The Story of the Dinosaurs in 25 Discoveries: Amazing Fossils and the People Who Found Them

10	WHO’S THE BIGGEST OF THEM ALL?
PATAGOTITAN

The tragedy of the biggest sauropods is that they’re so scrappy. Argentinosaurus—often cited as being about 100 feet long and in the range of 80 tons—is only known from a relatively paltry collection of vertebrae, ribs, and an incomplete femur. Bruhathkayosaurus, a dinosaur that may have been as big or even bigger than Argentinosaurus, was only known from limb, hip, and tail elements, and those fossils disappeared (much like the near-mythical dinosaur giant Amphicoelias, estimated to be 190 feet long from a long-lost piece of vertebra).

—BRIAN SWITEK, 2014

THE BIGGEST DINOSAUR?

In 2014, pictures began to appear in the newspapers and online of these enormous dinosaur limb bones, with a man lying on the ground next to it for scale (figure 10.1A). The articles hinted that paleontologists in Argentina had found the largest dinosaur ever to live, but most scientists greeted the news with caution. Again and again the media hyped new discoveries as “the biggest” or “the longest” to generate attention for their stories, but when the detailed work was finally done years later, the claim turned out to be false or exaggerated.

Figure 10.1

The titanosaur Patagotitan: (A) a limb bone with a human for scale as it was unearthed in Patagonia; (B) replica of Patagotitan in the American Museum of Natural History as it stands today; (C) the replica nicknamed “Maximo” in Stanley Field Hall of the Field Museum in Chicago, first installed in 2018. ([A] Courtesy of J. Carballido; [B] courtesy of M. Wedel; [C] courtesy of B. McGann)

However, this discovery was truly exceptional and not just media hype. David Attenborough did a BBC special in 2015 that showed its enormous size and had many shots of the enormous bones, including some with Attenborough in the field at the locality in Argentina. By early 2016, the American Museum in Natural History in New York exhibited a huge replica of the Argentinian giant in the Miriam and Ira D. Wallach Orientation Center, the entrance lobby to their fourth-floor halls displaying fossil vertebrates (figure 10.1B). At 122 feet, it was so long that it didn’t completely fit inside that enormous room. Its head stuck out through the door frame into the elevator lobby adjacent to the room to the east. It hadn’t even been formally named or described yet; it was just called “The Titanosaur.” In the spring of 2018, the Field Museum of Natural History in Chicago followed suit, moving their famous Tyrannosaurus rex specimen “Sue” to another hall and replacing it with another cast of the Argentinian giant (figure 10.1C). Even though Sue was impressive, the museum staff explained that it looked tiny in the giant central Stanley Field Hall of the Field Museum (which had once housed their replica of Brachiosaurus/Giraffatitan; see chapter 9). The giant titanosaur was not dwarfed by this enormous space.

Finally, on August 9, 2017, the long-ballyhooed dinosaur was formally described in the Proceedings of the Royal Society of London B: Biological Sciences, a very prestigious journal (and free online). José Carballido, Diego Pol of the American Museum, and nine other Argentinian coauthors named it Patagotitan mayorum. The genus name comes from Patagonia, where it was found, plus “titan,” and the species honors the Mayo family, on whose ranch it was found. The first bones were found in 2008 by one of the ranch shepherds, Aurelio Hernandez, on the La Flecha ranch, which is in the Patagonian desert about 250 kilometers (150 miles) west of Trelew. The Mayo family put out a call to the paleontologists at the Museum of Paleontology Egidio Feruglio, who sent out seven expeditions from January 2013 to February 2015. Over those two years, the crews found more than 200 bones of sauropods, mostly from six partial sauropod skeletons composed of about 130 of those bones. There were also 57 isolated theropod teeth. Three or more of the partial skeletons make Patagotitan one of the most complete sauropods known, with a few neck vertebrae, a nearly complete backbone, many parts of the tail, nearly all the forelimbs and hind limbs, front shoulder girdle and parts of the hips, plus numerous ribs. Like most sauropods, however, there was no skull attached to the neck, so it was another “headless wonder,” and the replica mounts have borrowed a skull from a different titanosaur.

The individuals represented by the three most complete partial skeletons did not all die at the same time, and they come from at least three different layers in the same quarry. The way they are disarticulated, with some bones missing, plus the abundance of broken and shed predator teeth suggests that the carcasses came to rest on a river bottom or sand bar and were scavenged by predators and rearranged by currents before being buried by the next flooding event that covered them with a layer of sand and mud.

MEASURING GIANTS

So how big was Patagotitan? In the original press releases, Carballido and colleagues gave a length estimate of 40 meters (131 feet) and a weight estimate 77 metric tonnes (85 tons), but by the time the specimen was finally published, those dimensions had shrunk to 37 meters (122 feet) in length and a weight of 69 metric tonnes (76 tons). Other authors estimated that it came in at 33.5 meters (110 feet) and weighed about 45.4 metric tonnes (50 tons).

Another huge titanosaur, Argentinosaurus (figure 10.2), had long been considered the largest known land animal, but it was known from much less complete material: a few enormous vertebrae of the back and hip (figure 10.2B), and some of the hind limb bones. The reconstructed skeleton (figure 10.2A) in the Museo Carmen Funes in Plaza Huincul, Argentina, is 40 meters (130 feet) long. However, it is so incomplete that estimates of its size range from 26 meters (85 feet) to 30 meters (98 feet) to 30–35 meters (98–115 feet) in length, and weight estimates from different scientists range from 60–88 metric tonnes (66–97 tons), 80–100 metric tonnes (88–110 tons), and 83.2 metric tonnes (91.7 tons). These size estimates might put Argentinosaurus back at Number One if we use the smaller numbers for Patagotitan.

Figure 10.2

(A) Reconstructed skeleton of Argentinosaurus in the Museo Carmen Funes in Plaza Huincul, Argentina. For a long time, it was the largest relatively complete dinosaur known. (B) A single titanosaur vertebra showing their immense size. (C) Compare this to a vertebra of Giraffatitan, which was the largest dinosaur known for a long time. ([A] Courtesy of F. Novas; [B] photograph by the author; [C] courtesy of M. Wedel)

Earlier in 2014, before Patagotitan, the news was filled with the announcement of another giant Argentinian titanosaur. It was named Dreadnoughtus after the giant World War I battleships, or “dreadnoughts,” that feared no other smaller ship, or “dreaded nothing.” The media naturally touted this animal as the largest ever found, claiming that it passed Argentinosaurus. It was at that time the most complete of the giant titanosaurs, with 45–70 percent of the bones preserved (depending on which index of completeness is used). Some of the forelimbs are preserved, part of the back, much of the hind legs and pelvis, and part of the tail—but no neck or head. Ken Lacovara and colleagues, who named and described it, estimated that it was about 26 meters (85 feet) in length, and its weight was estimated around 22–38 metric tonnes (24–42 tons).

Why are these numbers so widely divergent? Partial skeletons, especially with material that consists only of a few vertebrae, often have no bones in common that would allow direct comparison. The length estimate is highly dependent on how large and how many vertebrae were in the neck and tail, and none of the specimens have a complete neck or tail yet (most don’t have a skull either). As science writer Brian Switek said in 2014 when the first news of Patagotitan appeared in the media:

A great deal of a sauropod’s length was in the tail, and how long that tail really was hinges upon how many vertebrae were in that part of the spine. The trouble is that complete dinosaur tails are very rare in the fossil record, and some of those precious fossils even suggest that the number of vertebrae in a dinosaur species’ tail could slightly vary from one individual to the next. When you’re dealing with dinosaurs that had vertebrae measured in feet, not inches, the number of tail vertebrae paleontologists reconstruct can make a big difference for a size estimate. And given that the new bonebed contains only 150 bones between seven individuals, the length of the sauropod’s tail—and other parts—is going to have to rely on what we know about other species.

There is an even bigger problem with estimating weight. All we have are skeletons, so we really don’t know how fat or skinny the soft fleshy parts of the living animal were. Even with a complete skeleton, we can only get a rough mass estimate, usually by taking a key limb bone dimension and comparing it to the same bones of animals of known weights. For Patagotitan, Carballido and colleagues used the circumference of the humerus (upper arm bone) and femur (thighbone) and got a weight estimate of 69 metric tonnes, with a range from 59 to 86 metric tonnes, whereas the fragmentary femur of Argentinosaurus estimates a weight of 66–97 metric tonnes (and that femur is partially reconstructed in plaster, so the measurement is not entirely based on real fossil bone).

The second technique involves constructing a three-dimensional model of the living animal and calculating its volume and thus its mass. In the old days, this meant sculpting an actual physical model, but more recently virtual models have been developed that can quickly estimate volume and mass. Carballido and others got a smaller range of weight estimates for Patagotitan this way, ranging from 45–77 metric tonnes. As Brian Switek put it in 2014:

Weight is another matter altogether. Determining a dinosaur’s body mass not only relies on filling in missing bones based upon close relatives, but also a particular researcher’s perception of whether the dinosaur in question had a beefy build or was leaner. That’s why paleontologists are familiar with shrinking sauropods.

In addition to difficulty estimating weights are problems estimating density. Like birds and apparently many dinosaurs, sauropods had many weight-reducing air sacs in the body (especially along their spine, as the bones suggest). In this case, sauropods might not have been as heavy as would be suggested by taking a simple model of uniform density and calculating its weight. Charlotte Brassey of Manchester Metropolitan University points this out:

They were so unusual, with long, slender necks and tails, and an air-filled skeleton, that we have no convincing analogues in the modern animal kingdom. And by virtue of being extremely large, we need to extrapolate our understanding of how animals function far beyond the upper limits of living land animals, such as elephants. The more we need to extrapolate, the less confidence we have in our reconstructions.

These are just the most recently discovered and relatively complete species. There are many more just known from a few huge bones. For example, among Argentine titanosaurs, the thighbones of Antarctosaurus are the only bones known (figure 10.3), but they are larger than those of Argentinosaurus and about the same size as those of Patagotitan. Another Argentine titanosaur, Argyrosaurus, is in the same ballpark. As discussed in chapter 7 for Apatosaurus and Brontosaurus, all these huge titanosaurs may be part of a highly variable population rather than being distinct genera and species. Those from the same time interval in Argentina should not be given new names until this is ruled out.

Figure 10.3

Thighbones of Antarctosaurus from the Cretaceous of Argentina, the largest such bones ever found until recently—no other part of this dinosaur is known. Francisco Novas for scale. (Courtesy of F. Novas)

Another problem with these huge but incomplete specimens is that many of them have been lost. For example, Bruhathkayosaurus (South Indian Sanskrit for “huge heavy body” plus the Greek for “lizard”) was found in Upper Cretaceous beds in India and described in 1989. When it was first described, its size was estimated at 175–220 metric tonnes (190–240 tons), then later shrunk to 139 metric tonnes (153 tons). It consisted of some hip bones, a partial thighbone and shin bone, a forearm, and a few pieces of vertebrae. However, the shin bone was 2 meters (6.6 feet) long, 30 percent larger than the shin of Argentinosaurus, and the thighbone was also larger. So was it really bigger than any of the Argentine fossils? It’s hard to know now because all we have are the simple published line drawings of the bones and a few measurements. The fossils themselves were completely destroyed when flash floods inundated the basement storage area and washed them away.

And then there is the mystery of Amphicoelias fragillimus. This name was based on a single enormous vertebra found by Cope and named in 1877. His one figure of the specimen shows that it was the incomplete portion of a vertebra (the neural arch) of enormous dimensions, possibly 2.7 meters (8.8 feet) tall if Cope’s measurements are accurate! If you compare it to more completely known dinosaurs and estimate its size, you come up with lengths around 40–60 meters (130–200 feet) and weights around 112 metric tonnes (135 tons). That would have dwarfed any of the current record holders.

However, the specimen itself is now lost. According to Ken Carpenter, it was probably stored in Cope’s house with no preservatives or hardeners (rarely used in those days) and deteriorated so badly that it crumbled to dust before Osborn bought Cope’s collection for the American Museum, or after it reached the museum. Lots of people are skeptical of Cope’s measurements because it is difficult to know what landmarks he used when measuring. There may be a typographic error in printing the size or scale (I’ve found a few of those in older scientific papers, so it’s not rare). In 2015, Woodruff and Foster made a strong argument that not only was the name Amphicoelias fragillimus invalid but doubt whether we can really say that it was as large as it was once claimed. Cope named another species, Amphicoelias altus, in 1878, which is about the same size and shape as Diplodocus, and probably just another junior synonym of that dinosaur.

LANDS OF THE TITANS

Throughout this chapter, we have been talking about titanosaurs. This is not just a nickname but an actual group of dinosaurs with their own distinctive characteristics and history. They are distinct from the diplodocines, the camarasaurs, and the brachiosaurs discussed in the three previous chapters. Most of these other groups of dinosaurs flourished in the Jurassic and then vanished, but the titanosaurs were far more diverse and largely flourished throughout the Cretaceous all over Gondwana and Laurasia (figure 10.4).

Figure 10.4

Family tree of sauropods, including titanosaurs, brachiosaurs, diplodocines, camarasaurs, and others mentioned in this book. (Redrawn from several sources)

Brachiosaurs are distinctive because of their long forelimbs and high shoulders and long necks adapted for feeding higher than any other animal. Diplodocoids tended to have slender bodies with long necks and extremely long tails. By contrast, some titanosaurs had relatively shorter necks and tails, and relatively small heads with large nostrils and crests formed by their nasal bones. Most of them had small teeth shaped like little spatulas (broad at the tip, narrow at the root), although a number had teeth shaped like pencils (similar to the teeth of diplodocoids). Although it was dominated by titanosaurs, South America also had diplodocoids, including the odd-looking Amargasaurus, which had paired spines down the back of its neck and spine (figure 10.5A).

Figure 10.5

Argentine sauropods: (A) the spiny diplodocine Amargasaurus, with paired spikes down the back of its neck and spine; (B) the armored Saltasaurus, a smaller sauropod from the Cretaceous, with numerous armor plates in its skin; (C) a nest of eggs of Saltasaurus from Auca Mahuevo, Argentina. ([A] Courtesy of Wikimedia Commons; [B] courtesy of N. Tamura; [C] courtesy of L. Chiappe)

Titanosaurs also had very broad shoulders and hips with a wider stance than other sauropods, and this can be recognized from their trackways. They tended to have stocky forelimbs, sometimes longer than their hind limbs (although not as disproportionate as in brachiosaurs). Like all other sauropods, they walked on the tips of very long hand and foot bones (metacarpals and metatarsals). Most sauropods have a few stumpy remnants of finger and toe bones as well, and maybe a claw on their thumb, but many titanosaurs lost the bony parts of their fingers and toes completely, replacing it with cartilage. Apparently, they walked on the blunt “stumps” of their metacarpals and metatarsals (covered by pads of cartilage and keratin, as most animals have covering the bones of their fingers and toes). One of them (Saltasaurus) had bony plates on its back shaped like large dishes (figure 10.5B), and many of the titanosaurs that are well enough preserved show small dish-like pieces of armor (osteoderms) in their skins that made it harder for a predator to bite into them.

The most diagnostic feature of many titanosaurs, however, was how the centra of the tail vertebrae are convex on the rear surfaces. More advanced titanosaurs have a distinctive peg-and-socket joint in their vertebrae, so even a single vertebra can be identified as titanosaur.

The name Titanosaurus was first coined by British paleontologist Richard Lydekker in 1877, who described a gigantic isolated tail vertebra from India. For the next century, the genus Titanosaurus became a taxonomic “wastebasket” for large sauropods of unknown relationships. Today the genus Titanosaurus is considered invalid because the original specimens are not diagnostic enough to tell how it is related to or distinct from more complete fossils that have been discovered more recently. But the name of the group Titanosauria is still valid, even if the genus it was based on is no longer in use.

Not all titanosaurs were as huge as Patagotitan, Argentinosaurus, Antarctosaurus, Argyrosaurus, and Dreadnoughtus. The armored Saltasaurus (figure 10.5B) is known from almost complete skeletons and was relatively small for a sauropod: only 8.5–13 meters (28–42 feet) long and about 2.5–7 metric tonnes (00.0–7.6 tons) in weight. It had a barrel-shaped body, shorter limbs, and a relatively short neck compared to most sauropods. Saltasaurus is known not only from good juvenile and adult bones but also from nesting grounds full of eggs at Auca Maheuvo in Argentina (figure 10.5C). Each nest had about 25 eggs, roughly 12 centimeters (5 inches) in diameter. These eggs were buried under dirt and vegetation to hide them from predators and to keep them warm and protected. Some of the eggs even had the tiny bones of embryos inside them as well as preserved egg membranes. The embryos had skin impressions, showing that they had a mosaic of tiny armor plates on their bodies even before they hatched.

Titanosaurs were by far the most diverse and widespread group of sauropods, spreading to all the continents when they began their evolutionary radiation in the Early Cretaceous. They clearly replaced diplodocines and brachiosaurs, and they may have competed with and displaced those groups ecologically as well. Once they began evolving, titanosaurs reached a diversity of more than 100 genera known from the Cretaceous (although many of these names are dubious because they are based on fragmentary nondiagnostic fossils).

Titanosaurs were also unusual in that they lived nearly everywhere in the Cretaceous and have been found on every continent that has rocks of the right age and environment. The most primitive titanosaur of all is Andesaurus, named in 1991 for specimens found in lower to middle Cretaceous rocks in the Argentinian foothills of the Andes. Although the fossils are incomplete (as always), Andesaurus wasn’t small just because it was early and primitive. It was about 15–18 meters (49–59 feet) in length. Andesaurus was found in beds that also contained the one of the largest predators of all time, Giganotosaurus, so they had some fierce rivals to contend with (see chapter 15).

The next most advanced group of titanosaurs is the Lognkosauria, which include most of the giants described in this chapter: Patagotitan, Argentinosaurus, plus the enormous Futalognkosaurus (only slightly smaller than these other giants and known from more complete skeletons). Most Lognkosauria are known from South America (the previous genera plus Mendozasaurus, Puertasaurus, Traukutitan, Quetecsaurus, and Drusilasaura), and there are some African fossils. Keep in mind that Africa was attached to South America in the Early Cretaceous, and the South Atlantic didn’t really begin to open until the Late Cretaceous, so dinosaurs from Argentina were not that far as the crow flies from dinosaurs in southern Africa.

There are primitive titanosaurs on many continents, such as the numerous genera from Asia, including China (Jiangshanosaurus, Jiutaisaurus, Qingxiusaurus, Ruyangosaurus, Zuchengsaurus), Pakistan (Pakisaurus, Balochisaurus, Sulaimanisaurus, Marisaurus, Khetranisaurus), India (Isisaurus and the original Titanosaurus), and Gobititan from Mongolia. Other regions are represented by Savannasaurus from Australia, Paralititan from Egypt, Rukwatitan from Malawi, Lohuecotitan from Spain, Macrurosaurus from England, Hypselosaurus from southern France, and Paludititan and Magyarosaurus from Romania.

Magyarosaurus is particularly interesting because it is a dwarfed titanosaur, only 6 meters (20 feet) long. It is thought that this dwarfing occurred because most of western Europe was flooded with high sea levels, forming a series of small islands. On islands, it is common for larger animals to become dwarfed because they no longer need large body size to cope with big predators, and large body size is a disadvantage when animals need to survive on the much smaller food supply of most islands. This is well documented from pygmy mammoths during the Ice Ages on many islands, from the Channel Islands of California to Crete and Cyprus in the Mediterranean, as well as dwarfed hippos on Madagascar, Cyprus, Crete, and many other places. Apparently this was a phenomenon in larger dinosaurs as well.

The most advanced titanosaurs are called the Eutitanosauria and include the rest of the huge diversity of this group. These include such groups as the small armored saltasaurs mentioned previously, and the aeolosaurs (wind reptiles) of South America, plus the Argyrosauridae of Egypt. Finally, there is the large group of titanosaurs known as the Lithostrotia. Lithostrotians include many South American groups such as Saltasaurus, Neuquensaurus, as well as Rapetosaurus from Madagascar (a small sauropod but known from one of the most complete skeletons in the group) and its relative Isisaurus from India. Other lithostrotian titanosaurs include Malawisaurus from Africa, Nemegtosaurus from Mongolia, Diamantinasaurus from Australia, and Tapuiasaurus from Brazil. Clearly the many branches of titanosaurs could get around very easily because they appear over and over again in just about every corner of the world. In December 2011, titanosaur remains were found in Antarctica, and a few years earlier in New Zealand.

But what about North America? You might have noticed its conspicuous absence in our list of cosmopolitan titanosaurs. North America is different from the other regions in that the best-known Late Cretaceous faunas from Montana and Wyoming and Alberta are dominated by duckbills and horned dinosaurs, and lack sauropods. One might get the impression that sauropods vanished from North America in the Cretaceous once the brachiosaurs and camarasaurs and diplodocines died out in the Late Jurassic or Early Cretaceous. But that impression is wrong: they just stuck to the southern parts of North America in the Cretaceous, and for unknown reasons never moved to northern Wyoming and Montana and Alberta.

Only one titanosaur is known: the huge Alamosaurus, a close relative of the Argentinian Saltasaurus. It got its name after the Uppermost Cretaceous Ojo Alamo Sandstone in the San Juan Basin of northwestern New Mexico. (No truth to the rumor that it is named after the Alamo mission in San Antonio, site of the famous battle for Texan independence.) Although it is known only from fragmentary remains, the available parts suggest that it was almost as large as the South American giants: 28–30 meters (92–98 feet) long, and about 73 metric tonnes (80 tons). It was first discovered in 1921 in New Mexico by Charles W. Gilmore of the Smithsonian. Since then, bones of Alamosaurus have been found in the North Horn Formation of Utah, several formations in the Big Bend region of Texas, and even a few scraps in southwestern Wyoming—but none have been found in the Lance Formation of northeastern Wyoming, the Hell Creek Formation of central Montana, or the many Cretaceous formations of Alberta.

Why was this? Paleontologists speculate that there may have been some sort of climatic or geographic boundary between northern Wyoming and southern Wyoming, although nothing is obvious—ankylosaurs, duck-billed dinosaurs like Kritosaurus, horned dinosaurs like Torosaurus, and large theropods seem to have ranged from New Mexico to Montana to Alberta without a problem.

Others have argued that Alamosaurus is a late immigrant to North America and never spread all the way north. After all, it is a member of the most advanced titanosaurs, the Saltasauridae, from the latest Cretaceous of Argentina, and it only shows up in the latest Cretaceous of North America as well. Throughout most of the 80 million years of the Cretaceous, there are no titanosaurs or any other kinds of sauropods anywhere in North America.

Finally, Alamosaurus has figured in the debates about the extinction of the dinosaurs as well. Specimens in New Mexico and Texas have been found just a few meters below the boundary that marks the end of the Cretaceous and the mass extinction that occurred then. There are even some specimens in Texas from a unit that spans the Cretaceous-Cenozoic boundary, although there is no clear evidence of Alamosaurus living after the Cretaceous extinctions that wiped out all the other nonbird dinosaurs. For a while, however, debates about where to place that boundary in the rock sequence of New Mexico had scientists claiming that there were Cenozoic dinosaurs including Alamosaurus. More careful work since then has shown no clear evidence that Alamosaurus survived the catastrophe.

The titanosaurs were not only the largest radiation of sauropods and the most cosmopolitan of all sauropod groups but also the largest land animals the earth has ever seen—and among the very last nonbird dinosaurs on earth. With this, we leave the sauropods behind and look at the next group: the predatory dinosaurs, or theropods.

FOR FURTHER READING

Colbert, Edwin. Men and Dinosaurs: The Search in the Field and in the Laboratory. New York: Dutton, 1968.

Curry Rogers, Kristina, and Jeffrey Wilson. The Sauropods: Evolution and Biology. Berkeley: University of California Press, 2005.

Farlow, James, M. K. Brett-Surman, and Robert Walters. The Complete Dinosaur. Bloomington: Indiana University Press, 1999.

Fastovsky, David, and David Weishampel. Dinosaurs: A Concise Natural History, 3rd ed. Cambridge: Cambridge University Press, 2016.