Diggin' For Dinosaurs - Clayton Lake & Petrified Forest

So far, scientists have catalogued tracks from at least four different kinds of dinosaurs. Most of the prints were left by large, two-legged plant eaters, perhaps hadrosaurs or iguanodonts. Others may have been made by the ornithopod Tenontosaurus, whose fossils have been found in Texas, Montana, and Utah. The largest of these tracks would fill a small kitchen sink. Adults and juveniles alike left prints, suggesting that entire dinosaur families milled across this area. About a quarter of the prints, with pointed heels and clawed toes, were made by carnivorous theropods.

The tracks that have gotten the most attention recently lie off the public walkway. These are six small prints accompanied by several shallow, linear depressions. Scientists once interpreted these as the prints of a pterosaur, or flying reptile, in the process of taking off.

But new studies suggest that the tracks actually were left behind by a crocodile-like creature, perhaps as it touched the shallow bottom. The size, shape, and spacing of the prints support this theory, which explains the linear depressions as marks of a crocodilian tail dragging across the mud.

By exploring Clayton Lake, palaeontologists have deepened their understanding of where and when different kinds of dinosaurs lived. For example, scientists have found that rocks just a bit older than the 100-million-year-old ones here are dominated by the tracks of sauropods, the giant plant-eating dinosaurs. Sauropods make up most of the trackways of that age in Texas, Oklahoma, and Arkansas. But at Clayton Lake and other slightly younger sites, there are no sauropod tracks, only those of other plant-eating dinosaurs. This observation suggests that sauropods temporarily went extinct in North America - only to return around 30 million years later, perhaps by migrating from South America.

Petrified Forest National Park, Arizona

Navajos call them "male rains," the brief but violent downpours that pelt the bruise-colored hills of the Painted Desert in northeastern Arizona, sending torrents into easily eroded arroyos and washes. The sand, clay, and volcanic ash that form these colorful badlands were laid down more than 220 million years ago, and their erosion reveals one of the world's greatest treasure troves of fossils from the late Triassic period, when dinosaurs and early mammals were just beginning to roam the planet.

Most famously, the region contains one of the largest concentrations of petrified wood anywhere. Along several short trails in Petrified Forest National Park, visitors can amble among stone logs that record the forms of trees that once grew here. At the south end of the park, the Giant Logs Trail behind the Rainbow Forest Museum, and Long Logs Trail about half a mile north, both allow a close-up look at petrified logs. Along the Giant Logs Trail, some multicolored trunks are as thick as a person is tall, while a few of the trees seen from the Long Logs Trail are more than 150 feet tall. Stripped of bark and branches by an ancient flood, they were quickly buried and resisted decay. Silicon seeping in from the ash slowly and precisely replaced their woody tissues; trace quantities of iron, manganese, and other minerals lent them brilliant shades of red, yellow, and purple.

Continued in: Diggin' For Dinosaurs - Petrified Forest National Park

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