Monday, February 04, 2008

Ridiculously Large Cretaceous Theropods

Different lineages of theropod dinosaurs independantly achieved gigantism several times throughout the Mesozoic. For the most part, these enormous theropods were not directly related. In a few cases, a single genus in a family of otherwise normal-sized theropods grew ridiculously large. Oddly, the groups represented here grew to their maximum sizes during the Cretaceous period. While theropods in the Jurassic (like Allosaurus and Saurophalanx) grew quite large, they could not complete with the behemoths of the Cretaceous.

Spinosaurus aegypticus may have been the largest terrestrial carnivore to ever walk the Earth. While its remains (sketched above by Stromer when he described the beast in 1915) were destroyed during WWII, a new snout specimen described by Dal Sasso et al. (2005) estimated the total body length may have been between 16 and 18 meters, making it significantly larger than Giganotosaurus, another monster theropod on this list. Other members of the Spinosauridae, like Baryonyx and Irritator, reached respectable lengths (30 feet, 36 feet) but they were dwarfed by their ultimate decendant.

Giganotosaurus was the largest member of the Carcharodontosauridae, an enormous Gondwanna branch of the Allosauroidea. Its body was built like a typical allosaur, but its head was long, tall, and built for killing sauropods. This 50-foot monster was the first dinosaur unearthed to beat out Tyrannosaurus rex for "biggest carnivorous dinosaur," and rightly deserves the title. While the tyrant lizard king is a more compact killing machine, Giganotosaurus was leaner and longer, focusing more on speed than strength. While this gigantic lizard couldn't crush bone, it would have delivered a fatal tearing bite to any prey item foolish enough to stand its ground.

Discovered by Barnum Brown in 1902, Tyrannosaurus rex was, for a very longn time, the largest carnivorous dinosaur known. While it no longer retains that title, it does get to keep another: strongest carnivorous dinosaur known. Tyrannosaurus rex could shatter bone with its bulldog grip, which left the arms with nothing to do, apparently. Although huge, Tyrannosaurus was compact for its size, with a short midsection, thick but short neck, and rounded muzzle. Compare Tyrannosaurus' skull to Giganotosaurus', and the differences are readily apparent. Tyrannosaurus seems to have gone through several growth phases, during which its diet and mode of prey capture may have changed. Thomas Holtz has wondered if this single species dominated the carnivore scene at the end of the Mesozoic. This would have made Tyrannosaurus rex the most powerful and most flexible carnivorous dinosaur known!

In a family of lightweight, midsized theropods, Deinocheirus really stuck out in a crowd. The Ornithomimidae is usually characterized by Struthimimus, Gallimimus, and Ornithomimus. The only known remains of their gigantic cousin--a pair of fearsome-looking arms, demonstrate that one of their ranks towered over the rest. If you scale the arms of Deinocheirus to one of its smaller brethren, you get an monsterous ostrich mimic that would have rivaled Tyrannosaurus in size. It could be that Deinocheirus' arms were out-of-proportion to the rest of its body, or that it wasn't an ornithomimosaur at all, but until more complete remains are found, Deinocheirus will remain the largest of the ostrich mimics.

Like Deinocheirus, this next dinosaur is a giant among much smaller cousins. Gigantoraptor, known from a fairly complete Mongolian skeleton, was about 30 feet long. While this doesn't seem very large in comparison to the other dinosaurs on this list, it's more than three times the size of Gigantoraptor's next-largest family member, Citipati. Aside from its strange size increase, Gigantoraptor is interesting for bucking a common trend among large carnivorous dinosaurs. In small theropods, like Coelophysis, Ornitholestes, and even Struthiomimus, the tibfib is longer than the femur. The metatarsals are also quite long. This usually means that the animal was a fast runner. The larger the theropod, however, the closer the ration becomes. In Tyrannosaurus, for example, the tibfib and femur are about the same length, and the metatarsals are quite short. Even in Albertosaurus, the bone length ratios are more similar to ornithomimids than big ol' T.rex. Larger theropods, then, were slower and did more walking than running. Gigantoraptor, however, despite its large size, retains its smaller cousin's leg proportions, implying that Gigantoraptor was the most able-legged runner of all the giant theropods.
While many theropods became very large in order to hunt larger game, one family increased in size due to a basic change in diet. The Therizinosauridae is a bizarre group of maniraptor theropods which switched from meat to vegetation during the Early Cretaceous. Very quickly, their anatomy changed to accomodate this change. In addition to an enlarged gut, retrovated pubis, hypertrophied pedal digit I, widened pelvis, and renovated pectoral girdle, therizinosaurs also began growing. By the end of the Cretaceous, Therizinosaurus was 40 feet long and may have converged to some degree with the lifestyle of sauropods, which had since disappeared from Laurasia. Not much of Therizinosaurus' skeleton is known, but it is most famous for its enormous, scythe-like manual claws. Whether it used the claws for defense, digging, or reaching tall branches, we may never know.

The smallest "giant" on our list may also be the deadliest. Utahraptor was a massive dromaeosaur, and may have reached between 18 and 25 feet long. While virtually none of its bones are known (a short caudal series, maxilla, and giant claws), scaling the beast to its close cousin Deinonychus results in a raptor that's almost twice as large! Utahraptor lived alongside Deinonychus during the Early Cretaceous, and neither animal had to compete with tyrannosaurs, which did not impede on their territory until the Late Cretaceous. It may have hunted resident ankylosaurs and ornithopods, as well as North America's remaining brachiosaur, Cedarosaurus.


ScottE said...

Love that last image with the human in position for scale.

He's waving at us! (As well as being blissfully unaware that sudden, painful death is bearing down upon him.)

Cameron McCormick said...

Have you seen the publication:

Therrien, Francois & Henderson, Donald M. 2007. My Theropod is Bigger than Yours...Or Not: Estimating Body Size from Skull Length in Theropods. Journal of Vertebrate Paleontology 27(1):108–115,