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Not only was Ornithomimus feathered, but the dinosaur’s fluffy coat changed as it aged. Lovely art by Julius Csotonyi.

Another week, another feathery dinosaur. Since the discovery of the fluffy Sinosauropteryx in 1996, paleontologists have discovered direct evidence of fuzz, feather-like bristles and complex plumage on over two dozen dinosaur genera. I love it, and I’m especially excited about a discovery announced today. In the latest issue of Science, University of Calgary paleontologist Darla Zelenitsky adds another enfluffled species to the dinosaurian ranks. Even better, the specimens raise hopes that many more dinosaurs might be preserved with their feathery coats intact.

Zelenitsky’s downy dinosaurs are not newly discovered species. Ornithomimus edmontonicus was initially described by famed bone hunter C.H. Sternberg in 1933, and it is one of the characteristic Late Cretaceous species found in Alberta, Canada’s fossil-rich Horseshoe Canyon Formation. In Sternberg’s time, these dinosaurs were thought to be scaly, but recent finds of so many feathery dinosaurs has raised the likeliehood that the “ostrich mimic” dinosaur was at least coated in some sort of dinofuzz.

A family tree of Saurischian dinosaurs, showing lineages within this group with direct evidence for feathers. From Zelenitsky et al., 2012.

The prediction of fluffy Ornithomimus came from the spread of feathers on the coelurosaur family tree. The Coelurosauria is a major dinosaur group that encompasses tyrannosaurs, compsognathids, ornithomimosaurs, alvarezsaurs, oviraptorosaurs, deinonychosaurs and birds. To date, evidence of feathers has been found in every coelurosaur lineage except one–the ornithomimosaurs. The spread of feathers hinted that some sort of plumage was present in the common ancestor of all coelurosaurs and therefore should have been inherited by the ornithomimosaurs, but, until now, no one had found direct evidence.

A trio of Ornithomimus skeletons have finally confirmed what paleontologists expected. Zelenitsky enthusiastically explained the details to me by phone earlier this week. In 1995, when Zelenitsky was a graduate student, paleontologists uncovered an articulated Ornithomimus with weird marks on its forearms. No one knew what they were. But in 2008 and 2009 a juvenile and an adult Ornithomimus turned up with preserved tufts of filamentous feathers. “When we found these specimens,” Zelenitsky said, “we made the link to the 1995 dinosaur.” All those strange marks on the arms of the previously discovered Ornithomimus, Zelenitsky and colleagues argue, are traces of longer, shafted feathers.

Even though paleontologists expected feathery Ornithomimus, the discovery was still a surprise. “I was in disbelief,” Zelenitsky said. “They’re the first feathered dinosaurs from the Americas, and the first ornithomimosaurs with feathers, as well. It was shocking to say the least.”

But there’s more to the find than simply adding another species of fluffy dinosaurs to the list. The fact that the adult and juvenile animals had different kinds of plumage adds new evidence that coelurosaurs changed their fluffy coats as they aged. “The one juvenile was completely covered in filamentous type feathers,” Zelenitsky said. What the adults looked like comes from the two other specimens. One adult skeleton, lacking forearms, preserves fuzzy feathers, and “the second adult had markings on the forearm.” Together, the specimens indicate that adult Ornithomimus were mostly covered in fuzz but developed more complex arm feathers by adulthood.

Sex is probably behind the plumage change. “We infer that because these wing feathers are not showing up until later in life, they were used for reproductive purposes,” Zelenitsky said. Perhaps adult Ornithomimus used flashy arm feathers to strut their stuff in front of potential mates. Then again, based upon the resting and brooding postures of other theropod dinosaurs, adult Ornithomimus could have used their proto-wings to cover their nests. We don’t know for sure, but the developmental change appears to be another example of dinosaurs undergoing significant changes as they approach sexual maturity. This discovery, and others like it, will undoubtedly play into the ongoing discussion about the role of sexual selection in dinosaur biology and evolution.

Best of all, the new study indicates that paleontologists may soon find more feathered dinosaurs in unexpected places. The Ornithomimus skeletons were found in prehistoric river deposits composed of sandstone. Since almost all feathered non-avian dinosaurs have been found in fine-grained sediment–such as those around Liaoning, China–paleontologists thought that coarser-grained sandstone deposits were too rough to record such fine details. Now we know better. “That’s the really exciting part of it,” Zelenitsky says. If traces of dinosaur feathers can be preserved in sandstone, the twist opens up the possibility that paleontologists might find fluff and feathers with a greater array of dinosaurs–including the tyrannosaurs, deinonychosaurs, therizinosaurs and other coelurosaurs of North America. The trick is recognizing the traces before they’re destroyed during excavation and preparation. Rock saws and airscribes can all too easily obliterate the delicate fossils. A word to researchers–keep your excavation tools sharp, and your eyes sharper.

Reference:

Zelenitsky, D., Therrien, F., Erickson, G., DeBuhr, C., Kobayashi, Y., Eberth, D., Hadfield, F. 2012. Feathered non-avian dinosaurs from North American provide insight into wing origins. Science. 338, 510-514

Tail vertebrae from a previously known Alamosaurus specimen (A), compared with a newly-discovered Alamosaurus tail vertebra (B) and a tail vertebra from the large titanosaur Futalognkosaurus (C). From Fowler and Sullivan, 2011.

Alamosaurus was an unusual sauropod. What makes it so remarkable is not so much its appearance—the dinosaur seems to be a fairly typical member of a group called titanosaurs—but when and where it lived. Even though North America once hosted multiple, coexisting genera of sauropods during the Late Jurassic, that diversity was eventually lost until, about 100 million years ago, there were none left on the continent. By this time the horned dinosaurs and hadrosaurs were the primary herbivores on the landscape. Then, after a 30 million year absence, sauropods returned to what is now the southwestern United States in the form of Alamosaurus. A new study suggests this dinosaur may have been one of the biggest ever.

Among the various dinosaur superlatives, the title of “biggest dinosaur” is always the most hotly contested. It’s also among the most difficult to assign. Despite their size, many of the contenders for biggest dinosaur, whether they’re going for the all-time title or just in a particular slice of prehistory, are known from only partial remains. Some degree of estimation is often required to envision the whole animal. Then there’s the problem of what characteristics make a dinosaur the largest of its kind—is it just length? Or do weight and height also get figured in? Many of the contenders for largest sauropod ever are estimated to have been about 90 to to 110 feet long, and so far, no dinosaurs have exceedingly outstripped that range. (I am not counting the supposed giant Amphicoelias since the original material was lost long ago and no additional remains have been confirmed.) This range may represent some kind of upper limit for sauropod body size due to constraints of structure or biology.

According to the new paper by paleontologists Denver Fowler from the Museum of the Rockies and Robert Sullivan of the State Museum of Pennsylvania, newly-discovered fossils from the latest Cretaceous of New Mexico may indicate that Alamosaurus reached the top tier of titanosaur size. The material in question consists of two partial vertebrae and a partial femur found at different field sites, and the assignment of these bones to Alamosaurus is based upon the fact that this sauropod is the only one presently known from the Late Cretaceous deposits where the remains were found. (Though it should be noted that the fragmentary nature of this specimen and others from around the same time makes comparisons between dinosaurs and estimates of actual diversity difficult.) The fact that the bones were found at different field sites means that the three bones came from different individuals of different sizes and ages, but comparison of these bones with those of other sauropods can provide a rough idea of how large Alamosaurus grew to be.

The two vertebrae described by Fowler and Sullivan appear to indicate that Alamosaurus could reach roughly the same size as the titanosaurs Futalognkosaurus and Puertasaurus. Both of these dinosaurs from South America are estimated to have been within the roughly 90 to 110 foot size range that many other big sauropods fall into, although the fact that paleontologists have not yet found complete skeletons of either dinosaur means we can’t be really sure just how big they got. This makes estimating the actually upper size range of Alamosaurus problematic. The vertebrae Fowler and Sullivan describe are certainly larger than those previously assigned to Alamosaurus, but accurately estimating the size of the dinosaur requires reliance on previous estimates of other partially known specimens. Alamosaurus appears to have been in the upper sauropod size class—and was probably among the biggest ever to have lived in North America—but additional fossil material from this dinosaur and other giants will be needed to figure out exactly how enormous they became.

There was one other wrinkle to this story that caught my attention. A Montana State University press release about the study stated that a tyrannosaur tooth was found near another Alamosaurus vertebrae that was being excavated by the same team. Whether this is an indication of predation or scavenging by the tyrannosaur remains to be seen—teeth are resilient and easy to transport—but the association is a further confirmation that Alamosaurus shared its habitat with Tyrannosaurus rex. The two dinosaurs have been found in the same deposits before, such as Utah’s North Horn Formation, and the occurrence of the two dinosaurs in New Mexico makes me wonder exactly how a large tyrannosaur would go about hunting an enormous sauropod. Clashes of titanic dinosaurs were not restricted to the Late Jurassic of North America or the Cretaceous of South America. At the close of the Cretaceous, prehistoric New Mexico may have been the setting for confrontations between the largest herbivore and carnivore ever to live in North America.

References:

Fowler, D., & Sullivan, R. (2011). The first giant titanosaurian sauropod from the Upper Cretaceous of North America Acta Palaeontologica Polonica DOI: 10.4202/app.2010.0105

A particularly spiky restoration of Ampelosaurus, a Late Cretaceous sauropod from Europe. Image from Wikipedia.

For the past century, paleontologists have been trying to figure out one of the most puzzling disappearing acts in the fossil record.

In both Europe and North America, the Jurassic was the heyday of the sauropod dinosaurs. After the beginning of the Cretaceous period 145 million years ago, however, the number of these dinosaurs dwindled and they ultimately disappeared. It was only much later that other sauropods from elsewhere reestablished the presence of these dinosaurs in Europe and North America.

But, as explained in an in-press Palaeogeography, Palaeoclimatology, Palaeoecology paper by paleontologists Philip Mannion and Paul Upchurch, this great “sauropod hiatus” is probably an illusion. Over the past few years new discoveries have begun to fill in the sauropod gap on both continents. Sauropods may be missing in North America only between about 90 to 75 million years ago, and they are absent in Europe during two short intervals between 95 and 83 million years ago. That is quite a different picture than the near-total elimination of sauropods at the end of the Jurassic I learned about as a kid.

Of course, there are a few uncertainties about with these dates. Some of the 80- to 70-million-year-old sauropod remains from North America have been found to actually belong to hadrosaurs, and these fossils have been difficult to get dates for. Even so, recent discoveries of tracks and bones have extended the range of sauropods in Europe and North America through at least the first 50 million years of the Cretaceous, and scrappy remains between 90 and 75 million years ago may further fill in the gaps.

Frustratingly, though, the recovery of better fossil remains from the latter half of the Cretaceous may be a difficult task. The reason why explains the apparent sauropod gap.

Sauropod fossils are found primarily among inland deposits, perhaps indicating that these dinosaurs preferred inland habitats. During the middle of the Cretaceous, there was an increase in coastal deposits in both Europe and North America. (In North America, for example, the Western Interior Seaway split the continent in half and created long stretches of coast in what is now the middle of the continent.) The new research by Mannion and Upchurch shows that the number of sauropod species rises and falls with the amount of known inland habitat, meaning that the rarity of Cretaceous sauropods may indicate a lack of well-sampled, inland fossil sites from the right age. In other words, sauropods may have moved away from the better-sampled coastal areas during the mid-Cretaceous and only spread out again when the coastal areas receded.

The closure of the great sauropod gap also has important implications for where the last of these dinosaurs in both Europe and North America came from. In North America, for instance, Alamosaurus is recognized as one of the very last sauropod dinosaurs and has been thought to be a descendant of a South American ancestor. A paper about the same topic published in the same journal just last year supported this hypothesis, but Mannion and Upchurch dispute the idea that Alamosaurus was an immigrant from the south.

Alamosaurus was a titanosaur, a group of sauropods which were recognized mainly thanks to discoveries made from the southern hemisphere. As Mannion and Upchurch note, this has created the impression that titanosaurs are a group of southern dinosaurs and that the titanosaurs of the Late Cretaceous of Europe and North America must have migrated from the south. Based upon what we now know, however, titanosaurs had spread around the world by the Early Cretaceous. In the case of Alamosaurus, its ancestors may have come from Asia or may have been present already in North America during an earlier time, although the origin of the European titanosaurs such as Ampelosaurus and Lirainosaurus is less clear.

There is still much that is unknown about the history of North American and European sauropods during the Cretaceous. Nevertheless, the new study does point researchers in directions of future research. If inland fossil sites during the mid-Cretaceous can be identified and systematically sampled, paleontologists may find the missing dinosaurs.

References:

Mannion, P., & Upchurch, P. (2010). A re-evaluation of the ‘mid-Cretaceous sauropod hiatus’ and the impact of uneven sampling of the fossil record on patterns of regional dinosaur extinction Palaeogeography, Palaeoclimatology, Palaeoecology DOI: 10.1016/j.palaeo.2010.12.003

A restoration of Alamosaurus. From Wikipedia.

The Late Jurassic was the heyday of sauropod dinosaurs in prehistoric North America. Apatosaurus, Diplodocus, Barosaurus and Brachiosaurus were among the titans found in the 156- to 146-million-year-old Morrison Formation. But after this slice of geologic time, North American sauropods all but disappear.

There have been just a few discoveries of Cretaceous sauropods in North America. The recently described Abydosaurus was found in the 127- to 98-million-year-old Cedar Mountain Formation of Dinosaur National Monument, and the existence of the Late Cretaceous sauropod Alamosaurus has been known for nearly a century, but the post-Jurassic record of North American sauropods is sparse and discontinuous. Sauropods thrived in South America and elsewhere, but in North America their diversity declined, they disappeared about 100 million years ago, and Alamosaurus showed up on the continent about 30 million years later, just in time for the curtain call of the non-avian dinosaurs. In a study just published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology, scientists Michael D’Emic, Jeffrey Wilson and Richard Thompson went back to the record of these animals in an attempt to better outline their history on this continent.

The proper identification of sauropod remains is key to figuring out the ups and downs of sauropod evolution in North America. As D’Emic and colleagues propose, some specimens previously thought to belong to sauropods should actually be attributed to other kinds of dinosaurs and therefore widen the gap between the species that hung on during the Early Cretaceous and those that reappeared on the continent toward the close of the period. More specifically, the authors of the new study looked at putative sauropod fossils of Campanian age—the time period just before that of Alamosaurus—to see whether there was an yet-unknown species of sauropod in North America during that time.

The specimens the scientists investigated were two neck vertebrae and one back vertebra from a single animal discovered in the circa 75-million-year-old rock of Arizona’s Santa Rita Mountains. They do not look very much like sauropod vertebrae. Instead they most closely resemble the vertebrae of hadrosaurs such as Gryposaurus, a species already well known from the southern United States during this time period. The same can be said of similar partial vertebrae found from the same span of time throughout the southwest. If this new study is correct, then, there is no sign that sauropods made it back to North America until the arrival of Alamosaurus a few million years later.

As outlined by the authors of the study, there was a 30-million-year gap in which sauropods did not exist in North America. The question that remains is where Alamosaurus came from. The authors propose that its lineage could have traveled to North America from Asia thanks to an east-west connection between the continents during the last 35 million years of the Cretaceous that allowed the dispersal of tyrannosaurs, horned dinosaurs, hadrosaurs and other lineages present on both continents during the Cretaceous. Given the last-minute appearance of Alamosaurus, however, this seems unlikely. A more plausible explanation may be that it migrated north from South America when a north-south landbridge opened up around five million years before the end of the Cretaceous. Either way, the arrival of Alamosaurus was not so much a triumphant return as a fleeting hint of a landscape once dominated by long-necked giants.

References:

D’Emic, M., Wilson, J., & Thompson, R. (2010). The end of the sauropod dinosaur hiatus in North America Palaeogeography, Palaeoclimatology, Palaeoecology, 297 (2), 486-490 DOI: 10.1016/j.palaeo.2010.08.032

The recovered bones of Seitaad (top) and a restoration of how they were articulated when found in the ground (bottom). From the PLoS One paper.

Even though the first dinosaurs had evolved by 228 million years ago, it was not until the early Jurassic (about 201 million to 176 million years ago) that they were established as the dominant large vertebrates on land. It was during this time that various groups of dinosaurs diversified and began to be adapted in ways which made them quite different from their ancestors, and among these groups were the sauropodomorphs. These were the early relatives of the immense sauropod dinosaurs of the Jurassic and Cretaceous, and an unexpected discovery from southern Utah has shown that these dinosaurs were more widespread that was previously understood.

During the heyday of the early sauropodomorphs, many of the world’s landmasses were still connected, which allowed the dinosaurs to travel between continents that are separated by oceans today. Their remains have been found Africa, Antarctica, Asia, North America and South America, but in some places they are more rare than in others. In North America, especially, the bones of sauropodomorphs are difficult to come by, but as reported in the journal PLoS One by paleontologists Joseph Sertich and Mark Loewen, a new partial skeleton greatly adds to our understanding of these dinosaurs from this part of the world.

The remains, found in the approximately 190-million-year-old rock of southern Utah’s Navajo Sandstone, primarily consist of portions of the hips, partial hindlimbs, most of the forelimbs and shoulders, and several vertebrae and ribs. From its preservation and the geological details surrounding the bones, it appears that the animal died and was subsequently buried by a collapsed sand dune, hence its name Seitaad ruessi, for a sand monster in Navajo lore (Seitaad) and naturalist Everett Ruess (ruessi) who disappeared in southern Utah in 1934. It was preserved articulated within the rock, a body without head or tail.

What is most significant about this fossil, however, is that it is the best-preserved sauropodomorph yet found from the western United States. Paleontologists have been finding fragments of them for years, but this is the first time that enough has been found to compare the dinosaur to its relatives from elsewhere in the world. When Sertich and Loewen did so they found that Seitaad was  most closely related to either Plateosaurus from Europe and its close relatives or Adeopapposaurus from South America and its kin. The trouble was that most of the comparisons made for these dinosaurs so far have relied upon characteristics of bones not preserved in this particular specimen (such as the skull). But it is most certainly a variety of sauropodomorph that probably spent much of its time walking on two legs (like its distant cousin Aardonyx).

Joseph J. W. Sertich, Mark A. Loewen (2010). A New Basal Sauropodomorph Dinosaur from the Lower
Jurassic Navajo Sandstone of Southern Utah PLoS One, 5 (3) : 10.1371/journal.pone.0009789