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In this restoration by Emiliano Troco, a Sauroniops feeds on a juvenile Spinosaurus. (And yes, all the dinosaurs in this image are fluffy.) Image courtesy Andrea Cau.

Earlier this year, paleontologists Andrea Cau, Fabio Dalla Vecchia and Matteo Fabbri described a strange, 95-million-year-old skull scrap from an unknown dinosaur. Acquired by a commercial collector from Morocco’s Kem Kem beds and later donated to Italy’s Museo Paleontologico di Montevarchi, the bone showed signs that it belonged to a carcharodontosaurid–massive cousins of the familiar Allosaurus. There was something odd about the fossil. The bone was a frontal–situated at the top of the skull just above and in front of the dinosaur’s eye opening–but, unlike the same bone in related species like Carcharodontosaurus, a small dome protruded from the middle of the specimen. No caracharodontosaurid has been found with a dome before.

The skull fragment of Sauroniops, showing where it would have fit on a carcharodontosaur skull (human skull for scale). Image courtesy Andrea Cau.

While a single piece of skull isn’t much to go on, Cau and colleagues nevertheless were able draw on the dome and other subtle features to determine that the frontal didn’t belong to any previously known dinosaur. Still, at the conclusion of their brief Acta Palaeontologica Polonica report, the scientists cautioned against naming a new species from an isolated skull bone. “Although the combination of features present in [the frontal] is unique and should support the institution of a new species,” Cau and coauthors concluded, “pending more complete specimens we feel it would be inappropriate to erect a new taxon.”

Cau, Dalla Vecchia and Fabbri quickly changed their minds. While the rest of the dinosaur remains unknown, after reanalyzing the frontal the paleontologists decided that it was truly unique enough to merit establishing a new name. The subtly-domed dinosaur is now known as Sauroniops pachytholus–the genus name a tribute to the demonic Sauron of the Lord of the Rings series, and the species name for the thick dome on the dinosaur’s head.

I emailed Cau to ask why he changed his mind about the dinosaur so quickly. During the year between the time the two papers were submitted, Cau replied, several papers were published showing that carcharodontosaurids–such as the high-spined Acrocanthosaurus from North America–had frontal bones that were so distinct that they could be use to tell one theropod genus from another. That inspired Cau to take another look at the domed specimen from Morocco.

Ultimately, Cau wrote, “the collected data showed that the unique morphology of our specimen was as diagnostic as those available from the type specimens of other African carcharodontosaurids (e.g., the holotypes of Eocarcharia [a single postorbital bone], Carcharodontosaurus iguidensis [a single maxilla], Veterupristisaurus [a single caudal vertebra]).” If all these dinosaurs were based on isolated bones, Cau explained, “then there are no real objections for erecting Sauroniops even from a single frontal.”

Frustratingly, though, the limited material means that we only have the barest outline of what Sauroniops was like in life. The size of the frontal, compared to the bone in other carcharodontosaurs, indicates that the dinosaur probably exceeded thirty feet in length. The carnivore was probably just as big as the better-known Carcharodontosaurus, which it lived alongside, but such estimates always await the test of more fossils.

And then there’s the dome. Why did such a large theropod have a prominent bump on its head? In other theropod lineages, such as the abelisaurids, bumps, knobs and horns are common forms of ornamentation. Perhaps the same was true for Sauroniops–thanks to Acrocanthosaurus and the sail-backed Concavenator, we know that carcharodontosaurs showed off with visual signals. Then again, Cau and coauthors speculate that the dome might have been a sexual signal or might have even been used in head-butting behavior. I think the last hypothesis is unlikely, especially since we don’t know what the microstructure of the dome looks like and there’s no evidence of pathology, but it’s still a distant possibility.

So Sauroniops has a name and a family. Like its cousins Kelmayisaurus and Shaochilong, though, we don’t know very much about this dinosaur’s appearance or biology. The lone frontal is a tantalizing glimpse at a dinosaur that paleontologists will have to hunt down in the deserts of Morocco. With some luck, and a lot of persistence, we may eventually become better acquainted with the dome-skulled dinosaur.

For more on this discovery, see Cau’s blog post at Theropoda.

References:

Cau, A., Dalla Vecchia, F., Fabbri, M. 2012. Evidence of a new carcharodontosaurid from the Upper Cretaceous of Morocco. Acta Palaeontologica Polonica 57, 3. 661-665

Cau, A., Dalla Vecchia, F., Fabbri, M. 2012. A thick-skulled theropod (Dinosauria, Saurischia) from the Upper Cretaceous of Morocco with implications for carcharodontosaurid cranial evolution. Cretaceous Research, in press. DOI: 10.1016/j.cretres.2012.09.002

Despite being famous for its size, Spinosaurus is mostly known from fragments such as this bit of upper jaw. We don’t really know how large this carnivore was. Photo by FunkMonk, image from Wikipedia.

How big was Spinosaurus? The croc-snouted, sail-backed theropod was heralded as being even bigger and more menacing than Tyrannosaurus rex thanks to Jurassic Park III, placing Spinosaurus among the ranks of Giganotosaurus and Carcharodontosaurus as challengers to the vaunted title of the biggest flesh-eater to ever walk the earth. Depending on who you ask, Spinosaurus was about 41 to 59 feet long, making it as large as–if not larger than–old T. rex.

Asking “Which dinosaur was the biggest?” isn’t very helpful, though. “Bigness” isn’t something scientists actually measure. Consider the contemporaneous sauropods Apatosaurus louisae and Diplodocus carnegii. So far as we know, both grew to about 80 feet long, but Apatosaurus was a much bulkier dinosaur. Which is the more important feature for deciding which dinosaur was bigger–mass, length or a combination of the two? In this case, Apatosaurus would seem to win out through its combination of bulk and length, but what if you have two dinosaurs that are about the same size, but the shorter one seems to be stouter than the longer one? What then?

Dinosaur comparisons are especially fraught when dealing with partial skeletons and scientific estimates. How hefty we think a dinosaur was depends on the techniques we use to reconstruct mass. Paleontologists can come up with a probable range that encompassed the variation of a dinosaur species, but, sadly, we can’t weigh an Apatosaurus or Carcharodontosaurus to find out if we’re on the mark.

Length would seem to be a better option for comparing dinosaur size. With a little mathematical work to fill in the extent of cartilage and soft tissues between dinosaur bones, paleontologists can turn to the fossils themselves to gauge dinosaur size. Only, many of the largest dinosaurs are only known from scrappy skeletons.

Very few dinosaurs are known from complete skeletons. This is especially true of the largest dinosaurs. With the exception of specimens like the T. rex “Sue“, one of the most complete large dinosaurs ever discovered, many giants are only known from bits of skull, spine and limbs. Despite being touted as an absolute giant, for example, very little of Spinosaurus has been described. We don’t know how long this theropod truly was–paleontologists can only estimate using more complete dinosaurs as guides for what to expect. And even in relatively compete dinosaur skeletons, few specimens are found with complete tails. The delicate bones near the tip of the tail, especially, are rarely found.

Paleontologist Dave Hone examines how tails–or lack thereof–contributed to dinosaur size in the latest issue of Journal of Vertebrate Paleontology. In his survey of museum collections and the literature, Hone only identified a few dinosaur specimens with tails complete enough to fully comprehend how the organ contributed to the dinosaur’s size. Specimens of the ankylosaur Dyoplosaurus, ceratopsian Centrosaurus and tyrannosaur Gorgosaurus, among others, have complete tails, while individuals of dinosaurs such as the sauropodomorph Lufengosaurus and the oviraptorosaur Caudipteryx have tails missing five vertebrae or less.

When Hone examined these informative fossils, he found that dinosaur tails complicated the question of how long certain varieties of dinosaur were. Tails varied in their proportions among members of the same evolutionary lineage–one species of dinosaur may have a very short tail while its closest known relative may have an exceptionally long tail. And, not surprisingly, individuals of the same species varied in their tail lengths. In essence, statements such as “Spinosaurus was 45 feet long” are rough estimates that are significantly complicated by both variation and a lack of complete tail specimens. On his blog, Hone explained that these estimates affect how we envision dinosaurs and study their biology:

This is not a facile question, aside from the obvious public interest (when was the last time you saw a report on a new dinosaur that didn’t suggest how long it was, if only in terms of double decker buses?). Total length is a measure that’s been used by various researchers (myself included) over the years as a proxy for the mass of dinosaurs. If we’ve been over- or underestimating these values it could potentially affect our results quite a bit, so knowing whether or not these measures are right is worth checking.

This problem isn’t unique to dinosaurs. Natural variation even complicates length estimates of extant species. Take crocodiles, for example. For a long time, herpetologists thought that you could multiply a crocodile’s skull length by seven to get a fairly accurate estimate of the animal’s full stretch. Simple enough. But this rule appears to break down among the biggest individuals, particularly thanks to variations in their tail length. Researchers face the same problem with other reptiles. In estimating the size of extinct, giant monitor lizards, for example, paleontologists consider the length of the snout to the lizard’s “vent” at the base of the tail. This is because tails are variable, and may make an individual animal longer or shorter based on how it is reconstructed. Considering size from the tip of the nose to the base of the tail is a less unwieldy way of measuring size and comparing individuals.

What’s a paleontologist to do? Hone suggests cutting the tail out of dinosaur length estimates. While total length figures will never go out of fashion in popular articles and books, researchers may be better served by estimating the snout-vent length, or similar measurement, that allows for more accurate estimates of dinosaur size. As Hone states, dinosaur bodies from the snout to the back of the hip seems to vary less than tails, so this measurement may present more reliable estimates for dinosaur size. Hone is not saying that paleontologists should totally abandon measurements of total length for dinosaurs, but instead suggests that “snout-sacrum length” would be a better measurement that would coincidentally bring examinations of dinosaurs into line with studies of other tetrapods. The “My dinosaur is bigger than yours” contests will never end, but Hone’s paper suggests a new way of measuring the size of the contestants.

For more, see Hone’s two posts, as well as Ed Yong’s commentary.

Reference:

David W. E. Hone (2012): Variation in the tail length of non-avian dinosaurs, Journal of Vertebrate Paleontology, 32:5, 1082-1089 DOI: 10.1080/02724634.2012.680998

Spinosaurus was named for its long neural spines. What would you call it? Photo by Kabacchi, image from Wikipedia.

Dinosaur names are important. Each moniker is a title that encompasses the various bones and specimens that paleontologists use to bring dinosaurs to life. When I write Tyrannosaurus rex, for instance, the name instantly conjures up an image of a hulking, deep-skulled bone-crusher that roamed western North America during the last two million years of the Cretaceous. A dinosaur’s name conveys a lot of information.

Some names are more mundane than others. Allosaurus is one of my favorite dinosaurs, but her name translates to “different lizard.” Not very inspiring. Alternatively, some dinosaur names can be hard to pronounce. I always pause before I say Amphicoelias to make sure I don’t butcher the sauropod’s name. And, then again, some dinosaur names are unintentionally funny. Pantydraco, anyone?

Just as there are people who are put off by dinosaur feathers, though, some folks are irritated by what they deem “dinosaurs with dumb names.” One of my neighbors over at WIRED, humorist Lore Sjöberg, wrote a brief whine featuring a list of dinosaurs that he thinks should be renamed for dignity’s sake.

Now, there are some dinosaur names that I’m not totally enamored with. While I understand the dinosaur’s symbolic status, Bicentenaria argentina doesn’t exactly roll off the tongue, and the same goes for the unevocative Panamericansaurus (yes, named after Pan American Energy). Then there are the names that appeal to the more puerile portion of my sense of humor. Read the name Texasetes too fast and you may get the dinosaur confused with a part of the male anatomy (not to mention the actual debate over whether the name of Megalosaurus should really be “Scrotum“), and you should always be careful with the pronunciation of Fukuiraptor unless you’re actually trying to insult the allosaur.

But what baffles me is that Sjöberg didn’t pick any of these names. Instead, his list includes the likes of Spinosaurus and Giraffatitan. I get his beef with dinosaurs named after places (Albertosaurus, Edmontosaurus, etc.), and I agree that Gasosaurus was comically unimaginative, but Iguanodon? The second dinosaur ever named, and one of the most iconic prehistoric creatures named for the clue in its teeth that led Gideon Mantell to rightly hypothesize that the dinosaur was an immense herbivore? I have to wonder whether Sjöberg would consider “Iguanasaurus– the original proposed name for the dinosaur–to be a step back or an improvement.

I just don’t get Sjöberg’s contention that Giraffatitan is “terrible” because–*gasp*–the sauropod wasn’t actually a big giraffe. Strict literalism only in naming dinosaurs, please. And, really, what would Sjöberg suggest as a replacement for Spinosaurus? When Ernst Stromer found the theropod, the most distinctive thing about the dinosaur was its enormous vertebral spines. What would you call it? Suchomimus–a cousin of Spinosaurus–is a little more poetic, but I like Stromer’s choice just fine.

There’s no reason to focus on the negative, though. There are plenty of awesome dinosaur names. Yes, yes, Tyrannosaurus rex will always be the best, but I still get a kick out of saying the names of the enigmatic sauropod Xenoposeidon, the dromaeosaur Pyroraptor, the stegosaur Miragaia, the ceratopsian Spinops, and the oviraptorid Khaan (“KHAAAAAAN!“). Not every dinosaur name is easily pronounced (say Willinakaqe ten times fast) or truly encapsulates the nature of the animal, but at least paleontologists aren’t naming species after online casinos. Not yet, anyway.

When I was a young dinosaur fan, Spinosaurus was one of my most favorite dinosaurs. What could be more fantastic than a giant predatory dinosaur equipped with a bizarre sail? But Spinosaurus as I knew it during the 1980s—imagine a fin-backed Allosaurus—looked significantly different from the dinosaur as we know it today. The reason for the big change is largely attributable to the discovery of a different, related dinosaur in England.

In 1986, Alan Charig and Angela Milner described a very strange, crocodile-snouted dinosaur they called Baryonyx. The Cretaceous creature turned out to be the key to identifying what is now one of the most famous dinosaur groups, the spinosaurs. Paleontologists had been finding pieces of spinosaurs for over a century, but often the teeth of these dinosaurs were confused for those of crocodiles, and the original Spinosaurus fossils were destroyed during Allied bombing of Germany in WWII. When Baryonyx was discovered, however, paleontologists began to recognize the similarities between it, older discoveries and similar dinosaurs that were soon found in South America, Africa, Asia and Australia. Some, such as Suchomimus and Spinosaurus from Africa, had sails, while others—including Baryonyx—did not, but the initial discovery formed the basis for the great spinosaur makeover. (Even before new Spinosaurus material was found, the relationship between it and other spinosaurs like Baryonyx was used to restore the predator with heavy-clawed hands and an elongated snout.) In the above video, created by London’s Natural History Museum, paleontologist Angela Milner explains how the dinosaur was discovered and why Baryonyx is so peculiar compared to other predatory dinosaurs.

A hump-backed Spinosaurus, restored by R.E. Johnson and from Bailey 1997.

Spinosaurus and Ouranosaurus were among the most prominently ornamented of all dinosaurs. Both dinosaurs—a carnivore and herbivore, respectively—had elongated neural spines sticking out of many vertebrate along their backbones, which created prominent skeletal sails. In life, these structures are thought to have been covered by a thin layer of flesh, but in 1997 paleontologist Jack Bowman Bailey proposed an alternative idea. These dinosaurs were not sail-backed, Bowman hypothesized. They were hump-backed.

Superficially, the high-spined dinosaurs appeared to be analogues of two other strange prehistoric creatures. The carnivorous Dimetrodon and the herbivorous Edaphosaurus were synapsids, our own distant cousins, that lived between approximately 280 million and 265 million years ago. Both had the skeletal rigging for prominent sails on their backs and lived in a dry, arid landscape roughly similar to the kind of habitat Spinosaurus and Ouranosaurus inhabited much later. But Bailey argued that paleontologists had selected the wrong set of analogues. Bison were a better choice.

Bailey used basic anatomical comparison to set the stage for his idea. Illustrating the skeletons of Ouranosaurus, Dimetrodon and a bison side by side, Bailey noted that the back spines of the dinosaur were most similar to the thick, flattened spines near the shoulder region of the bison and were generally unlike the spindly backbone spires of Dimetrodon. (The elongated neural spines of the bison were so high, in fact, that Bailey wondered, “If bison had become extinct prior to the emergence of our own species, would they be interpreted today as sailbacked mammals?”) The resemblance led Baily to propose that the sails were sites for the attachments of powerful ligaments and large muscles.

Bison-backed dinosaurs would have been obligated to take up a different posture to handle all that extra bulk. If Spinosaurus had a thick hump, Bailey hypothesized, then it probably walked on all fours instead of balancing on two legs like other large theropods. “Thus, it seems unlikely that Spinosaurus was an agile cat-like sprinter like many short-spined theropods (e.g., Allosaurus),” he wrote, “but perhaps used the huge mass of its bear-like body to overpower young or weak prey, or perhaps to steal the kills of smaller more agile predators.” Restored by R. E. Johnson in one of the paper’s illustrations, Bailey’s vision of Spinosaurus looks like an enormous, hunch-backed crocodile.

Spinosaurus and Ouranosaurus were not the only dinosaurs Bailey thought might have humps. Bailey also viewed the elongated neural spines of dinosaurs such as the large theropod Acrocanthosaurus, the ceratopsian Protoceratops, the plate-backed Stegosaurus and others to infer the presence of large and small humps among many dinosaurs. These structures might have allowed dinosaurs to store up large amounts of energy in harsh environments, or maybe they allowed dinosaurs to maintain high, constant body temperatures (something that Bailey did not think dinosaurs were capable of without some specialized anatomical equipment, like a hump). The idea seemed plausible to some. A few months later, in a news report printed in Science, paleontologist Paul Barrett was cited as being in favor of Bailey’s notion. More recently, a 2007 National Geographic feature on “Extreme Dinosaurs” also counted Hans-Dieter Sues as supporting the idea, and a sketch by paleontologist Jason Poole showed a typical, sail-backed Spinosaurus standing next to a hump-backed one.

Beyond these notes, however, the idea that dinosaurs were bison-backed has not caught on. Spinosaurus, Ouranosaurus, and other dinosaurs Bailey cited are most often depicted with sails or other relatively thin structures, such as the fin-like projection at the hips of the recently-described predator Concavenator. There are a few reasons for this.

At the time Bailey wrote his paper, Ouranosaurus and Spinosaurus were thought to have lived in hot, dry, arid habitats where big sails would have caused them to overheat in the hot sun. A hump, in Bailey’s alternative view, would have acted as a “heat shield” in the Cretaceous environments. But paleontologists now know that these dinosaurs lived in lush, swampy environments and probably did not require protection from the desert-like environment Bailey based his ideas on. This also means that the dinosaurs would not have needed humps to store extra energy to make it through harsh dry seasons, thereby undermining the idea that Spinosaurus and Ouranosaurus were like desert lizards that store resources for tough times. (Additionally, if Spinosaurus and Ouranosaurus really did have heat-shield humps, then it is strange that other dinosaurs from the same ancient environments did not share the same adaptation.)

The dinosaurs were also relatively unique in the shape of their elongated spine rows. In terms of maximum spine height compared to the rest of the body, the dinosaurs considered in the study had sail or hump heights intermediate between those of Dimetrodon and bison, and the long spines of Spinosaurus and Ouranosaurus jutted up over a greater length of the back than in the mammals. Whereas the elongated spines of bison typically peaked between the shoulderblades and quickly became reduced in size, the highest points of the dinosaur backs were set further back along the spine and had a more gradual slope to them. This is probably because the elongated spines of bison are sites for muscle and ligament attachments that connect to the neck and head, whereas there is no indication that Ouranosaurus, Spinosaurus, or the other sail-backs needed extra support and power in the neck region. (If this were the case, and dinosaur humps contained muscles to support the head and give the neck more power, then it is odd that huge-headed dinosaurs like Tyrannosaurus did not have a similar adaptation.) Nor is there any indication that Spinosaurus had a body adapted to walking on all fours, although Ouranosaurus likely shared the ability of its hadrosauroid relatives in being able to switch between two- and four-legged locomotion.

Why Spinosaurus and Ouranosaurus had long rows of elongated spines is unknown. The structures supported fleshy banners that almost certainly played roles in display and species recognition—these dinosaurs practically carried billboards on their backs—but beyond that, it is difficult to say. Reconstructing soft tissues on extinct animals is very difficult, and doubly so when there are no solid modern analogues for the structures in question. Though Bailey pointed to the humps of mammals, for example, the elongated spines of bison, mammoths, prehistoric deer and other creatures were related to providing support for the head and strength to the neck, which was apparently not the case with Spinosaurus and Ouranosaurus. Desert lizards with fat tails don’t appear to be good analogues, either. Spinosaurus and Ouranosaurus were fundamentally different, and they remain among the most bizarre dinosaurs yet discovered.

References:

Anonymous (1998). Dino Fins More Like Humps? Science, 279 (5354), 1139-1139 DOI: 10.1126/science.279.5354.1139d

Bailey, J.B. (1997). Neural Spine Elongation in Dinosaurs: Sailbacks or Buffalo-Backs? Journal of Paleontology, 71 (6), 1124-1146