If the fossil remains of any longstanding, stable hybrid zone were examined, together with the fossils of the parent populations crossing to produce it, neither would change much with time. The fossils of the parents would stay the same, right up through the strata (recall the example in Section 3 of two types of hybridizing flickers that have remained distinct since Egyptian times). The fossils of the hybrids would vary from one individual to another within any particular stratum sampled, but they would typically show no progressive change over time in successive strata (as would be the case with the hybrid flickers). The non-progressive, spatially localized variation typical of a hybrid zone does not imply the existence of change in the participating parental forms in the fossil record. In fact, it implies stability of those forms over time.
Moreover, there is good reason to suppose hybrids will be rare in fossil samples even in cases where hybridization has been longstanding and intense. For suppose specimens were randomly sampled from any single stratum, corresponding to some particular time in the past, over all geographic regions where the two parents and their hybrids at that time existed. In most cases, since parental ranges are typically much larger than hybrid zones, the samples would come from areas occupied by parental individuals. Hybrid fossils would be common only within the relatively small region corresponding to the hybrid zone. But recall from Section 2 that hybrid zones move from one place to another, even on the short timescale of human observation. This means a hybrid zone would be expected to move extremely extensively over geological time so that the remains of hybrids would be spread widely hither and thither across the landscape. So hybrid remains would be spread thinly over regions occupied the great majority of the time by parental individuals. Therefore among fossil remains there would be a large percentage of pure individuals and only a smattering of hybrids. To a paleontologist looking at fossils, the parental types would seem morphologically discrete despite the former occurrence of ongoing hybridization.
Human Origins: Are we hybrids?
On the Origins of New Forms of Life
Cat-rabbit Hybrids: Fact or fiction?
Georges Cuvier: A Biography
Prothero: A Rebuttal
Branches of Biology