The Mysterious Marshosaurus

Eastern Utah’s fossil-filled Cleveland-Lloyd quarry is best known for its fossils of Allosaurus. From the time the site was opened in 1929 to the present, the scattered remains of at least 46 Allosaurus have been collected from the roughly 147-million-year-old slice of Late Jurassic rock. But Allosaurus has not been the only dinosaur found there. Rare pieces the huge herbivores Barosaurus, Camarasaurus, Camptosaurus and Stegosaurus have been picked out of Cleveland-Lloyd, as have the remains of other predatory dinosaurs such as the early tyrannosauroid Stokesosaurus, the massive Torvosaurus, the well-ornamented Ceratosaurus and a poorly known theropod named Marshosaurus.

Up until about a year ago, I had never heard of Marshosaurus. Allosaurus, Ceratosaurus and, to a lesser extent, Torvosaurus were traditionally promoted as the predators of the Late Jurassic in North America. That’s why I was surprised to see the restored skull of Marshosaurus set into an explanatory display in the Carnegie Museum of Natural History during the reception at last year’s Society of Vertebrate Paleontology conference. The skull looked slender and quite unlike the deep skulls of the bigger Morrison theropods I had previously learned about. What was this dinosaur?

Marshosaurus was not a new dinosaur that had slipped under my radar. Quite the opposite. In 1979 paleontologist James Madsen, Jr. named and initially described the dinosaur on the basis of a virtually complete pelvis and a few elements of the upper jaws found in the Cleveland-Lloyd quarry. Madsen acknowledged that this wasn’t much to describe a new genus from and lamented that there was simply not enough funding to sift through, prepare and study the dozens of other bones at the site that might belong to the new, relatively small dinosaur. Nevertheless, the known parts of the theropod were clearly different from those of other dinosaurs found at the site, including small Allosaurus, and so Madsen gave the creature the title Marshosaurus bicentismus in honor of the famous paleontologist Othniel Charles Marsh and the bicentennial anniversary of the United States of America.

Parts of Marshosaurus later turned up in other dinosaur bonebeds. Additional skull material, a partial vertebral column, and other portions of the skeleton were found at northeastern Utah’s Dinosaur National Monument in a slightly geologically older part of the Morrison Formation called the Salt Wash Member. (The Cleveland-Lloyd site is part of the stratigraphically higher Brushy Basin Member.) Taken together, the collected remains of Marshosaurus seem to represent an approximately 18-foot-long predator which was in a lower weight class than the giant Allosaurus and Torvosaurus of the same environments. What variety of theropod Marshosaurus was, however, has been unclear until recently.

In his 2010 revision of the dinosaur Megalosaurus, paleontologist Roger Benson included Marshosaurus in his analysis of theropod relationships. Benson found Marshosaurus to be a relatively basal member of the Megalosauroidea—a large and varied group of predatory dinosaurs which presently includes the sail-backed spinosaurs in one subgroup and dinosaurs such as Torvosaurus and Megalosaurus in another. This would mean that Marshosaurus would be an early and archaic member within this large group which generally represents the form of the megalosauroids before the big split between the Spinosaurus and Torvosaurus lineages. Further analyses will test these hypothesized relationships, and perhaps additional Marshosaurus material will be identified from places like Dinosaur National Monument and the Cleveland-Lloyd quarry in the future. We still know very little about this dinosaur. For one thing, how did this relatively small carnivore make a living alongside so many other more imposing predators?

References:

Benson, R. (2010). A description of Megalosaurus bucklandii (Dinosauria: Theropoda) from the Bathonian of the UK and the relationships of Middle Jurassic theropods
Zoological Journal of the Linnean Society, 158 (4), 882-935 DOI: 10.1111/j.1096-3642.2009.00569.x

Madsen, J. 1979. A second new theropod dinosaur from the Late Jurassic of east central Utah. Utah Geology 3(1): 51–60.

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.