The Lost World
The dying dinosaurs of Castle Valley were gathered up in the embrace of a lazy river, laid to rest under a blanket of blue silt and--98 million years later--unearthed by paleontologists from what time had transformed into the arid badlands of central Utah.
The fossilized remains offer scientists the first detailed glimpse into a mysterious epoch of primordial North America--an 80-million-year gap in the fossil record that spans the age of dinosaurs from the end of the Jurassic Period, 145 million years ago, to the end of the Cretaceous Period, 65 million years ago, when the animals abruptly became extinct.
It was a time, scientists believe, when flowers first conquered the world.
As new species of blooming angiosperms spread across the globe, edging out other, more primitive species, they transformed more than the plant kingdom. They also shaped the evolution of the creatures that depend on them, forcing the demise of some species and the rise of others as the food supply changed dramatically.
Until this most recent set of fossil finds, however, scientists had little more than speculation to guide them through this period.
“This gap has been like a twilight zone in the age of dinosaurs,” said Richard Cifelli, the University of Oklahoma expert who led the excavation. “We are fairly confident this is the time when flowering plants took over, and that appears to have had a major effect on terrestrial animals.”
Since 1990, Cifelli and his colleagues have exhumed the fossilized bones of more than 6,000 specimens encompassing 72 species of dinosaurs, reptiles, fish and primitive mammals from the sedimentary beds of the Cedar Mountain Formation east of Castle Dale, in Emery County, Utah. Their work was supported by the National Science Foundation and the National Geographic Society.
Most of the fossils are species new to science, according to research published recently in the Proceedings of the National Academy of Science. Only 26 of the 72 have been previously recorded.
Among the discoveries are the oldest known tyrannosaurus in North America, the world’s first duck-billed dinosaurs, and the last of the long-necked, treetop-grazing sauropods. The researchers also found America’s first known snake, the first known specimen of a Gila monster, and the world’s first marsupial, as well as the first examples in North America of horned dinosaurs.
The Berkeley Geochronology Center reliably dated the fossils by determining the age of nearby volcanic ash deposits with radioactive argon.
The finds together offer an unusually comprehensive look at the world as it existed about 98 million years ago. There is a wide variety of different specimens of the same period--from tiny seed-gnawing, rodent-like mammals and the ancestors of modern loons to freshwater sharks, frogs and salamanders.
For researchers who usually must reconstruct an entire species based on a single tooth or a thigh bone, this makes the discoveries especially useful in trying to understand the forces shaping the evolution of life.
“They are all from the same [layer in the rock] and they are all the same age as far as we know. While we have not filled in all of this huge gap in the fossil record, we now have one important benchmark,” said Cifelli, who is curator of vertebrate paleontology at the Oklahoma Museum of Natural History.
The collection, with its detailed picture of the ecology of the middle of the Cretaceous Period, not only offers new insight into the relationship between plants and animals, but also helps buttress a long-standing theory about the origin of many dinosaurs in North America.
So many of the new fossils belong to creatures that had close relatives or direct ancestors in Mongolia, China, Europe and Africa that scientists are convinced that a land bridge linked North America and the Asian mainland during the middle Cretaceous. But at least one species, they suspect, may have drifted north from South America.
The appearance of such a range of new species in the fossil record of the period also coincides with the disappearance of many earlier dominant animals, such as the titanic long-necked sauropods like the brontosaurus that, each weighing as much as a herd of modern elephants, were among the largest animals to ever walk the Earth.
The huge herbivores were replaced by relatively small dinosaurs. Indeed the one sauropod species that appears to have survived into the mid-Cretaceous Period is barely the size of a pony, the fossils show.
“What happened to the stuff that was here before? What about all those sauropods? What caused the turnover and why?” Cifelli said.
The prevalence of fossil flowering plants and pollen in the deposits may be a clue.
The researchers concluded that with the advent and expansion of flowering plants there was “a profound reorganization of North America’s terrestrial ecosystem.”
Researchers believe the towering sauropods were so effective at gnawing down trees that they cleared the way for faster-growing, flowering ground cover. Those plants in turn fostered the development of new grazing species such as the duck-bills and the horned dinosaurs, which thrived on fodder close to the ground.
As the trees died off, so did the long-necked dinosaurs that relied on them for sustenance.
