ones. The anatomy of the scleral rings may not be a clear-cut predictor of behavior.

That isn’t to say that the scleral rings can’t tell us some important things about the eyes of extinct animals. Hall and collaborators remarked that the inner diameter of the scleral ring corresponds to the diameter of the cornea—an essential measurement for figuring out how much light can enter the eye. The problem is that another measurement—axial length, or the distance from the front to the back of the eye—is essential for gauging the vision of dinosaurs, but no known specimen has the preserved soft tissue anatomy required to figure this out. Until other anatomical markers of eye shape and size are found, our inferences about whether dinosaurs were active during the night or day will be weak. “[I]t is not yet possible to reconstruct the activity patterns of most fossil archosaurs with a high degree of confidence,” Hall and colleagues concluded.

Schmitz and Motani issued a rebuttal in the same issue of Science. In defense of their paper, Schmitz and Motani reject the criticisms as based on what they consider to be “unscreened data, untenable assumptions, and inappropriate methods” and affirm that their methodology properly categorized dinosaur behavior on the basis of what is known about modern animals. Regarding anatomical minutiae such as the axial length of the eye, Schmitz and Motani suggest that the outside border of the scleral ring is correlated with axial length and therefore can be used as a proxy to reconstruct an animal’s visual capabilities. Altogether, Schmitz and Motani affirm that “the inference of nocturnality in dinosaurs from scleral ring and orbit morphology is sound.”

A good deal of this disagreement deals with methods of statistical comparison and analysis that, I must admit, are over my head. Still, there remain important questions about the way skeletal anatomy relates to soft tissue anatomy. When dealing with animals that have been extinct for millions and millions of years, can we accurately reconstruct the shape and important features of their eyes? Some skeletal features definitely correspond to soft-tissue structures, but interpreting the capabilities of those reconstructed eyes is a more difficult task and the central point of contention. I have little doubt that there were dinosaurs that were active at night, in the heat of the day, and at dawn and dusk, but the trick lies in accurately figuring out which ones were which.

References:

Schmitz, L., & Motani, R. (2011). Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology Science, 332 (6030), 705-708 DOI: 10.1126/science.1200043

Hall, M., Kirk, E., Kamilar, J., & Carrano, M. (2011). Comment on “Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology” Science, 334 (6063), 1641-1641 DOI: 10.1126/science.1208442

Schmitz, L., & Motani, R. (2011). Response to Comment on “Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology” Science, 334 (6063), 1641-1641 DOI: 10.1126/science.1208489

A restoration of the island hadrosauroid Tethyshadros by Nobu Tamura. Image from Wikipedia.

Everyone knows what a “duck-billed” dinosaur was. This bit of shorthand has been permanently grafted onto the hadrosaurs—the widespread group of herbivorous dinosaurs with elongated skulls and what appear to be duck-like beaks.

The title made perfect sense during the early 20th century when these dinosaurs, such as Edmontosaurus and Parasaurolophus, were thought to be amphibious creatures that dabbled in the water for soft plants and escaped into Cretaceous lakes when predators came near. If the dinosaurs looked like monstrous ducks, then they must have acted like ducks. But that vision of paddling hadrosaurs was discarded decades ago. These dinosaurs were terrestrial animals, and discoveries of well-preserved hadrosaur beaks have indicated that the mouths of these dinosaurs were not so duck-like, after all. One beautifully preserved Edmontosaurus skull on display at the Natural History Museum of Los Angeles shows that the tough beak of this dinosaur ended in squared-off, almost vertical croppers and not a duck-like, spoon-shaped bill. The so-called duck-billed dinosaurs didn’t look like mallards at all. And one of the strangest variations in beak shape was found in a small, island-dwelling hadrosauroid described in 2009.

On the basis of a nearly complete and articulated skeleton, paleontologist Fabio Dalla Vecchia named the dinosaur Tethyshadros insularis. The name is a testament to where the dinosaur lived. During the time of Tethyshadros, around 71 million years ago, an ancient sea called Tethys covered most of southern Europe. This oceanic incursion created chains of islands, and it was on one of these islands—where Italy sits today—that Tethyshadros lived. More than that, the isolation of the dinosaur on the island might have been responsible for the dinosaur’s relatively small size (about 13 feet long) compared to its distant, North American cousins such as Edmontosaurus—it’s an example of a phenomenon called insular dwarfism that has been documented for other prehistoric herbivores, including dinosaurs.

But one of the most peculiar aspects of Tethyshadros was its beak. Instead of a long, low duck bill, the upper beak of this dinosaur was a ridged structure jutting out in a shape roughly reminiscent of a snowplow. And rather than being smooth, the margin of the upper beak was pointed, with the middle point being the largest. This general type of serrated beak has been seen before in iguanodontian dinosaurs—the stock from which hadrosaurs evolved, with Tethyshadros being closer to hadrosaurs than to the iguanodontians—but never before in such an extreme shape. Why Tethyshadros had such a strange beak is a mystery. As paleontologist Darren Naish wrote in his detailed summary of this new dinosaur, “Did [the beak spikes] help Tethyshadros to bite at specific food items? Were they for grooming? For display? The mind boggles.”

References:

Dalla Vecchia, F. (2009). Tethyshadros insularis, a new hadrosauroid dinosaur (Ornithischia) from the Upper Cretaceous of Italy Journal of Vertebrate Paleontology, 29 (4), 1100-1116 DOI: 10.1671/039.029.0428

About 110 million years ago, an ankylosaur settled on the bottom of a Cretaceous sea. This was no place for a dinosaur. No dinosaurs were adapted to a marine lifestyle, and the heavily armored ankylosaurs were probably the least suited to paddling around in the water. Yet, almost a year ago, shovel operator Shawn Funk found an ankylosaur in the marine, Early Cretaceous sediments at a Suncor mine in northern Alberta. How did the dinosaur get there?

Donald Henderson, curator of dinosaurs at the Royal Tyrrell Museum, explained how this dinosaur died, was preserved and was discovered in a recent lecture for the Royal Tyrrell Museum Speaker Series. Almost everything about the discovery was lucky. The dinosaur just happened to settle in a place where sediment quickly covered its body; the carcass was not torn apart by scavengers; the shovel operator who stumbled across the ankylosaur recognized that he found something potentially significant and the discovery of the dinosaur in the mine meant that paleontologists had lots of heavy machinery on hand to help excavate the skeleton.

But the strangest aspect of the find is the ecological context of the dinosaur. This ankylosaur must have lived along the coastline of the great Western Interior Seaway which once split North America into two. But that was many, many miles away from where the skeleton was found. Exactly how the dinosaur died is unknown, but as Henderson notes, the carcass undoubtedly floated upside-down through the sea. The gases from decomposition gave the body enough buoyancy to travel—what paleontologists commonly refer to as a “bloat and float” scenario.

A parent Massospondylus attends to its hatchlings. Art by Julius Csotonyi.

Two years ago, paleontologist Robert Reisz and colleagues revealed that the Early Jurassic dinosaur Massospondylus started off life as an awkward little thing. An exceptional set of eggs recovered from South Africa in 1976 contained the well-preserved skeletons of these baby dinosaurs, and the infants did not look very much like their parents. A roughly 20-foot-long adult Massospondylus had an extended neck and a long, low skull and it walked on two legs. But a baby of the same dinosaur had a short neck, a big head for its body, and it walked on all fours. The change between baby and adult was fantastic, and now, in a new PNAS paper, Reisz and colleagues provide an even more detailed look at how Massospondylus started life.

In 2006, Reisz and collaborators located the site where the Massospondylus eggs had been discovered in South Africa’s Golden Gate Highlands National Park. They found more eggs and baby dinosaurs, but not just that. About 190 million years ago, this place was a nesting ground that multiple Massospondylus used from one season to the next.

The paleontologists have discovered bones, eggshell fragments and ten egg clutches—the largest has 34 eggs—within a six-and-a-half-foot swath of siltstone. These nest sites were not all found in the same level, demonstrating that this particular place was used multiple times by Massospondylus moms. Despite the fact that this place was a nesting ground, however, there does not appear to be any evidence that the parent dinosaurs made special accommodations for the eggs—no clear sign of bowl-shaped depressions or other hints of nest construction were found.

Exactly how much parental care adult Massospondylus offered their babies is unknown. Crocodylians and many birds—the closest living relatives of dinosaurs—often attend their nests from the time the eggs are laid and guard their offspring for at least a short interval after their babies hatch. Massospondylus may have done the same, and small tracks found in siltstone blocks indicate that hatchling dinosaurs remained in the nesting site after emerging from their eggs. The tiny hind- and fore-foot tracks are about twice the size of what would be expected for a newly-hatched Massospondylus, and so it seems that the babies stayed at the site until they doubled in size, at least.

The setting of the nesting site allowed all these intricate details to be preserved. In the time of Massospondylus, the site was a relatively dry habitat near the margin of a prehistoric lake. Relatively gentle flooding events covered up the nest site with fine-grained sediment, and afterwards the area dried out. This was a regular, seasonal cycle, and the bad timing of some expectant dinosaur parents resulted in the good fortune of the paleontologists.

With this new data point, Reisz, Evans, and co-authors looked at the big picture of dinosaur reproduction to see which traits might be widely shared and which might be specializations. It seems that communal nesting sites that were used over and over again was an old, common aspect of dinosaur behavior. And, regarding sauropodomorphs specifically, the Massospondylus site may provide some insight into the evolution of different reproductive behavior among its larger sauropod cousins. Evidence from some sauropod nesting sites has been taken to suggest that exceptionally large long-necked dinosaurs did little more than lay eggs and leave their offspring to fend for themselves. What the Massospondylus site might indicate is that the “lay ‘em and leave ‘em” strategy was not the ancestral state for these dinosaurs, but instead was a reproductive specialization related to increasing body size.

So far, this is the oldest known dinosaur group nesting site. Similar sites created by hadrosaurs and sauropods are about 100 million years younger—a vast expanse of time. Potentially earlier nest site finds have not been well studied. One such Late Triassic site in Argentina has yielded multiple infant and juvenile specimens of the sauropodomorph Mussaurus. I asked David Evans, a paleontologist at the Royal Ontario Museum and one of the co-authors of the new study, about the possibility that the Mussaurus locality is an even older nesting ground. “[E]vidence of any form of extensive nesting site [at the Mussaurus localities] is very scant,” he said, but noted that “given our luck in South Africa, I would not at all be surprised if there are a bunch of nests similar to what we have [found] at the Mussaurus localities too—someone just needs to look and document.”

References:

Pol, D., & Powell, J. (2007). Skull anatomy of Mussaurus patagonicus (Dinosauria: Sauropodomorpha) from the Late Triassic of Patagonia Historical Biology, 19 (1), 125-144 DOI: 10.1080/08912960601140085

Reisz, R., Evans, D., Roberts, E., Sues, H., & Yates, A. (2012). Oldest known dinosaurian nesting site and reproductive biology of the Early Jurassic sauropodomorph Massospondylus Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1109385109