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Diggin' For Dinosaurs - Purgatoire River Trackway
At the bottom of a canyon in southern Colorado, where the Purgatoire River cuts through shale and limestone deposits that were laid down some 150 million years ago, visitors can see one of the most remarkable dinosaur sites in the world. With more than 1,300 footprints embedded in the ground, representing the travels of more than 100 animals, the Purgatoire site is the largest mapped dinosaur footprint area in North America.
Although local residents knew of the trackway since at least the 1920s - some called it "Elephant Crossing" - not until 1935 was the site brought to the attention of scientists. A schoolgirl named Betty Jo Riddenoure told her science teacher about the tracks, and this prompted the first scientific expedition to the site. In 1938, palaeontologists decreed that the tracks had been made by brontosaurs, making them the first such tracks ever reported. But perhaps because of their remoteness, the Purgatoire tracks were largely forgotten for nearly half a century, until researchers at the University of Colorado studied and mapped them in the early 1980s.
Most of the tracks now skirt the edge of the Purgatoire River. But the presence of fossilized plants, mollusks, fish, and crustaceans suggests that in the Jurassic, the area was a lake basin perhaps six miles wide. Dinosaur Lake, as palaeontologists call it, sported a semiarid climate similar to the savannas of East Africa today, and, judging by the alternating deposits of shale and limestone, the lake experienced seasonal or longer term fluctuations in water level. Scientists say the shale layers were laid down as mud in times of high water, while the limestone layers were deposited as coarser sediments along the lakeshore when water levels were low.
Because their tracks all appear in the limestone, the dinosaurs apparently liked to wander the edge of the lake and perhaps wade in shallow water. Today their tracks appear as clearly defined cavities in four different layers of hard limestone, stretching about a quarter mile along the river. About 60 percent of the tracks are believed to have been left by Apatosaurus visiting the lake. These plant eating animals stood 14 feet or taller at the shoulder and weighed 33 tons or more. Their roundish footprints, some two to three feet long, look like a series of highway potholes; they undoubtedly gave rise to the "Elephant Crossing" nickname.
The rest of the footprints are smaller and clearly belong to a different animal: Each is an oblong base topped with three rather sinister looking claw marks. Those tracks, say the scientists, were left by smaller theropods, meat eating dinosaurs like Allosaurus. The size of most two legged dinosaurs can be readily estimated using such tracks. Hip height is generally about four to five times the track length, so the theropods that roamed about Dinosaur Lake stood from two to eight feet tall at the hip. Those animals, although they weighed much less than the sauropods, were quite fast and ferocious and probably preyed on the huge plant eaters.