[1] The fossil record can give an appearance of faunal turnover due to multiple causes including evolution, migration, or changing preservational biases.
This group of species is replaced higher in the formation by a different ornithischian fauna characterized by the presence of Lambeosaurus and Styracosaurus.
The appearance of several new, rare species of ornithischian at the very top of the formation may indicate that a third distinct fauna had replaced the second during the transition into younger, non-Dinosaur Park sediments, at the same time an inland sea transgresses onto land, but there are fewer remains here.
[1] Later research came to find that the supposedly distinct dinosaur faunas at different levels of the formations were more similar than had been previously thought.
[5] While the dinosaur fauna of the lower and middle sections Two Medicine was apparently diverse, the quality of preservation was low and few of these remains can be referred to individual species.