Some of you know exactly what I'm asking: Are living species that resemble ancient fossil species ("living fossils" like the coelacanth and ginkgo) the same as fossil types that disappar in the rocks for millions of years and then return (Lazarus taxa like the calcareous sponges, which vanished from the fossil record for a while after the Permian extinction)?
The "living fossil" is a popular idea while the Lazarus taxon is a scientific one. People have the popular idea that living fossils haven't changed at all over the ages. Scientists have never quite believed that, yet never quite rejected it, because they had no way of knowing for sure. However, now we've become pretty sure: genetic studies of living species and closer attention to the fossils both demonstrate that even the most conservative-looking, unchanging organisms evolve at the same speed as everything else. For more details see the story "The Falsity of Living Fossils," reported by Ed Yong for The Scientist this week.
This new understanding slightly clarifies how we think about Lazarus taxa. Note that the word "taxa" means groups of organisms at any level of taxonomy, not just the species level. Most often a Lazarus taxon is a genus (a group of related species) or a family (a group of related genera). That is to say, paleontologists never assumed that the fossils across the gap in time were the same speciesagain, because they had no way to be sure. Today, though, we can be pretty sure that a Lazarus taxon, even a Lazarus species, always represents separate species at two different times. That suggests to me that fossils millions of years apart, even if they look just the same, should be given a closer look and named different species, or at least be considered separate populations. Perhaps the paleontologists should make some adjustments.