The question is, "What makes sauropod dinosaurs so unique among the dinosaurs?
Sauropods, such as Diplodocus and Apatosaurus (formerly called Brontosaurus) were immense four-footed dinosaurs. By their fossil skeletons, researchers know that their skeletal design features were "uniquely" equipped to support their massive weight.
Clearly sauropod dinosaurs had all or nothing design features without small heads, hollow vertebrae, long necks, pillar legs, specially shaped pelvis bones, and fast growth rates, that were all at the same time, or these sauropods could not have existed.
Unlike other dinosaurs, sauropods had hollowed vertebrae and small heads. These design features lightened the load placed on their necks. Fredric Heeren wrote in "Nature" that sauropods' long necks allowed them a wide reach through the treetops. Some of the largest may have needed up to a ton of vegetation a day. Heeren wrote, "That kind of feeding required long necks, which would have been impossibly heavy if they were built with solid vertebrae. But large sauropods had vertebrae riddled with holes. These air-filled, or pneumatic, bones weighed only about 35% as much as solid ones." (Heeren, F. (2011), "Dinosaurs: Rise of the Titans", Nature, pages 155-161)
The Bible's history provides a framework that best explains the most significant facts. Sauropods and other dinosaurs were created by God on the sixth day of creation week (Genesis 1 : 24-25).
First, sauropods were intentionally created at one time, not morphed from other creatures. This is why no single undisputed ancestor exists. Sauropods have a all or nothing design feature. The "unique" sauropod body structure demands that sauropods were specially created, not evolved. The fact that they were fossilized on many continents demands a worldwide cataclysmic mud burial. Unlike the evolution guesswork, there observations confirm the biblical record of Genesis, the Creation, and the global Flood in Noah's day. Amen !!!