UKHAA TOLGOD, Mongolia - On the first afternoon here, fossil-hunters struck out across the parched sand to the rock outcrops along the bleached brown ridges and down into the broad basin. They walked their separate courses at paces as if set to geologic time.
With every step, their figures diminished into the expanse of empty silences and far horizons that is the Gobi Desert, where only camels, nomads, and hardy paleontologists seem at home.
It has been the paleontologists' boast, never disputed, that this particular forbidding stretch of the Gobi holds the world's richest and most diverse deposits of dinosaur and early mammal remains from 80 million years ago, a critical time for life in the Cretaceous geologic period.
Nearly everyone this afternoon had something new to talk about. The haul included two nests of dinosaur eggs, the remains of several ancient lizards, and the skull and skeleton of a small mammal, most likely a previously unknown species that lived in the shadow of the formidable dinosaurs.
"Sometimes you go days, whole expeditions, without finding something," said Gina D. Wesley-Hunt, a paleontologist at the Smithsonian Institution. "We found 10 skulls just today, early mammals and lizards."
Happy and relieved by the finds, Michael J. Novacek, a paleontologist at the American Museum of Natural History who has directed the expeditions since their start in 1990, declared, "This place is still rich."
For every month in the field, paleontologists know from experience, the study of fossils requires at least another 11 months of tedious indoor work back at their universities and museums. But one of the immediately evident changes in Gobi research is the increasing number and variety of small mammals turning up in excavations.
This development is expected to bring new insights into the formative epoch of mammalian life in the Cretaceous Period, which lasted from 140 million years ago until the extinction of nonavian dinosaurs 65 million years ago.
In the fossil beds at Ukhaa Tolgod, laid down in the late Cretaceous and entombing not only dinosaurs but a broad sampling of its life, paleontologists estimate that over recent years they have found 1,000 mammal skulls. Mr. Novacek said this amounts to 90 percent of all the recovered mammal specimens from the Cretaceous.