There has been some debate, though, on whether or not the massive, short-armed theropods (like T. rex, Giganotosaurus, Albertosaurus, and Allosaurus) could run very fast because if they fell, their short arms would not break their fall and they would be badly injured (James Farlow, 1995). This meant that these large theropods were slow, lumbering animals.
Dr. Bruce Rothschild, of the Arthritis Center of Northeast Ohio, has found evidence of 14 fractured ribs in an Allosaurus that reflect healed injuries that were probably received in falls. These were most likely belly flops that happened while running (as reported in the April 16, 1998 New Scientist).
An x-ray analysis of the Allosaurus fossil indicated that the Allosaurus ribs near the scapula (the shoulder bone) were cracked and had healed. The Allosaurus was capable of recovering after many severe forwards tumbles that probably occurred while it was running. So the suggestion that perhaps the large short-armed theropods were not capable of running because they couldn't recover after a fall apparently wasn't so, at least for Allosaurus - this Allosaurus did recover many times after bad tumbles.
In 1995 James Farlow of Indiana-Purdue University argued that a large T. rex could run no faster than 20 mph (32 kph), because if it did, a fall would probably be so severe as to kill it. T. rex weighed about 6 tons and was up to 20 feet (6 m) tall but Allosaurus was slightly smaller, about 3 tons and 16.5 feet (5 m) long. Farlow says that Rothschild's analysis is consistent with his theory since Allosaurus was smaller than T. rex (its smaller mass would make the impact much less powerful so the animal may have been able to recover after a running fall). So T. rex may or may not have been a fast runner.
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