(Left) Cruciate ligaments of the right knee, adapted from an image by Bruce Blaus, Wikimedia Commons, CC BY SA 4.0. (Right) Achilles tendon attaching the gastrocnemius muscle to the heel bone of the right foot, adapted from the DataBase Center for Life Science, Wikimedia Commons, CC BY-SA 2.1 JP.
Bones and Skeletons
Bones are what give your body its shape. Bones that meet at a joint are attached to each other by tough connective tissue called ligaments, like the anterior cruciate ligament (ACL) connecting your femur to your tibia at your knee joint.
Skeletal muscles attach to your bones through tendons, like the thick Achilles tendon attaching your calf muscles to your heel bone. When you flex (contract) a muscle, it pulls the bones it is attached to toward each other, causing movement at one or more joints.
Hard tissues like bones and teeth are much more likely to fossilize than softer tissues like muscles, ligaments, tendons, cartilage, and skin.
Human skeleton courtesy of DataBase Center for Life Science, CC BY 4.0 via Wikimedia Commons. 3D scan of SUE (c) Field Museum, used with permission. (Skeletons are not to scale.)
SUE and You
Walking along the base of a cliff of the Hell Creek Formation in South Dakota in 1990, Sue Hendrickson noticed bone fragments on the ground. Looking up, she saw larger bones protruding from the eroding cliff. It took a crew of six more than two weeks to excavate the fossilized bones of the Tyrannosaurus rex that would come to be known as SUE. SUE was acquired by the Field Museum in 1997 and first displayed to the public in 2000. SUE is perhaps the largest and most complete T. rex skeleton in the world (approximately 90 percent complete by bulk).
SUE lived to approximately 30 years of age, which was quite old for a T. rex. SUE’s soft tissues did not fossilize, so we don’t know whether SUE was male or female.
Although SUE lived 67 million years ago and weighed 8,000 kg more than you do, you can see many commonalities between your own skeleton and SUE’s. In particular, T. rex and human legs both have a single bone attached to the hip (the femur) and two bones attached to the ankle (the tibia and fibula), and T. rex and human arms both have a single bone attached to the shoulder (the humerus) and two bones attached to the wrist (the radius and the ulna). Scientists believe this is because we share a common ancestor with T. rex, perhaps a fish that first walked on land approximately 400 million years ago, which already had pectoral fins evolving to include primitive forms of a humerus, radius, and ulna.
T. rex had small arms relative to the rest of its body. Paleontologists are still unsure why T. rex arms evolved to be so small, but T. rex is not the only dinosaur species to evolve small arms. For example, Gualicho shinyae, which was discovered by Akiko Shinya in Argentina in 2007, is also a bipedal meat-eating dinosaur (a theropod, like T. rex) with small arms and two-fingered hands, like T. rex. But G. shinyae lived approximately 93 million years ago and is only distantly related to T. rex. This has led paleontologists to conjecture that the evolution of small arms on theropods may be an example of convergent evolution, where common environmental characteristics lead to similar designs evolving in different species at different places and times.
You can learn more by
inspecting a 3D model of SUE,
reading the Field Museum’s introduction to SUE,
studying the Field’s detailed educator’s guide to dinosaur evolution, or
browsing other educational resources provided by the Field.