After two centuries of paleontological harvest, the evidence seems stranger than any fable, and continues to get stranger. Dozens of new species emerge each year; China and Argentina are hot spots lately for startling new finds. Contemplating the bizarre specimens recently come to light, one cannot but wonder what on earth Nature was thinking of. What advantage was conferred, say, by the ungainly eight-foot-long arms and huge triple claws of Deinocheirus? Or, speaking of arms, by Mononykus's smug dependence on a single, stoutly clawed digit at the end of each minimal forearm? Guesses can be hazarded: The latter found a single stubby claw just the thing for probing after insects; the former stripped the leaves and bark from trees in awesome bulk. A carnivorous cousin, Deinonychus, about the size of a man, leaped on its prey, wrapped its long arms and three-fingered hands around it, and kicked it to the death with sickle-shaped toenails.
Tiny Epidendrosaurus boasted a hugely elongated third finger that served, presumably, a clinging, arboreal lifestyle, like that of today's aye-aye, a lemur that possesses the same curious trait. With the membrane they support, the elongated digits of bats and pterosaurs enable flight, and perhaps Epidendrosaurus was taking a skittery first step in that direction. But what do we make of such apparently inutile extremes of morphology as the elaborate skull frills of ceratopsians like Styracosaurus of the horizontally protruding front teeth of Masiakasaurus knopfleri, a late Cretaceous oddity recently uncovered in Madagascar by excavators who named the beast after Mark Knopfler, the lead singer of the group Dire Straits, their favorite music to dig by?
Masiakasaurus is an oddity, all right, its mouth bristling with those slightly hooked, forward-poking teeth; but, then, odd too are an elephant's trunk and tusks, and an elk's antler rack, and a peacock's tail. A difficulty with dinosaurs is that we can't see them in action and tame them, as it were, with visual (and auditory and olfactory) witness. How weird might a human body look to them? That thin and featherless skin, that dish-flat face, that flaccid erectitude, those feeble, clawless five digits at the end of each limb, that ghastly utter lack of a tail--ugh. Whatever did this creature do to earn its place in the sun, a well-armored, nicely specialized dino might ask.
Dinosaurs dominated the planet's land surface from some 200 million years ago until their abrupt disappearance, 135 million years later. The vast span of time boggles the human mind, which took its present, Homo sapiens form less than 200,000 years ago and began to leave written records and organize cities less than 10,000 years in the past. When the first dinosaurs--small, lightweight, bipedal, and carnivorous--appeared in the Triassic, the first of three periods in the Mesozoic geologic era, the Earth held one giant continent, Pangaea; during their Jurassic heyday Pangaea split into two parts, Laurasia and Gondwana; and by the late Cretaceous the continents had something like their present shapes, though all were reduced in size by the higher seas, and India was still an island heading for a Himalaya-producing crash with Asia. The world was becoming the one we know: The Andes and the Rockies were rising; flowering plants had appeared, and with them, bees. The Mesozoic climate, generally, was warmer than today's, and wetter, generating lush growths of ferns and cycads and forests of evergreens, ginkgoes, and tree ferns close to the Poles; plant-eating dinosaurs grew huge, and carnivorous predators kept pace. It was a planetary summertime, and the living was easy.
Not that easy: Throughout their long day on Earth, there was an intensification of boniness and spikiness, as if the struggle for survival became grimmer. And yet the defensive or attacking advantage of skull frills and back plates is not self-evident. The solid-domed skull of Pachycephalosaurus, the largest of the bone-headed dinosaurs, seems made for butting--but for butting what? The skull would do little good against a big predator like Tyrannosaurus rex, which had the whole rest of Pachycephalosaurus's unprotected body to bite down on. Butting matches amid males of the same species were unlikely, since the bone, though ten inches thick, was not shock-absorbent. The skulls of some pachycephalosaurs, moreover, were flat and thin, and some tall and ridged--bad designs for contact sport. Maybe they were just used for discreet pushing. Or to make a daunting impression.
An even more impractical design shaped the skull of the pachycephalosaurid Dracorex hogwartsia--an intricate sunburst of spiky horns and knobs, without a dome. Only one such skull has been unearthed; it is on display, with the playful name derived from Harry Potter's school of witchcraft and wizardry, in Indianapolis's Children's Museum. Duck-billed Parasaurolophus walkeri, another late Cretaceous plant-eater, sported a spectacular pipelike structure, sweeping back from its skull, that was once theorized to act as a snorkel in swimming. But the tubular crest had no hole for gathering air. It may have served as a trumpeting noisemaker, for herd communication, or supported a bright flap of skin beguiling to a Parasaurolophus of the opposite gender. Sexual success and herd acceptance perpetuate genes as much as combative prowess and food-gathering ability.
Dinosaurs have always presented adaptive puzzles. How did huge herbivores like Brachiosaurus, Apatosaurus, and Diplodocus get enough daily food into their tiny mouths to fill their cavernous guts? Of the two familiar dinosaurs whose life-and-death struggle was memorably animated in Walt Disney's 1940 Fantasia (though in fact they never met in the corridors of time, failing to overlap by fully 75 million years), T. rex had puzzlingly tiny arms and Stegosaurus carried on its back a double row of huge bony plates negligible as defensive armor and problematic as heat controls. Not that biological features need to be efficient to be carried along. Some Darwinian purists don't even like the word "adaptive" as carrying a taint of implied teleology, of purposeful self-improvement. All that is certain is that dinosaur skeletons demonstrate the viability, for a time, of certain dimensions and conformations. Yet even Darwin, on the last page of The Origin of Species, in summing up his theory as "Natural Selection, entailing Divergence of Character and the Extinction of less-improved forms," lets fall a shadow of value judgment with the "less-improved."
In what sense are living forms improvements over the dinosaurs? All life-forms, even such long-lasting ones as blue-green algae and horseshoe crabs and crocodiles, will eventually flunk some test posed by environmental conditions and meet extinction. One can safely say that no dinosaur was as intelligent as Homo sapiens, or even as chimpanzees. And none that are known, not even a heavyweight champion like Argentinosaurus, was as big as a blue whale. One can believe that none was as beautiful in swift motion as a cheetah or an antelope, of as impressive to our mammalian aesthetic sense as a tiger. But beyond this it is hard to talk of improvement, especially since for all its fine qualities Homo sapiens is befouling the environment like no fauna before it.
The dinosaurs in their long reign filled every niche several times over, and the smallest of them--the little light-boned theropods scuttling for their lives underfoot--grew feathers and became birds, still singing and dipping all around us. It is an amazing end to an amazing evolutionary story--Deinonychus into dove. Other surprises certainly lurk within the still unfolding saga of the dinosaurs. In Inner Mongolia, so recently that the bones were revealed to the world just this past spring, a giant birdlike dinosaur, Gigantoraptor, has been discovered. It clearly belongs among the oviraptorosaurs of the late Cretaceous-90-pound weaklings with toothless beaks--but weighed in at one-and-a-half tons and could have peered into a second-story window. While many of its fellow theropods--for example, six-foot, large-eyed, big-brained Troodon--were evolving toward nimbleness and intelligence, Gigantoraptor opted for brute size. But what did it eat, with its enormous toothless beak? Did its claw-tipped arms bear feathers, as did those of smaller oviraptorosaurs?
The new specimens that emerge as tangles of bones embedded in sedimentary rock are island peaks of a submerged continent where evolutionary currents surged back and forth. Our telescoped perspective gives an impression of a violent struggle as anatomical ploys, some of them seemingly grotesque, were desperately tried and eventually discarded. The dinosaurs as a group saw myriad extinctions, and the final extinction, at the end of the Mesozoic, looks to have been the work of an asteroid. They continue to live in the awareness of their human successors on the throne of earthly dominance. They fascinate children as well as paleontologists. My second son, I well remember, collected the plastic dinosaur miniatures that came in cereal boxes, and communed with them in his room. He loved them--their amiable grotesquerie, their guileless enormity, their unassuming small brains. They were eventual losers, in a game of survival our own species is still playing, but new varieties keep emerging from the rocks underfoot to amuse and amaze us.
X-FACTOR
Bull horns, tiny arms
WHEN 82-67 million years ago WHERE Argentina
[ILLUSTRATION OMITTED]
Consider the evolutionary hand dealt to Carnotaurus, or "meat-eating bull": a big, bad, but seemingly underequipped predator, as if nature had set out to design a perfect killing machine but ran out of funding. Powerful jaws and long, agile legs suggest a highly mobile hunter prowling the lakeshores of what is now Patagonia.
Its skull (left), constructed like a battering ram, features a stout pair of horns. Yet accompanying this formidable hardware are tiny arms (even more stunted than the famously puny arms of Tyrannosaurus rex) and surprisingly small teeth. Some scientists, like University of Chicago paleontologist Paul Sereno, envision Carnotaurus and its kin as dinosaurian hyenas--fleet of foot and short-snouted to track down and gnaw on carcasses. "Who needs arms for that?" he asks.
X-FACTOR
Trombone crest
WHEN 76 million years ago WHERE North America
[ILLUSTRATION OMITTED]
The tubular bone sweeping back from a Parasaurolophus walkeri skull has inspired a variety of theories about its function. Weapon? Breathing tube? Hypersensitive nose? None of the above? Aided by computer modeling, scientists now think it was used to generate sounds like a trombone, though it also may have played a role in sexual display.
MASIAKASAURUS
X-FACTOR Inscrutable teeth
WHEN 70-65 million years ago WHERE Madagascar
The mouth of Masiakasaurus speaks to how this German-shepherd-size meat-eater survived in the river basins of northwestern Madagascar, near the end of the dinosaurs' reign. But what is it saying? Stony Brook University paleontologist David Krause led the team that found the remains, including part of the lower jaw (fossil shown below). Masiakasaurus has long, conical front teeth with hooked tips that curl out of its mouth--unique among theropods--while its back teeth are more typically blade-like and serrated. So how did it use such a specialized mouth? "Our best guess is the teeth up front were used to stab small prey, perhaps mammals, lizards, and/or birds," says team member Scott Sampson of the University of Utah, "and the teeth at the rear of the jaw were then used to tear up the kill." Despite its formidable dentition, Masiakasaurus was likely prey itself for crocs and other large carnivores, like the 20-foot-long theropod Majungasaurus, with which it shared territory. Against such monsters, its best defenses would have been speed and agility.
[ILLUSTRATION OMITTED]
SPINOSAURAUS
X-FACTOR
High-spined giant
WHEN 97 million years ago WHERE North Africa
In 1912 a collector for German paleontologist Ernst Stromer emerged from the Egyptian desert with the remains of the biggest predatory dinosaur the world had ever seen. The creature may have measured more than 50 feet long--arguably still the largest terrestrial carnivore known. It possessed crocodile-like teeth and a row of enormous spines (some six feet long) projecting from the vertebrae, which prompted Stromer to name the beast Spinosaurus. Scientists have argued over the function of the spines ever since. The debate offers insight into one of the key questions paleontologists face: How do you reconstruct a flesh-and-blood dinosaur from just a few bones?
One way is to piece together clues by comparing the new specimen with more complete skeletons already in hand. Scientists also draw inferences from the extinct animal's environment and the way living creatures function with analogous skeletal equipment. Naturally, the less one has of a specimen, the more speculative become the reconstructions--and the more heated the controversy surrounding them. Over the years, many scientists have argued that Spinosaurus's vertebral projections were connected by a fleshy membrane. Some living lizards employ similar "sails" for sexual display. Perhaps Spinosaurus too sported a sail to win the attention of mates, as some paleontologists hypothesize about Amargasaurus (pages 32-4). The sail may also have helped Spinosaurus regulate body temperature, serving as a radiator to cool the blood, much as a car radiator cools the water circulating through an engine.
Then again, perhaps the various renderings of a sail-bearing Spinosaurus have all missed the mark. As Smithsonian paleontologist Hans-Dieter Sues points out, other related dinosaurs, such as Baryonyx, regulated their body temperature just fine, sans sail. Sues seconds a notion put forth a decade ago by Jack Bowman Bailey of Western Illinois University: The spines instead supported a structure similar to a bison's hump. "If Spinosaurus had puny, slender spines, they might have supported a sail, but these were very massive,' says Sues. "It makes more sense that Spinosaurus's spines were embedded in a lot of muscle and tissue"
Other scientists argue that humps are usually found on herbivores in arid environments, while Spinosaurus, a carnivore, appears to have been living in a coastal mangrove forest. Paleontologist Paul Sereno ascribes to the theory that the spines supported a sail for sexual display. Suchomimus, a closely related predecessor, had much smaller vertebral spines. Millions of years later, says Sereno, "Spinosaurus took that trait to the extreme."
TUOJIANGOSAURUS X-FACTOR Shoulder spikes
WHEN 161-155 million years ago WHERE China
[ILLUSTRATION OMITTED]
With a thorny tail and rows of bony plates along its back, Tuojiangosaurus, like its better known cousin Stegosaurus, resembles a Jurassic tank. What grants this ponderous Chinese herbivore admission to the ranks of the truly bizarre, however, is the long, tapering spike thrusting out from each shoulder. "The shoulder spikes [fossil at left] would have helped protect its vulnerable flanks, which would have been right at the level of an attacking allosaur" says University of Maryland paleontologist Thomas Holtz. As for the plates
on its back, their purpose is a matter of much debate, says Susannah Maidment, a paleontologist at Cambridge University. Early armored dinosaurs were covered with small scutes to protect against a predator's bite, a trait passed on more or less unchanged to some of their descendants. But in others such as Tuojiangosaurus, the scutes gave way to plates along the backbone, which perhaps made the animal look bigger, but offered little protection. A large theropod, says Maidment, would have been able to chomp through them "like potato chips."
DEINOCHEIRUS X-FACTOR Enormous arms, giant claws
WHEN 70 million years ago WHERE Mongolia
[ILLUSTRATION OMITTED]
Whose arms are these? The question has puzzled paleontologists for nearly four decades. Unearthed in Mongolia near an assortment of predatory dinosaur remains, the reconstructed limbs each measure eight feet long, tipped with ten-inch claws. But few other parts of the original find have ever been recovered. Paleontologists dubbed the creature Deinocheirus ("terrible hand") and tried to match the bones with those of known species. Anatomically, Deinocheirus's forearms and hands seem most similar to those of ornithomimids, a group of fleetfooted dinosaurs that resembled ostriches and used their arms for grasping prey. Yet when scientists try to estimate Deinocheirus's size based on a general ratio of arms-to-body size in this group, the calculations project a massive animal 40 feet long (silhouette), putting it among the largest known theropods, such as Carcharodontosaurus. Or perhaps it's a smaller dinosaur with outlandishly long arms (modeled below). "The body is a mystery,' says Thomas Holtz. "It might not be an ornithomimid at all. But then what is it?"
X-FACTOR Shovel-like mouth, 600 teeth
WHEN 110 million years ago WHERE North Africa
Does natural selection ever paint a species into a corner, leaving it with anatomy so specialized that a slight change in its environment pushes it over the edge into extinction? Consider the 50-foot-long diplodocoid (a branch of the sauropod group) Nigersaurus--an anatomical sideshow with a mouth shaped like a vacuum cleaner, hundreds of tiny teeth, a boom of a neck, and skull bones thin to the point of translucence. How did ir survive with such a preposterous eating apparatus? Paleontologist Paul Sereno, a National Geographic explorer-in-residence, picked up the quizzical beast's trail in the mid1990s in northeastern Niger. Nigersaurus's oddest feature is its broad, straight-edged muzzle, which allowed the business end of its mouth to work very close to the ground, "like a lawn mower," says Sereno. Its 600 teeth, each about the size of a toddler's incisor, were tightly aligned, with a single row of more than 50 in operation in each jaw at any one time. A CT scan exposed up to eight "replacements" stacked up behind each tooth, so that new teeth immediately filled in for worn ones. Despite its impressive battery of teeth, Nigersaurus hada weak bite. Where the jaw muscles attach to the skull, the bone is as thin asa paper plate.
"Its mouth appears designed for nipping rather than chomping or chewing" says Sereno, pointing to wear patterns that suggest the teeth slid by one another like a pair of shears. With nearly 80 percent of the animal's skeleton recovered, a portrait emerges of a finely tuned eating machine designed to crop mouthfuls of soft plants growing near the rivers that coursed through what is now the Sahara's southern flank. Its long neck would have allowed Nigersaurus to mow down an entire field of plants without taking a single step.
A number of these features, seen in the extreme in Nigersaurus, show up over millions of years in other diplodocoids, which thrived on nearly every major land mass. That, says Sereno, suggests that this feeding strategy emerged in primitive form much earlier. But could this evolutionary trend toward the perfect eating machine ultimately have led to the extinction of the lineage? "Selection can favor specialization that can be advantageous over the short run, but create vulnerabilities over the long haul," says University of Chicago evolutionary biologist David Jablonski. We'll never know for sure whether Nigersaurus fell victim to its own outlandishness. But while it lasted, this fern-eating giant was a bizarre and beautiful success.
[ILLUSTRATION OMITTED]
DRACOREX X-FACTOR Spiky head and snout
WHEN 67-65 million years ago WHERE North America
[ILLUSTRATIONS OMITTED]
Bristling with spikes and pointed knobs, the skull of a Dracorex looks like something forged in medieval legend. Yet the creature it belonged to probably resembled a flower-eating wild pig more than the fire-breathing "dragon king" its name implies. Found by amateur fossil hunters in the South Dakota part of the Hell Creek formation, Dracorex has been assigned by paleontologists Robert Bakker and Robert Sullivan to the plant-eating pachycephalosaur family. Many species in this group had thick domes atop their skulls, which, like a bighorn sheep's horns, may have allowed them to use their heads as battering raras. Could Dracorex, with its relatively flat skull, also have been a headbanger? Why not? asks Bakker. He points to the giant forest hog of Africa, which uses a similarly flat skull and long muzzle to defend against predators and ram its rivals in violent clashes over mates. Endowed with flesh and skin, Dracorex stares out from this magazine's cover.
EPIDENDROSAURUS X-FACTOR Tiny body, elongated finger
WHEN 160 million years ago WHERE China
[ILLUSTRATIONS OMITTED]
At the diminutive end of the extreme dinosaur spectrum perches tiny Epidendrosaurus, a sparrow-size theropod with grossly oversize hands. Described in 2002 by paleontologists at the Chinese Academy of Sciences, it is the smallest known dinosaur, excluding birds, though scientists remain unsure whether the bones and impressions (below), discovered in siltstone in Inner Mongolia, are those of an adult ora juvenile. A more vexing question" What was it doing with its disproportionate hands, with third fingers longer than the other two digits combined? The closest modern comparison is to the aye-aye, a lemur that inhabits Madagascar's dense forest canopies and uses a similarly elongated finger to probe holes in trees for grubs and insects. It is also possible, says Thomas Holtz, that Epidendrosaurus might have had birdlike plumage. "But until we find more specimens, we can't say."
STYRACOSAURUS X-FACTOR Massive horned frill
WHEN 75 million years ago WHERE North America
Like an armor-laden knight, Styracosaurus would have cut an imposing figure on the forested river plains in what is now Alberta, Canada. Multiple individuals of these rhino-size herbivores have been identified in the same bone beds, suggesting they traveled in herds. Horned dinosaurs are a well-understood group, says Hans-Dieter Sues, and since Styracosaurus lived near the end of this lineage, we can trace the evolutionary paths that led to it. "Its ancestors began with a little bump over their nose and then developed a little bit of a frill at the back of the skull" says Sues, "but Styracosaurus takes these traits to the top." The bump on the nose in ancestral species evolved into an enormous spike that would have given Styracosaurus a potent weapon to fight off predators and fend off rivals. Meanwhile, the skull frill enlarged and added a profusion of horns, which probably let other styracosaurs identify it from a distance. Some scientists have suggested blood pumped into the skin covering the frill could have caused it to change color, possibly to attract mates or to scare enemies. "These extreme traits just didn't suddenly appear" says Sues. "There were compelling reasons why the), were selected and pushed down the evolutionary line."
[ILLUSTRATIONS OMITTED]
* Troodon or False? How much did you learn in school about dinosaurs that was all wrong? Take our quiz at ngm.com and find out.
John Updike is the author of 22 novels and numerous collections of stories, poems, and essays. His latest novel is Terrorist. Ira Block has photographed 32 features for NATIONAL GEOGRAPHIC. Pixeldust Studios and Renegade 9 provide animation and visual effects for film and TV. This collaboration is their first contribution to the magazine.
Source Citation
Updike, John. "Extreme dinosaurs: a bizarre gallery of Mesozoic monster prompts John Updike to ask: what has evolution wrought?" National Geographic Dec. 2007: 32+. Academic OneFile. Web. 31 Dec. 2009.
Gale Document Number:A172435354
No comments:
Post a Comment