What's the largest clade we could realistically be missing from the fossil record?

Largest could have many definitions, including species diversity, geographic range, age, and a nebulous "weirdness" factor (how different it is from anything else in the fossil record).

I guess I'm asking if there is a way to calculate the probability of missing a smaller clade like "turkey vultures" from the fossil record, vs missing something huge like "rodents"?

How would the answer differ between different definitions of a "big clade"? How about between vertebrates and invertebrates?