Via Science:

While examining fossilized shark feces (main picture) collected from southern Brazil, researchers noticed a strange cluster of oval-shaped objects. Taking a closer look, they realized they had found a tapeworm egg case bearing an uncanny resemblance to those produced by modern pests today. The eggs featured operculums (indicated by the blue arrows in inset), or small teapot lid-like flaps characteristic of tapeworm eggs, which helped the researchers identify the finding. Such discoveries, they write in PLOS ONE, are exceptionally rare; the older the fossil, the less chance of finding signs of the tiny, fragile parasites that may have once colonized it. 

Also… informative interview with Dr. George Poinar, a Paleontologist from the Department of Zoology at Oregon State University, by CBC’s Quirks and Quarks.