Some good-sized sauropod pectoral region casts, with your humble author for scale

What’s the Biggest Dinosaur?

The mystery of the biggest dinosaur is one that, surprisingly, doesn’t concern most palaeontologists. That might seem unusual. What’s the point of these gigantic animals if we don’t figure out, first and foremost, which one was the biggest of them all?

It would be hard to deny that no one, palaeontologist or layperson, is completely unimpressed with the biggest dinosaurs known to science. Getting a sense of how big some of the true giants were reawakens that sense of childhood wonder that we’re all born with, but only a lucky few ever keep. But palaeontologists don’t study dinosaurs simply because they’re big. There are dozens of experts out there who pin their careers on fossil animals ranging from the size of an eagle to the size of a sparrow.

It’s the thrill of uncovering another fragment of a primordial world we’ll never fully see that drives scientists out into the quarries with their hammers and brushes. Dinosaurs that are the oldest or most basal of their clade, or are the first to be found on another continent, or have some anatomical feature we’d never expected or seen, are usually the ones that perk up the ears of specialists the most, because they fill in crucial gaps in our knowledge of dinosaur evolution. Fossils that point to where certain traits and features in animals come from are the real trophies. Finding a titanic fossil colossus is certainly interesting, and leads to questions about the ecosystem this animal lived in, but it doesn’t give us a lot of new insight into the crucial evolutionary steps of the dinosaur family tree.

As a little side-note before we move on, it’s worth remembering that the animal with the greatest mass to ever live is still the modern blue whale (Balaenoptera musculus). This animal, while perhaps not being as long as the longest dinosaurs, still outweighs them by an astounding amount, with historical records showing blue whales being up to 200 tons or more in weight. Giants like this are exceedingly rare nowadays thanks to the scourge of whaling, and today this wonderful creature is still endangered. Here’s hoping that the most massive animal yet known can bounce back and live on in the future.

If you want to know what the largest dinosaur was, well… There’s the quick and easy answer and the long complicated headache of an answer. If you want to take the first option and just leave it there, it’s generally safe to say that Argentinosaurus is the biggest dinosaur ever discovered. If that’s not quite satisfying, then follow me down the giant rabbit hole.

While there’s some real titans throughout the dinosaur tree, including the giant 50 foot hadrosaur Shantungosaurus, all the top contenders are sauropods- the four-legged long-necked dinosaurs. Most of the really big sauropods belonged to a group called the titanosaurs, which are found mostly from the mid-to-late Cretaceous of the southern hemisphere. There’s a bunch of enormous titanosaurs that are of roughly similar size, but are surprisingly obscure outside the hardcore dino-fandom. Notable species include Patagotitan, Dreadnoughtus, Puertosaurus, Notocolossus, Alamosaurus (perhaps my favourite of the bunch- a rare example of a giant late Cretaceous sauropod, and it even coexisted with Tyrannosaurus!), and the aforementioned Argentinosaurus. This last species, at over 100 feet long and at least 50 tons or more, is the biggest dinosaur that we have decent evidence of (Bonaparte & Coria, 1993). It’s an unfortunate stroke of fate that most of the biggest dinosaurs are known from relatively scant material, making their exact size hard to estimate and open to a lot of debate.

Some of the biggest sauropods we’ve yet discovered (plus a puny human), all roughly to scale. By Nicholas Carter

While the titanosaurs seem to have the market cornered on gigantic sizes, other sauropod lineages produced enigmatic titans of their own. The late Jurassic Mamenchisaurus of China is usually most regarded for its immensely long neck, but one species, Mamenchisaurus sinocanadorum, may have reached lengths of 115 feet and weighed up to 75 tons (Russell & Zheng, 1994)). This is close to the same maximum size estimates for Argentinosaurus. At the same time, in the United States, rare behemoths may have also been walking the earth. One particular specimen of the normally stout yet modest Apatosaurus may have weighed up to 72 tons, more than triple the weight estimates for most individuals of this dinosaur (Wedel, 2013). A specimen of the long and slim Supersaurus seems to have been in the 105-115 feet range itself (Lovelace et al. 2007). Its close cousin, Barosaurus, is usually found to measure a respectable 85 feet or so, but one lone Barosaurus neck vertebra seems to have come from a creature that was a staggering 164 feet, possibly the biggest dinosaur ever found by a substantial margin (https://svpow.com/2016/09/16/how-horrifying-was-the-neck-of-barosaurus/). Horrifying.

An estimate of the giant Barosaurus, with an Argentinosaurus. Also roughly to scale. That little dinosaur skulking around down there? A regular Barosaurus. By Nicholas Carter

These are just a few examples of creatures that we actually have skeletal fossils of, and anyone could theoretically go and look at them now. But, as I said earlier, there’s still loads of debate amongst paleontologists over which size estimates are accurate. Even when you have most of the skeleton, it can be tricky to figure out the weight of the animal you’ve got, let alone one or two isolated bones. Sauropods had strange, hollowed-out neck bones with a string of air sacs along them, so that affects things. But then there’s the nearly-mythical dinosaurs, of whom we barely have even the scantest evidence of their existence.
The most famous one is Edward Drinker Cope’s massive sauropod vertebra collected from Colorado in 1877. The vertebra was incomplete, but what was recovered was reported by Cope to be about a meter and a half tall, suggesting a total height of between 2 and 3 meters when complete. That’s a single vertebra taller than most people. If this animal had proportions similar to the well-known contemporary Diplodocus, as most paleontologists assumed, it would have been, unbelievably, over 200 feet long. This would have potentially made it the largest animal to have ever lived. Cope named this monster Amphicoelias fragillimus, placing it in the same genus as a diplodocid species he named from better material, Amphicoelias altus (Cope, 1878b). Needless to say, there’s been no end of controversy around this vertebra ever since, with some paleontologists arguing that Cope mixed up or even forged his measurements and drawings, and others faithfully believing he was right and had discovered a true colossus. Unfortunately we can’t look at Cope’s vertebra to verify the facts, as it’s been lost for over a century, likely having crumbled into dust during the move the saw most of Cope’s fossils shipped off after his death to the waiting arms of our old friend Henry Fairfield Osborne. In a recent study, however, paleontologist Kenneth Carpenter took a hard look at the Amphicoelias fragilimus description and compared the vertebra to those of various rebbachisaurs- sauropods with relatively short necks and tall vertebrae (Carpenter, 2018). In this paper, Carpenter stated what many in the sauropod business had been murmuring for years- the vertebra of Amphicoelias fragillimus was legitimate as reported by Cope, but instead of belonging to a gargantuan diplodocid, it actually came from a pretty darn big rebbachisaurid. This animal, re-named Maraapunisaurus fragilimus by Carpenter, was a more realistic 90-100 feet long or so- less than half the length of what was previously thought, but by far the biggest rebbachisaurid- certainly no slouch in the size department. There’s other examples out there of mysterious giants, from oversize trackways in Western Australia to the dubious Indian titan Bruhathkayosaurus.

With all these size statistics being thrown around, there’s a few important things to remember. Think about the size variations that exist in living animals today. Heck, look at people. We come in all sorts of shapes and sizes based on genetics, lifestyle, and other factors. We can get a decent idea of the average size of a species by looking at many different individuals, but with these gigantic sauropods we only have a small handful at best from each species. There’s another fact that really throws a wrench into our size rankings- most sauropods we’ve excavated, especially the beloved mounts in museums around the world, weren’t fully grown when they died. Some individuals may not have even been close to reaching their full potential. That’s right, our museum halls and collections are filled with mostly juvenile and sub-adult sauropods. Paleontologists suspect that, for some reason, the environments these animals inhabited didn’t favour them living to full skeletal maturity, and only rare exceptions made it past the gauntlet and grew into true giants. Perhaps even dinosaurs that we think of as being relatively modest in size had the capacity to grow well beyond our expectations. Who knows how different things would be if we could get a good picture of the full possible size of these animals. Perhaps beasts like the biggest titanosaurs, the 70+ ton Apatosaurus, and that terrifyingly giant Barosaurus wouldn’t be seen as monstrous outliers, but standards for their species.

It might seem unrealistic to some readers that these dinosaurs could ever have grown so massive without being crunched under the weight of their own bulk. Large land mammals have come and gone since the end of the Cretaceous, but even the biggest fossil land mammals have nothing on the biggest sauropods. Whales of course are a different story, but as anyone who’s seen a beached whale will tell you, they sadly don’t do very well out of the water. Sauropods, however, were well equipped for the challenges of getting huge. Relatively low metabolic rates meant that they didn’t cook themselves in the heat that big bodies produce. Their legs were adapted for load-bearing and had huge amounts of cartilage in the joints. Their vertebrae were full of hollow spaces and air sacs, and they probably had minimal body fat and other bulky soft tissues. It could be that the only constraint on how big sauropods could get was their external environment, the amount of resources they could harvest to fuel their growth. I can’t help but wonder- if we could resurrect one of these animals and give it all the food, water, and space it could ever want, what would we see?

If you’re still with me, hopefully it’s clear by now that the question of the biggest dinosaur isn’t as easy to answer as you might think. But neither should it really matter, in my opinion. Many of these creatures would look basically the same size standing shoulder-to-shoulder. Open up any academic description of a new large sauropod and you’ll find the authors usually spend a lot more time going over the fine anatomical details of the animal- slight angles of bony projections from the vertebrae, the length to width ratio of the foot bones, or the exact placement of the shoulder girdle, all in language that most of us would need a course in Latin to understand.

If it’s ever conclusively proven what dinosaur species was the biggest, the only thing the person or people who described it will get is ego points. No awards are handed out for finding the biggest fossil animals, only for making groundbreaking contributions to science. Even if these giants might have made the world shake from time to time.

By Nicholas Carter

References:

Bonaparte J, Coria R (1993). “Un nuevo y gigantesco sauropodo titanosaurio de la Formacion Rio Limay (Albiano-Cenomaniano) de la Provincia del Neuquen, Argentina”. Ameghiniana (in Spanish). 30 (3): 271–282

Carpenter, Kenneth (2018). “Maraapunisaurus fragillimus, N.G. (formerly Amphicoelias fragillimus), a basal Rebbachisaurid from the Morrison Formation (Upper Jurassic) of Colorado”. Geology of the Intermountain West. 5: 227–244.

Cope, Edward Drinker (1878b). “A new species ofAmphicoelias”. American Naturalist. 12 (8): 563–564.

Lovelace, David M.; Hartman, Scott A.; Wahl, William R. (2007). “Morphology of a specimen of Supersaurus (Dinosauria, Sauropoda) from the Morrison Formation of Wyoming, and a re-evaluation of diplodocid phylogeny”. Arquivos do Museu Nacional. 65(4): 527–544.

Russell, D.A., Zheng, Z. (1993). “A large mamenchisaurid from the Junggar Basin, Xinjiang, People Republic of China.” Canadian Journal of Earth Sciences, (30): 2082-2095.

Wedel, M. 2013. A giant, skeletally immature individual of Apatosaurus from the Morrison Formation of Oklahoma. The Annual Symposium of Vertebrate Palaeontology and Comparative Anatomy 2013:45.


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