Fossils from a couple of two-legged, meat-eating dinosaurs have also been found, one of which was a six-foot-long (1.8-meter-long) dromaeosaur, a fast, fearsomely clawed carnivore.
Other finds include a small, stocky Rhabdodon with large blunt teeth for grinding up vegetation, and an ankylosaur, a heavily built, squat plant-eater with a big bony club on the end of its tail for whacking predators.
Prehistoric turtles and crocodiles account for the bulk of the other fossils recovered from Lo Hueco, the dig team reports.
The fossil creatures were found grouped together in clay and silt sediments, suggesting a river created the dinosaur graveyard.
"Flooding maybe was responsible for the accumulation of carcasses," Sanz said.
The site may provide further clues to understanding the demise of the dinosaurs some 66 million years ago, he added.
Many experts believe their sudden extinction during the so-called K-T event (or Cretaceous-Tertiary event) was foreshadowed by a steady decline in dinosaur diversity, until a massive meteorite strike delivered a final death blow.
(Read related story: 'Dinosaur Killer' Asteroid Only One Part of New Quadruple-Whammy Theory" [October 30, 2006].)
The newfound fossil cache appears to contradict that theory, Sanz said.
"This site seems to indicate, as some [other] scientists have suggested, that dinosaurs were at their maximum level of diversity during the K-T biotic crisis," Sanz said.
He pointed out, however, that the fossils don't represent the very last of Europe's dinos, since the remains date to some four million years before the extinction event.
(See an interactive feature on what killed the dinosaurs.)
Darren Naish, a paleontologist based at the U.K's University of Portsmouth, said the new discoveries support recent evidence that Europe, toward the end of the dinos' reign on Earth, was much richer in dinosaurs than previously thought.
"Having so many dinosaurs together at the same site is a big deal," Naish said of the site.
"This group of dinosaurs living in the same place in the same environment hadn't been established before," he added.
And while the dinosaurs so far identified at Lo Hueco are reasonably well known, "they are all animals for which we could do with more complete specimens," he said.
The titanosaur skull remains are especially interesting, Naish commented.
"The skulls of sauropod dinosaurs are comparatively rare, probably because they're quite fragile," he said.
"There has been a long controversy as to what the heads of titanosaurs looked like. We don't have much information on the European ones in particular."
The finds may also reveal more clues to titanosaur body armor, Naish added.
"We're still quite unsure as to how the armor of titanosaurs was distributed across the animal," he said. "For instance, was it scattered over the back or was it aligned in rows?"
Fossils in the direct path of the high-speed rail route have now been removed, Sanz, leader of the dig, said.
Excavation of the rest of the site is due to continue next spring.
Many smaller fossils still await examination in the lab, Sanz added, including plant remains, fish scales, freshwater clams, and individual teeth.
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