LUMBERING Diplodocus with its long neck and massive body is an iconic dinosaur. It belongs to a group known as sauropods, which includes the biggest animals ever to walk the Earth. Some of these monsters weighed more than 120 tonnes, but equally impressive was the length of their necks. At up to 15 metres, they were more than six times as long as a giraffe’s.
How did sauropods evolve such extraordinarily long necks? That’s the sort of question you might expect palaeontologists to discuss after dinner. Which is exactly what Mathew Wedel and Mike Taylor did one night at Taylor’s house. Hauling out their laptops to look at other impressive necks, they noticed a striking pattern: in all other land animals, living and extinct, necks top out at about the same length. For everything excluding the sauropods, there seemed to be a limit of about 2.5 metres. That piqued the pair’s curiosity even more. So they decided to take a closer look and have now published their findings ().
Sauropods represent an early innovation in the evolution of dinosaurs. The first dinosaurs appeared about 240 million years ago and soon split into three groups. One, the ornithischians, evolved into plant-eaters; their descendants include Triceratops and the duck-billed hadrosaurs. A second, the theropods, became predators, later splitting into several lines including tyrannosaurs, birds and a few long-necked herbivores. The third group was sauropods, which by 210 million years ago had evolved into giants that lumbered on four massive legs and sported hugely long necks.
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Taylor, at the University of Bristol, UK, and Wedel from the Western University of ÎçŇą¸ŁŔű1000ĽŻşĎ Sciences in Pomona, California, reasoned that the secret of the sauropods’ remarkable necks must lie in traits inherited from their ancestors. The earliest dinosaurs were bipeds about a metre from nose to tail, with the neck accounting for about a third of their length. They were probably omnivores, using their teeth to grab leaves or small animals, or to rip flesh from larger prey, but not to chew. While ornithischians evolved massive teeth for grinding vegetation, both sauropods and their predatory cousins the theropods retained the habit of gulping down their food. That was the first clue.
All mouth
Essentially sauropods opened their mouths wide and pulled in all the vegetation they could hold, relying on gut bacteria to extract the nutrients as food passed through their long digestive system. “The head was basically a cropping device – all mouth, like Cookie Monster or Pac-Man,” says Wedel. “The size of the head is the size of the bite.” So the head typically weighed in at just 25 kilograms in the 10-to-15-tonne Diplodocus, for example, and about 50 kilograms for the 25-to-40-tonne Giraffatitan. That was light enough to sit on the end of a long neck, which probably moved up, down and around as sauropods devoured all the vegetation they could reach without moving their massive bodies.
A small head leaves little room for brains, of course, but that doesn’t seem to have been a disadvantage for sauropods. Wedel thinks that if they needed it they could have carried more weight in the brains department while still retaining their long necks. At less than half a kilogram, he says, “the brain of Diplodocus was so small that you could double or triple its size and it would have negligible impact”. Big brains clearly weren’t important for sauropod survival; the group was extremely successful without them, thriving for almost 150 million years, right up to the mass extinction of all the dinosaurs 65 million years ago.
The need to chew was, however, a limiting factor in neck evolution in other lineages. Chewing comes with a high overhead – jaws, teeth and muscles to process the food before swallowing. The larger the animal, the more it eats, and the larger that food-processing overhead becomes. This explains why Triceratops and other ornithischians tended to have massive heads atop short, muscular necks. The same is true of large mammalian herbivores such as elephants, which inherited their food-chewing habit from the earliest mammals – small creatures that needed to grind up their food to make it easy to digest in their small guts. Some mammals did evolve longer necks, but this heritage helped impose what Wedel calls an “internal limit” on length. The largest land mammal on record, Paraceratherium, an extinct cousin of the rhinoceros, had a neck that stretched some 2 to 2.5 metres. The giraffe’s, at a similar length, is the longest of any living mammal.
As well as needing to support heavy heads, mammals have another handicap: almost all possess just seven neck vertebrae. Sauropods had no such constraint. Like the most primitive dinosaurs that gave rise to them, they had a variable number of neck vertebrae, typically 12 to 17. What’s more, these had a design feature that made them the ideal scaffold for a long neck: they are full of air spaces. “Most of the time 60 to 70 per cent of the cross-section is air, and in some cases it is as much as 89 per cent,” says Taylor. The vertebrae are a bony mesh of hollow chambers with rigid walls as thin as eggshell in places, maximising the strength-to-weight ratio. Mammals, by contrast, only have air-sac structures in their skulls, such as the sinuses.
Solid bone is heavy, weighing about 1.8 grams per cubic centimetre. “Put air inside the bones and you have lightened the skeleton, and you can fly or become very big,” says Martin Sander at the University of Bonn in Germany, who has studied the biology of sauropod gigantism (). A 1.4-metre long vertebra from the 11 to 12-metre neck of Sauroposeidon, for example, is so full of air cavities that it weighs just 0.2 grams per cubic centimetre. We don’t know how much soft tissue sauropods had on their necks, but Taylor estimates that the record-breaking 15-metre-long neck of Supersaurus weighed a mere 6 tonnes. That is only about 40 per cent of the weight of the tree trunk it resembled (see diagram).
“Put air inside the bones and you have lightened the skeleton, and you can fly or become very big”
Even so, such a load would have needed a massive, stable body as a counterbalance. Although their ancestors walked on two legs, sauropods normally used all four, their huge bodies balancing their long necks. By contrast, a 6-tonne neck would leave a 1.6 tonne male giraffe flat on its face, and for bipeds the problem of balance is greater still. That helps explain the limit of about 2.5 metres in the longest-necked bipedal dinosaurs – members of three unusual groups of herbivorous theropods, the therizinosaurs, ornithomimids and oviraptors. Only one other dinosaur-age reptile seems to have exceeded this limit. A huge pterosaur, Arambourgiania, had a neck measuring perhaps 3 metres, but that is an estimate based on a single vertebra. Besides, pterosaurs were hollow-boned flyers that walked on all four legs on the ground. Today the longest-necked bird is the ostrich, which can manage only a thin, metre-long neck.
Taylor and Wedel have identified one final secret behind the sauropod neck. It lies in the way they breathed. Mammals inhale by inflating their lungs and because these need fresh air in each breath, the volume of the trachea must be much smaller than the volume of the lungs, limiting the size of the neck. Birds inhale by inflating internal air sacs that pull fresh air through the trachea and their relatively small lungs, then push it back through the lungs and trachea as they exhale. Because the air sacs are larger than the lungs, this lets birds inhale far more air than mammals in each breath, so they can have proportionally longer necks. “The bird-style lung is much more efficient than mammal lungs, and there’s good reason to think that dinosaurs had them” at least as far back as the origins of the sauropods, says Taylor. With these structures on a much larger scale than birds, they could have inhaled fresh air all the way down their immense necks.
Unfortunately, little sauropod soft tissue has fossilised and so there is no hard evidence of air sacs. What the fossil evidence does make clear, however, is that neck size and shape varied considerably among species. “No two sauropods have the same kind of long neck,” says Kristi Curry Rogers of Macalester College in Saint Paul, Minnesota. “It looks like necks are plastic structures that change pretty easily and quite readily.” This suggests they were adapting to something in the environment, such as the nature and distribution of foliage. That makes sense since a key advantage of a long neck is to enable foraging without moving around too much. “A long neck is good because it saves you energy,” says Sander, and the savings increase with size. Curry Rogers is in no doubt that for sauropods the combination of massive bodies and necks several metres long was a recipe for success. “They arrived at it really early, and stuck with it until they went extinct.”
Had an asteroid impact not wiped out the dinosaurs, “we would probably still have sauropods, and they would probably still be recognisable”, says Wedel. As it is, nothing has taken their place. That reflects one of nature’s hidden laws: what you can evolve into depends on where you start. Birds cannot grow as large or as long-necked as sauropods because they are wedded to being bipedal. Mammals have necks limited to giraffe size because they have solid bones and need massive teeth, jaws and muscles to chew. Sauropods alone had the fortuitous combination of characteristics that allowed them to smash the limits to neck length experienced by other land animals. Evolution shows no sign of producing anything else quite like them.
Beasts of the deep
Plesiosaurs never approached the extreme size of sauropods but, living in a marine environment with water to support their weight, they did evolve extraordinarily long necks. The most extreme example was Albertonectes, a fossil of which had a neck that accounted for 7 metres of its 11.2 metre length (excluding the skull, which is missing). That impressive neck included a record 76 vertebrae. Palaeontologists have long argued about its function, but today’s consensus is that it was “a stealth device” that enabled the plesiosaur’s head to approach a shoal of fish without giving away the fact that there was a large predatory reptile attached to it.
This article appeared in print under the headline “The neck generation”
