Pre-Historic Nookie!

[Anomalocaris]

Actually, when examined over the time span when speculations occur, interbreeding between different but closely related species that resulted in some fertile offsprings is probably not rare at all.  In fact when speciation occurs in place, there is probably an extended period when inter-species breeding occurs, gradually tailingin both in frequency and in percentage resulting in fertile offsprings, as selection pressure continues to drive the species further apart genetically, until such time as their interbreeding becomes so unlikely to be fruitful in terms of permepruating their genes and therefore so likely to be a waste of energy detrimental to genetic survival, that it pays evolutionarily when behavioral or biochemical barriers against further courtship evolve.