Protoceratops andrewsi , Andrews s ancestral horn-face. Ptotoceratops and its close relatives must have swarmed over Asia, because their bones and nests of eggs are the commonest dinosaur fossils found in the widespread dune beds. But not one Protoceratops has ever been reported from the rich beds of the American Judith and Laramie Deltas. The horned-dinosaur story shows how paleontologists can trace the major evolutionary events. Rarely do fossils yield a complete evolutionary sequence from mother to daughter to granddaughter species.
Evolution is too bushy to permit such a straightforward story, too full of side branches.
As clans evolved, the ever-branching species spread over the continents. Since fossils come from a few small areas, it is impossible to follow every stage of an evolutionary line. But it s possible to make out an overall progression of uncles and nieces, even when parent-daughter sequences cannot be found. In other words, the fossil evidence suggested that the horned dinosaurs were created fully-formed. But rather than draw the obvious conclusion, they considered it an evolutionary puzzle.