T is for Torvosaurus

At ScienceOnline 2013, marine biologist and Deep Sea News blogger Al Dove asked me about the reputation of Allosaurus. The 150 million year old hypercarnivore has often been treated as a smaller, weaker forerunner of the later Tyrannosaurus, as if the bones of Allosaurus testify “The one who comes after me is more powerful than I.” In the dinosaur books I grew up reading, Allosaurus was presented as the Jurassic theropod prototype which reached an apex in Tyrannosaurus, just before the non-avian dinosaurs were snuffed out. Al wanted to know whether such representations were accurate.

As I told Al, I hate to see Allosaurus get shortchanged. While the typical Allosaurus skeleton you’re bound to see in a museum is going to look small compared to Tyrannosaurus, there are rare, fragmentary skeletons which hint that Allosaurus could have grown to be 40 feet long or so – about as large as our favorite tyrant dinosaur. (These scrappy remains have often been given different names, such as “Epanterias” and “Saurophaganax.”) And while the bite of Allosaurus wasn’t as devastating as that of Tyrannosaurus, differences between two species of the Jurassic predator indicate that the skull of Allosaurus was being modified in a way to put more power into each chomp. While the older Allosaurus “jimmadseni” had a relatively narrow cranium, the back of the skull in the later Allosaurus fragilis was expanded to provide more room for powerful jaw muscles (similar to the difference seen between Tyrannosaurus and its close, slim-skulled relative Tarbosaurus).

And Allosaurus was extremely successful. The dinosaur is the most commonly-found carnivore in the Jurassic Morrison Formation. Within eastern Utah’s Cleveland-Lloyd Dinosaur Quarry, for example, the remains of at least 46 Allosaurus have been found in one place – totally outstripping the counts of all other dinosaurs in the same quarry. But the prevalence of Allosaurus also highlights a Mesozoic mystery.

Allosaurus wasn’t the only giant carnivore stalking Jurassic floodplains. The Cleveland-Lloyd quarry, to pick just one site, has also yielded the partial remains of two other big predators – a remarkably large Ceratosaurus, and pieces of Torvosaurus. While both of these predators shared the same habitats as Allosaurus, they were apparently much rarer. Why this should be so is an enigma, made all the more complicated by the fact that there is comparatively little of these other carnivores to work with. Torvosaurus, especially, was a huge hunter that paleontologists know relatively little about.

While Allosaurus and Ceratosaurus were both discovered and named during the late 19th century “Bone Wars”, Torvosaurus tanneri was a more recent discovery. Paleontologists Peter Galton and James Jensen named the theropod in 1979 from remains uncovered in Colorado’s Dry Mesa Quarry (the place where Supersaurus also was found). The initial description was short and preliminary, but the find established that there was a third type of large predatory dinosaur running around the Late Jurassic landscape. Allosaurus and Ceratosaurus belonged to their own distinct lineages (allosaurids and ceratosaurids, respectively), while Torvosaurus was a megalosaurid more closely related to Megalosaurus itself from prehistoric England.

Based upon the various skeletal parts found at Dry Mesa, Galton and Jensen estimated that Torvosaurus reached approximately 33 feet in length. (Brooks Britt reviewed the same remains in 1991 and also estimated that the theropod was about 30 feet long.) Barring exceptionally large Allosaurus, this would have made Torvosaurus the biggest carnivore of the Morrison Formation. And Torvosaurus may have grown even larger still.

In 1992, Robert Bakker and coauthors proposed a new genus of giant Morrison Formation predator found at the classic Como Bluff, Wyoming locality. Named Edmarka rex, the dinosaur was represented only by a large cheekbone – the jugal – and a few other skeletal pieces that were quite similar to Torvosaurus. Bakker and coauthors proposed that the details of the jugal were different enough to establish Edmarka as a different genus, but this hypothesis hasn’t been picked up by other paleontologists. “Edmarka” was probably a big Torvosaurus – an individual that possibly reached the 40-foot-long size range of Tyrannosaurus and the biggest Allosaurus.

Large specimens of Torvosaurus have been found in Portugal, too. (In fact, the finds there were the inspiration for the Dinosaur Revolution scene starring Allosaurus and Torvosaurus.) Although both Allosaurus and Torvosaurus are often thought of as classic Morrison Formation dinosaurs, parts of the same two dinosaurs have been found in the Late Jurassic exposures of Portugal’s Lourinhã Formation.

In addition to a tibia that likely belonged to Torvosaurus, part of an upper jaw, a partial femur, and a few other bones attributable to this dinosaur have been found by Octávio Mateus and colleagues. A size estimated based on the femur fragment indicated that the dinosaur was about 36 feet long – comparable to the Dry Mesa material – but the jaw fragment showed a curious difference compared to the North American equivalent. The specimen from Portugal had anywhere from one to three fewer teeth in the same section of upper jaw, and the tooth sockets in this animal were larger. Additional specimens are needed to be sure, but Torvosaurus from prehistoric Portugal may have had fewer, but larger, teeth than their North American counterparts.

Despite these scattered finds, though, we know relatively little about Torvosaurus. No one has yet found a complete skeleton, and so how large this carnivore was relies on estimation based on more complete, closely-related dinosaurs. The predator’s paleobiology is equally enigmatic. We don’t know much of anything about how Torvosaurus captured and consumed prey, or even what sort of dinosaur meals it targeted, much less how the huge dinosaur coexisted with abundant Allosaurus and equally-rare Ceratosaurus. Even though North America’s Morrison Formation has been intensely studied for over a century, there is still a great deal we don’t yet understand about the Jurassic ecosystem. Dinosaur mysteries go far beyond identifying bones. To understand the past, we need to carefully sift through rock and bone to reconstruct how fantastic animals such as Torvosaurus actually lived.

Previous entries in the Dinosaur Alphabet series:

S is for Segisaurus

R is for Rapetosaurus

Q is for Qiaowanlong

P is for Pelecanimimus

O is for Ojoceratops

N is for Nqwebasaurus

M is for Montanoceratops

L is for Leaellynasaura

K is for Kileskus

J is for Juravenator

A-I at Dinosaur Tracking.

References:

Bakker, R., Siegwarth, J., Kralis, D., Filla, J. 1992. Edmarka rex, a new, gigantic theropod dinosaur from the Middle Morrison Formation, Late Jurassic of the Como Bluff outcrop region. Hunteria. 2, 9: 1-24

Britt, B. 1991. Theropods of Dry Mesa Quarry (Morrison Formation, Late Jurassic), Colorado, with emphasis on the osteology of Torvosaurus tanneri. Brigham Young University Geology Studies. 37: 1-72

Galton, P., Jensen, J. 1979. A new large theropod dinosaur from the Upper Jurassic of Colorado. Brigham Young University Geology Studies. 26, 2: 1-12

Mateus, O., Antunes, M. 2000. Torvosaurus sp. (Dinosauria: Theropoda) in the late Jurassic of PortugalTorvosaurus sp. (Dinosauria: Theropoda) in the late Jurassic of Portugal, in I Congresso Ibérico de Paleontologia/XVI Jornadas de la Sociedad Española de Paleontología. pp. 115-117.

Mateus, O., Walen, A., Antunes, M. 2006. The large theropod fauna of the Lourinhã Formation (Portugal) and its similarity to the Morrison Formation, with a description of a new species of Allosaurus, in Foster, J., and Lucas, S. Paleontology and Geology of the Upper Jurassic Morrison Formation. New Mexico Museum of Natural History and Science Bulletin. 36: 1-7