​Titanosaurs were the last surviving group of sauropods to see the end of the dinosaur’s reign, at the end of the Cretaceous. Such a cast of a fossil specimen is currently on display at the American Museum of Natural History. The cast measures 122 feet long, and paleontologists suggest the creature weighed around 70 tons.

But how did this dinosaur group end up with such formidable heft?

In a paper published today in Royal Society Open Science, a group of researchers examined the temporal and phylogenetic body sizes of various sauropods. They created 3D models to reconstruct the bodies and gauge how they evolved through time.

“As a result of devising these models we were able to ascertain that the relative size of sauropods’ neck increased gradually over time, leading to animals that were increasingly more front-heavy relative to their ancestors,” said Karl Bates, of the University of Liverpool’s Department of Musculoskeletal Biology, in a statement.

Sauropods descended from smaller dinosaurs that walked on two legs, according to University of Liverpool. Their tails were long, but they had small chests and forelimbs. Eventually, their chests grew larger, as did their forelimbs, and their neck lengthened. This resulted in Jurassic-era sauropods, such as Diplodocus and Apatosaurus.         

“What’s important to remember about studies like this is that there is a very high degree of uncertainty about exactly how these animals were put together,” said co-author Vivian Allen in a statement, who is from the Royal Veterinary College London. “While we have good skeletons for many of them, it’s difficult to be sure how much meat there was around each of the bones. We have built this uncertainty into our models, ranging each body part form emaciated to borderline obesity, and even using these extremes we still find these solid, trending changes in body proportions over sauropod evolution.”

According to the study, the shift to titanosaurs was marked by a craniad shift that was also paired with further neck enlargement. Titanosaurs included the Argentinosaurus and Dreadnoughtus.