When Tyrannosaurus Chomped Sauropods

Tyrannosaurus rex never crunched into Stegosaurus. Despite what Walt Disney’s animators so dramatically depicted in Fantasia, the two dinosaurs were separated by about 83 million years. The same is true for Apatosaurus, Diplodocus, Barosaurus, Brachiosaurus and Camarasaurus—all of these 150-million-year-old icons flourished during a time when tyrannosaurs were tiny, fuzzy creatures that could have tackled only much smaller fare. It wasn’t until millions of years later—when the famous Morrison Formation dinosaurs were long gone—that tyrannosaurs became gargantuan apex predators.

But this doesn’t mean that Tyrannosaurus never ate sauropods. Discoveries in New Mexico, Utah, Texas and Mexico have not only placed sauropods back in southwestern North America at the very end of the Cretaceous, but rare bits of tooth and bone have confirmed that Tyrannosaurus prowled many of the same places. Tyrannosaurus almost certainly preyed on titanic sauropods.

The potential tyrannosaur prey goes by the name of Alamosaurus. This dinosaur, which may have reached lengths of 100 feet or more, marked the return of sauropods to North America after a 30-million-year hiatus. Even though sauropods were the dominant herbivores in North America during the Late Jurassic, and though various forms persisted through the Early Cretaceous, the entire group vanished from the continent about 100 million years ago. Horned dinosaurs and hadrosaurs eventually replaced the long-necked herbivores, but the disappearance of sauropods in North America doesn’t mean that they went extinct on a global scale. Sauropods persisted on other continents, most prominently South America, and sometime around 70 million years ago Alamosaurus, or the precursor of Alamosaurus, trod northward to arrive in the American Southwest. This was the southern limit of Tyrannosaurus.

Most Tyrannosaurus skeletons—and certainly the most famous ones—have been found in Montana and South Dakota. But in 2005, paleontologists Scott Sampson and Mark Loewen described a partial Tyrannosaurus skeleton found in Utah’s North Horn Formation. Since Alamosaurus bones had already been found at the same site, this cinched the connection between predator and prey.

Unfortunately, dinosaur fossils found in the North Horn are frequently scrappy and brittle. Much remains unknown about the dinosaurs that lived in Utah at the very end of the Cretaceous. The record of Alamosaurus and Tyrannosaurus in Texas and New Mexico is also quite fragmentary, but, in a press release that accompanied his recent paper about the size of Alamosaurus, paleontologist Denver Fowler mentioned that his team found a tyrannosaur tooth in association with an Alamosaurus vertebra at a New Mexico site. Was the tooth just washed into that position during burial, or might the connection show that the tyrant was feeding on the sauropod’s carcass? For the moment, that’s unclear, but the coincident burial reinforces the ecological connection between the animals. Tyrannosaurus to the north might have been specialists in taking down Edmontosaurus and Triceratops, while their southern cousins had the option of long-necked fare.

References:

Sampson, S., Loewen, M. (2005). Tyrannosaurus rex from the Upper Cretaceous (Maastrichtian) North Horn Formation of Utah: biogeographic and paleoecologic implications Journal of Vertebrate Paleontology, 25 (2), 469-472 DOI: 10.1671/0272-4634(2005)0252.0.CO;2

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Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.