Scientists think titanosaurs grew rapidly and didn’t slow down until they reached their massive adult size. “When we compare their life-long growth rates to [those of] living animals, it comes closest to the growth rates we see in whales. It’s incredibly fast,” says Rogers.
Titanosaurs, unlike whales, lacked the benefit of consuming protein-rich and nutritious milk provided by their mothers, Rogers adds. Instead, these dinosaurs foraged for their own food. “Maintaining their high growth rate is one of the ways most sauropods differ from their carnivorous dinosaur relatives, which seem to have interrupted their growth more often as they aged.”
And while titanosaurs were the largest sauropods, Carrano points out that their sauropod ancestors were already quite large. “Evolving to be 70-80 tons from a 20-30 ton ancestor probably didn’t require as much evolutionary innovation,” he says. He explains that all the structures and systems were already in place to be successful at scale.
“Sauropods developed long necks very early on from their primitive cousins, the sauropodomorphs,” says Skye Walker, field assistant at the Elevation Science Institute for Natural History Exploration, which conducts fieldwork in Montana. and Wyoming. “It allowed them to have varied diets, giving them access to a range of nutritious foods,” she says.
Due to their growing size, sauropods developed “pneumatic” air sacs in their bones, to lighten their skeleton. “These air sacs were made up of soft tissue connected to the lungs,” Walker explains. “This made their weight easier to bear and allowed for a more efficient supply of oxygen throughout the body. Unlike mammals, sauropods had this to thank because there were almost no limits to their size.”
Predatory dinosaurs had pneumatic air sacs just like modern birds, Carrano says, and these likely evolved from a common ancestor. However, they are also thought to have evolved independently in other groups, including pterosaurs and sauropods. “It would have increased breathing capacity and lightened those huge bones without sacrificing their strength.” Carrano explains that sauropods also had short feet and pillar-like limbs to support their enormous weight. “These are all characteristics taken to extremes in larger titanosaurs,” he says.
Titanosaurs also had hidden adaptations in their joints. Armita Manafzadeh, a postdoctoral fellow studying biology at Yale University, points out that small non-avian dinosaurs like T. rex had snug joints in which their bones fit together precisely, much like ours. In contrast, titanosaurs had joints with huge volumes of spongy cartilage at the ends of their bones. This difference in joint structure, she says, particularly at key limb joints such as the hip, is believed to be an adaptation to better support the animal’s massive body weight.
However, according to Carrano, what is less clear is how titanosaurs managed to overtake their sauropod ancestors. Maybe they just had more time to evolve bigger bodies, after their Jurassic predecessors left. On the other hand, they may have acquired new innovations – tweaking their existing anatomy so they could grow even bigger. “But there’s not a huge difference that makes the answer obvious. It’s also possible that they benefited from the availability of new foods, especially flowering plants, that didn’t exist in the Jurassic,” says Carrano.
The jury is still out on whether titanosaurs could have gotten even bigger if the dinosaurs hadn’t gone extinct.
“I think there could have been a bit bigger sauropods, maybe, but not dramatically,” Carrano says. He says it’s helpful to think in terms of “orders of magnitude,” which technically refers to change by factors of ten, to think of major changes in size. For example, going from one ton to 10 tons is a big deal, he says, much more so than going from 10 to 20 tons. “There is a change of scale in the first but not in the second.”