The biggest dinosaur ever may have been twice the size we thought

2020-06-10

FOR a century, visitors to Chicago’s Field Museum have marvelled at a display featuring two African bush elephants, frozen mid-fight. In the past couple of years, however, this awesome spectacle of the largest living land animals has been overshadowed by an enormous skeleton. As impressive as the elephants are, they look like squabbling children beside Patagotitan, a 100-million-year-old sauropod dinosaur that was as long as a blue whale, taller than a giraffe and probably outweighed each elephant 10 times over.

Since 2014, when news first broke of its discovery, Patagotitan has frequently been described as the most massive animal ever to walk the Earth. Such superlatives captivate us. Even if you aren’t a dinosaur fan, it is awe-inspiring to think that the skeleton in the Field Museum belongs to a creature that is as big as they get.

Except it isn’t. Weighing up such giants isn’t simple, but new calculations indicate that other dinosaurs from the same family – the aptly named titanosaurs – were at least as massive. In fact, Patagotitan might not even come close to claiming the heavyweight title. Some palaeontologists now believe that the ground once trembled under the mass of a near-mythical dinosaur that was twice as heavy as Patagotitan. How they have reached these conclusions is a story of monumental discoveries, lost treasures, academic showmanship and clay models.

Sauropods first appeared in either the Late Triassic or early Jurassic, about 200 million years ago. Dinosaurs in the group, which includes Diplodocus and Brachiosaurus, are known for having large bodies, long necks and tails, and tiny heads. Yet their most defining characteristic is their size (see “How to grow really big”). For more than a century, people have wanted to know just how big they could get. Until recently, an answer has proved elusive, not least because sauropods often leave surprisingly light footprints in the fossil record. “The bigger they are, the less of them we find,” says Mathew Wedel at the Western University of Health Sciences in California. “Burying a whale-sized land animal isn’t easy.”

“Sauropods often leave surprisingly light footprints in the fossil record”

So when, in the late 1980s, palaeontologists in Argentina excavated a few bones of a truly enormous sauropod, it wasn’t clear what the complete animal looked like or how large it was. They named it Argentinosaurus and continued t0 work – and to great effect. In the past two decades, the fossil beds of Argentina have yielded a string of remarkable discoveries from the same group of sauropods, the titanosaurs. First, in 2000, came Futalognkosaurus, an animal known from three specimens that together preserve about three-quarters of the skeleton, excluding the skull – sauropod skulls were so fragile that they rarely fossilised. In 2005, Dreadnoughtus began to emerge from the ground in the form of two specimens, also representing around three-quarters of the skeleton. Finally, in 2013, work began to uncover bones at a site in Chubut province. At least six specimens of Patagotitan were found, which together encompass most of the skeleton.

“Titanosaurs were very fragmentary and poorly known,” says Gregory Paul, an independent researcher based in Baltimore, Maryland. “Now we’re getting enough skeletal material to get some good reconstructions.” Rebuilding these skeletons still requires someone who understands sauropod anatomy, he says, because there is plenty of variation and potential for error. This is especially true when it comes to the torso, where the bulk of a sauropod’s weight is concentrated. Rebuild the skeleton incorrectly – which even some museums do, according to Paul – and you will overestimate the weight of the sauropod. But get the anatomy right and you can calculate its body mass with reasonable precision.

A few months ago, Paul published his latest estimates for a range of sauropods. To do so, he fashioned 30-centimetre-long models from clay, measured their volumes from the amount of water they displaced and then scaled up. Futalognkosaurus came out at about 29 tonnes and Dreadnoughtus at about 31 tonnes. Patagotitan was much heavier, weighing between 50 and 55 tonnes. These estimates are very close to others published recently based on virtual sauropod models built using computer software. In other words, says Paul, a consensus is beginning to emerge on exactly how big these titanosaurs were. There is just one problem.

An alternative method for estimating body mass in land animals is based on the idea that it correlates with the sturdiness of their limbs. After all, it is the limbs that must bear the weight of anything that stalks the land. That is why, however you measure them, not even the most enormous dinosaurs yet discovered are anything like as big as the giants of the sea: blue whales, which can weigh in at upwards of 150 tonnes, remain the biggest animals we know to have existed on Earth.

By measuring the circumference of a thigh bone and plugging this into a “scaling equation”, you can get reasonably accurate body mass estimates for animals as distinct as orangutans and kangaroos, says Nicolás Campione at the University of New England in Armidale, Australia, who swears by the technique. However, when applied to sauropods, it gives results wildly different from those obtained with anatomical models. For instance, it almost doubles the weight of Dreadnoughtus to 59 tonnes, and makes Patagotitan about a third heavier at 69 tonnes. What is going on?

“I don’t think the scaling equations are wrong,” says Wedel. “I think they’re imprecise.” The main problem is the margin of error, which can be 30 tonnes or more for a gigantic sauropod. Despite its imprecision, the method is popular among dinosaur palaeontologists because it is easy to use, even without a good understanding of sauropod anatomy. They aren’t necessarily concerned by its shortcomings. Biologically and behaviourally speaking, a 30-tonne sauropod was probably similar to a 60-tonne one, says Campione, and pinning down body mass more precisely arguably has limited scientific value.

Kenneth Carpenter at Utah State University Eastern in Price, who has attempted to measure sauropod body mass with great accuracy, agrees that such efforts aren’t particularly important scientifically. He points out that palaeontologists who claim to have found the largest and heaviest sauropods are invariably male. This sentiment is echoed by Kristi Curry Rogers at Macalester College in Saint Paul, Minnesota. “I just don’t get that excited by these claims, even though I work on these giant dinosaurs myself,” she says. “I’d rather put more focus on the palaeobiology and less on the sauropod showmanship.”

Yet try telling an inquisitive child it doesn’t matter exactly how big the biggest ever land animal was – or, indeed, anyone’s inner child. That is why people take notice when someone like Wedel says that Patagotitan was almost certainly not “the largest dinosaur that ever lived”, as the Field Museum claims. He thinks that the half dozen Patagotitan specimens were approximately the same size and weight as the largest specimens of two other Argentinian titanosaurs we know from just a few bones: our old friend Argentinosaurus and Puertasaurus, which was discovered in 2001. What’s more, this could be very significant. “Maybe they’re hitting the limits of what you can bury,” he says. To put it another way, sauropods might have come much larger than Patagotitan, but such gigantic beasts might be so difficult to fossilise that we will never find their remains. Or will we?

Mythical monster?

Way back in 1878, Philadelphia-based palaeontologist Edward Cope received delivery of a fossil from a quarry in Colorado. It was part of a sauropod vertebra – a spine bone – and it was astonishingly large: the fragment alone was 1.5 metres tall. Cope named the dinosaur Amphicoelias fragillimus. He then, somehow, misplaced the fossil, or perhaps it was so fragile – as the name suggests – that it crumbled to dust. Ever since, A. fragillimus has been considered almost legendary. Some simply refuse to accept that the bone was as large as Cope claimed, because that would make the dinosaur unfeasibly huge. In 2014, a pair of researchers suggested that there was a typo in his description – that he mistakenly labelled its height as 1500 millimetres instead of 1050 millimetres. Carpenter finds that implausible, pointing out that the specimen’s collector, Oramel Lucas, discussed the fossil’s size in his correspondence. “It is an independent corroboration of what Cope published,” he says.

In 2018, Carpenter estimated that the complete vertebra stood 2.4 metres tall. The equivalent bone in Patagotitan is 1.4 metres tall. He also suggests an explanation for its enormousness. Researchers now recognise a distinct group of Diplodocus-like sauropods with unusually large vertebrae. Carpenter believes A. fragillimus belongs in this group; these dinosaurs had thigh bones about 1.2 times as long as their vertebrae, whereas in some sauropods they are almost twice as long. This would put the thigh bones of Cope’s specimen at 2.9 metres – not outrageously longer than Patagotitan thigh bones, all of which are around 2.4 metres long. That isn’t implausible, according to Wedel.

Paul has taken Carpenter’s reappraisal of Cope’s dinosaur and built on it. In his recent study, Paul estimated its body mass at between 80 and 120 tonnes, making it about twice as heavy as Patagotitan. Carpenter says such a wide range is little better than a guess, and Paul accepts that his estimate is necessarily vague, given that it is based on one fossilised vertebra that is now lost. Nevertheless, when Carpenter attempted a similar estimate 15 years ago, he calculated that the missing giant might have weighed 120 tonnes.

“Sauropods might have come much larger than Patagotitan, but such gigantic beasts might be too difficult to fossilise”

Wedel reserves his judgement. He says that, without discovering many more fossils, we can’t be sure just how big the biggest ever land animals were – however frustrating that might be to some museum-goers. “There’s a widespread perception that we understand dinosaurs a lot better than we do,” he says. “Palaeontologists are nowhere near done. We are just getting started.”

newscientist.com, 10 June 2020
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