Argentinosaurus: The largest the land has ever known

File:Argentinosaurus skeleton, PLoS ONE.png
A reconstruction of the skeleton of Argentinosaurus from the Museo Municipal Carmen Funes, Plaza Huincul, Neuquén, Argentina.
Image Credit: William Irvin Sellers, Lee Margetts, Rodolfo Aníbal Coria, Phillip Lars Manning, http://www.ploscollections.org/article/info:doi/10.1371/journal.pone.0078733;jsessionid=441A913F8D576BBA46BF0960D01599FD

35 metres long. Longer than a Blue Whale!

21.5 metres tall. Tall enough to look into a 6th story window.

80 tons. Heavier than the Space Shuttle “Endeavour”!

These aren’t the measurements of a building, they are not the dimensions of a ship, and they are not the specifications for a new jumbo jet. They belong to a single, now extinct animal. The largest to have ever walked on this world.

Its name is Argentinosaurus huinculensis (meaning “Argentina lizard of Huincul”). It was first discovered in the Neuquén Province of Argentina (hence the name) in the 1987 by a local rancher named Guillermo Heredia then revealed to the world in a paper written by Bonaparte & Coria in 1993. Remains of Argentinosaurus skeletons are fragmentary with only 13 intact bones having ever been found. These include vertebrae, ribs and a single incomplete femur all of which are huge in themselves. Even a single vertebrae is about 1.5 metres (5 feet) tall. But with only fragments such as these how can palaeontologists estimate Argentinosaurus’ enormous size? Well, one way that the weight can be estimated is by taking measurements of the minimum circumference of the upper arm bone (humorus) and thigh bone (femur), ideally from the same individual animal, then plugging them into a mathematical formula. This method can produce a range of estimates however, especially if we don’t have a humorus and femur from the same animal. So 80 tons may be an over or under estimation depending on who you talk to. The current estimated range of possible weights is between 77-110 tons! As for the height and length, it’s a matter of comparing the Argentinosaurus fossils with fossils of more complete close relatives and then determining the size based on the proportions of these relatives, taking into account the varying size, shapes and relative weights of the bones. This has problems too as it is based on assumptions about similarities close relatives. So the estimated proportions may not be 100% accurate, but it’s the best that palaeontologists can do at present.

File:Argentinosaurus 9.svg
A size comparison between an average person and Argentinosaurus. The bones shown are among the only ones palaeontologists have ever found. Image Credit: User:Slate_Weasel, https://commons.wikimedia.org/wiki/File:Argentinosaurus_9.svg

Argentinosaurus lived during the Mid-Cretaceous period, roughly 100-95 million years ago, and belonged to an order of dinosaurs known as the sauropods. The sauropods are among the most popular and recognisable dinosaurs out there and are typically characterised by long necks, long tails, elephant like bulk and of course by their large size. They first evolved during the Late Triassic/Early Jurassic period (roughly 200 million years ago) and the best known members (i.e. Diplodocus, Brachiosaurus and Brontosaurus) are from the Late Jurassic, around 155-150 million years ago. However by the time of Argentinosaurus the sauropods were a totally different group than how they were in the Jurassic. The Diplodocids, Brachiosaurids and Brontosaurs were long gone and in their place rose a group of Sauropods known as the Titanosaurs. The Titanosaurs were the largest herbivores of the Cretaceous, dominating the Southern Hemisphere (and bits of the Northern Hemisphere) for pretty much all of it, right up until the meteorite impact that marked the end of the non-avian dinosaurs 65 million years ago.

I’m sure the main question that a lot of people will have regarding Argentinosaurus is; How could a land animal have possibly grown this big? Well the reasons why Argentinosaurus could reach this size is due to the remarkable biology of sauropods, and dinosaurs as a whole. Firstly they had special bird-like lungs known as a unidirectional lung. In our mammalian lungs oxygen is extracted from the air we breathe in once as it cycles through the lungs, with the same air then being breathed out. However in birds, theropod dinosaurs and sauropod dinosaurs such as Argentinosaurus some of the air breathed in is stored in cavities, known as air sacs, which are attached to the lungs. These are specialised in these dinosaurs so that when they breathe out the air sacs push the stored air through the lungs again, giving them a second chance to extract oxygen. This increases the lungs efficiency and results in more energy being produced via respiration, enough to sustain larger body sizes. Secondly the bones of these large dinosaurs were more lightweight than you might expect. This is because they also contain air sacs that penetrated through the bones giving them an air filled honeycomb like structure. This hollows out the bones which lightens them, while allowing them to retain their strength. For example air sacs in the neck vertebrae of sauropods such as Argentinosaurus lightened the whole neck and reduced the force required to support them. These air sacs also provided a large surface area that allowed any excess heat to be dissipated. Those air sacs really were the key and without them an animal as big as Argentinosaurus wouldn’t have been possible. Other features of sauropods that allowed them to get very big were long necks and four column-like legs. Their necks allowed them to access vegetation that was out of reach of other animals and could also reach a wide area without them needing to move a lot. This means they could eat more food (and so generate more energy) while not wasting energy moving. Meanwhile their columnar legs, like those of an Elephant, were very effective at supporting and spreading weight out equally.

File:Argentinosaurus BW.jpg - Wikimedia Commons
Argentinosaurus huinculensis as it may have look in life.
Image Credit: Nobu Tamura, http://spinops.blogspot.com/

Argentinosaurus was, unsurprisingly, the largest herbivore in its environment and would have needed to have spent the majority of its life ploughing through vegetation to sustain itself. But even with its huge size it still would’ve needed to watch for predators. This was especially the case when they were young. An Argentinosaurus hatchling was comparatively tiny and wouldn’t have received much parental care or protection. So to survive they would have had to grow pretty quickly and their estimated growth rate was quite something. It is estimated that at its peak growth rate an Argentinosaurus could have gained 40kg in a day. That’s just over 6 stone! (for context that’s heavier than a Dalmatian, or even two). Even as an adult large predators such as Mapusaurus (a large predatory theropod dinosaur belonging to the Carcharodontosaurids) would have been a threat. However they may not always have hunted Argentinosaurus in the traditional sense. Instead they may have employed a tactic known as “flesh grazing”. This is where a predator bites chunks off an herbivore without killing it. An Argentinosaurus would have been so big that even a group of 12 metre long Mapusaurus may have preferred to attack Argentinosaurus in this way rather than risk exhaustion and death trying to bring it down. So in effect, to a Mapusaurus each Argentinosaurus would’ve been like a walking butcher shop!

While Argentinosaurus is generally considered by many to be the largest land animal ever, this is still a matter of debate. This is because other sauropod candidates have been put forward as the largest. One is called Bruhathkayosaurus (….yeah I can’t pronounce it either!) which is a Late Cretaceous (70-66 million years ago) titanosaur known from a few bones unearthed in India. Another is Amphicoelias, which has a whopping size estimate of 180 feet in length. However this is based on one set of measurements, made over a century ago by the palaeontologist Edward Drinker Cope, of a single vertebrae which has since mysteriously disappeared. Because that vertebrae was the only known fossil of Amphicoelias to date there’s no way to test the validity of this measurement. Another possible contender is Patagotitan, another mid-Cretaceous titanosaur from South America. Recent estimates have put Patagotitan at just a fraction smaller than Argentinosaurus. However almost all of these titanic titanosaurs share a problem. They are all reconstructed from fragmentary remains. This means that their size estimations often change as more material is found. Furthermore, different palaeontologists come up with different estimates. For example the most recent size estimates of Argentinosaurus and Patagotitan come from a study published in February of this year conducted by the palaeontologist Greg Paul. From his own measurements he revised the size estimations of many large sauropods and concluded that, as of right now, Argentinosaurus was slightly larger than Patagotitan. The debate will rage on until enough complete remains of the two are found to give more accurate estimations. To be honest, these two sauropods were so similar in size that I suspect that while the largest Argentinosaurus were the biggest land animal ever, some large Patagotitan may well have been bigger than many Argentinosaurus.

The sauropod dinosaurs were marvels of biological engineering, with multiple biological features combining to allow them to exist. Argentinosaurus pushed those features to their absolute limit. Its effect on the world around it, being a key cog in the ecosystem that it lived in, and its impact on the imagination of dinosaur lovers everywhere should not be underestimated. In the history of life on Earth there had been nothing quite like it before, there has been nothing like it since, and it’s quite possible that there won’t be anything like it ever again!

References/Further Reading

Bonaparte & Coria 1993. The paper that first described Argentinosaurus

J. F. Bonaparte and R. A. Coria. 1993. Un neuvo y gigantesco saurópodo titanosaurio de la Formación Rio Limay (Albanio-Cenomaniano) de la Provincia del Neuquén, Argentina [A new and huge titanosaur sauropod from the Rio Limay Formation (Albian-Cenomanian) of Neuquén Province, Argentina]. Ameghiniana 30(3):271-282

Paul 2019: A study conducted by Gregory Paul that rexamined the bones of the largest dinosaurs, concluding that Argentinosaurus was the largest

Paul, G. (2019). “Determining the Largest Known Land Animal: A Critical Comparison of Differing Methods for Restoring the Volume and Mass of Extinct Animals.” Annals of Carnegie Museum 85(4): 335-358, 324.

• Pages 114-117 of “The Rise and Fall of the Dinosaurs” by Dr Steve Brusatte gives a fantastic overview of the reasons why sauropods could grow so big. I highly recommend this book for anybody whose into dinosaurs as it details the group as a whole, the different types of dinosaur, how they lived, how they evolved and how they went extinct.

Brusatte, Steve, “The Rise and Fall of the Dinosaurs”, 2018, Macmillan, chapter 3, pg 114-117.

An article on Live Science by Laura Geggel on the topic of the worlds largest dinosaur.

Geggel, Laura, “What’s the World’s Largest Dinosaur?”, Live Science, January 27th, 2019, https://www.livescience.com/34278-worlds-largest-dinosaur.html

A National Geographic article, by Laelaps, discussing the biggest dinosaurs

Laelaps, “Biggest Dinosaur Ever? Maybe. Maybe Not.”, National Geographic, https://www.nationalgeographic.com/science/phenomena/2014/05/18/biggest-dinosaur-ever-maybe-maybe-not/

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