It is the dinosaur version of grave robbing: fossil poachers plundering a paleontological dig, frequently smashing ancient skulls and stealing valuable teeth, claws and feet.

Often, all that remain are shards of fossilized bone and a wrecked, irreplaceable scientific record. And in cases where poachers excavate an entire skeleton and spirit it away to illicit entrepreneurs or collectors, it is as if the bones, buried for millions of years, were being dug up only to be hidden away again in private collections.

“This is huge,” said Catherine A. Forster, a paleontologist at George Washington University who is president of the Society of Vertebrate Paleontology. “It isn’t just one or two specimens. A fair proportion of very good fossils just disappear from knowledge, and few are ever seen again.”

And while some scientists hoped that a high-profile legal case in New York last year over the $1 million sale of a rare Mongolian dinosaur would curb the illegal digging, that does not appear to have happened. Mark A. Norell, chairman of paleontology at the American Museum of Natural History in New York, said a visit to the Gobi Desert over the summer made clear that poaching continues “in a big way.”