In 1890, the fossil-hunter Othniel Charles Marsh described a new species of dinosaur from Colorado. He only had a foot and part of a hand to go on, but they were so bird-like that Marsh called the beast Ornithomimus – the bird mimic. As the rest of Ornithomimus’ skeleton was later discovered, Marsh’s description seemed more and more apt. It ran on two legs, and had a beaked, toothless mouth. Despite the long tail and grasping arms, it vaguely resembled an ostrich, and it lent its name to an entire family – the ornithomimids—which are colloquially known as “ostrich dinosaurs”.
Now, the bird mimic has become even more bird-like. By analysing two new specimens, and poring over an old famous one, Darla Zelenitsky from the University of Calgary has found evidence that Ornithomimus had feathers. And not just simple filaments, but wings – fans of long feathers splaying from the arms of adults. (More technically, it had “pennibrachia” – a word for wing-like arms that couldn’t be used to glide or fly.)
Look up any dinosaur, and chances are you will soon come across an estimate for how long it was. And chances are that estimate is wrong. That’s because, as Dave Hone from University College Dublin points out, our knowledge of dinosaur tails is woefully inadequate.
After searching through papers, museum collections, photos, and the minds of his colleagues, Hone found that among the thousands of dinosaur specimens that have been found, there are “barely two dozen complete tails”. These range from animals like Spinosaurus, where virtually no tail fragments have been found, to others where skeletons are missing an unknown number of vertebrae from the tips. Even in complete skeletons, Hone’s research showed that closely related species, and even individuals, can vary greatly in the length and number of bones in their tails.
This matters since tails are factored into estimates of the animals’ lengths, and lengths are often used to estimate mass. As I wrote in my Nature piece on Hone’s work, “If tails are telling tall tales, other important measures could be inaccurate.” Head over there for the rest of the story.
Image by Ballista
The largest mounted dinosaur skeleton in the world towers over visitors in the central hall of Berlin’s Museum of Natural History. It belongs to Giraffatitan, an animal formerly known as Brachiosaurus (the big one from the opening act of Jurassic Park). From the bones, we can tell how long and tall Giraffatitan was, but how much did it weigh?
The dinosaur’s flesh has long decayed, but Bill Sellers from the University of Manchester has developed a new way of reconstructing its physique and estimating its weight. By laser-scanning the skeleton, and wrapping skin around its virtual bones, he calculated that this particular Giraffatitan weighs in at a hefty 23,200 kilograms, or 23.2 tonnes. And no matter what the university’s press office would like you to believe (more on this later), that’s virtually identical to the best current estimates.
There are two typical approaches for estimating the weight of fossil animals. You can compare the lengths of certain bones with those of known animals, assume that its mass scales accordingly. This is the predictive regression approach, and it can be unreliable. Skeletal features can vary greatly and they may not relate to weight in the same way between different animal groups. The alternative is the volumetric approach: you draw an outline of its body, estimate how much volume it took up, and multiply that with its predicted density. It’s better, but drawing the outline is both subjective and laborious.
Sellers has devised a third option. He scans an entire skeleton and his software automatically stretches a virtual skin over the outline as tightly as possible. This estimates the volume of the animal, albeit of an emaciated unrealistic individual.
When Sellers tested this technique with 14 mammal skeletons, from a wild boar to an African elephant, he found that it underestimated the weight of all the species. You’d expect that – after all, the virtual reconstruction doesn’t include any muscles or organs. The point is that the technique consistently underestimated the animals’ weight by around 21 per cent. You can just take the former, multiply it by 1.21 and get the latter.
The relationship between the predicted and actual weight is remarkable in its reliability. Sellers thinks that this is because most of the missing volume in the virtual models is from the limbs muscles, which make up a fairly fixed proportions of a mammal’s mass, regardless of its size.
Sellers then applied his method to Berlin’s famous Giraffatitan and got a value of 23.2 tonnes. Obviously, it’s not clear if a technique that was calibrated against large mammals would apply to dinosaurs, or other groups like reptiles or birds. Sellers acknowledges this, and plans to test his technique in a wider range of animal groups.
There are other potential issues. Mike Taylor from the University of Bristol and SV-POW has estimated the weight of Giraffatitan before, and thinks that around 70 per cent of its volume comes from the torso. And reconstructing the torso is very difficult for large dinosaurs, because the ribs are often poorly preserved or distorted. Taylor also says that using a single density value isn’t that appropriate for brachiosaurs. “The very long neck likely had a density no more than half that of the legs,” he says.
For the moment, it’s encouraging that the new estimate is very close to previous ones. You might not get that from the press release (and probably most of the resulting coverage). It leads with “Dinosaurs lighter than previously thought”, and follows with “Previous estimates of this Brachiosaur’s [sic] weight have varied, with estimates as high as 80 tonnes, but the Manchester team’s calculations – published in the journal Biology Letters – reduced that figure to just 23 tonnes.”
While it is true that the weight of Giraffatitan and Brachiosaurus have varied wildly over the years, the most recent estimates have been nowhere near the cherry-picked 80-tonne figure. Indeed, in 2009, Taylor concluded that Giraffatitan weighed 23,377 kilograms, or 23.3 tonnes. Sellers’ new estimate shaves off a mere 177 kg from that figure – around 2 humans from a dinosaur that weighed as much as 300.
Taylor used the volumetric method to get his result. If his result was exactly the same as the new figure, one might question whether Sellers’ method adds anything new. It does, however, have several benefits. “It requires no irreproducible judgements on the part of the person using it, and it’s ground-truthed on solid data from extant animals,” says Taylor. It’s also automated. If it truly works for dinosaurs, we can weigh these extinct beasts as quickly as the laser-scanner can be wheeled around a museum. Even with the caveats, Taylor says “It’s an important new method which I expect to see widely adopted.”
Reference: Sellers, Hepworth-Bell, Falkingham, Bates, Brassey, Egerton & Manning. 2012. Minimum convex hull mass estimations of complete mounted skeletons. Biology Letters http://dx.doi.org/10.1098/rsbl.2012.0263
Image from Berlin Natural History Museum postcard
HT Matthew Cobb for the story tip
More on sauropods
Yutyrannus, by Brian Choo
Meet the largest feathered animal in history – an early version of Tyrannosaurus rex, clad in long, fuzzy filaments. This newly discovered beast has been named Yutyrannus huali, a mix of Mandarin and Latin that means “beautiful feathered tyrant”. And its existence re-opens a debate about whether the iconic T.rex might have been covered in feathers.
“This is a tremendously important fossil. Paleontologists have been waiting for a gigantic feathered theropod to turn up for some time,” says Lindsay Zanno from the Field Museum. Larry Witmer from Ohio University, agrees. “The big thing is the one-two punch of being huge AND feathered,” he says.
For the longest time, artists could only speculate about what dinosaurs looked like. Sure, we could reconstruct their silhouettes from their bones, but the colour of their skin was a mystery. That’s not quite true anymore. Thanks to some well preserved fossils and some ingenuous detective work, scientists have started to assign the right palettes to these prehistoric reptiles.
The latest species to get this treatment is Microraptor. This Chinese dinosaur was about the size of a crow. Its body was covered in feathers. Long plumes on both its arms and legs gave it a distinctive four-winged, baggy-trousered look, and may have allowed it to glide or fly. And thanks to a new study by Quanguo Li form the Beijing Museum of Natural History, we know that Microraptor was probably black and certainly shiny.
It was a iridescent dinosaur, with the same metallic sheen that you see on today’s hummingbirds, peacocks, and swallows. If you travelled back in time and stumbled across Microraptor, you might think that you’d found a Cretaceous starling.
Thomas Libby filmed rainbow agamas – a beautiful species with the no-frills scientific name of Agama agama – as they leapt from a horizontal platform onto a vertical wall. Before they jumped, they first had to vault onto a small platform. If the platform was covered in sandpaper, which provided a good grip, the agama could angle its body perfectly. In slow motion, it looks like an arrow, launching from platform to wall in a smooth arc (below, left)
If the platform was covered in a slippery piece of card, the agama lost its footing and it leapt at the wrong angle. It ought to have face-planted into the wall, but Libby found that it used its long, slender tail to correct itself (below, right). If its nose was pointing down, the agama could tilt it back up by swinging its tail upwards.
The sickle-shaped “killing claws” of dinosaurs like Deinonychus and Velociraptor have captured the imagination for decades. They were held aloft from the second toe, and were far bigger than the neighbouring claws. In Jurassic Park, Alan Grant tells an annoying child that the dinosaurs used their claws to disembowel their prey with slashing motions. That seems unlikely – they didn’t have a suitable cutting edge. Others have suggested that they were used for climbing onto larger prey.
But neither idea made sense to Denver Fowler from Montana State University, who has put forward a very different idea about how these animals used their infamous claws. He compared the feet of extinct dinosaurs like Deinonychus to those of living dinosaurs like eagles, hawks and other birds of prey. Both groups are known as “raptors” and Fowler thinks that they share more than their nicknames.
In his vision, which he calls the “ripper” model, Deinonychus killed small and medium-sized prey in a similar style to a hawk or eagle dispatching on a rabbit. Deinonychus leapt onto its target and pinned it down with its full body weight. The large sickle-shaped claws dug into its victim, gripping tightly to prevent it from escaping. Then, Deinonychus leant down and tore into it with its jaws. The killer claws were neither knives nor climbing hooks; they were more like anchors.
It’s a simple idea, but a potentially important one, for it casts Deinonychus’s entire body into a new light. Fowler thinks that it flapped its large feathered arms to keep its balance while killing a struggling victim. And its feet, which were adapted for grasping prey, would have given its descendants the right shape for perching on branches. Fowler says, “It really helps to make sense of the weird anatomy of these little carnivorous dinosaurs.”
In the Field Museum of Natural History, there is a piece of bone about the size of a child’s torso. It’s an osteoderm – a scale-like bone embedded in the skin of an animal. In this case, the owner was Rapetosaurus, one of the giant armoured dinosaurs known as titanosaurs. The osteoderm probably sat on the animal’s back, and the standard interpretation is that they provided a sturdy defence.
But Kristina Curry Rogers from Malacaster College thinks that these “skin bones” did something else. She was the scientist who discovered Rapetosaurus, and she has been analysing two new osteoderms from northwest Madagascar, where the titan used to live. The larger of these is the biggest one ever found – 57 centimetres in length, 27 across and 19 deep.
It’s also hollow. When Rogers put the bone in a CT scanner, she found a big cavity inside it, big enough to hold five litres. She now thinks that the osteoderm was a store for minerals that the dinosaur could draw upon when times got tough.
We now know that birds evolved from small, feathered dinosaurs. It’s easy to think that since birds are still around today, they must have come after their dinosaur* cousins, but that’s not true. In the Cretaceous period, dinosaurs were still around while their descendants flitted through the skies. And some dinosaurs made meals of their flighty relatives. Jingmai O’Connor from the Chinese Academy of Sciences has uncovered the remains of a small dinosaur called Microraptor that has the bones of small bird in its gut.
O’Connor analysed the fossil with Xing Xu, a Chinese scientist who has made a career from discovering beautiful feathered dinosaurs. Microraptor is one of his most important finds. This tiny animal, about the size of a pigeon, had four wings, with long feathers on both of its legs as well as its arms. It was, at the very least, a very competent glider, if not a true flier.
Some of you know Brian Switek, ace blogger who covers all things fossil. What you may not have realised is that Brian Switek hates your childhood dreams and is out to crush them by making all badass prehistoric predators seem a bit rubbish. This is the latest volley in his ongoing campaign.
Last night, I decided enough was enough and drastic action was needed to preserve the predators that stalked our young imaginations, in the face of Switek’s tawdry “facts”. After all, you could prove just about anything that’s true using facts.
Thus was born the #GRAWR hashtag on Twitter, a collection of trivia about prehistoric animals based solely on how awesome they would be, and leaving aside silly notions like evidence. I collected some of my favourites (you should see a stream of tweets below; if not, check on a different browser): Read More