Tuesday, February 1, 2011
Horns of plenty: when it comes to reconstructing spiny dinosaurs, too many bones can be as confusing as too few.(includes related articles on the ceratopsids horned dinosaurs).
You can never know too much about a dinosaur. Consider the ceratopsids, large-headed, barrel-chested plant eaters that roamed the lowlands of North America near the end of the dinosaur era. Ceratopsids all had fairly standard-issue bodies, but their heads were decked with kaleidoscopic combinations of horns, hooks, studs, and spurs.
Most flamboyant of all were the centrosaurs, slightly older, rhino-sized ceratopsid cousins of the bigger, better-known horned dinosaur Triceratops. Why so many different kinds of headgear? Did they belong to animals of different ages? Different sexes? Different species? It's hard to tell.
The reason we know about all that variation is that ceratopsids lived and died in vast herds, leaving behind bone beds that give paleontologists like me a lot of information to work with. It would be wonderful if those beds were bulging with good fossils -- perfectly preserved records of animals of both sexes, all ages, and every single species. Instead we have to settle for second or third best. A paleontologist might find a complete, well-preserved skeleton of one animal -- but if that animal was a half-grown male, it might say little about adult females (just as the skeleton of a Texas Longhorn bull calf might give a misleading picture of a Jersey milk cow). Alternatively, a paleontologist might have to piece together a single skull from pieces of many poorly preserved ones. At best, the result would be a fairly good composite of a creature that never existed as an individual; at worst, it would be a mishmash of young and old, male and female.
Scientists who study dinosaurs can afford to be wishy-washy about the models we propose. The artist who make models of those dinosaurs, on the other hand, must choose. How big? What pose? Horns or no horns? In the centrosaur models that follow, part of a traveling exhibit called Ceratopsians: Life and Times of the Horned Dinosaurs, artists contracted by the Dinamation International Society worked with paleontologists to give the bones of these creatures a new lease on life. Pairing the models with the skulls on which they are based shows how much imagination goes into building dinosaurs from the bones up. But in many cases the bones themselves are reconstructions, too -- and works of just as much hard thought and imaginative daring as the models they inspired.
With its deep skull and hefty nasal horn, Centrosaurus is the classic centrosaur. It is also one of the best known, thanks to bone beds being excavated in Dinosaur Provincial Park in Alberta, Canada, by paleontologists from the Royal Tyrrell Museum of Palaeontology. Field teams have discovered thousands of bones packed in river sediments, along with teeth from crocodiles and meat-eating dinosaurs. Apparently a Centrosaurus herd drowned while crossing a river, and scavengers feasted.
The jumbled-up bones from such mass deaths show that Centrosaurus probably reached adulthood in three to four years, as fast as large mammals do today. Fossils of young centrosaurs lack most of the horns, spurs, and other skull decorations that fused to the frill in adults. Since those decorations are the easiest way to tell one kind of centrosaur from another, it's risky to try to identify centrosaur fossils from the remains of young animals alone.
Skull from the Canadian Museum of Nature, Ottawa
Since its discovery, this contemporary of Centrosaurus has been a shadowy and controversial member of the centrosaur clan. Monoclonius was named on the basis of a few bones, skull fragments, and teeth found in Montana in 1876 by the prodigious dinosaur hunter Edward Drinker Cope. Since then several skull fragments have been discovered, but only one complete skull of the creature has turned up. It shows that Monoclonius resembled Centrosaurus but had a smooth frill (Centrosaurus frills have short barbs called epoccipitals curling forward over the frill at the back of the skull).
Many paleontologists, in fact, think Monoclonius and Centrosaurus wee the same animal. The so-called Monoclonius skull, they say, actually belonged to a young Centrosaurus whose frill decorations hadn't developed yet. But others hold that the disputed skull was unmistakably adult (it's big for one thing) and that the two dinosaurs were different animals.
When it was first identified in 1986 by the paleontologist Peter Dodson, Avaceratops was the first new ceratopsid discovered in thirty-six years. Check out the brow horns. No other known centrosaur had them, but there is evidence that ceratopsid ancestors from Asia and North America did. One branch of the family, the centrosaurs, gradually lost them, while another branch -- the line leading to Triceratops and its closest relatives -- kept them and carried them to breathtaking extremes.
So far only two Avaceratops skulls have been found, one with a frill but no horns, the other with a frill and horns but no face. The skull shown in the photograph, and the model based on it, are composites based on those specimens. Paleontologists are hedging their bets. If better fossils turn up, this odd creature might prove the key to a peculiar byway in centrosaur evolution. Otherwise it could disappear from the textbooks.
This creature is another puzzler. For a long time Brachyceratops was known mainly from several partial skeletons identified in 1914. The skulls were plain and primitive looking -- but they also came from young animals. If their owners had grown up, would they have blossomed into something a little fancier -- maybe a young Einiosaurus or Achelosaurus (see page 30)? Or were they just quirky, old-fashioned, dowdy dinosaurs?
More-mature-looking fossils discovered since then may help to resolve the identity crisis. Meanwhile, paleontologists and model makers have an excellent idea of what a young Brachyceratops looked like. They just don't know what it really was.
This outrageous-looking creature had the longest nasal horn of any known ceratopsid (the bony core alone is twenty inches long). It also sported six huge spikes sprouting from its frill, as well as a fearsome assortment of thornlike spurs on its cheeks, brows, frill, and forehead.
Little of this formidable-looking headgear would have been much use in combat. Most of its probably was just for show, like ceremonial armor -- handy for bluffing down opponents and impressing females. If the bluff failed, the huge nasal horn could also be employed for ritual combat or defense. There is no way to tell whether styracosaurs were as garish as the model suggests, but it's reasonable to suppose that they and other ceratopsids evolved bright pigments to highlight their fancy trim.
The nasal horn of Styracosaurus was the last of its kind. Curiously, centrosaurs that came after Styracosaurus -- Einiosaurus, Achelosaurus, and Pachyrhinosaurus -- became less and less spiny. The transition from horn-heavy to hornless is shown on the following two pages. It can be traced for ten million years in the fossil record, one of the clearest evolutionary sequences known for any dinosaur.
EINIOSAURUS AND ACHELOSAURUS
When these two creatures from Montana burst on the scene in 1995, paleontologists could hardly believe it. It's rare to see a clear-cut example of missing links. Yet there they were, looking for all the world like a tidy transition between the spiky Styracosaurus and the spineless Pachyrhinosaurus. Einiosaurus, found in rocks about seventy-two million years old, came equipped with a pair of frill spikes and a nasal horn. The seventy-million-year-old Achelosaurus also had two spikes, but instead of a horn, its nose sported a roughened patch of bone called a boss -- a smaller version of the one on Pachyrhinosaurus.
The name Achelosaurus comes from Achelos, a Greek river god who turned himself into bull and lost a horn in a fight with Hercules. In centrosaurs the evolutionary shape-shifting was probably due to changes in geography. Einiosaurus and Achelosaurus lived at a time when rising oceans reached right up to the mountains, drowning low coastal plains and splitting the ceratopsids that lived there into isolated herds. Separate populations no longer share their genes, so each group can evolve in a different way. In this case the change took two million years -- an instant in geologic time.
As its many well-preserved fossils show, Pachyrhinosaurus was a superlative beast. It was the biggest centrosaur, the last to die out, and the most far flung. A Pachyrhinosaurus skull has been discovered on the North Slope of Alaska -- 350 miles from the position, at the time the animal lived, of the geographic north pole.
Perhaps most unusual of all is what Pachyrhinosaurus lacked: a horn. Young pachyrhinosaurs had small, bladelike nasal horns, but those crumbled away as the animals grew up, leaving adults with a massive rough boss, or buttress of bone, covering the top of the skull. In living animals the boss probably was capped by a knob of keratin, the same protein that makes up hair and fingernails.
Some paleontologists, however, suspect the "knob" of keratin may have been a spike -- possibly a big one. Rhinoceros horns, they point out, are made of keratin anchored in bony bosses on the animals' skulls. No such horn has ever been identified from a centrosaur, but keratin from beaks and claws of other dinosaurs has been discovered, and fossil-hunters are keeping their eyes peeled.
The skull in the photograph belonged to one unlucky animal. The bone under the eye was eaten away by infection; even the lower rim of the eye socket is missing. The abscess may have started as a wound from a fight, probably with another member of the herd. But whatever caused it, the infection wasn't fatal. The skull was found in a bone bed; its owner survived long enough to perish along with the herd in one common catastrophe.
ROOTS OF A LINEAGE
In sheer numbers and variety, the horned dinosaurs called ceratopsids were a North American success story. But they probably didn't start out there. Most paleontologists think they arose in Asia, the home of smaller, more primitive frilled dinosaurs called protoceratopsids. A new fossil I have been studying gives the "immigrant makes good" story an unexpected twist.
The fossil, which has not yet been given an official name, was found near the border between Arizona and New Mexico by Doug Wolfe of the Mesa Southwest Museum in Mesa, Arizona, and his son Christopher. In life it probably looked something like a baby Triceratops, with horns jutting out above each eye. It came from rocks between ninety-one million and ninety-two million years old.
That makes it older than Turanoceratops, a creature from Uzbekistan that had been considered the ancestor of all North American ceratopsids. The Wolfes' find may be an even stronger candidates, because, while clearly related to Turanoceratops, it is even more primitive. The key is in its teeth. Later North American ceratopsids all had specialized double-rooted teeth, which locked together efficiently for chewing. Turanoceratops had some double-rooted teeth and some older-style, single-rooted ones, By contrast, all of the new fossil's teeth were single-rooted.
The new horned dinosaur doesn't overturn the idea that horned dinosaurs originated in Asia, but it does show that fairly advanced ones roamed North America earlier than anyone had thought. Perhaps brow horns originated in North America, and Turanoceratops, Asia's only known ceratopsid, represents a short-lived lineage of reverse immigrants. The fossil also shows that brow horns evolved before double-rooted teeth did, something paleontologists had wondered about.
Kirkland, James. "Horns of plenty: when it comes to reconstructing spiny dinosaurs, too many bones can be as confusing as too few." Earth Dec. 1997: 26+. General OneFile. Web. 1 Feb. 2011.
Gale Document Number:A19931906