Trilobites and Their Ancestors

Trilobites are a problem for the theory of evolution because they appear suddenly in the fossil record without any trace of ancestry. In fact, hundreds of thousands of trilobites have been found, however, there has yet to be found any animal that can be declared an ancestor of the trilobite kind.
" What bothered Darwin about the fossil record more than anything else was the pattern of paleontology that we've been talking about...the oldest fossils you see are both diverse and complex, [such as] fabulously complicated things like trilobites."

Dr. Andrew Knoll, Paleontologist and Professor of Biology, Harvard University

Is this a problem with the fossil record, or is this evidence of a problem with the theory of evolution?