Complex sentence: Its fossilized remains were first discovered in the southern United States (Louisiana), and were initially believed to be some sort of reptile, hence the suffix- "saurus", but later found to be a marine mammal.
Split into simpler sentences: Its fossilized remains were first discovered in the southern United States (Louisiana). The American fossils were initially believed to be some sort of reptile, hence the suffix- "saurus", but later found to be a marine mammal.