What Is a Titanosaur? The Largest Animals That Ever Walked

Titanosaurs are the last major group of sauropods and the only one to survive to the end of the Cretaceous, by which time the other long-necked lineages had largely disappeared. They were most diverse in the southern continents, particularly South America, which reflects the breakup of the supercontinent and the separate evolutionary histories that followed, and their fossils are found on every continent including Antarctica. Their skeletons differ from earlier sauropods in several respects, with wider-set limbs producing a distinctive broad trackway, more extensively pneumatic bones, and in many species bony armour plates embedded in the skin, which is unusual among sauropods and was confirmed by specimens found with the plates in place. They ranged enormously in size, and the group includes not only the giants but also dwarf forms from European islands, where restricted resources produced animals a fraction of the size of their mainland relatives through the same process that produces island dwarfism today.

Turning a partial skeleton into a body mass involves several methods that disagree with each other:

• •Limb bone scaling, which uses the circumference of the weight-bearing bones and a relationship calibrated on living animals, and which is quick and depends on those specific bones being present
• •Volumetric reconstruction, which builds a three-dimensional model of the animal and multiplies by an assumed density, and which depends heavily on how much flesh the modeller puts on
• •Density assumptions, which matter enormously since extensive air sacs make a sauropod considerably less dense than a mammal of the same volume
• •Extrapolation from fragments, which is how several famous estimates were produced and which carries very large uncertainty
• •Convergence between independent methods, which is the main check available and which has improved confidence for the best-known specimens
• •Published ranges that are frequently reported as single numbers, which is where most of the public confusion about which dinosaur was largest comes from

Several species compete for the title and the competition is unsatisfying because the best candidates are known from the worst material. Argentinosaurus is represented by a small number of vertebrae and limb elements and has been estimated between roughly fifty and a hundred tonnes depending on the method, which is a range wide enough to make the claim hard to assess. Patagotitan is known from far more material, including multiple individuals, and yields a better-constrained estimate somewhat lower than the upper Argentinosaurus figures. Dreadnoughtus is among the most complete very large sauropods and its original estimate was subsequently revised downward substantially when the method was reconsidered, which is a useful illustration of how much the assumptions matter. Several other claimants rest on material that has been lost or on measurements that cannot be checked, including one famous case known only from a drawing of a vertebra that disappeared. The honest position is that several animals were in the same size class and ranking them precisely is beyond the evidence.

The interesting question is not which was largest but why the ceiling sits where it does. Bone strength scales with cross-sectional area while weight scales with volume, so a larger animal needs disproportionately thicker limbs, and beyond a certain point the limbs become impractical. Feeding becomes limiting, since intake must scale with mass and a head can only be so large, which sauropods addressed by not chewing at all and processing food in a large fermenting gut. Heat becomes limiting, since a large body generates heat and sheds it through a relatively smaller surface, which the extensive air sac system may have helped with. Growth becomes limiting, since an animal hatching from a small egg must grow through several orders of magnitude before reproducing, and the faster that growth the more food is required. Reproduction is the one constraint sauropods escaped compared with mammals, since laying many small eggs avoids the burden of carrying enormous young, and that is part of the standard explanation for why no mammal has come close.

This is the last surviving sauropod group, diverse in the southern continents, broad-limbed and in many species armoured, and it includes both the heaviest land animals and island dwarfs. Mass estimates from limb circumference and from volumetric models disagree, and density assumptions matter because air sacs make these animals far lighter than their volume suggests. The best size candidates are known from the worst material.