In humans, skin is the biggest organ--an average adult's skin weighs as much as a gallon of milk. The skin of an adult Mamenchisaurus weighed about as much as a small car.
Very little dinosaur skin fossilized, so what we know about sauropod skin comes from impressions made when it pressed into mud or sand that then hardened and turned to stone. These impressions show that sauropod skin had small bumps and scales that didn't overlap. Some sauropods had bony growths in the skin called osteoderms. But no sauropods had hair or feathers.
On the Surface
What would it have felt like to touch a dinosaur? Sauropod skin was almost certainly dry and warm. Because dinosaurs had no sweat glands in their skin, they didn't perspire. They were covered with small scales that protected the dinosaur's body and prevented evaporation of water from inside. These scales were bumpy and knobby and did not overlap like snake scales do.
What Color Were Sauropods?
You're used to seeing dinosaurs colored gray, green or brown. Those colors are just guesses--we don't really know what color sauropods were. Skin impressions and fossils reveal that sauropods had scales, but they don't tell us their color. Many living reptiles, including birds, are brightly colored, so it's possible sauropods were, too.
The nonsauropod dinosaur group known as ankylosaurs (an-KIE-luh-SAWRS) are often called armored dinosaurs because they had bony pieces, or osteoderms, that grew from within the skin. One group of sauropods also produced osteoderms: the titanosaurs (tie-TAN-uh-SAWRS). These animals probably didn't use them to fend off predators--their size alone was likely protection enough. Paleontologists aren't sure what purpose the osteoderms might have served.
Bones that Regulate Temperature?
Each spike or ridge along the back of an alligator is made by a piece of bone embedded in the skin--an osteoderm. One purpose these osteoderms serve is to help the animal regulate body temperature. It's possible some dinosaur osteoderms did the same.
When this fossil was found in India in 1922, scientists knew it was an "osteoderm"--a bony growth in the skin. But they didn't know what kind of dinosaur it came from. Recently, researchers compared this fossil to other osteoderms and determined it belonged to a titanosaur. They aren't sure where the osteoderm was located on the dinosaur's body or what its function might have been.
Some sauropods might have evolved camouflage patterns that would help them blend into their surroundings or hide their true shape and size. Today, even animals as big as giraffes and killer whales sport camo.
When animals choose mates, it's often what's on the outside that counts. Evolution might have favored sauropods with brightly colored necks and tails, which may have attracted mates.