Complex sentence: Amphicyonids began to decline in the late Miocene, and largely disappeared in the Pliocene; the reasons for this are unclear, with the most blamed factor being competition with other carnivorans, though no direct evidence for this has been found.
Split into simpler sentences: Amphicyonids began to decline in the late Miocene, and largely disappeared in the Pliocene. The reasons for this are unclear: possibly it was due to competition with other carnivorans, but no direct evidence for this has been found.