The size of dinosaurs, especially long-necked sauropods, fuels our fascination with the creatures and helps make movies, such as Jurassic Park, classic blockbusters. Yet no one ever made an Oligocene Park, perhaps because our own kin, the mammals, rarely achieved anything close to the sauropods’ size supremacy on land.

One key to the dinos’ evolutionary ability to scale-up may have recently been found in their knees.

Paraceratherium, also known as Indricotherium (Dmitry Bogdanov, Wikimedia Commons)

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The leg joints of massive mammals, such as the extinct 12-ton Paraceratherium, needed to withstand tremendous pressures. During the Oligocene Epoch 23 to 34 million years ago, Paraceratherium was the largest known land mammal ever at 16 feet tall. Yet just the necks of many sauropods, such as Brachiosaurus, were twice that length, and many dinosaurs weighed two to three times as much as Paraceratherium.

Sauropod joints handled the weight of the massive animals differently than the giant mammals, and that may have been one reason the sauropods could grow larger, according to a study published in PLOS ONE.

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In the study, biologists examined how, as they evolved into larger forms, mammals developed bones with joints that fit into each other more exactly, a trait known as congruence. By fitting together better, the mammal bones reduced the possibility of stresses in unexpected parts of the joint. However, at the same time, the mammals developed thinner cartilage pads between their bones.

The study’s authors suggested there may have been a limit where the benefits of increasingly congruent bones proved insufficient without thick cartilage cushions. At that point, the mammals couldn’t evolve any larger because their joints may have lacked the physical ability to deal with the gigantic mammals’ weights.

Millions of years before mammals hit this size ceiling, the long-necked dinos may have avoided the limit by keeping a thick layer of cartilage between their leg bones. As the sauropods grew larger, their cartilage pads grew along with them and continued to cover a larger surface of the knee joints. These larger cartilage pads could expand evenly across the bone joint and spread out the tremendous weight of the sauropods.

TOP IMAGE:  Brachiosaurus altithorax (Богданов, Wikimedia Commons)