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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 fused vertebrae of an archosaur with a diagram of where they fit on the living animal. From Cisneros et al., 2010.

When we envision prehistoric life, we often picture long-extinct animals in the most healthy state possible. Each restored individual is the acme of its particular species—be it Allosaurus or a woolly mammoth—but we know that things in the natural world are never so clean and neat. Not only do individual animals of any species vary from one another thanks to heredity, but injury and disease is ubiquitous. Just as animals break bones and contract ailments today, so did creatures during the past. (To their credit, some paleoartists such as Michael Skrepnick have deliberately illustrated individual animals with interesting pathologies.) A 245-million-year-old fossil from South Africa illustrates the long fossil record of a pathology that is still with us today.

There was not much left of the animal described by Juan Carlos Cisneros and colleagues; just a set of three vertebrae from the tail of an archosaur (the group containing crocodiles, pterosaurs, dinosaurs and their close relatives). The bones appear to have belonged to a crocodile-like creature that lived just a few million years after the mass extinction which marked the end of the Permian, and they had become fused together. This was not their normal condition—something must have happened to this animal to cause these three vertebrae to become fused.

When the scientists performed a neutron tomography scan—a type of 3-D scan similar to an X-ray—they found no evidence of fractures, traumas, or tumors which could account for the pathology on the bones. Neither did they find evidence consistent with the hypothesis that the animal had been born with this condition. After examining the list of potential causes, a particular kind of inflamed bone growth called spondarthritis appeared to be most consistent with the pathology seen on the vertebrae. If this identification is correct, it would be the oldest record of spondarthritis in the fossil record, with the next oldest cast being seen in an individual of the circa 147-million-year-old sauropod dinosaur Camarasaurus.

Just how the archosaur became afflicted with spondarthritis is unknown. As the authors point out in their description, even when doctors can examine people who suffer from this same condition, it can be difficult to uncover the cause. What can be determined, however, is that the pathology was probably painful for the archosaur and constrained the movement of its lower back and tail. Whether this pathology contributed to the death of this animal is unknown, but it certainly didn’t provide it with any benefits!

References:

Cisneros, J., Gomes Cabral, U., de Beer, F., Damiani, R., & Costa Fortier, D. (2010). Spondarthritis in the Triassic PLoS ONE, 5 (10) DOI: 10.1371/journal.pone.0013425

The early Jurassic rock - with a dinosaur track on the top surface - in place at the NJSM. Photo by Jason Schein.

The New Jersey State Museum (NJSM), where I am a research associate, has a new dinosaur exhibit, and it has been placed outside for all passers-by to see. It’s an enormous chunk of rust-red rock recently removed from a quarry in Woodland Park, New Jersey, and on its top is the track of a predatory dinosaur which strode across a mudbank around 199 million years ago.

The acquisition of the fossil was a coup for the NJSM. The site where the footprint was found is being transformed by K. Hovnanian Homes in preparation for the construction of new condos, and it is feared that this important site—which contains a intricate geologic record of the time spanning the very end of the Triassic to the beginning of the Jurassic—might be destroyed forever. NJSM scientists and other geologists have been picking over the site on a nearly daily basis to recover significant fossils before they are lost. The three-ton rock was one of the recent discoveries, and the developer agreed to donate it to the museum for display.

So what kind of dinosaur made the track? From the overall anatomy of the footprint it is clear that it was made by a medium-size theropod dinosaur, and the track can be classified with similar trace fossils under the name Eubrontes. Beyond that, however, it is impossible to tell. Traditionally the dinosaur Dilophosaurus has been taken as an appropriate stand-in for these kinds of tracks, but without fossil bones we can’t know for sure.

Regular readers know that I can’t resist cheesy dinosaur movies, and a new SyFy feature set to debut late next month will be the latest stinker to be heaped on the pile of bad dino cinema.

Called Triassic Attack, this direct-to-video schlock features the reanimated skeletons of a pterosaur and a Tyrannosaurus that set about chomping up the boneheaded attendees of a local college. It just figures, doesn’t it? The film is called Triassic Attack, but both of its monstrous stars were Cretaceous creatures. With a title like that, I was hoping that one of the crocodile-like rauisuchians—such as the toothy predator Prestosuchus—or the early predatory dinosaur Herrerasaurus might make an appearance. No such luck, apparently.  

Two bite-marked toe bones from Tyrannosaurus rex. Close-up views of the bite-marks are on the right. From Longrich et al., 2010.

For a Tyrannosaurus rex, there was nothing more dangerous than another Tyrannosaurus rex. From a relatively young age these dinosaurs tussled by biting each other on the face—possibly spreading parasitic microorganisms as they did so—and a few fossil scraps have suggested that some tyrannosaurs may have killed or eaten members of their own kind. This latter kind of fossil forensic evidence—bite-marked bones and teeth embedded in skeletons—has been very rare. A study just published in PLoS One presents new evidence that confirms that Tyrannosaurus rex was certainly capable of cannibalism.

As described by paleontologists Nicholas Longrich, Jack Horner, Gregory Erickson and Philip Currie, at least four Tyrannosaurus rex bones bear toothmarks made by a large carnivorous dinosaur. They are several foot bones and an upper arm bone from four different individual animals. The bite traces they bear are not just punctures into the bone, but U- and V-shaped gouges which suggest that the feeding dinosaur was biting onto the body of the Tyrannosaurus and pulling the flesh off the bones. This is consistent with a set of 13 other bones bearing similar toothmarks, including parts of horned dinosaur and hadrosaur skeletons.

That Tyrannosaurus rex is the most likely culprit in each case rests upon the fact that there was no other creature capable of inflicting that kind of damage in each locality from the end of the Cretaceous. The toothmarks were inconsistent with damage done by crocodiles, the predatory lizards in the area were far too small, and the only predatory dinosaur of suitable size to make such bite marks was Tyrannosaurus rex itself.

The collection of bite marks most likely represents feeding rather than combat. The marks are in places and positions that appear to be impossible for fighting animals, and since the bite-marked bones show no evidence of healing it is most probable that the damage was done after the individual animals died. The fact that the bite marks were found primarily on limb and toe bones hints that the feeding Tyrannosaurus was a scavenger that came by after most of the soft tissues had been removed from the dead Tyrannosaurus. There would not have been very much meat on the upper arms and toes of Tyrannosaurus, and so the authors of the new study hypothesize:

Tyrannosaurus therefore seems to have been an indiscriminate and opportunistic feeder, feeding not only on herbivorous dinosaurs, but also on members of its own species. The traces described here likely result from opportunistic scavenging, and were probably made after most of the flesh and organs had been removed from the carcass.

Furthermore, that four traces from different specimens have already been found hints that Tyrannosaurus may have regularly fed upon its own kind. Considering how rare fossils are to start with, and how much rarer carcasses destroyed by predators are, that scientists have found so many traces already suggests that Tyrannosaurus-on-Tyrannosaurus scavenging was relatively common. It is impossible to know whether these Tyrannosaurus were actually victims of predation or died from some other cause—such as wounds from a fight with another Tyrannosaurus—but the damaged bones show that a hungry Tyrannosaurus would not let a good carcass go to waste.

For more on tyrannosaur feeding, see these posts:

Did Giant Predatory Dinosaurs Eat Bones?
Tarbosaurus: A Predator and a Scavenger With a Delicate Bite

References:

Longrich, N., Horner, J., Erickson, G., & Currie, P. (2010). Cannibalism in Tyrannosaurus rex PLoS ONE, 5 (10) DOI: 10.1371/journal.pone.0013419