Final answer:
Lewis and Clark were prepared to encounter various kinds of unknown wildlife, including potentially prehistoric animals, during their expedition. They discovered a myriad of wildlife and documented the customs of the native tribes they encountered, but they did not actually find prehistoric creatures like mammoths or mastodons.
Step-by-step explanation:
On their expedition, Lewis and Clark were curious and prepared for the possibility of encountering prehistoric megafauna, as their conception of the West was filled with mysteries and the unknown. The journey through the newly acquired territory in the United States brought them into contact with various wildlife, although they did not encounter the large prehistoric animals like mammoths or mastodons that had gone extinct thousands of years prior. Instead, they encountered and documented numerous plants, animals, and the customs of indigenous tribes.
Historical records from the Lewis and Clark expedition do not provide clear evidence that they specifically expected to find any one type of prehistoric animal, but given the era's limited knowledge and wild speculation about the unexplored West, it is likely that they were open to discovering a wide array of new species, possibly even unknown or prehistoric creatures. They did have knowledge of large extinct species such as mammoths and mastodons from discoveries in other parts of the country, and they may have wondered whether similar creatures still roamed in the uncharted territories.