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The peculiar dinosaurs of Laramidia: weird horns and more

The discovery of two new dinosaur species in the American West – which, 76 million years ago, was part of a continent called Laramidia – has scientists thinking about odd horns and why certain species didn't appear to spread out across all Laramidia.

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The researchers say the ornate bone structures the two animals displayed likely evolved to attract females and ward off competitors for their attention, rather than for defense.

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"Most of these features would have made lousy weapons to fend off predators," Dr. Sampson says.

Why the species stayed put

If dinosaurs on Laramidia were provincial rubes compared with later large animals, the difference might have something to do with food availability, Sampson and his colleagues suggest.

With high temperatures, relatively high carbon-dioxide levels in the atmosphere, and perhaps a more oxygen-rich atmosphere as well, plants would have thrived in ways that would make a backyard gardener swoon. If plant abundance could support more kilograms of critter on a given patch of land, large numbers of large animals wouldn't need to roam an entire continent to forage for cheap eats.

A variation on that theme suggests that dinosaur metabolism was low enough that they didn't need much food, and so it was easier for them to live off the land they occupied and not wander far afield.

North-south differences in species also could stem from dinosaurs' suspected high fertility compared with comparably large mammals such as rhinos or elephants, Dr. Holtz suggests. While a single pair of large placental mammals might produce one or two offspring every one or two years, a mating pair of dinosaurs could produce eggs by the dozen or two on shorter time scales.

More offspring means fewer parental pairs needed to maintain a viable population.

Whatever the mix of explanations, they don't explain the reason the north-south differences appeared in the first place, Sampson adds. There is no obvious physical barrier that would have separated the populations. And the climate is thought to have been similar enough south to north to rule out dramatic changes in plant types that might present a preferred diet to each broad assemblage of large animals.

The clear species divide in the fossil record "just doesn't make sense at some level," he says.

The lure of Laramidia

Indeed, Holzt says, the real significance of the new finds and the lost continent they come from is that they represent a rapidly growing assemblage of remains that is allowing researchers to delve ever more deeply into evolutionary and environmental changes dinosaurs experienced during their heyday.

As Laramidia evolved, the region hosting the new fossils developed the right combination of ingredients for fossil studies there: volcanic activity that can supply the radioactive elements for precise dating of fossil beds; mountain-building and erosion to supply the sediment to entomb the animals and preserve their fossils; and relatively little rain to destroy them. All in a location relatively easy for paleontologists to reach.

That combination of things make the Grand Staircase-Escalante National Monument "a really good spot for finding a succession of dinosaur faunas," Holtz says.