It is true that the presence of Triceratops and
Edmontosaurus suggests a paleoenvironment which
excluded lambeosaurs. But the Scollard seems to be the
kind of relatively dry inland environment which
favored Hypacrosaurus and Saurolophus in the late
Horseshoe Canyon period. Based on environmental and
faunal differences, Lehman considered the Scollard
part of a Leptoceratops biozone distinct from the
Triceratops zone of MT etc. I suspect that by Scollard
times Edmontosaurus and other taxa moved into more
upland habitats left vacant by the waning of
lambeosaurs and others.