Stegosaurus Week: Playing the Stegosaur Name Game

A reconstruction of Hesperosaurus on display at the Museum of Ancient Life at Thanksgiving Point, Utah.

Measuring diversity in the fossil record can be a tricky task. Short of inventing time travel, there will be always be some uncertainty about how many species of dinosaur existed at any one place and time, and as we learn more about the fossil record it may turn out that what we once thought to be distinct species or genera really belonged to already-known taxa (or vice versa). Stegosaurs are not immune from such lumping and splitting, and in his contribution to the stegosaur issue of the Swiss Journal of Geosciences, paleontologist Ken Carpenter used the debate over Hesperosaurus to dig into what distinguished this armored dinosaur from Stegosaurus.

Earlier this week I wrote about a new study describing skin impressions and other soft-tissue traces of the stegosaur Hesperosaurus mjosi. What I did not mention was that some paleontologists have proposed that this dinosaur was actually a smaller species of the more famous Stegosaurus genus, which would make its name Stegosaurus mjosi. Carpenter, who was one of the scientists who named Hesperosaurus in 2001, disputes this, but notes that whether the contentious stegosaur falls into one group or the other relies on more than anatomy alone.

Back in the Bone Wars era, when Stegosaurus was first described, paleontological rivals E.D. Cope and O.C. Marsh were in uncharted territory as far as taxonomy was concerned. The bits and pieces of the fossil animals they found had not been seen before, so it is not surprising that they created a vast accumulation of names to label them all (to say nothing of the competition between them that likely influenced their scientific practices). Given what we know now, though, any paleontologist who applied a new name to every bone scrap they found would be derided by the paleontological community. The naming of a new species—or the synonymy of two old ones—must be explained in minute detail, but even then different scientists have different perspectives on how different two fossils have to be in order to be designated as two different species.

That different species of dinosaur actually existed is immediately obvious. Tyrannosaurus rex and Stegosaurus stenops were so different from each other that it is at once apparent that they were two distinct species of dinosaur. Where a scientist’s personal views come into play are cases where there are two groups of animals which are only slightly different from each other. Do these two groups represent different growth stages of the same animal, different populations of the same species, different species of the same genus, or well-distinguished genera which can readily be told apart? Since, as Carpenter notes, dinosaur taxonomy is based on the comparison of bones alone, disputes can easily arise over how much variation a species had and what falls outside that range.

As for Hesperosaurus, the debate over its validity has been greatly influenced by the material O.C. Marsh used to create the name Stegosaurus armatus in 1877. The fossils were very scrappy, and compared to skeletons discovered since the 19th century, are not very useful in distinguishing these bones from other better-established Stegosaurus species such as S. stenops and S. ungulatus. This means that almost any restoration of the first species Marsh described, Stegosaurus armatus, is going to be a composite of other specimens and therefore obscure the defining characteristics of Stegosaurus as seen in the other species. As a result, it would be possible to lump almost any dinosaur with characteristics similar to the sparse materials Marsh found into the genus Stegosaurus, and it was on that basis that Hesperosaurus was proposed to be a unique species of Stegosaurus.

As Carpenter (and, in the same volume, Peter Galton) argues, however, Stegosaurus armatus is not the best dinosaur to use for determining differences between Stegosaurus species. If the more complete Stegosaurus stenops is taken as representative of the genus, it clearly differs in enough characteristics from Hesperosaurus for both to be considered separate genera. In fact, the differences between them have only become more apparent since more complete specimens of Hesperosaurus have become known.

Overall, I think Carpenter makes a solid case for Hesperosaurus; when compared to the better-known species of Stegosaurus, it was clearly a very different animal. Nevertheless, the fact that two groups of animals were easily distinguishable from each other does not tell us whether we should group them as different species or genera. That is something that is proposed, debated and revised according to the ideas of scientists, and there is no doubt that paleontologists will continue to play the dinosaur name game as research continues.

References:

Carpenter, K. (2010). Species concept in North American stegosaurs Swiss Journal of Geosciences, 103 (2), 155-162 DOI: 10.1007/s00015-010-0020-6

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A Strange Sail-Backed, Bristly-Armed Dinosaur

A life restoration of Concavenator by artist Raul Martin. From Ortega et al., 2010.

When I logged on to Facebook Wednesday morning, one of the first things I saw was a cryptic status update from University of Maryland paleontologist Thomas Holtz. He speculated that the paleo community at large would be “duly impressed” by something set to debut later in the day, but what was it? I jokingly replied that it would have to be something pretty impressive to outshine the weird raptor Balaur bondoc, but Holtz was right. Described in this week’s issue of Nature, Concavenator corcovatus is one of the strangest dinosaurs ever found, and possibly one of the most significant.

On a superficial level, Concavenator looks very familiar. Discovered in the approximately 130-million-year-old rock of Spain, this dinosaur was a carcharodontosaurid, or an early relative of the giant Giganotosaurus and a somewhat distant cousin of Allosaurus. What made it unique, however, were a series of elongated, upward-pointing neural spines near its hips. This dinosaur did not have a sail running the length of its back, like Spinosaurus, nor did it have a more uniform set of elongated neural spines, like the carcharodontosauid Acrocanthosaurus, but instead had two neural spines that jutted up high right in front of its hips followed by a series of shorter—but still elongated—spines at the base of its tail. This kind of arrangement—a short, tall sail near the hips—had been proposed for a very incompletely known dinosaur named Becklespinax before, but with the mostly complete remains of Concavenator we now know that at least some predatory dinosaurs had this weird decorative arrangement.

With two (and possibly three, if Becklespinax turns out to belong to the same group) carcharodontosaurids with strange structures on their backs, sails, humps, or fins may very well be found on other members of this group. But, despite its flashy sail, the most impressive aspect of Concavenator is much more subtle. Arrayed in a line along its ulna—one of the two bones that make up the forearm—were a series of round, raised bumps. This is not the first time paleontologists have seen such a feature. In 2007 it was announced that Velociraptor had these same structures, and they looked identical to the quill knobs on the arms of birds where arm feathers attached. The question was: what was a dinosaur so far removed from the origin of birds doing with quill knobs?

During the past two decades, a flood of new fossils has confirmed that birds evolved from one lineage within the diverse, feather-covered group of theropod dinosaurs called coelurosaurs. Almost every lineage within this group has at least one feathered representative, but Concavenator was not a coelurosaur. As a carcharodontosaurid, its lineage last shared a common ancestor with the coelurosaurs back in the Middle Jurassic, and the knobs on its arms represent the first evidence of a body covering other than scales on a theropod outside the coelurosaurs. Just what these knobs supported is as yet unknown. Perhaps they were feathers, or maybe they were a kind of bristle that was structurally similar to feathers. Skin impressions from other parts of the dinosaur show that it was not entirely covered by such structures, meaning that Concavenator may have had a mosaic of scales and feather-like structures on its body.

Even better, the discovery that Concavenator had a type of filamentous body covering reinforces the emerging hypothesis that dinosaurs as a whole may have sported a variety of such structures. Within the past decade paleontologists have found at least two examples of ornithischian dinosaurs (Psittacosaurus and Tianyulong) with feather-like bristles on their backs. These animals were all the way on the other side of the major evolutionary divide in the dinosaur family tree—about as distantly related to birds as possible while still being dinosaurs—yet they, too, had unique body coverings that were similar in structure to the fuzzy precursors of feathers seen in some coelurosaurs. If ornithischians had bristles, coelurosaurs had feathers, and carcharodontosaurids had similar structures, then it is possible that feather-like body coverings were a common trait for dinosaurs that might go all the way back to their origins. Either that, or these structures independently evolved in different lineages multiple times during evolutionary history. Regardless of which hypothesis turns out to be correct, we need to rethink what we thought dinosaurs looked like, and I expect that we are going to see the discovery of further evidence in the years to come that many dinosaurs were feathery, bristly creatures.

Post-script: There is some debate as to whether the knobs on the ulna of Concavenator are truly quill knobs or are another feature associated with muscle attachments. For more details on this angle of the story, see the posts by Darren Naish and Mickey Mortimer.

Ortega, F., Escaso, F., & Sanz, J. (2010). A bizarre, humped Carcharodontosauria (Theropoda) from the Lower Cretaceous of Spain Nature, 467 (7312), 203-206 DOI: 10.1038/nature09181

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Stegosaurus Week: A Rare Look at Soft Tissue

The skeleton of the Hesperosaurus known as "Victoria". From Christiansen and Tschopp, 2010.

Dinosaur skin impressions are pretty rare, and, even among the known collection of these soft-tissue traces, not all dinosaurs are equally well-represented. There are plenty of skin impressions from hadrosaurs, but stegosaurs are among the dinosaurs in which the skin texture is still largely unknown. Now, as reported by paleontologists Nicolai Christiansen and Emanuel Tschopp, an exceptional specimen from northern Wyoming gives scientists a first look at the skin and other body coverings from a North America stegosaur.

The individual described by Christiansen and Tschopp, nicknamed “Victoria,” is an approximately 150-million-year-old, nearly complete skeleton of the stegosaur Hesperosaurus mjosi. Discovered in 1995, it came from the well-known Howe-Stephens quarry site, where soft tissue impressions of other Jurassic dinosaurs have been found before. Based on the state of the skeleton, it appears that the dinosaur died, was partially buried, and then completely buried by a second flow of sediment, with the best preserved elements being found on the dinosaur’s right side.

The soft-tissue impressions found in association with the skeleton were scattered around the section of the ribs just before the hips and on one of the large armor plates on the dinosaur’s back. The preservation was not complete, but rather shows bits and pieces within these areas. Even so, enough of the skin impressions were preserved to show what the skin of Hesperosaurus was like. Overall it consisted of the same kind of honeycomb scale pattern seen in hadrosaurs, horned dinosaurs and another stegosaur from Asia called Gigantspinosaurus. Rather than being uniform, however, the scale pattern differed over the dinosaur’s body, with larger, domed scales surrounded by the smaller tubercles found on skin impressions from its back.

Among the most remarkable aspects of Victoria’s remains were the soft tissue impressions from the plate. For decades paleontologists have debated what the plates would have looked like, how they were arranged, and what function they might have had, and while this new specimen probably won’t resolve the ongoing discussions about the purpose of stegosaur plates, it appears to show a relatively smooth plate covering marked by shallow grooves. That this preserved material is really from a kind of plate sheath cannot be confirmed without any doubt, but Christiansen and Tschopp make the case that this interpretation is the most consistent with the structure of the material and the existing hypothesis that stegosaur plates were probably covered in this kind of material. If further remnants of these plate sheaths can be found, they can help paleontologists better understand the anatomy of these armored dinosaurs and better test ideas about the function of their plates.

The report was published in the Swiss Journal of Geoscience as part of the proceedings from the Symposium on Stegosauria held last year. The papers cover a range of topics, from new species to the bite mechanics of Stegosaurus, and several of this week’s Dinosaur Tracking posts will feature new findings presented at the meeting. Stay tuned for more on this bizarre group of dinosaurs.

References:

Christiansen, N., & Tschopp, E. (2010). Exceptional stegosaur integument impressions from the Upper Jurassic Morrison Formation of Wyoming Swiss Journal of Geosciences DOI: 10.1007/s00015-010-0026-0

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Stegosaurus Week: The Many Postures of Kentrosaurus

The skeleton of Kentrosaurus on display at the Museum f?r Naturkunde in Berlin. From Wikipedia.

Since the early days of paleontology, the posture of dinosaurs and the range of motion they were capable of have been contentious subjects for paleontologists. During the 19th century, especially, the general view of what dinosaurs would have looked like changed no less than three times, and investigations into how these animals moved continue to this day. Among the spate of recent studies on dinosaur flexibility, posture and motion is a new paper by Heinrich Mallison which used the Jurassic stegosaur Kentrosaurus to investigate some of the hypotheses surrounding this armored dinosaur.

Most of what we know about Kentrosaurus comes from the approximately 153-million-year-old Tendaguru Formation in Tanzania. It was there that the German paleontologist Edwin Hennig found numerous isolated bones and elements of disarticulated Kentrosaurus skeletons—in addition to the bones of many other dinosaurs—during the early 20th century; he was also lucky enough to find one partial skeleton of the stegosaur that was suitable for mounting. This specimen, reconstructed with sprawling limbs and a dragging tail, was on display at the Museum f?r Naturkunde in Berlin for decades. When it was taken apart to restore it in a more accurate posture in 2005, scientists made laser scans of each bone in order to create a digital restoration. It is this digital Kentrosaurus that formed the basis of Mallison’s new study—the closest thing a paleontologist has to a living dinosaur to examine.

In addition to its normal posture and range of motion, Mallison’s study looks at several controversial, little-studied ideas about this dinosaur and its kin. According to Hennig, Kentrosaurus had a squished, lizard-like posture and could not use its spiky tail for defense. In the 1980s, however, paleontologist Robert Bakker went to the opposite extreme, restoring stegosaurs with an erect posture that would have allowed them to pivot and swing their formidable tails at attacking predators. Additionally, Bakker proposed that Stegosaurus and its kin could have adopted a “tripodal” posture in which they reared back to rest on their tails, too, and were much more dynamic animals than envisioned by Hennig and other early 20th-century paleontologists.

Although Mallison stresses that the findings based upon his model are provisional, Kentrosaurus appears to have used different postures for different reasons. While walking, it would have held its limbs erect, but when threatened it was capable of flexing its forelimbs out into a sprawling position to help support itself as it swung its tail at an offending predator. In the latter circumstance, Kentrosaurus would have also been able to extend its neck to look backwards at an attacking dinosaur, though shifting position to keep a predator in view may have created blind spots that would have left this armored dinosaur vulnerable to multiple predators. As far as feeding was concerned, Kentrosaurus was indeed capable of rearing back to rest on its tail, though how often it would have done so—and what sort of food it would have been able to reach by doing so—is unknown. Overall, Kentrosaurus was not as stiff as Hennig proposed. Quite the contrary—this stegosaur was capable of altering its posture to suit a variety of circumstances, and it is likely that at least some of its relatives had similar abilities.

References:

Mallison, H. (2010). CAD assessment of the posture and range of motion of Kentrosaurus aethiopicus Hennig 1915 Swiss Journal of Geosciences DOI: 10.1007/s00015-010-0024-2

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The Dinosaurs of Industry

A sketch of Brachiosaurus from R.S. Lull's 1920 textbook Organic Evolution.

Since the time of their discovery in the early 19th century, dinosaurs have been pop-culture superstars. Beyond their scientific identities, they have a celebrity that has remained strong from decade to decade, and given their notoriety it is no wonder that they have been so often used as metaphors and symbols.

More often than not, dinosaurs have been used as icons of stagnation. They were creatures that seemed “too big to fail”—only to have their gargantuan size turned against them. This belief stemmed from uncertainty about the extinction of the dinosaurs. At the beginning of the 20th century, many naturalists thought that dinosaurs were either out-competed by mammals or became so big and grotesque that they could no longer adapt to changing environmental conditions. Either way, they ultimately failed because they were too large and ponderous to react appropriately in the face of new challenges, and so they became the perfect icons of big business. Jay S. Miller, in a 1913 issue of the Business Philosopher, put it this way:

But why was the dinosaurs, with all its size and strength, finally compelled to succumb to its weak and apparently helpless rivals [the mammals]?

The answer is easy. It was their degree of adaptability to changed conditions.

In spite of its seeming advantages, the dinosaur possessed little ability to respond to changed conditions. Just so long as its environment was favorable and congenial it continued to flourish. But when its surroundings began to change and become less favorable it failed to adapt itself to these changes and was necessarily slowly but surely exterminated.

The lesson behind all this was that, to survive in business, being able to swiftly adapt to new conditions was key. Better to be like the small mammals than the powerful dinosaurs. A May 1919 issue of The Shoeworkers’ Journal similarly admonished cordwainers to be more like mammals and less like dinosaurs. Of dinosaurs, the article’s author, Victor McCone, said:

They planned nothing. They were contented.

They produced nothing. They were contented.

They achieved nothing. They were contented.

They voted “no” on life above the dead line.

Once again, mammals showed the potential of mental agility and innovation, leading McCone to offer his readers a choice:

Will you be a man or a dinosaur? Are you going to be chained down by beef and boneheadedness? Or will you cultivate every personal excellence, all the skill you have no matter what you are doing, and rise out of the cellar of life? It is up to you.

A century later, these disparaging perspectives of dinosaurs seem rather silly. Dinosaurs were not a homogeneous group of large, lazy, and stupid creatures that died out one by one. They were a very diverse group of organisms, one lineage of which left living descendants, and they were done-in by a cataclysmic event that wiped out a variety of organisms (including some groups of mammals). If we look back even further, we can see that the kin of the first mammals were pared back by an even worse extinction, yet it would be ludicrous to say that the origin of mammals was delayed because their ancestors were so short-sighted and slothful that they ceded ground to the more agile dinosaurs. Ultimately, any use of dinosaurs as a metaphor or symbol for human endeavor tells us more about the way we view dinosaurs than what they were actually like.

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Who Wrote the Worst Paleo-Poetry?

The skeleton of Ceratosaurus, from The Dinosaurs of North America.

I am by no means a connoisseur of poetry, but I have to admit that I can’t think of any decent poems about dinosaurs or paleontology. The poems that do exist can be almost painful to read, and, as Sarah Zielinski documented on our Surprising Science blog a few months ago, bad geological poetry has a long tradition going back to the early 19th century. Don’t just take my word for it; listed below are snippets from some dinosaur doggerel.

Edward Hitchcock – “The Sandstone Bird” (1836)

[In which the reanimated "Sandstone Bird" laments the sorry state of the modern world]

Oh how unlike Iguanodon next me
In dignity, yet moving at my nod.
The Mega-Plesi-Hylae- Saurian tribes-
Ranked next along the grand descending scale:
Testudo next below the Nautilus
The curious Ammonite and kindred forms,
All giants to the puny races here,
Scarce seen except by Ichthyosaurian eye,
Gone too the noble palms, the lofty ferns,
The Calamite, Stigmaria, Voltzia all:
And Oh! what dwarfs, unworthy of a name,
Iguanodon could scarce find here a meal!
Grow on their graves! Here, too, where ocean rolled,
Where coral groves the bright green waters graced,
Which glorious monsters made their frolic haunts,
Where strange Fucoides, strewed its very bed,
And fish of splendid forms and hues, ranged free,
A shallow brook troop, where only creatures live
Which in my day were Sauroscopic called,
Scarce visible, now creeps along the waste.

Charles H. Sternberg – “The Permian Beds of Texas” (1911)

[On an Edmontosaurus "mummy" Sternberg and his sons had found]

The glory of this specimen—
He lies there as he floated in
With bloated body on the wave.
The gas escapes he found his grave,
As he sinks to his long rest,
Skin clinging fast to bone and breast.

Samuel Ward Loper – “A Modern Dinosaur” (1911)

A startling evolution,
Onrushing through the street;
A mighty, roaring monster,
And dangerous to meet -
Like something supernatural
With fiercely blazing eyes,
And breath of vilest odor
That all around it lies

Bert Leston Taylor – “The Dinosaur” (1911)

Behold the mighty Dinosaur,
Famous in prehistoric lore,
Not only for his weight and strength
But for his intellectual length.
You will observe by these remains
The creature had two sets of brains—
One in his head (the usual place),
The other at his spinal base.
Thus he could reason a priori
As well as a posteriori.

Carl Sandburg – “The Dinosaur Bones” (1921)

The dinosaur bones are dusted every day.
The cards tell how old we guess the dinosaur bones are.
Here a head was seven feet long, horns with a hell of a ram.
Humping the humps of the Montana mountains.

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Dinosaur Sighting: Star-Spangled Theropod

A patriotic theropod in the vicinity of Beloit, Wisconsin. Photo submitted by David Rice.

Today’s Dinosaur Sighting comes to us from David Rice, who spotted this star-spangled theropod dinosaur in the vicinity of Beloit, Wisconsin. As David pointed out in his e-mail, the top half of the dinosaur is reminiscent of a tyrannosaur, but the feet have weird lumps which look like the sickle claws of the “raptors;” maybe it is some kind of hybrid. Whatever the theropod is meant to be, though, it is hardly the only patriotic dinosaur around—in previous posts we’ve featured a Stegosaurus covered in stars and stripes and “George Washasaurus.”

Have you stumbled across a dinosaur in an unexpected place? If you have, and have a photo of the encounter, send it to us via dinosaursightings@gmail.com!

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