How Big Did Dinosaurs Get? The Sauropods and the Limits of Size
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The heaviest land animal alive is the African elephant, at around six tonnes. The heaviest that has ever lived on land was a sauropod dinosaur, and the best estimates for the largest of them, the Argentine giants of the Cretaceous, are between sixty and eighty tonnes, with necks that could reach a fourth-floor window. Getting that big takes more than eating; it takes solving problems of breathing, circulation, heat and structure that no mammal has solved, and working out how the sauropods did it has explained why they had no successors.
The contenders
The largest dinosaurs known from good skeletons are the titanosaurs of South America: Argentinosaurus, described in 1993 from a few vertebrae and a leg bone, and Patagotitan, found in 2012 and known from several individuals, both estimated at around 37 metres long and 60 to 70 tonnes. Rivals rest on scraps: Maraapunisaurus, known from a single vertebra drawn in 1878 and since lost, might have been bigger, or might have been a misreading. Among the earlier Jurassic sauropods, Brachiosaurus stood tallest, at about 13 metres, and Diplodocus was the longest for a century at 27 metres.
Mass estimates are uncertain by a factor of two, because the soft tissue has to be guessed. The methods are to scale up from the circumference of the leg bones, which must carry the weight, or to build a three-dimensional model of the body and calculate its volume. The two rarely agree, and every headline claim of a new record should be read with that in mind.
How a body that size works
A sauropod scaled up from an elephant would have collapsed under its own weight, overheated and suffocated. The sauropods avoided all three with features inherited from small ancestors and refined over a hundred million years:
- •Hollow bones: the vertebrae and ribs were riddled with air sacs, as in birds, cutting the weight of the skeleton by a third or more without losing strength
- •A bird-like lung: air sacs throughout the body pushed air through the lungs in one direction, far more efficient than a mammal's in-and-out breathing and able to ventilate a nine-metre neck
- •A small head: sauropods did not chew; they cropped leaves with peg-like teeth and swallowed, so the head at the end of the long neck weighed almost nothing
- •Egg laying: a sauropod hatched at around ten kilograms and grew to tens of tonnes, so no mother ever had to carry a giant, unlike a whale or elephant
- •Columnar legs: straight, pillar-like limbs with the bones stacked vertically, so that muscle did not have to hold the joints against the weight
The neck
The neck is the sauropod's defining feature and its most argued-over one. Mamenchisaurus, from China, had a neck nearly half its total length, and the necks of the largest titanosaurs reached ten to twelve metres. Whether they were held high, like a giraffe's, or swept horizontally like a boom is unresolved: a high neck would need blood pressure roughly double a giraffe's to reach the brain, and a heart to match, while a low neck would let the animal sweep a wide arc of vegetation without moving its body, which for a sixty-tonne animal is a considerable saving. The likeliest answer is that different sauropods did different things, and that the neck's main purpose was to reach food others could not while standing still.
Why nothing has matched them
Mammals have never come close. The largest land mammal, the rhinoceros relative Paraceratherium of thirty million years ago, weighed perhaps twenty tonnes, a third of the biggest sauropod. The reasons trace back to the list above: mammals have solid bones, tidal lungs, heavy chewing heads and live young that must be carried and born, and each of those imposes a ceiling. A whale escapes the limits by living in water, where the sea carries its weight, and the blue whale at 150 tonnes is heavier than any sauropod. On land, the sauropod body plan appears to be the only one that has ever worked at that scale, and it needed the specific inheritance of air-sac breathing from the early dinosaurs to work at all.
It also needed a world to feed it. Sauropods lived in warm, high-carbon-dioxide climates with lush conifer and fern growth, and they ate constantly, perhaps a few hundred kilograms of vegetation a day, digested slowly in a vast gut. When that world ended, so did they.
The takeaway
The largest dinosaurs were the titanosaur sauropods of Cretaceous South America, around 37 metres long and 60 to 70 tonnes, ten times an elephant. They reached that size with hollow, air-filled bones, bird-style one-way lungs, small unchewing heads, egg-laying and pillar legs, a combination no mammal possesses, which is why nothing on land has approached them since.