Madrid. Tyrannosaurs had unique ligaments that provided additional strength to their feet, allowing them to move quickly over great distances.
It is the finding of a new investigation published in Vertebrate Anatomy Morphology Paleontology.
“For a long time, people were drawn to the amazing power and ridiculously small arms of Tyrannosaurus rex and its relatives, but the legs, and especially the feet, were also highly specialized,” Professor Thomas R. Holtz of the University of Maryland. “This new study helps show that, even at the microscopic level, tyrannosaurs were adapted to both running long distances and accelerating rapidly.”
Proportionally, tyrannosaurs had longer legs than any other large carnivorous dinosaur, but the uniqueness of their legs wasn’t limited to their long stride. The large midbone of its foot is triangular when viewed from the front or in cross section, tapering to a narrow ankle, a feature Holtz dubbed the “arctometatarsus” in the 1990s.
Previous studies by Holtz, Oklahoma State University’s Eric Snively, and other researchers have shown that a long arctometatarsus allowed for relatively fast forward locomotion, but the reason for this unusual shape remained a mystery.
Holtz’s new research has tested the hypothesis that the large ligaments strengthened the soles of tyrannosaurs’ feet near the toes in a way that would have been unique among large dinosaurs and not present in any modern animal.
University of Calgary researcher Anthony Russell demonstrated for the study that traction from ligaments and tendons can cause tyrannosaur bones to extrude, leaving rough, wavy surfaces on the bone. Snively identified rough surfaces in the T. rex fossils, but it remained possible that unfossilized cartilage or rapid growth could be responsible for the rough terrain.
Lead author Lara Surring, of Alberta Health Services, realized that researchers could probe the ligaments by training a scanning electron microscope (SEM) on the rough surfaces where bones touch in the tyrannosaurus Gorgosaurus. The authors then removed thin, translucent sections of metatarsal bones from a tyrannosaurus and a “control” dinosaur, the small carnivore Coelophysis.
SEM revealed pits in the rough surface of the bone, consistent with tight ligament junctions in modern animals. The internal bone structure of the tyrannosaurus showed mineralized ligaments that anchored the tendons within the bone. Coelophysis lacked such strong links.
The researchers discovered even more ligament attachments that bind the foot together, both externally and internally. The authors’ methods also allowed them to rigorously test for the presence of soft tissue in fossil animals such as tyrannosaurs. Soft tissues such as ligaments and tendons are critical to skeletal function, but they are rarely preserved in fossils. Finding evidence of these tissues helps to elucidate how these ancient animals operated as living things.
“With external and internal microscopy revealing its discolored soft tissues, one small step for a tyrannosaur becomes a modest leap in understanding a vivid past,” Snively said.
In addition to answering an old question, the intricacies of Tyrannosaurus feet also have relevance to human health.
People are among the best long-distance walkers and runners of any animal today, but ligament and tendon injuries are common, comprising approximately 30-50% of sports injuries.
Overexertion can pull on tendons and ligaments, so understanding how these structures attach to bone, even in extinct animals like dinosaurs, can help humans avoid such injuries.
“We are hopeful that learning how tyrannosaurs made skeletal adjustments to stay functional at the limits of animal size will eventually help us evaluate and improve human skeletons after injury or aging,” Surring said. “This research is another step in that direction.”
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