Had their fossilised bones not emerged from the ground, no one would believe that they had ever existed. But it’s hard to miss thigh bones that are as tall as a human and thoracic vertebrae so huge that you need a forklift to move them.
Ever since sauropods — the giant dinosaurs with the extra-long necks — were described for the first time in the 1800s, they have presented scientists with plenty of fodder for debate.
These iconic dinosaurs were the largest animals to ever live on dry land, weighing in at sizes of up to 100 tonnes, more than the weight of 15 elephants. They were long considered large, stupid and slow, and were thought only to have existed briefly during the
Jurassic period some 145 million years ago — basically, a blind alley in evolution. But over the past decade, intriguing finds and technological advances have shed new light on the lives of these behemoths. With the discovery of more than 60 new species, scientists have realized that these huge herbivores were hardly slow and doomed; in fact, they were one of the planet’s longest-running, success stories.
“The research is still at an early stage, but in the next decade, we will have a completely different impression of these animals,” says paleontologist Mark Norell of the American Museum of Natural History in New York.
Indeed, new findings show that sauropods came in many shapes and successfully adapted to all the world’s continents except Antarctica. From the time they came into existence some 225 million years ago until the sudden extinction of all dinosaurs 160 million years later, they thrived. In comparison, the large mammals of the modern world will need to survive for another 100 million years to match the sauropods’ longevity.
Since 2004, an international group of sau-ropod specialists have been studying the massive dinosaurs, and may have finally solved the mystery of their incredible growth. In short, it turns out that sauropods became the most efficient herbivores that the world has ever seen. Everything from their digestive system to their lungs was designed to obtain the largest amount of kilojoules while expending the least amount of effort.
Sauropods grew up fast
Early on, scientists dismissed the theory that environmental factors such as extremely nutritious food or high oxygen content in the atmosphere could explain sauropods’ explosive growth. The living conditions of plants and animals actually were worse than they are today.
Instead, they focused on the creatures’ biology. Sauropods clearly evolved from smaller ancestors, and there are only two ways they could have become so large: either by growing over a longer period of time than their predecessors, or by growing at a faster pace.
If a sauropod were to have grown at an even rate, like reptiles do, it would have needed more than 60 years to become fully grown and sexually mature. But by studying the microscopic structures preserved in fossilised bones, scientists discovered that sauropods grew much more quickly; the growth rate of the dinosaurs resembles that of modern large mammals, which become fully mature in the course of a few decades.
But fast growth is not enough. The largest modern land animals, elephants, stop growing long before they reach 10 tonnes. Even the largest land mammal that ever existed, the six-metre-tall Indricotherium rhino, which lived in Asia some 30 million years ago, weighed around 15 tonnes. But, over millions of years, a line of animals will push evolution as far as it can. In other words, animals will become larger over time as long as their environment supports it. Sauropods were able to develop into giants, reaching weights of up to 100 tonnes, because they had plenty of room to grow.
However, their increasing size created challenges, namely the need to consume a sufficient amount of kilojoules to meet the huge energy demands of maintaining such enormous bodies. When scientists began to focus on how much sauropods needed to eat in order to reach their phenomenal size, the pieces of the puzzle fell into place. As it turns out, sauropods combined evolutionary traits from mammals, birds and lizards in a unique way.
They kept the primitive traits of lizards, which have lots of very small young and don’t chew their food, but simply rip it off and swallow it. But they also developed a more modern trait seen in mammals — a high metabolism — and sophisticated lungs like those seen in birds. These traits all contributed to fulfilling a crucial goal: extracting as much energy as possible as easily as possible.
The advantages of a long neck
Although sauropods’ distinctive long necks were designed to reach food, they were not (as previously assumed) used to reach the leaves of the tree canopy. Data indicates that sauropods actually held their heads horizontally most of the time.
Fossilised feces also show that sauropods consumed as much grass as they did leaves. Scientists have thus concluded that their necks primarily swung from side to side, and that the extravagant length enabled the dinosaurs to graze over huge areas of ground without having to move their large bodies. By contrast, a dinosaur with a shorter neck would need to move around much more — using more energy — to cover a similar area. Their long necks probably enabled the sauropods to outcompete other herbivores at ground level while also giving them the additional advantage of being able to reach higher leaves in the trees.
The record for having the longest neck as compared to body size is held by the 18-metre, 13-tonne Mamenchisaurus, which lived 160 million years ago in China. At 9.3 metres, its neck accounted for more than half the length of its body, giving Mamenchisaurus an unprecedented reach. Compared with the rest of its body, its head was tiny, weighing only 25 kilograms; a long neck generally can’t support a large head. Likewise, sauropods couldn’t develop a massive chewing apparatus, which would have required extensive binding surfaces to the powerful jaw muscles and large molars.
Sauropod teeth were designed to tear off food, not chew it. Scientists conclude that they didn’t waste time processing plant matter — they just constantly gorged themselves. A midsize sauropod like Mamenchisaurus likely consumed 500 to 600 kilograms of food a day, while its most massive cousins ate more than a tonne.
A large body also requires an elongated digestive system, another potential reason for sauropods’ rapid growth rates: A long digestive system extracts more nutrients from food.
An innovative lung design
While mammalian thoracic vertebrae are heavy and close-set, the thoracic and cervical vertebrae of sauropods featured cavities filled with air sacs, which would reduce the weight considerably and enable the neck to be so long.
The air sacs also had another function. In birds, they form part of the lungs, which are inflexible. In mammals’ lungs, air enters into and exits from the same place, but in birds, the air sacs blow the air in a circular motion.
Air enters in one side of the lung but exits from the other. This allows birds to constantly absorb oxygen. Scientists maintain that similar air sacs in sauropods functioned as bellows, enabling the creatures to empty their long necks of air. This lung design meant that the sauropods needed to breathe less and could conserve their energy for other purposes — such as growing to a massive size.
The final vital trait for sauropods’ success was the number of young they could have in each reproductive cycle. Scientists have unearthed hundreds of fossilised sauropod eggs, including some that have tiny embryos. The finds reveal that sauropods laid up to 40 eggs at special nesting sites to which they frequently returned. Despite their record-breaking size, sauropods laid relatively small eggs, about the size of grapefruits.
This supports the theory that newly hatched sauropod babies had to have grown incredibly quickly to reach their adult size. Scientists think that they doubled their weight in five days. By comparison, a newborn human takes some five months to double its birth weight.
After two months, the sauropod young weighed 16 times more than when they hatched. And the fast growth continued: After about 20 years they reached sexual maturity, and were fully grown around 30, topping out at 10,000 times larger than when they hatched.
Such growth was vital to survival, as young sauropods were extremely vulnerable to predatory dinosaurs. Although most died before obtaining their nigh-invulnerable adult size, the population held steadily. A female sauropod could lay 2, 000 to 3,000 eggs in her lifetime, and since only two or three hatchlings needed to reach reproductive age to maintain the population, enough sauropods survived to hold their dominant position.
The return of the giants
This evolutionary cocktail of traits has disappeared from the modern gene pool, but sauropods have taught us much about the mech-anisms behind explosive growth. Most scientists are certain that mammals will never grow as large — some may boast similar growth rates, but mammals lack bird-like lungs that constantly absorb oxygen, they chew their food too much and they expend too much time and energy raising a small number of relatively large young.
However, experts haven’t ruled out the possibility that giants may again roam the Earth some day. As University of Bonn paleontology professor Martin Sander says, “Wait 500 million years and a couple of mass extinctions, and then we will see what happens.”
Facts about Sauropods
THIN, WIDELY SPACED RIBS – Scientists long thought that sauropods lived in swamps, where the buoyancy of the water helped support their giant bodies. But their ribs were thin, light and widely spaced — characteristics similar to land-based animals.
COUNTERBALANCING TAIL – The extra-long tail helped sauropods maintain their balance by serving as a cou nterweig ht when they moved their heads laterally. It may also have been used for communication.
SLOW HEARTBEAT – The larger the animal, the lower the pulse. A mouse’s heart beats 700 times a minute, a human heartbeat clocks in around 72 beats per minute and an elephant’s heartbeat is 28 bpm. Sauropod hearts may have beat as slowly as 5 to 7 times a minute.
PURPOSEFUL TEETH – Teeth types varied among sauropods, but within an individual they were all alike: Diplodocus had small, pencil-shaped teeth similar to a rake, which could be used to rip leaves off branches. Others, like Camarasaurus, had larger, spoon-shaped teeth that were good for breaking twigs and branches.
TINY HEAD – In proportion to their size, sauropods had very small heads — around 25 kilograms. The animals’long necks could not have supported a heavy head, and as sauropods didn’t chew their food, they had no need for large molars and massive chewing muscles.