Cool Evolution Find: Fossil Virus

Not in stone of course, but in crocodilians' genomes. ERV covers well. Question for evolution, as she puts it: if this virus was good enough at what it did to work its way into a genome, why does it apparently have no descendants today? Two possibilities come to mind: 1) its descendants are still around, but we haven't discovered it yet (remember Nannarrup hoffmani? A whole new genus of metazoans found 7 years ago in Central Park, New York?). 2) the chance of becoming an ERV do not correlate over time with fitness.