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Dawn of the Dinosaurs

Paleontologists probe the majestic reptiles’ origin and rise

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2:32pm, May 6, 2011
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Any 10-year-old knows how the dinosaurs met their end: A huge meteorite slammed into Mexico’s Yucatán Peninsula 65 million years ago, blasting the planet beyond anything imagined by Bruce Willis in Armageddon.

But neither kids nor Hollywood have spent much time thinking about how dinosaurs appeared in the first place. “We know a heck of a lot more about the extinction of dinosaurs than their origins,” says Randall Irmis, a paleontologist at the Utah Museum of Natural History and the University of Utah in Salt Lake City.

Lately, though, new discoveries have begun to flesh out the script of dinosaurs’ earliest days. From ghostly footprints in a Polish quarry to the bones of a pint-sized predator in Argentina, these findings tell a more complete and nuanced tale of dinosaur genesis. Dinosaurs, it turns out, were not predestined to rule the planet for more than 130 million years.

Instead, dinosaurs appeared on the scene tentatively, perhaps as early as 250 million years ago. Paleontologists are now unearthing fossils so primitive that they may be close to the common ancestor at the root of the dinosaur family tree — the “ur-dinosaur,” as Paul Sereno of the University of Chicago puts it. Ingloriously, that creature probably looked something like a chicken: two-legged, scurrying around in the corners, snapping up plants, insects, small animals and whatever else passed its way.

Other new findings come not from dinosaurs themselves, but from some of their closest relatives. Just as a long talk with an estranged cousin can help fill in family history, these discoveries are illuminating how dinosaurs evolved alongside other animals.

“We’re really in a renaissance in our understanding of early dinosaur history,” says Stephen Brusatte, a paleontologist at the American Museum of Natural History in New York City.

In the beginning

A famous extinction event may have wiped most dinosaurs out, but another extinction, just as spectacular, made way for their inauspicious rise.  It occurred 252 million years ago, at the boundary between the Permian and Triassic periods of geologic time.

Scientists aren’t sure what caused the Permo-Triassic extinction; there’s no impact crater to serve as a “smoking gun” like there is for the close of the dinosaur era. But Earth was roiled by huge environmental changes, from massive volcanic eruptions in Siberia to radical shifts in ocean chemistry. For whatever reason, 90 percent of marine species and 70 percent of land species went extinct.

That left the slate clean for dinosaurs to arise in the early part of the Triassic. “You had your status quo basically wiped out, and new groups had the opportunity to originate and flourish in this post­apocalyptic world,” Brusatte says.

Beyond dinosaurs, those new groups included the ancestors of creatures that would look familiar today, such as lizards, frogs and salamanders. These animals — in particular, the crocodile-like creatures known as crurotarsans — were far more abundant than the first dino pip-squeaks. “If you were standing in the Triassic, you would say these crocodile-like animals, not dinosaurs, would expand and be dominant” in the eons to come, says Brusatte.

Triassic Park was not exactly Jurassic Park. But dinosaurs were around, and new research hints at when they first appeared and what they looked like.

Fossil footprints from Poland’s Holy Cross Mountains provide the oldest clue. Last fall Brusatte and his colleagues, including University of Warsaw paleontologist Grzegorz Niedźwiedzki, reported finding three sets of tracks, the oldest dating back 250 million years. That’s right after the Permo-Triassic extinction.

Only several centimeters long, these tracks were made by a four-legged creature no bigger than a house cat. But certain characteristics of the footprints are distinctly dinosaur-like, Brusatte says.

Among other traits, the outer digits — the first and fifth toe — are smaller than the others, and the long bones of the foot bunch together more closely than in non-dinosaurs. The trackways are also narrow, as if they were made by an upright-walking creature instead of a sprawled-out crurotarsan, the team reported last year in the Proceedings of the Royal Society B. If confirmed, the tracks would be the oldest dinosaur evidence anywhere, and would suggest that dinosaurs evolved even earlier than scientists had thought, hard up against the end of the Permo-Triassic extinction.

Yet not everyone is convinced that the footprints are definitive evidence of dinosaurs. At the time the Polish trackways were formed, there were plenty of other dinosaur-like creatures roaming the landscape, points out Max Langer, a paleontologist at the University of São Paulo in Brazil.

Written in bone

Researchers usually say that the earliest dinosaurs lived 230 million years ago, because that’s the approximate age of the oldest known actual bones. These fossils come from the high desert of northwestern Argentina, near the border with Chile. There, a rock formation known as the Ischigualasto serves as a veritable treasure trove of the earliest dinosaurs, where for decades paleontologists have dug up and identified a boatload of new species.

Several of these earliest animals remain a puzzle. Scientists aren’t quite sure how to classify some of the most ancient species.

The dinosaur family tree includes three main branches: theropods, typically two-legged, meat-eating animals; sauropodomorphs, typically the four-legged, long-necked plant eaters; and ornithischians, the “bird-hipped” dinosaurs that included famous groups like the stegosaurs and the horned dinosaurs. Sauro­podomorphs and ornithischians went extinct 65 million years ago; a few theropods survived the meteorite impact and gave rise to modern birds.

Problems arise when researchers try to assign very primitive dinosaurs to one of these three branches. For instance, one of the Argentinean dinos, Herrera­saurus, was a two-legged meat eater about the size of a crocodile. Some researchers call it an early theropod, but others say it falls even deeper back in the family tree, somewhere just outside being either a theropod or a sauro­podomorph.

Not that such classifications were significant at the time; if you were prey in the early Triassic, says Langer, “it probably wouldn’t matter if you were being chased by a basal theropod or a basal sauro­podomorph.”

Another Argentinean creature, the smaller Eoraptor, has a similar identity crisis. Upon its discovery in the 1990s, it was labeled an early theropod. But in January in Science, it was reclassified as a sauropodomorph, in part because researchers hadn’t seen certain diagnostic features in its skeleton, such as a twisted thumb, until they re-examined it recently. Eoraptor may thus belie its fierce-sounding name and be something of a mellower, occasional plant muncher.

Paleontologists changed their minds about Eoraptor after they unearthed yet another close relative: Eodromaeus, a small, fleet theropod whose name means “dawn runner” (SN: 2/12/11, p. 10). Eoraptor and Eodromaeus may belong to different dinosaur branches, but they still look a lot like each other, says their discoverer, Ricardo Martinez of the Universidad Nacional de San Juan in Argentina. In turn, Eoraptor resembles yet another plant eater from Ischigualasto, reported on by Martinez and a colleague in 2009 in PLoS ONE.

Clues from cousins

Just as faded, black-and-white pictures of ancestors start to look the same, fossils of these early dinosaurs sometimes resemble one another to the point of indistinction. All of them begin to look like an unprepossessing chicken. Still, scientists say that knowing which creatures appeared when can fill in vast yawning blanks on the dinosaur ancestry chart.

Sometimes, information about a more distant relative can help flesh out the family tree. That’s why paleontologists are also looking at fossils of animals that weren’t true dinosaurs but were very closely related to them.

Technically, a dinosaur is defined as any member of the group whose lineage includes both the horned dinosaur Triceratops and the modern house sparrow (because of those theropods whose descendants fly around today as birds). Today’s crocodiles and lizards are not descended from the common ancestor of Triceratops and the sparrow, and hence are not dinosaurs.

Many other creatures in the Triassic were also reptiles but not dinosaurs. Among these are a group known as silesaurids: mostly four-legged, mostly plant-eating creatures. For paleontologists, silesaurids are turning out to be nearly as exciting as dinosaurs, because silesaurid bones help illuminate crucial differences between dinosaurs and their relatives deep in the reptilian past.

Last year, scientists reported finding the oldest known silesaurid, a creature that lived 244 million years ago in what is now Tanzania. Less than 10 million years after the Permo-Triassic extinction, this animal had already evolved characteristics of later silesaurids, such as a beak-shaped lower jaw and teeth shaped like leaves. That evolution shows that the silesaurid and dinosaurs have not been closely related for a long, long time, says team leader Sterling Nesbitt, now at the University of Washington in Seattle.

The Tanzanian creature is also the oldest dinosaur relative ever found. During the Triassic, the Earth’s continents were united as Pangaea, with one southern landmass known as Gondwana and one northern one known as Laurasia. Africa was part of Gondwana, which is also where most of the Triassic dinosaur fossils have been found. So dinosaurs and their close relatives may have originated in Gondwana before spreading to other parts of the world, says Irmis.

That concept was bolstered in 2009, when Nesbitt, Irmis and colleagues described a theropod that lived 213 million years ago in what is now New Mexico (SN Online: 12/11/09). Despite coming more than 15 million years after the “dawn” dinosaurs of Argentina, this creature — called Tawa after the Hopi name for the Pueblo sun god — still has a lot to tell paleontologists.

That’s because the rocks that contain Tawa look quite a bit like those from Argentina 230 million years ago. The New Mexican fossil might represent a creature that evolved in an environment and landscape similar to the home of the earliest dinosaurs known, or it might be the descendant of a long-distance migration. “Although Tawa was found in the Northern Hemisphere, it probably was a lineage that originated in the Southern Hemisphere,” Irmis says.

Tawa’s skeleton also shares many features with Herrerasaurus and other primitive dinosaurs, and so it helps paleontologists classify those earlier creatures as true theropods. Tawa mixes earlier dinosaur characteristics with later ones, allowing researchers to see when certain features, such as air sacs within the spinal column, evolved.

Another dinosaur unearthed recently in New Mexico is similarly helping paleontologists understand the relationships among early theropods. At 205 million years old, this animal, called Daemonosaurus, is younger than Tawa yet still retains certain characteristics from its ancestors.

Known from a single buck-toothed skull, Daemonosaurus has relatively large bones at the tips of its jaws, as Herrerasaurus did, mixed with the vertebral air sacs that generally characterize later dinosaurs. Once again, paleontologists can use such findings to untangle which features appeared when in theropod history. A team led by Hans-Dieter Sues of the Smithsonian National Museum of Natural History in Washington, D.C., described Daemonosaurus online in April in the Proceedings of the Royal Society B.

Dino dominance

For all the recent insight into the dawn of the dinosaurs, one question still looms: How did such specimens come to dominate the planet? Some­how these runty chickenlike creatures managed to give rise to the formidable T. rex, Triceratops, Stegosaurus and more.

One clue may lie in yet another mass extinction. Around 200 million years ago, at the end of the Triassic period, something traumatic happened on Earth. As Pangaea began splitting apart, great quantities of magma poured out along the planetary seam that would become the middle of the Atlantic Ocean. Atmospheric carbon dioxide levels spiked, and the chemistry of the oceans shifted dramatically.

Life suffered. Crurotarsans and other non-dino reptiles were some of the worst-hit creatures, losing large fractions of the species living at the time. Their disappearance opened up ecological niches — and may have been the lucky break dinosaurs needed to move into new roles, many scientists think. (Similarly, mammals got the chance to rise to prominence after most of the dinosaurs were wiped out.)

New findings may yet revise this part of the story. Martinez and his colleagues are working on a detailed analysis of plants and animals throughout the Ischigualasto rocks and are finding that dinosaur fossils might not be quite as rare there as once thought. And dinosaurs didn’t automatically and rapidly move into open ecological niches, Martinez says; sometimes, after the competing groups disappeared, dinosaurs took several million years to expand and diversify into those roles.

Other times, dinosaurs simply did better in some parts of the world than others. In North America, for instance, sauropodomorphs and ornithischians didn’t even show up until after the Triassic was over, Irmis reports in an upcoming issue of Earth and Environmental Science Transactions of the Royal Society of Edinburgh.

Researchers are now pushing to see how many more early dinosaurs they can find, and where. In Argentina, Martinez is working on another new find, details of which he won’t reveal. And Niedźwiedzki and others continue working in Poland, looking for more footprints and perhaps actual fossil bones of Triassic dinos.

No matter what, expect new findings in the years to come. “If the fossils can wait for us for 230 million years,” says Martinez, “we can wait several years more.”


Fitting in
Bones belonging to the earliest dinosaurs can help researchers build the dino family tree. Today the tree is divided into three main branches (note that the pterosaurs and crurotarsans are archosaurs but not dinosaurs).

Click here to view larger image.

Ornithischians
After ornithischian tooth remains were reidentified as coming from crocodile-like creatures and not dinosaurs, the evidence for this group in the late Triassic was whittled down to just three body fossils from a small area of southern Gondwana. These creatures, known as the "bird-hipped" dinosaurs because of their hip structure, were probably small, just over a meter long — at least at first. Size increased during the early Jurassic, and over time horned dinos like Triceratops and Einiosaurus (shown) arose.

Theropods
Theropods — the two-legged, meat-eating dinosaurs made famous by Tyrannosaurus rex — probably date to around 230 million years ago. Though the name means "beast foot," most theropods alive during the Triassic were small and slender, around 2 meters in body length. Mightier creatures are known from the Jurassic, during which theropods became more common and varied (Cryolophosaurus shown). Other dinos went extinct around 65 million years ago, but some small, nonflying theropods gave rise to modern birds.

Sauropodomorphs
Sauropodomorphs came in two forms — the big bipedal ominvores and the bigger, four-legged, barrel-chested, long-necked herbivores (Brachiosaurus shown). Evidence for these creatures dates to about 230 million years ago in South America, and by about 200 million years ago they could be found across South America, Europe, Africa and Asia. Sauropodomorphs have the best fossil record among the dinosaur branches in terms of diversity and abundance, but relationships within this branch remain controversial.

Credits: Tree: M.C. Langer et al/Biological Reviews 2010, adapted by T. Dubé; Dinosaur illustrations: Julius T. Csotonyi

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